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Lateral design with mass timber: examination of structural wood in high-rise timber construction

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Lateral design with mass timber: examination of structural wood in high-rise timber construction

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Content LATERAL DESIGN WITH MASS TIMBER: Examination of Structural Wood in High-Rise Timber Construction By Isik Goren Presented to the FACULTY OF THE SCHOOL OF ARCHITECTURE UNIVERSITY OF SOUTHERN CALIFORNIA In partial fulfillment of the Requirements of degree MASTER OF BUILDING SCIENCE May 2019 ACKNOWLEDGMENTS First, I want to express my gratitude for the Building Science department at the School of Architecture of the University of Southern California. This would not have been possible if it wasn’t for the Fulbright Program. I will be forever thankful not only for the scholarship and the support, but also being introduced to a brilliant group of scholars and lifelong friendships. I want to express my thankfulness for my committee. Prof. Goetz Schierle, with his endless amount of knowledge and answering every question of mine with patience and with his love of teaching. Prof. Karen Kensek, her vast amount of effort, energy and discipline that guide me through all the way from the beginning, and her excellence in teaching that turns confused students into graduate scholars. Prof. Kyle Konis, always showing support and his invaluable comments that contributed to my thesis. Prof. Santosh Shahi, always giving his great support and time, and directing me to the right path by showing me how to ask the right questions. I want to thank Ms. Zarmine Nigohos from Skidmore, Owings & Merrill for her kind interest and support throughout the process. I want to thank my classmates and friends, whom I have learnt a lot from, whom made my years at USC special and helping me call Los Angeles home. I want to thank my family, and my mom and dad. From thousand miles away, I always felt their love, and their support throughout the whole year, every day and night. Last, I want to thank my brother, by being my best friend and joy of life. COMMITTEE MEMBERS Chair: G. Goetz Schierle, PhD, FAIA Title: Professor Affiliation: University of Southern California, School of Architecture Email: schierle@usc.edu Second Committee Member: Karen Kensek, LEED BD+C Title: Professor of Practice Affiliation: University of Southern California, School of Architecture Email: kensek@usc.edu Third Committee Member: Kyle Konis, Ph.D., AIA Title: Assistant Professor Affiliation: University of Southern California, School of Architecture Email: kkonis@usc.edu Fourth Committee Member or Advisor: Santosh Shahi, Ph.D. Title: Lecturer Affiliation: University of Southern California, School of Architecture Email: santoshbshahi@yahoo.com ABSTRACT As cities are getting denser and larger, tall buildings are becoming prototype of large and expanding cities around the world. Among the conventional building materials used in the last century, timber is increasingly becoming more prominent with its engineered production and smaller ecological footprint. Although mass timber in high rise construction is getting more common, there still is not enough information on how an all-timber high rise will be affected by the fire, wind, or seismic forces. In addition, many designers are resorting to the use of reinforced concrete or steel lateral load resisting systems in mass timber buildings instead of using wood exclusively. Lateral system design alternatives with timber for high rise construction were developed, focusing on timber braced frame building systems. A timber braced frame exoskeleton was analyzed in different locations in the world that are dominated by lateral loads. Skidmore, Owings & Merrill’s Dewitt-Chestnut Apartments in Chicago, Dewitt Chestnut Apartments, was taken as the benchmark building. SOM’s ongoing research: “Timber Tower Research Project” had already applied the concept of converting this building to timber, which then was used to develop a new timber structural. Based on the research by SOM, further investigation on the building was made in order to design the lateral system from wood, with the assumption that the building was being relocated to Los Angeles as a region dominated by seismic forces, focusing initially on downtown Los Angeles, CA. The results showed how an efficient lateral design topology can be achieved in terms of strength, carbon footprint, cost, and material quantity. Further application of the load resisting frames and their variation in typology can be studied in different locations that are subjected to different lateral forces. The workflow aimed to be used as a guideline to design and improve mass timber lateral systems, which can serve as a more sustainable alternative to steel and concrete. KEYWORDS: Tall timber structures, mass timber building systems, lateral design, braced frames, multi-objectivity HYPOTHESIS A developed design methodology for mass timber braced frames can produce improved results for seismic resilience for a lateral system design for mass timber and create an efficient and effective alternative for the high rise building market. RESEARCH OBJECTIVES • Explore how to use structural wood and mass timber with specific requirements for lateral design • Evaluate and determine design topologies that are both reseilient against earthquakes and effective in terms of architectural and practical constraints. • Assess the performance of lateral design topologies in high rise wood construction • Develop a tool/workflow for minimizing weight, cost, carbon, and displacement for lateral loads in a mass timber structure • Create a set of guidelines or a prototype workflow for structural design and fabrication methodology TABLE OF CONTENTS 1. INTRODUCTION……………………………………………………………………………………..…………....1 1.1 High-Rise Buildings……………………………………………………………………………….....…....1 1.2 Challenges in Construction and High-Rise Building Structures.………… …………………..….…….….2 1.3 Material: Wood and Mass Timber……………………………………………………………………........5 1.3.1 Material Sustainability and Potentials of Wood…………………………………...…………...5 1.3.2 Mass timber and earthquakes ………………….………………………………………...….....9 1.4 Summary…………………………………………………………………………………………………..9 2. LITERATURE REVIEW…………………………………………………………………………………….…...10 2.1 High Rise Buildings………………………………………………………………...……...….………....10 2.2 Structural Challenges: Forces and Lateral Loads in Vertical Systems ………………………...................12 2.2.1 Vertical/Gravity Loads…………………………………………………………….……........12 2.2.2 Horizontal (Lateral) Loads: Wind and Earthquake ………………………………….……......13 2.2.3 Forces and Loads in Building Design Considerations. …………………………….................16 2.2.4 Systems Selection in High Rise Building and Lateral Load Resisting Frames. ……………....18 2.3 Braced Frame Principles ……………………………………………………………………….……..….18 2.3.1 Load Distribution ………………...…………………………………………………….….....20 2.3.2 Types of Braced Frames ……………...……………………………………..….………….....20 2.4 Mass Timber and Structural Utilization……………………………………………………….………….23 2.4.1 Performance of Wood and Mass Timber as a Structural Material……………...……..……....25 2.5 Development of Structural Systems and Optimization Methods……….………………….…….……….27 2.5.1 Optimization of Shape, Size, Cross-Sectional Area and Topology Placement……….……….27 2.5.2 Topology Optimization in Lateral Design…………………….…………….…………….......28 2.6 Summary…………………………………………………………………………………………………28 3.METHODOLOGY………………………………………………………………………………………………....29 3.1 Case Study Analysis: Timber Tower Research Project………………………………………....….…......30 3.2 Timber Tower Design Analysis ……………………...…………………………………………....……..32 3.2.1 Form and Layout Attributes of Dewitt Chestnut-Timber Tower …………………….….…....32 3.2.2 Structural System Overview……………………………………………….....….…………...32 3.2.3 Gravity Load Design……………………………………….…………….…………………...33 3.2.4 Lateral Loads at TTRP…………………………………………….……………...….…….....34 3.3 Research Methodology Overview…………………………………………………………..…………....35 3.4 Bracing Type Selection: Development of Layouts……………………………………...…….………….37 3.4.1 Location of Braced Frames…………………………………………………………….……..38 3.4.2 Types of Braced Frames……………………………………………….…...............................39 3.4.3 Placement of Braced Frames………………………………………….…................................40 3.5 Design Development Of Bracing Layouts And Modeling …………………………….…........................41 3.5.1 General Considerations and Principles…………………….....................................................43 3.6 Bracing Layout Number 1 ……………………………….........................................................................43 3.7 Bracing Layout Number 2 …………………………….…………………….…………………..….…....44 3.8 Bracing Layout Number 3 …………………………………………………….……………..…………..44 3.9 Bracing Layout Number 4 ……………………………………….…........…….…………..…………….45 3.10 Gravity Load Modeling …………………………….............………………….………..…………...…46 3.11 Robot Structural Simulation For The Baseline Model And Braced Frame Typologies............................48 3.11.1 Creating Load Case Simulations in Robot Structural Analysis………….……...…...…........52 3.12 Summary…………….………….…...….…...……………..…...……….…...……….…...…………....53 4.RESULTS, EVALUATION AND DESIGN IMPROVEMENT ……...………………………………………....54 4.1 Introduction…………………………………………………………………………………..…………..54 4.2 Data…………………………………………………………………………………………..…………..56 4.2.1 Braced Frame Scenario 1 Calculation Results ………………………………….….…………57 4.2.2 Braced Frame Scenario 2 Calculation Results …………………...……………..………….....58 4.2.3 Braced Frame Scenario 3 Calculation Results …………………...………..………………….59 4.2.4 Braced Frame Scenario 4 Calculation Results …………………...…………..…………….....60 4.3 Comparison Charts ……………………………………………………………………..………………..60 4.3.1 Braced Frame Scenario 1 Chart…………………………………...………….……………….61 4.3.2 Braced Frame Scenario 2 Chart…………………………...……………….............……….…62 4.3.3 Braced Frame Scenario 3 Chart……………...…………………...………………..……….…64 4.3.4 Braced Frame Scenario 4 Chart ………………..………………...………………..……….…65 4.4 Comparison of Results and Design Parameters ……………………………………………….………....65 4.4.1 Displacement ……………………………………………….……..…………………….…...66 4.4.2 Connection and Cost……………..……………………………….……..………………..…..67 4.4.3 Carbon……………..……………………………….……..……………………………….....67 4.5 Discussion per Architectural Parameters………………………………….……………………………...68 4.6 Evaluation of Workflow, Validation and Selection……………………….………………………….......71 4.6.1 Interoperability ……………………………………….……….…………………………......71 4.6.2 Limitations and Assumptions ………………………...……….………………………….......76 4.7 Summary……………………………….………………………………….…………………………......76 CHAPTER 5: DESIGN ITERATION, VALIDATION AND CONNECTION RECOMMENDATIONS…………….…………………...……….…………………….……………………….......77 5.1 Introduction…………………………………...........……………………...…………………………......77 5.2 Hand Calculations………………………………...…………………….…………………..…………....77 5.3 Frame Analysis in Enercalc ………………………………...……………………….……….………......77 5.4 Design Iteration …………………………………...……………………….…………….....…………....88 5.5 Sectional Member Properties and Connection………………………….……………...……...………….92 5.6 Connection Methods…………………….…..…...……………………….……………....………….......93 5.7 Connection Strategies and Recommendations……………………………….………...………...……....93 5.8 Summary …………………………………………………………………………………………..…….94 6. CONCLUSION, SCOPE, AND FUTURE WORK………..…………………………………………….……….95 6.1 Introduction………………………………………………………………………………........................95 6.2 Current Limitations ………………………………………………………………………………...........96 6.2.1 Problems with the Existing Workflow………………………………………………..............97 6.2.2 Material Properties for Software Use…………………………………………………............98 6.2.3 Standardized Libraries…………………………………………………………………..........98 6.2.4 Plug-Ins …………………………………………………………………................................99 6.2.5 Revit and Robot Interoperability Issues and Robot Errors……………………………..........100 6.2.6 Code Updates…………………………………………………………………………..........100 6.3 Future work…………………………………………………………………………………..................100 6.3.1 Validation………………………………………………………………...............................101 6.3.2 Fabrication and Physical Testing……………………………………………........................102 6.3.3 Connections……………………………………………………………………....................102 6.3.4 Create Guidelines……………………………………………………………........................102 6.3.5 Other Features…………………………………………………………….............................103 6.5 Conclusion…………………………………………………………………………...............................103 REFERENCES………………..……………………………………………………………………….....................104 APPENDIX A……………………………………………...…………………………………………......................107 APPENDIX B…………………………………………………………………………………………......................111 1 Chapter 1: INTRODUCTION Chapter 1 is an overview of the history and trends in high-rise building structures, along with the challenges the construction industry started facing in the last decades. These challenges are both structural issues that come with height and horizontal forces, and sustainability in terms of material properties and carbon emissions. The section then introduces a new material that has also started to be used in high-rise construction which is mass timber. Mass timber has the potential to mitigate the problems of the construction sector in the near future, as it is being a highly sustainable material, being beneficial in terms of embodied carbon, and also other benefits in seismic design. Chapter 1 also includes a brief summary of the general concerns of timber performance and how mass timber addresses them. The introduction concludes on the seismic behavior of timber and mass timber and how it can actually be used in lateral resisting design against wind and earthquake motions. 1.1 High-Rise Buildings The ability to reach unimagined heights has always been one of the defining achievements of human history. Even though in the span of history of buildings it is possible to see many outstanding examples of buildings with impressive heights and spans that is looked up with fascination even today, the definition of a high rise has changed over time. The total volumes of these megastructures were proportionally large, and the footprint of these buildings occupied a considerable amount of space. This was not only due to the construction methods or more preliminary application of structural engineering, but also the lack of modern materials such as steel and reinforced concrete. Timber, along with masonry, was a primary building material in the architecture around the world. The 19th century came with a lot of innovation and change in the human-made landscapes. In the 1800s, followed by the industrialism in western Europe and the U.S, the technological advancements greatly changed the shape of the construction industry (Vries, 2008). With a shift from society being more involved in heavy and light industry affiliated jobs, exponentially more people started to live in the cities, along with the overall increase in world population (Clark, 2019). The industrial revolution led to a major shift in the main construction material: steel (Vries, 2008). In the same century, cast iron had started to be seen in both infrastructure and civic architecture around the world. The Soho Cast Iron District in New York City is home to buildings from cast iron from the 19th century (“New York Preservation Archive Project,” www.nypap.org, n.d.) (Figure 1-1a and 1-1b). Figure 1-1a and 1-1b: Soho Cast Iron District of New York City houses buildings from the 19th century of cast iron and the district plan (“New York Preservation Archive Project,” www.nypap.org, n.d.). The use of cast iron led to initial attempts in multi-story buildings with this new material, Chicago was one of the first cities to have examples of multi-story cast-iron buildings. The Home Insurance Co. Building in Chicago, consisting of 12 stories and made with cast-iron, is considered the first skyscraper, although the building does not stand anymore (Sarkisian, 2016). 2 Within the second half of the 19th century, the first examples of steel construction and the use of steel in multi-story buildings had started to be seen in various cities in the United States. Following the end of 19th century, and during the 10 year period in the 1920s, New York has seen the construction of, (which will be regarded for a long while), the tallest buildings in the world (Dolkart, n.d.). It should not be forgotten that one of the biggest facilitators of the construction and utilization of high rises was the invention of modern elevators and their utilization in urban buildings (Petroski,2002). The commercialization of the elevator is as much, if not more, important than the use of structural steel in multi-story building construction and creating the new architectural typology called the “skyscraper.” On the other hand, wood as a structural material started to recede into the background in the high-rise building market in the same century in cities. The historical events show how devastating consequences a fire can make in the city. The Great Fire in the City of Chicago in 1871 devastated the city in many extents (Kogan, 2019). On the other hand, the fire gave the possibility for architects and engineers to start developing projects and experiment both with the materials and architectural typologies (Rayfield, 1997). Rethinking the design of the city and the buildings led the city to host the pioneers of modern architecture and first skyscrapers (Rayfield, 1997). That being said, the wood started to disappear from the picture of the modern tall building construction and the urban skyline. Being known to be a highly combustible material for centuries, the wood was considered risky for multi-story buildings, where the risks in terms of higher number of people and money. Considering the highrises being a new archetype, in the case of a fire, using a combustible material for a building hosting several times more people and goods than any other, would cause another disaster. The fire sprinkler systems were not being utilized by then, and there was no sufficient code for fire safety that these buildings could comply with (Prospal, 2010). The architects and engineered consulted on less combustible materials. In addition, wood was considered a weak material in its total structural capacity. The height and the dimensions of columns and beams were limited by the volume of the trees, whereas steel was able to reach bigger dimensions and larger spans. The need for bigger columns, in wood, meant bigger trees, which required a lot more labor in terms of extraction of trees and manufacturing. By the end of the century, the wood started to get less popular in dense urban areas based on code restrictions and practical concerns. Steel and concrete were cheaper, easier to fabricate, with better performance and reaching bigger dimensions in all directions. In the end, wood remained in the background from the modern and contemporary construction industry in the cities for last two centuries. 1.2 Challenges in Construction and High Rise Building Structures The industrialization and the urbanization are not the only phenomena of the last two centuries. The population growth has been exponential globally. In the last two centuries, the population grew by around 6 billion (Sarkisian, 2016). In 2017, the total world population was 7.6 billion people (“World Population Prospects: The 2017 Revision,” www.un.org, 2017). The world population is expected to increase around 1 billion in every 20 years and the total population of the world is expected to be 11.2 billion in 2100. (“World Population Projected to Reach 9.8 Billion in 2050, and 11.2 Billion in 2100,” www.un.org, 2017). The estimated population living in the cities will be around 2.5 by 2050 (“Around 2.5 billion more people will be living in cities by 2050, projects new UN report,” news.un.org, 2018). The population growth is reaching its limits and already causing many risks and posing many threats both to the environment and to the people itself (Mirkin, 2014). The resource depletion, followed by the resource scarcity is one of the most significant threats that come with it (Pimentel, 2006). The population growth is reaching its limits and causing risks, along with industrialization depletion of resources. The change in the climate and the increase in carbon emissions is one of the biggest challenges the world is facing in recent years (Martin, 2018). The built environment is highly responsible for the total amount of energy being used, along with carbon emissions caused by the construction and operational life of the buildings. Regardless of the height of the structure, the built environment is one of the main causes of carbon emissions and used energy around the world. In the U.S. almost 80% of the total electricity is used by the buildings and their occupants (“An Assessment Of Energy Technologies And Research Opportunities,” www.energy.gov, 2015). Again in the U.S., above 40% of all energy usage and carbon emissions are caused by the buildings as well (“An Assessment Of Energy Technologies And Research Opportunities,” www.energy.gov, 2015). In addition, around 76% of U.S. electricity is 3 consumed by and within the buildings as well (“An Assessment Of Energy Technologies And Research Opportunities,” www.energy.gov, 2015) (Figure 1-2). Figure 1-2: Energy used in buildings according to the primary use of energy. Heating, cooling, and lighting are the primary uses of the energy in the U.S. in 2014. (“An Assessment Of Energy Technologies And Research Opportunities,” www.energy.gov, 2015) The management of the population of these people in the built environment will be one of the biggest challenges of the city planners and the construction industry (Toorey, 2004). As stated above, the high rises are, and will continue to be the archetype of cities, accommodating thousands of people in a small footprint of land. They are also considered to be a solution for many of the threats posed by overpopulation, such as sprawl, consumption of natural habitat and agricultural lands, inefficient resource and energy use, transportation problems and material usage. The tall buildings use the urban space and habitat more efficiently, create more efficient transportation and service distribution, and if designed efficiently they can be sustainable in terms of material usage per square footage. Tall buildings can indeed have the potential to create an opportunity to mitigate the common environmental concerns, risks, and problems. Built in a dense urban area, they reduce the urban sprawl. They create more efficient districts in terms of energy distribution and transportation. In many terms, they can be considered efficient. However, on the other side, high rises are posing their own problems to this challenge: -Carbon: By their nature, the high rises use more material than low to mid-rise buildings, both in total quantity and most of the time, per square footage (based on the amplification of acting forces on high rises, operational energy usage increase, in need of using less sustainable or more unique products, and sometimes less efficient floor plans). The use of a material is directly linked to the amount of carbon emission of the building construction. High rises not only use the material in high quantities and masses but also use construction materials which both have a very high embodied carbon footprint: steel and concrete. Both steel and concrete emit far greater amounts of carbon dioxide during their production, especially when compared to wood (Buchanan, 1999). Including transportation and construction of these elements, the carbon footprint is many times more than a low or mid-rise building, as the material usage increases by height and volume as well. In general, high rises cause more carbon emission not only by their total area or mass but also per square ft/ per square m when compared with a baseline building. The use of special equipment, on-site labor, and additional construction costs make high rises a more costly building type. -Embodied energy and operational energy: In the 21st century, energy efficiency and sustainability gained significant attention in the construction industry. Along with the use of passive environmental strategies design for climatic 4 considerations, the active technologies has started to shift the architecture and urban landscape to reach a more energy-conscious future. Along with concepts and classifications like Net Zero Energy buildings and Passive Houses, there are initiatives and certifications which gained popularity all around the world such as LEED (Leadership in Energy and Environmental Design), LBC (Living Building Challenge) and similar accreditation systems. The state and nationwide goals that led to changes in the regulations show the criticality trends towards sustainable architecture strategies. Architecture 2030, California Building Code, Title 24 by California Energy Commission are just a few examples in the U.S. (“Building Energy Efficiency Program,” www.energy.ca.gov/title24, 2019). The embodied energy includes all the processes that the material needs energy for, starting from extracting the material, to manufacturing and machinery in the factories, transportation to site, on-site works, construction and the rest of the life span of the building, sometimes including the repair and replacement as well as demolition (Giordano et al., 2018). There is a conventional belief that embodied energy is not significant when compared to operational energy, as the buildings are expected last for a hundred years, or almost permanent, whereas this is not always the case (Ochsendorf, 2018). In addition, the use of high embodied energy structural materials in buildings might not be discounted for having a long life span, or can be compensated by establishing a net positive building either, since there may be other hazardous factors such as resource depletion in the time of the construction/life cycle of the building with the selected high emission materials. The embodied energy in buildings has historically not received enough attention as the operational energy of buildings and has been a less considered aspect in the built environment, even in energy-efficient buildings. Embodied energy is a major contributor to the total energy consumed, and the percentage of the embodied energy in total energy consumption gets even higher in energy efficient buildings, which have low operating energy (Ochsendorf, 2018). Whereas the operating energy in the energy use of buildings shows variable value, the embodied energy increases very slightly or (sometimes considered constant) and is in existence throughout the lifetime of the building (Ochsendorf, 2018) (Figure 1-3a and 1-3b). Figure 1-3a: The graph demonstrates the relationship between a constant embodied energy consumption versus changing operational energy consumptions in round number. (Stauffer, 2009). 5 Figure 1-3b: The graph is a more realistic illustration, as embodied carbon slightly increases by years, due to repair and other changes (Ochsendorf, 2018). The production of concrete and steel have a significant impact on total carbon dioxide emissions. Just the production of cement, the primary component of concrete, makes up around 7% of carbon emissions and the number is escalating (Design for Sustainability, MIT Open Courseware, 2004). Embodied energy should be given more emphasis in new construction, the choice of structural materials plays the highest role in the embodied energy and carbon footprint of buildings. Greater attention should be given to structural materials and carbon footprint. Especially in tall buildings, where the use of material /material mass is significantly higher than low rises, the use of materials with low carbon footprint becomes more important. 1.3 Material: Wood and Mass Timber The modern wood technology enabled panelized products, which enabled spans and strength that was thought it was not possible with traditional wood construction. The development of panelized wood systems are called engineered wood/timber, and, more commonly called as mass timber. Mass timber, from many aspects, outperforms the limited capabilities of traditional wood that has been used in light wood frame and post and beam construction. These prefabricated components make wood construction easy, cost- efficient while propose to maintain the life safety and serviceability of the building. With mass timber, architects and engineers have started to reach new heights, as they did with steel before. Mass timber has been an interest of structural material in various parts of Europe, mostly in Nordic countries, and the number of projects that are using mass timber is increasing in number in North America and the United States. By 2018, there were around 35 projects that utilize mass timber that is proposed to 21 jurisdictions around the U.S (“Media Brief: Tall Mass Timber Buildings” Mass Timber Code Coalition, www.forestbusinessnetwork.com, 2018). Besides the wood products’ structural performance being competitive with steel and reinforced concrete, mass timber is a product that became highly favorable due to its natural exposed look and aesthetically pleasing, warm character. 1.3.1 Material Sustainability and Potentials of Wood Wood, for many reasons, is remarkably a more sustainable choice as a structural building material. In consideration of initial embodied carbon stored in wood, it outperforms other structural building materials such as steel and concrete mainly. Wood is a natural product obtained from forests, and it does not require anything other nature itself to grow, which two main elements are the sun and the carbon dioxide. It releases oxygen while soaking up carbon dioxide, and as a result, has a negative embodied carbon footprint on contrary to the other building materials. Unlike 6 almost all of the other structural materials, timber is not a fossil fuel intensive material. Skidmore, Owings & Merrill’s conducted research, “Timber Tower Research Project” showed that wood is approximately 50% carbon by weight, and a “carbon sink.” The availability of the carbon calculator methods and tools enable the architect and engineers to estimate the total savings by utilizing wood as a structural material. There is a significant amount of research that has been done, showing how much in average carbon is stored in a wood building compared to a steel or reinforced concrete and how a building can save from greenhouse gas emissions. Although most of the existing research Although the there is tendency to ignore recurring embodied energy or the increase in the carbon footprint during the life cycle of a building, the validity of wood being a low carbon material does not change (in addition to considering the fact that recurring embodied energy mostly is not taken into account for the other building materials as well). In addition, during the manufacturing of wood products, renewable biomass is highly preferred, (around 75% by 2010) instead of fossil fuels (Industry Progress Report: Environment, Energy, Safety, www.awc.org, 2016). These include barks, residual fibers, and sawdust. Other research also emphasises the sustainability of the supply-demand chain. The results show that, based on mainly North America, estimate the duration (in minutes) of the given volume of wood to grow, to state the forestry can supply the resources very easily (Sarthe and Connor, 2010). The factors that affect the life cycle may vary on structural material and there might be essential differences in the raw material supply as well (Figure 1-4). Figure 1-4: Factors that affect energy use through the life cycle of a building with energy (Crawford and Cadorel, 2017). 7 Wood as a structural material is highly efficient during the manufacturing process. Both in U.S. and Canada, the logs are used almost at full capacity/efficiency. As wood is easier to obtain and manufacture, especially with engineered timber, the cost of panelized wood products can outperform other materials. In terms of foresty and wood products, the main concerns are centered around the deforestation and excessive use of natural habitats for wood construction and related industry products. It is emphasized that wood construction and cutting down forests for wood construction were not, and is not predicted to be the reason for the loss of forest lands in U.S. and Canada. During the last century, the main reason for the permanent loss of forest areas was due to mainly agricultural usage and opening land for real estate. (Ward and Patterson, 2011). Producing panelized mass timber products is fast, efficient and makes it ready to assemble at the construction site, having no downtime, and more efficient than steel and in situ concrete. The characteristic of wood being a lighter material also facilitates construction at the site. From an economic standpoint, faster construction, along with less construction waste is a lot more efficient in terms of cost. Fast and Epp’s accomplished mass timber projects were 25 percent faster than if they built it in concrete (Gafner, 2018). By utilizing tools such as EPA (United States Environmental Protection Agency)’s Greenhouse Gas Equivalencies Calculator, the carbon cost and benefits for the projects can be estimated. By utilizing the tools, it is easier to illustrate how much greenhouse gas emissions are avoided, or, in equal to how many years a house will be operated with the same amount of energy, or, equally how much cars off the road it will be taken, by replacing the structural material with wood. Timber structures are conceived to have a relatively low fire performance for a long time, due to how they perform during wildfires and fires that have occurred in the cities and inside the buildings. The performance of mass timber is facing similar preconceptions based on the data of light and heavy frame wood structures during a fire. On the other hand, the predictability of fire performance of mass timber products makes it highly favorable. Wood, especially in large cross-sections of mass timber panels has a char rate which can be predetermined according to the total cross-sectional area. The main determinant in the estimation of a specific building material or structural material is the fire performance Rating. The rating considers the fire performance in aspects as structural resistance, integrity, and insulation. The Fire Performance Rating is expressed in hourly resistance ratings, such as 1 Hr Rating. The full-scale panel tests are being done in engineered timber products, like CLT Wall panels, especially in the last 2 years (Barber, 2018). The engineered timber products have been tested for different objectives of performance in fire resistance design. These include interior finish and flammability, flame spread and smoke development; which both the exterior and interior elements have to undergo the tests. According to them A, B, C rating where C is the most flammable building material. B rating materials are most common in buildings. In most of the tests conducted in the last years, the existing and recent fire tests prove the fire performance of the CLT products have a fire rating of 2-hour rating noncombustible construction. The fire stops, sealing joints and prevent the passage of heat transfer and gases as well (Dagenais, 2014) (Figure 1-5). 8 Figure 1-5: Fire stops and joints to prevent the passage of heat transfer (Dagenais, 2015). The panelized timber products also undergo a char test which is the estimation of the char depth during the occurrence of a fire and estimating the strength. The char takes the role of a protective layer to the wood structural members, and with engineered timber, it is easy to estimate the “sacrificial char layer,” which will leave behind, the effective strength of the cross-sectional area (Figure 1-6). With predicting the behavior, the structural engineers can more easily estimate the effective strength of the mass timber during an occurrence of fire. Figure 1-6: Charring of timber and layering during a fire (Forest Products Laboratory (U.S.), www.onlinebooks.library.upenn.edu, 2018). 9 The important point is to consider the risks of delamination and char fall off, which puts the remaining effective strength at risk. As the timber loses its protective char layer, the remaining parts of the timber overall get a faster char rate. The use of proper adhesives is crucial to avoid the char fall offs and losing the effective structural strength (Figure 1-7). Figure 1-7: The remaining cross-sectional area after a fire test (Barber, 2018). There is increasing confidence for mass timber building construction and both in heights and innovation and the variety of panelized timber products being used. Since there is an increasing amount of code-approved buildings, the International Building Code allows up to 6 floors of wood frame construction for most occupancy types. As there are more case studies and reports that state the safety of successful wood projects, there is consensus from the sector to push the International Building Code. 1.3.2 Mass Timber and Earthquakes Timber construction, especially platform light frame timber construction in North America, in terms of seismic resilience against building damage, have been outperforming concrete construction, when looked at the historical data (Karacabeyli et al., 2004). Timber being a lightweight material and acting more ductile compared to concrete made the material comparably safer than unreinforced concrete structures. In California, dıuring Northridge and Loma Prieta earthquakes, most of the wooden buildings remained standing with minor damage, while concrete buildings, especially with unreinforced, saw collapses and major damages. The main hazard is usually related to fires after the earthquake, such as in the San Francisco earthquake. Although the historical evidence shows that the life safety objectives were met relatively higher in wood construction, both in a single story and multi-story wood frame buildings, the high rise timber structures is a new phenomenon with almost no experience and data on how they will behave in dynamic lateral forces, such as wind and earthquake. The historical earthquake data on wood performing well was mostly obtained from a limited amount of stories. In addition, most of the existing wooden or mass timber buildings, either utilize no bracing as a lateral system, or utilize bracing made of steel, moment frame out of concrete or steel. The wood, as a lateral load resisting system, is solely used as shear walls. The development of brace frames can open many new possibilities in the construction sector, not only in terms of the structure’s objective function to perform well, but also in terms of reducing carbon, cost, weight, and utilizing new material and assembly technologies in the high rise buildings. 1.4 Summary Among the structural challenges and disadvantages the existing high-rise building structures and materials pose, mass timber is emerging as a potential solution, based on the characteristic features of wood as a material, and combining it with engineered properties to alter the disadvantages. Timber’s potential and advantages become 10 enhanced with mass timber’s improvements over conventional timber. The chapter also includes timber and earthquake performance which links to the methodology utilizing mass timber in lateral designs. The following chapter demonstrates promising examples of high-rise buildings made out of mass timber or hybrid timber systems as a part of the literature review, where examples demonstrate high-level use of mass timber, but still incorporate conventional materials for lateral load resisting frames in general. 2. LITERATURE REVIEW The chapter reviews high-rise building systems and the forming of vertical systems. It is an introduction to the acting forces to buildings with taller heights and the loads that are created by different types of forces. The main scope is in lateral loads, so a further analysis of wind and earthquake loads is covered. Some approaches and design considerations in developing systems to resist these forces are reviewed, along with some existing or emerging methods in the structural design development stage. One of the main objectives of this chapter is to overview the lateral load resisting designs and mainly braced frames. 2.1 High Rise Buildings The first high rise building is considered to be Home Insurance Building, which was a 12 story building in Chicago (Sarkisian, 2016). Since the 19th century, to reach new heights has always been a challenge with different considerations, whether structural, constructional, architectural, or social. As buildings get taller, structural challenges included the serviceability and safety against lateral loads such as wind and seismic, prevention of possible failures and collapses and other factors such as fire safety (Sarkisian, 2016). The change in height and forms also required changes and innovations in structural systems and materials. The fires were a great determinant in shaping the high rise building construction and material usage (Sarkisian, 2016). Preceding with cast iron, steel dominated the high rise building industry in the 19th and 20th century, along with reinforced concrete. Steel was a construction material came to the forefront as it allowed to have more slender columns and beams in limited footprint area, although concrete had greater fire resistance. The second half of the century saw both advancements in steel and concrete by the spreading use of high strength concrete and reinforced concrete. Composite materials and systems became more common in the same period (Sarkisian, 2016). The increase in the high rise building construction was still accompanied with steel and concrete materials, and the number of buildings that exceeded 200 m of height (656 feet) in 2018 outnumbered all the preceding years (Figure 2-1). 11 Figure 2-1: The exponential increase can be observed in tall and super tall buildings over the course of years and number of buildings built (CTBUH Year in Review: Tall Trends of 2017, www.skyscrapercenter.com/year-in- review/2017, 2018). The definition of a tall building may vary according to different resources and is subjected to change with consideration of different parameters that define this typology. One of the ways to define is not only the height but also the height relative to the context (“Height Criteria,” www.ctbuh.org, 2018). A different general definition may take a 14+ story building as a high rise. At the same time, in a dense metropolitan zone such as Manhattan in NYC or Downtown Chicago, a 14 story height building may not be considered a tall building. Another commonly acknowledged parameter to distinguish a tall building is the verticality or proportion. A small footprint and a slender form are one of the main defining elements of a high rise/tall building. For example, there are numerous old buildings around the world which take up the size of an urban block and are a multi-story, but they may not be considered a high rise because of their height to footprint ratio. Although not as common as the previous two, there is another factor that is regarded as a factor that helps define and distinguish a tall building, which is the embracing and existence of architectural technologies, especially in forms of the resisting systems that a tall building would be affected by in terms of safety and serviceability (“Height Criteria,” www.ctbuh.org, 2018). These include but not limited to lateral load resisting braced frames, vertical transport systems and technologies and so on. The parameters listed above indicate that the given building can be considered tall, nevertheless, a simple definition of a tall building would be above 14 stories or more than 165 feet (50 meters) tall building. There can be additional categories defined within tall building structures such as supertall buildings and mega tall buildings which are above 984 feet (300 meters) and 1968 feet (600 meters) respectively. The number of supertall and mega-tall building around the globe are, again, respectively significantly less than average tall buildings. The current tallest building in the world that is currently occupied is the Burj Khalifa in Dubai, soaring 828-meter (2,717 ft) tall. The Jeddah Tower in Saudi Arabia is planned to be opened in 2020 and is planned to exceed Burj Khalifa by by 236 feet (72 meters) with the total height of 3280 feet (1000 meters) (“100 Tallest Completed Buildings in the World by Height to Architectural Top,” /www.skyscrapercenter.com, 2019). By 2017, in the building materials in high rise building construction, concrete was still the most dominant material by making up more than 50 percent of the sector (Figure 2-2). Figure 2-2: The average distribution of main structural material in high rise building construction in 2017; concrete was still the most common structural material (CTBUH Year in Review: Tall Trends of 2017, www.skyscrapercenter.com/year-in-review/2017, 2018). 12 2.2 Structural Challenges: Forces and Lateral Loads in Vertical Systems Just like any other structure, tall buildings are subjected to the exertion of forces, and in most cases, these forces are amplified and become a more significant determining factor in the design and construction process. These can be defined according to their direction such as the gravity forces and lateral forces. The gravity loads include dead loads and live loads of the building, whereas the two main lateral loads are wind and seismic. 2.2.1 Vertical/Gravity Loads Vertical loads, which can also be interpreted as gravity loads, are the dead loads and live loads applied on a given structure. The dead loads and live loads are two types of vertical loads and are a fundamental part of the structural design. Dead Loads: The dead loads are the structure itself, immovable, permanent loads applied to the structure. The weight of the structure, walls, floors, ceilings, partitions, and superimposed loads such as finishes, façade elements, MEP systems and other immovable elements during construction are considered dead loads (Sarkisian, 2016). Live Loads: Live loads, also considered as temporary loads are other structural and non-structural loads that may vary by the type of occupancy, location and amount of force during construction or during occupancy, so are accounted as unstable, mainly caused by the occupancy of the building. These may include occupants and their total number, other content such as furniture, vehicles, and everything in motion. Snow load is also considered a live load (Sarkisian, 2016). The averages of some example building components with their associated dead and live loads as follows (Sarkisian,2016); Superimposed dead loads (including non structural and semi permanent members) (Sarkisian,2016): Partitions: 1.0 kPa (20 psf) Ceiling: 0.15 kPa (3 psf) Live loads (standards depending on the type and usage of the building) (Sarkisian,2016): Office: 2.5 kPa (50 psf) Residental: 2.0 kPa (40 psf) It should be noted that, although for tall buildings there is a possibility to reduce live load during load design considerations, the reduction is limited to an extent only (Hallebrand and Jakobsson, 2016). A gravity load is the total amount of dead load and live load imposed on a building. With these loads combined, it creates compressive and tensile stress in the building elements such as the columns and beams. The rigid frame behavior under gravity load for single unit rigid frame also changes (Ambrose and Vergun, 1995) (Figure 2-3). 13 Figure 2-3: The rigid frame behavior under gravity load for a single unit rigid frame (a) and multi-unit frame (b) (Ambrose and Vergun, 1995). 2.2.2 Horizontal (Lateral) Loads: Wind and Earthquake Horizontal loads are the type of loads that are being exerted mainly with a horizontal direction to the structure. Lateral load is more common terminology for horizontal loads, thus it will be used in the following sections. Especially for tall buildings, of lateral loads, wind load and earthquake load are determinant in the behavior and performance of the structure. However, it should be noted that there are a variety of several other loads that are determinant in the lateral system design of the structure, such as the soil pressure to the substructure and basement walls, or again, the pressure of the earth to retaining walls near a sea body (Luebkeman, 1998). Wind loads and earthquake loads are by their nature more dynamic and complex than gravity load design for dead load and live loads (although both wind and earthquake loads are live loads, they are considered as a different category). Therefore, following code requirements based on international/uniform/ or state/local building codes streamlines the design process, especially with more complex, or taller structures. Wind Loads: One of the two most common lateral loads is the wind load and it is a fundamental part of lateral load design particularly in high rise buildings. Most of the time wind can be an even more effective in the design process than seismic loads, especially in tall buildings, since the ductility increases with the height of the structure, and the flexibility being the main determinant in the period of vibration. Although the wind loads may be unexpected in many directions, it is essentially a horizontal load and defined by its action direction on the building surfaces according to the direction of the force. (Mendis et al.,2007). The wind load is considered both static and dynamic, but in many cases, even for short term dynamic wind pressure, assumptions are made to replace the dynamic pressure to static force equations to facilitate the design process. Wind Motion And Building Behavior: Although the building attributes (stiffness, form, mass, height, etc.) and wind characteristics (wind velocity, wind direction) affect the behavior of the structure (possible deflections/displacements), there is default response classifications that are expected based on the directionality of the wind pressure. The 2 main classifications of the wind pressures on the affected surface are windward and leeward. 1. Windward: Direct pressure from the oncoming wind is initially exerted to the facing surface. In a conventional building, the windward surface, perpendicular to the wind will be facing the greatest force in the structure (Sarkisian,2016). 2. Leeward: The leeward forces are exerted on the opposite side of the along-wind force, creating a suction effect on the building surface. Whereas the windward faces are considered as positive pressured surfaces on the building, the leeward surfaces are considered to have a negative pressure around them. Along with the positive and negative forces, the drag effect creates the biggest and most significant in motion in tall buildings. Whereas the drag effect does not create a strong motion for low and mid-rise buildings. The development in building technologies and development of brace frames and outrigger trusses are not only important for seismic load design, but also for the motion and displacement caused by winds in high rise buildings. Although the thesis is focused on the brace frame design and optimization for seismic load dominated cases, it should not be forgotten that the existence of brace frames and outrigger in a wind induce motions is crucial (Sarkisian,2016). Estimating the building behavior and performance can be a complex procedure, considering the numerous conditions related to the atmospheric, site and building conditions. The site conditions include the setting of the building, such as the difference of the pressure the building get when it is located in a dense areas compared to the wind pressure in an open field, the effect of the soil conditions, extreme weather conditions such as storms, hurricanes, and tornados requiring specially designed constructions, the building feature like the mass, height, and aerodynamics of form. 14 However, regardless of atmospheric, site or building conditions, predicting building motion is essential for human safety and comfort, for serviceability and occupant response. The wind load estimation methods include a variety of handheld, empirical and computational methods and testings. Among them, there are variations of that depend on the static vs dynamic motion of the wind, and also changing factors according to the direction of wind response. Tall buildings, compared to low to mid-rise buildings, tend to behave in more sway, bending or displacement, which is caused mainly by horizontal loads. Thus they are mainly influenced by lateral loads such as the wind forces and require a different set of approaches. Structural design considerations for extreme lateral loads, such as wind forces, incorporate mega columns, outriggers and mega come and tube systems. These types of structural decisions are, most of the time, supported by mechanical systems and approaches such as damping systems (Günel and Ilgin, 2014). Different alternatives in material selection, such as diverting from concrete as a load resisting material is also a part of the discussion in resisting lateral loads in the last years. Although concrete is still the leading material in structural system design, the concerns that come with it, such as environmental harm and carbon emission also directs the industry and research towards other material selections. 1 ton (2205 lbs) of cement uses around 4000-7500 MJ energy and the amount of carbon released by the same amount is around 1-1.2 tons (Kovačević and Džidić, 2018). The lateral loads have a significant factor in shaping the building from the initial/conceptual design stages in the case of high rise buildings, thus defining the lateral system is a part of the initial structural design process. The main concerns are to provide the requirements of drift and stiffness control. Other structural and architectural objectives may initially be ignored at the schematic/conceptual design (Beghini et. al, 2013). Testing measures for model testings usually include implementing pressure taps on building models for analyzing pressure points on the surface (Figure 2-4). The pressure tap models are then tested on a wind-tunnel setup, where PIV (particle image velocimetry) help to simulate more valid results (Figure 2-5). Figure 2-4: Layout of pressure taps on the principal building model (units are in mm) (Zu and Lam, 2018). 15 Figure 2-5: PIV (Particle image velocimetry) set-up in a wind tunnel (Zu and Lam, 2018). Earthquake Loads: To better understand seismicity and earthquake forces, the following attribute/quality/phenomenons related to them should be well understood. These are seismic/earthquake intensity, earthquake magnitude, energy, and peak ground acceleration. The seismic intensity is a qualitative data (although it is based on Mercalli Intensity Scale and defined levels of intensity), whereas the magnitude, energy, and PGA are qualitative data obtained from the measurements during an earthquake. Seismic intensity can be described as the evaluation of observed damage to the people, and the built environment. The determination of the intensity of an earthquake depends significantly on the location and the affected region (Sarkisian, 2016). The Intensity Scale ranges from Intensity I to Intensity XIII and shows the increase in motion perception and aftermath respectively. As an example, whereas the Intensity I does not imply even a minor damage to the objects and the buildings, and describes the earthquake as almost non perceptible by the people; the Intensity XIII on the scale is described as an intensity so rare, and the damage is total, there is loss of “line of” sight, as well as total displacement of objects. An Intensity VII, for example, although rare, has been recorded in history many times. It can also show how an almost similar earthquake can create different consequences in different locations because of the variation of quality and construction methods of the buildings, (and preparedness over time and lessons learned). Whereas there is insignificant damage in well-designed structures, the scale describes there is considerable damage in poorly built structures. This can be seen in the aftermath comparison between the Northridge Earthquake and Adapazari earthquake. Magnitude: The developed scale to measure the earthquake “strength” force is of the most common methods used in earthquake engineering and still on use. The magnitude of the earthquake is consistent/correlated to the energy released by an earthquake. Although there are different (and arguably more precise) methods to measure the magnitude, or the strength of an earthquake, the following method developed in 1935 by Charles F. Richer from California Institute of Technology or else known as the Richter Scale. The following equation uses a logarithmic function to divide the “measured maximum amplitude” of the earthquake and dividing by a “calibrated” earthquake (Sarkisian, 2016). M=log10(A/Ao) A: measured maximum amplitude Determined by the oscillation measured by a seismograph Ao: calibration earthquake (measured amplitude of an earthquake of standard size). Usually taken as 0.001 millimeters (3.94x 10^-5 in) Most of the earthquakes that occurred in the last century are measured and expressed by the Richter Scale, with a whole and decimal number implies the magnitude of the event. 16 Although the Richter scale is the most widely used expression of earthquake magnitude, it should be noted that the duration of the earthquake is also a determinant in the damage caused, even though the measured maximum amplitude given above may be the same. Thus, the aftermath of two different earthquakes with the same M on the Richter Scale may be completely different from each other (Sarkisian, 2016). Energy: The energy estimation method in correlation with the magnitude M of an earthquake was developed by Beno Gutenberg and Charles F. Richter in 1956 (Sarkisian, 2016). An important factor of determination E is the difference between the radiated energy and the total energy released. This is due to the loss caused by the heat and non-elastic effects of the seismicity during an earthquake (Sarkisian, 2016). log10E = 11.8+1.5 M Where M is the magnitude and E is the energy. Peak Ground Acceleration (PGA): Peak ground acceleration is a term to describe the relative response to the ground movement. PGA can be expressed in g, % g, m/s^2 or ft/s^2. The stiffer the building is (and less vibration during an earthquake), the more it will move with less displacement to ground or foundation, whereas a building with less stiffness will have a higher displacement factor, thus the PGA will be higher. PGA is primarily affected by the soil conditions of the site (Sarkisian, 2016). California, being in the seismic zone, has witnessed a lot of earthquakes in the last century which caused substantial damage to life and properties. In the first half of the 20th century, Southern California was hit by a major seismic event centered in Long Beach. The Long Beach earthquake highly damaged the masonry buildings, almost all of them unreinforced. The affected buildings were not limited to residential uses, and included public buildings as well, such as schools. The positive impact of the aftermath is the consensus of new and updated construction requirements and the introduction of acts. In California, lateral force requirements were introduced depending on the structural system and seismic design coefficients had started to be used, and reinforcement became mandatory for masonry buildings. 2.2.3 Forces and Loads in Building Design Considerations The buildings, regardless of their location and construction type, are subjected to forces. The acting forces, when exerted on the structure become loads that are applied to the building that is subject to them. The forces applied on a building, are categorized in two main categories, based on the direction of action: SCode requirements offer a significantly conservative design load calculation and should be supported by other means, such as wind tunnel testing, especially for the taller buildings where the wind is a governing factor and forces are amplified (Sarkisian, 2016). In the U.S., the most common code requirements are in accordance with 2006 IBC (International Building Code) in accordance with ASCE (American Society of Civil Engineers), ASCE 7-10 and their section “Minimum Design Loads for Buildings and Other Structures (Sarkisian, 2016). The design for lateral loads usually follows a set of procedures depending on the complexity of the project and the character of the wind or seismic load conditions (Figure 2-6). 17 18 Figure 2-6: The figure shows the design development and calculation procedure for wind load design (Sarkisian, 2016). Whereas wind may have different provisions, the building as a structure and its components must be designed in consideration with ground motions and related provisions (Buyukozturk, 2013). 2.2.4 Systems Selection in High Rise Building and Lateral Load Resisting Frames Lateral load resistant design has a fundamental effect on the behavior of buildings in terms of safety and serviceability measures. An ideal lateral load resisting frame would not also keep the building in optimal stiffness both in the wind and seismic forces, but also would help the buildings stay in full function and preserve the occupants comforts in the buildings. Lateral load resisting frames and systems performance objectives are to minimize the lateral drift and sway that occurs when the building is subjected to lateral motions by increasing the overall stiffness of the building. The drift reduction is essential especially in resisting wind induced motions, as the performance goals for wind resistant design is to limit the sway (Schierle, 2018). Nevertheless, this does not indicate that lateral load resistance, such as braced frames is not essential in seismic design. Although earthquake resistant design favors the building designs which allows elastic motions, it is equally important to keep the lateral drift and the sway in permissible limits. The utilization of LLRF (lateral load resisting frames) is equally important to avoid the building sway too much that it affects the serviceability and occupant comfort in the building. As a result, the selection of lateral load resisting frames becomes equally important. The serviceability of the structure and the occupant comfort goes hand in hand with each other. The serviceability of a building includes architectural objectives and limitations on factors such as vibration control, amount of sway, and types of cracking and deflection. To limit the lateral drift, as stated before, is closely associated with serviceability and human comfort, in addition to collapse prevention. Human beings are highly sensitive to motion and motion sickness, which is one of the main resultants of too much horizontal motion and sway in a building. Even without general collapse of the building, too much sway and large displacements in the buildings may cause injuries or deaths, can cause facade displacement and collapse and affect the serviceability of the surrounding areas of the building. In extreme events, it can cause the building collapse as well. 2.3 Braced Frame Principles In lateral load design, the most common load resistings that are being used in construction are shear walls, moment frames and braced frames. Braced frames and moment frames are commonly utilized in high rise buildings against wind and earthquake forces. This section investigated the brace frame typologies as a part of the main scope of Chapter 3: Methodology. Brace frames have been a common part of lateral design since the last decades and have been used around the world. These systems were found to be capable of their structural efficiency and also creating architecturally interesting and aesthetic designs. Since then, there have been many examples that combine their architectural uniqueness and structural efficiency in buildings (Figure 2-7 and 2-8). 19 Figure 2-7: Examples of expressed braced frames around the world (Schierle, 2008). Figure 2-8: The structural engineering projects by Skidmore, Owings & Merrill from an exhibition in 2016, the tallest buildings of the firm demonstrated lateral load resisting frames they incorporated, such as braced frames. The image on the right shows a conceptual design done by the firm as well, aiming to combine structural efficiency and aesthetic in a building in San Francisco (Candiani and Sala, 2014). The tall buildings located in earthquake-prone regions, such as Japan, China, California and the west coast of the United States have to be designed with high performance goals to reduce earthquake-induced forces. With this in mind, it became apparent the elastic systems are not enough to dissipate energy in tall buildings subject to earthquake or forceful winds. In the late 19th and early 20th century, the braced frames have started to be recognized as an effective lateral load resisting elements. The early examples also adopted a set of various configurations and types/shapes of braced frames. The examples of the braced frame were mostly located on the cores of the buildings, not the exterior (Candiani and Sala, 2014). 20 The use of inner concrete in the second half of the 20th century allowed the steel frames to be expressed in the exteriors as well. This allowed the tall and super-tall buildings to mitigate large displacements and maximizes structural effectiveness while braced frames are expressed in the exposed structure. In comparison to other lateral load resisting systems, braced frames are less rigid than shear walls but stiffer than moment frames (Schierle, 2018). Hence, braced frames are efficient systems to resist lateral forces, and provide strength and stiffness. In addition, they are considered as advantageous as they can resist horizontal loads exerting on either side. 2.3.1 Load Distribution in Braced Frames Braced frames resist lateral loads through compression and tension in axial directions. The lateral load causes lateral shear in the axial action. Braced frames resist gravity load in beam bending and column compression and lateral load in axial compression and tension (Figure 2-9). Figure 2-9: Gravity load paths in diagonal, X and V bracings (Siddiqi, Hameed, and Akmal, 2014). Axial action of braces resists the lateral shear caused by the lateral load (Schierle, 2018). Because the lateral force is reversible, the axial force acting in columns, girders, bracings also reverse. Hence, the braces must act in both compression and tension, but it must be so efficient to resist the compression force. Under the action of gravity loads, columns shorten axially due to the compressive loads. Hence the diagonals are subjected to compression and beam will undergo axial tension due to the tying action. In the cases where diagonals are not connected at the ends of the beams, the diagonal members will not carry any force because no restraint is provided by the beams to develop force. Therefore, such bracing will not take part in resisting gravity loads (Schierle, 2018). 2.3.2 Types of Braced Frames Concentrically Braced Frames: CBF are the types where the bracing members join their center lines in the main joints in the structure (Figure 2-10). They provide strength and stiffness as they minimize residual moments in the frame. The concentrically bracing type may, on the other hand, be a restrictive aspect for other architectural considerations (Booth and Key, 2006). 21 Figure 2-10: shows the concentrically braced frames, respectively cross bracing (left) and simple diagonal bracing with changing placements (middle and right) (Rajan, 2019) Eccentrically Braced Frames And Knee Braced Frames: Eccentrically braced frames are the types of bracing where the bracing members' ends do not meet concentrically but are separated from each other. The eccentricity in bracing members allows the design to have more ductility compared to concentrically braced frames but still gives the stiffness to the structure in elastic motion (Figure 2-11) (Siddiqi, Hameed, and Akmal, 2014). Figure 2-11: The stiffeners may be needed for eccentric link elements in eccentrically braced frames (Siddiqi, Hameed, and Akmal, 2014). The gap between the ends of bracing elements behaves like a weak but ductile link, and it prevents yielding of other members before the weak link does. Some arrangements of eccentrically braced frames may be considered architecturally more advantageous as well, as they provide more space. There is no major difference between the elastic response behavior between eccentrically and concentrically braced frames, but eccentrically braced frames provide shear prevention to the structure, and to buckling occur from other elements in the structure (Booth and Key, 2006). Knee braced frames follow the same characteristics as eccentrically braced frames. An advantage of the knee braced frames is that, during an earthquake and possible damage, the knee braced could be replaced, whereas the link in eccentrically braced frames can not (Figure 2-12) (Siddiqi, Hameed, and Akmal, 2014). 22 Figure 2-12: A knee braced frame example with ductile knee braces on far right corners (Siddiqi, Hameed, and Akmal, 2014). Again, instead of the “link” in eccentrically braced frames, the yielding element in knee braced frames is the “knee brace.” When subjected to lateral forces, it yields to provide ductility, but usually remains elastic and stiff during seismic motions (Booth and Key, 2006). Diagonal Bracing: Diagonal braces are one of the most common types of bracing. The diagonal bracings can be listed as A bracing, V bracing, D bracing (or simple diagonal bracing) and K bracing and cross bracing. Some of them can also be eccentric or concentric. K-Braced Systems: Although K braced systems are commonly used to resist lateral loads, they are very efficient in seismic regions. During an earthquake, a non-uniform distribution of loads causes extra force applied to columns, which leads to a column failure, which are considered as a risky failure type as they might cause a general collapse in the building (Figure 2-13) (Siddiqi, Hameed, and Akmal, 2014). Figure 2-13: Some other topologies such as V bracing (on the left), inverted V bracing (in the middle) and K bracing (on the right) (Siddiqi, Hameed, and Akmal, 2014). X-Braced Systems: X bracing systems are preferred when there is a need for additional stiffness. Several codes in Europe and in the United States also limit the slenderness of each of the diagonal braces in this bracing type. These limits try to prevent each of the diagonal braces from worsening their strength and stiffness. The disadvantages of the X bracings occur when there is too much stiffness than needed, such as in responses to earthquake motions. From a 23 practical point of view, they are highly costly due to the increased material consumption and a number of connections and labor needed. They should be avoided as they are costly and have too many joints (Schierle, 2018). It is important to realize that the behavior of bracing systems changes according to their concentricity, placement, and type, thus the design of them should be selected after careful considerations of design requirements and lateral and gravity force distribution. 2.4 Mass Timber And Structural Utilization Multi-story residential buildings constructed with timber as the structural material has started to be seen in the last decade. The 14 story building, also known as the Treet in Bergen, Norway, is one of the tallest mass timber buildings in the world by 2015. The building was made possible to realize by a country’s major residential construction firm and also the largest glulam manufacturer of Norway, along with working with manufacturers from Estonia. The project utilizes glulam trusses and CLT shafts and interior walls (Abrahamsen and Malo,2014). The building exterior consists of large glulam trusses, providing the necessary stiffness to the structure. The building stories are a combination of glulam stories and concrete slabs, connected to the facade trusses, creating intermittent “power stories” which repeat in every 5 stories. The concrete slabs also help the building to increase the mass and the stiffness of the building against the region’s wind-induced dynamic motions (Figure 2-14a and 2-14b) (Abrahamsen and Malo,2014). 24 Figure 2-14a and 2-14b: Load bearing structural section and Global FEM model of Treet house in Bergen, Norway (Abrahamsen and Malo,2014). The fire performance requires significant research and testing for multi-story projects with mass timber as the main structural material. The European fire rating standard Eurocode 5 is applied for the given building. A widely used testing method is determining the effective cross section after charring, which is used in the project, the testing was approved by a third party reviewer (Abrahamsen and Malo, 2014). The analysis is made through Autodesk Robot Structural Analysis of the building, supported by Excel spreadsheets and manual calculations (Abrahamsen and Malo, 2014). The same software was used throughout Chapter 3 Methodology as well, for seismic induced dynamic motions of the subject of braced frame implementations (Figure 2-15). 25 Figure 2-15: Step by step visualization of the prefabricated elements erection process and the assembly (Abrahamsen and Malo,2014). Another mass timber building is a Brock Commons in University of British Columbia (UBC), Vancouver, BC, Canada is still known as the tallest wood building in the world, rising up to 18 stories. The building system incorporates cross- laminated timber (CLT) floor panels which span 2 ways and glulam columns (Figure 2-16) (Fast et al., 2016). Figure 2-16: Structural system of mass timber of floor plate and columns, and load resisting concrete cores at the UBC Brock Commons tower (Fast et al., 2016). The building has two concrete cores that extend the full height of the building. Along with UBC funding, the construction of Brock Commons was supported by the mass timber building and engineering industry in Canada (Fast et al., 2016). Due to its unexampled characteristic in the construction industry, the Brock Commons followed a different method in code compliance. The project located itself in a site-specific regulation given for this project and adopted the newest upcoming National Building Code of Canada (Fast et al., 2016). Different testings and building material selections were made to not have difficulty with passing fire performance ratings and acoustical performance benchmark. Cost efficiency had been one of the primary objectives of the project so that the prefabricated structural parts were to be installed quickly and economically at the site. The project utilizes the two-way spanning capability of CLT, which allowed the project to be designed without additional beams (Fast et al., 2016). Dlubal RFEM software was used for the analysis of the CLT members of the project, along with validation with hand calculations. In the analysis of CLT panels, not only the stiffness and bending but also the “rolling shear stresses” were also considered an affecting factor, especially in point supported members like in Brock Commons (Figure 2-17). The full-scale tests were conducted at FPI Innovations afterward (Figure 2-18) (Fast et al., 2016). 26 Figure 2-17: The procedure to approximate the rolling shear stresses based on area and geometry (Fast et al., 2016). Although the timber based lateral design was theoretically possible and feasible, due to construction time and costs to get the approvals were not a feasible option, so the project utilized two concrete cores against seismic and wind loads (Fast et al., 2016). Figure 2-18: Testing of CLT panels (Fast et al., 2016). Other considerations of the Brock Commons project was the prefabrication process in architecture to keep the costs minimally different than conventional building construction methods, glulam column shortening and shrinkage, progressive collapse of the building and dynamic wind-induced vibrations (Fast et al., 2016). The final design was also influenced by the full-size mock-ups and testing different connection types. The following section demonstrated some of the existing multi-story projects that have been completed or started in the last decade (Figure 2-19). 27 Fig 2-19: The precedent studies that take place in University of Southern California School of Architecture, including the well-known Brock Commons, Dalston Lane, Stockholm Wooden Skyscraper projects (Timber Source Book, Wahlroos-Ritter et al., 2018) 2.4.1 Performance of Wood and Mass Timber as a Structural Material Because wood is a very common structural material in North America, there are a vast number of wood structures that are subjected to earthquake loads and other forms of extreme level lateral loads. The seismic performance of wood is greatly affected by the material properties of wood at a cellular level and how wood construction comes together on-site. The performance of timber structures is affected by geographical factors and ground motion such as amplitude of the seismic force, the duration and the like. Nevertheless, the deformational characteristics of the timber as a structural material greatly affects how the building behaves against these external factors; such as stiffness, strength and ductility (“Forest Products Laboratory (U.S.)” | “The Online Books Page,” Onlinebooks.library.upenn.edu, 2018). Wood has a high strength to weight ratio and compared to concrete and structural steel the lightweight-ness is an advantage. In addition, the damping and ductility properties also, a characteristic of wood (“Forest Products Laboratory (U.S.)” | “The Online Books Page,” Onlinebooks.library.upenn.edu, 2018). Although the characteristics may vary in between different species of wood products, the average properties are consistent between species. The assessment of wood and timber buildings in previous earthquakes is an important resource to determine how the performance of wood is during lateral forces. The specific data on the following major earthquakes are obtained from the report (Karacabeyli and Reiner, 1999). The data obtained from Alaska Earthquake in with a magnitude of 8.4, shows that even though there were wooden houses slipped down the slopes, most of them protected their structural integrity, and, with a few exceptions most of the timber frame structures stayed undamaged. (Karacabeyli and Reiner, 1999). San Fernando Earthquake in 1971 affected some of the old wooden buildings in the region with minor damage or partial collapse. The damages also occurred mostly in cripple walls add on spaces such as porches, which aren’t directly linked to the initial design of timber frame structures itself. The modern timber houses were reported to perform relatively well in this earthquake (Karacabeyli and Reiner, 1999). 2.5 Development of Structural Systems and Optimization The system selection of vertical structures is dependent on many factors based on to provide similar purposes and functions to a building that center around safety, serviceability/operability, user comfort. As higher buildings require or encourage the utilization of new structural systems, increasing the efficiency of material usage, load resistance and cost by enhanced forms and topologies has started to become more important. The optimization term, which is mainly used in mathematics and engineering (such as mechanical engineering) also had started to be used in civil engineering and architecture. 2.5.1 Optimization of Shape, Size, Cross-Sectional Area and Topology Placement Topology optimizations are, especially in lateral bracing are focusing on optimizing material usage, weight/load concentration, and cost. Most of the time keeping the main objective as building behavior. The optimization of the 28 brace can be focusing on initial conceptual design or final sizing of the braces. The initial conceptual design may focus on the layout of material and higher level design decisions. The two main optimization techniques are discrete member and continuum methods, where each including a variety of parameters –design parameters- frameworks and design methods in their own division (Stromberg et. al, 2012). Mixed method and frameworks make use of mixing rules of discrete element method and continuum meshes. In both of these topology optimization methods, there is an objective function and set of design criteria.These can be listed as: maximum bending moment, stability, minimum compliance, critical buckling load (Neves et al., 1995), effective stiffness, limiting tip displacement (Baker) can all be considered as design criteria (Stromberg et. al, 2012). Also, there are many other design decisions that had to be given in these case studies as all of them were made based on conceptual designs and different representation techniques for built-up simulation models. (i.e.: solid areas such as to represent the deck, modeling floors as rigid elements) Mathematical derivations in optimization techniques in various stages and details of design include methods such as Principle in Virtual Work, Lagrangian multiplier method, a combination of PVW and Lagrangian multiplier method (based on cross-sectional areas for a statically determinate braced frame (This is a common example for topology optimization in brace frames). The knowledge of the shape and form of the building facilitates the design optimization process, as the lateral design layout is determined by the frame columns at the perimeter, and their spacing (Stromberg et. al, 2012). 2.5.2 Topology Development in Lateral Design: In the construction industry, topology optimization is significantly important in high rise buildings, where design is more complex and load exertion and distributions through an immense set of elements is more challenging, along with the new technologies and construction methods being applied mostly in high rise building structures. Brace Frame Optimization Methodologies Fully Stressed Design: Takes into consideration emphasizes a design method where each bracing member cross-sectional members are under a constant point load stress. Optimal Frame Geometry: Takes advantage of a symmetrical frame problem and applies a unit load and reaches internal forces in each member (Stromberg et. al, 2012). There are existing research that have been made on topology optimization methods; nevertheless, they are mainly focusing on steel structures in lateral design and topology optimization. 2.6 SUMMARY Buildings are subjected to a set of vertical and horizontal forces, and in tall buildings, these forces become amplified and pose a bigger challenge for safety, security, and serviceability. Seismic loads, which are nested under lateral loads, pose a great risk especially in earthquake-prone regions, like Los Angeles. Lateral load resisting frames are designed against these loads such as shear walls, moment frames and braced frames. There are methods of developing and optimizing these topologies, but they have been mainly conducted for steel frames. In Chapter 4, types of possible mass timber braced frame alternatives were developed and tested. The baseline building was an existing mass timber tower project, conducted by Skidmore, Owings & Merrill, called “Timber Tower Research Project.” The building was relocated to downtown Los Angeles and braced with timber frames against earthquake loads. 29 3. METHODOLOGY The chapter includes the review of the case study (baseline) building which is the Timber Tower Research Project, it is the conversion of Dewitt Chestnut Apartments in Chicago built by Skidmore, Owings & Merrill. The data was collected from the Research Project of SOM and the existing building, in terms of shape and layout attributes, material and building systems selection, and other building data that contributes to gravity load design. Simultaneously the braced frame system alternatives were reviewed and developed through an elimination method in the schematic design process, through a set of objectives and parameters. The Timber Tower Research Project was re-modeled in Autodesk Revit Structures, linked to Autodesk Robot Structural Analysis for baseline scenario simulation. Then 4 case studies were then modeled and implemented on the Baseline Building, and then all of them were linked to Robot structural simulation to test and evaluate against design considerations. The results were used to evaluate a different set of parameters and comparing the efficiency of the alternatives (Figure 3-1a and Figure 3- 1b). 30 Figure 3-1a: The research workflow shows the overall process from data collection to modeling design and testing and evaluation process. Figure 3-1b: The methodology diagram shows the steps in detail that have been covered in Chapter 3. 3.1 Case Study Analysis: Timber Tower Research Project The developed lateral design layouts were implemented on the existing Timber Tower Research Project conducted by Skidmore Owings and Merrill (abbrev.: SOM). The Timber Tower Research Project was held between 2013 until 2017. The first release of the project was made in June 2013 that can be accessed from the SOM database. Skidmore Owings and Merrill is an American firm which has accomplished landmark buildings in the U.S. and worldwide, focusing on high rises and innovative structural systems. The scope of the firm’s research project was to 31 use the potential of high rise buildings to set an example to overcome sustainability challenges the world is facing today. To make use of tall structures was uniquely important, as they have amplified drawbacks in terms of the amount of carbon emission they create, by using conventional tall building materials such as concrete and steel. For that reason, it was important to change the perception of traditional materials that can be used in high rises, in search of reducing carbon emissions caused by tall buildings. The firm wanted to make use of structural material that is sustainable, but not traditionally or practically used in high rise constructions. That is mass timber, wood, as the structural material. The significance of this research project came from the utilization of mass timber in the high rise building industry. To determine the benefits of using mass timber in the tall building industry, SOM resorted to a building to compare, that was going to be used as a benchmark, that had been designed with conventional tall building materials. The building SOM selected as a benchmark was one of their own buildings, that is, Dewitt Chestnut Apartments. The Apartments were built in 1966 and was designed by SOM’s main office in Chicago as a multi-story apartment building. The building was selected to re-design it from mass timber, then compare the results with the existing concrete structure. The selection of Dewitt Chestnut Apartments was important because in the time it was built, it was considered efficient in terms of material usage. Thus the comparison with mass timber system and the existing apartments will have a smaller range, thus avoid false claims on the result of the research if another prototypical building with less material efficiency was selected. The existing Dewitt Chestnut Apartments building is known to be very efficient and innovative in its time for both material usage and construction method against loads, utilizing “framed tube” system (“Timber Tower Research Project Final Report,” SOM, 2013). The challenge of Timber Tower Research Project was not just about to decrease the carbon emissions. The project team also had to come up with solutions for using mass timber in high rises, and the challenges it would create. The main issues they dealt with were about the characteristics of mass timber that it brings by being wood: Vibration, fire performance doubts, potential shortenings of wood frames, possible risk of uplift and other risks that wood creates by being a lighter material than concrete (Baker et al., 2014). The following section is an overview analysis of the Timber Tower Research Project along with existing reports. The analysis is a part of the scope of the methodology that includes converted building material attributes, gravity load analysis, proposed system selection and shape attributes (Figure 3-2a and 3-2b) (“Timber Tower Research Project,” www.som.com/ideas/research/timber_tower_research_project, 2019). Figure 3-2a and 3-2b: Existing Dewitt Chestnut Apartments on the left, and the Timber Tower from the SOM Research Project on the right (“Timber Tower Research Project,” www.som.com/ideas/research/timber_tower_research_project, 2019). 32 3.2 Timber Tower Design Analysis The Timber Tower Research Project design scope and objectives, project facts and material attibutes, form, dimension and structural system, detailed gravity load design, and existing load resisting frames and design are analyzed at this part. 3.2.1 Form and Layout Attributes of Dewitt Chestnut-Timber Tower The building geometry follows a simple and uniform rectangular geometry. It is 42 stories high, and about 395 feet (120 meters) tall. It follows the same footprint and plan dimensions of the existing concrete building. The regular plan dimensions are 80’-0” by 124’-6” (approx. 24.3 m by 38 m). Above ground, the building has 41 floors for residential purpose and a ground floor lobby area. The rooftop is utilized for other amenities and services (Figure 3- 3) (“Timber Tower Research Project,” www.som.com/ideas/research/timber_tower_research_project, 2019). Figure 3-3: In the proposed mass timber structure, the layout is kept as an open floor plan with floor structure spanning the entire length of the plan (“Timber Tower Research Project,” www.som.com/ideas/research/timber_tower_research_project, 2019). 3.2.2 Structural System Overview The main structural system of the TTRP is “Concrete Jointed Timber Frame,” designed and tested for the research project itself. The designed system still utilised timber as the main structural material, but makes use of concrete as well, in the connecting joints, the areas where the resultant stress was higher (“Timber Tower Research Project Final Report,” SOM, 2013). In the system, structural elements from mass timber and their connection joints were connected by cast in place concrete, which are reinforced by steel. The firm estimated this system would be just 30 to 35 percent of required embedded carbon, compared to the existing structural system. In the research report published for the Structural Congress in 2014, it is stated that the main aim of the project was not to replicate all the existing structural system components with timber, but rather create a sound alternative within the same architectural layout, that can be comprised of different arrangements and structural elements (Baker et al., 2014). The existing concrete building in Chicago had reinforced concrete flat plate (Thickness: 191 mm Span: 6.71 m), interior densely laid reinforced concrete columns and reinforced concrete framed tube (that also act as a resistance against high wind loads), the frame tube column spacing at the center was 1.68 meters. The TTRP utilizes Cross Laminated Timber 33 (CLT) for floor plates, Cross Laminated Timber for walls, and at the core towards the center of the floor plate (Figure 3-4). Figure 3-4: The timber tower floor area components, the typical floor structure and consisting building elements of the gravity load design (Timber Tower Research Project Final Report, SOM, 2013). 3.2.3 Gravity Load Design The building was designed to accommodate gravity loads and load paths with cross-laminated timber as the main material (Figure 3-5). 1. The first component of the gravity load system in TTRP is the timber floor panels, which are 8 inches thick. 2. The columns and walls: The timber built-up columns and shear walls deliver the loads to the floor below. The shear wall core is located at the center of the floor plan. They provide lateral resistance as well. 3. The timber shear walls are connected via link beams which are reinforced concrete, and also wall joints. The concrete joints that connect the shear walls to the plate also prevent the floor slabs from rotating. The spandrel beam at the perimeter also resists the torsion. They are made of concrete, utilizing concrete’s high strength. 34 Figure 3-5: A floor section from TTRP (not to scale) (“Timber Tower Research Project Final Report,” SOM, 2013). The gravity load reports consist of the validation and the testing of gravity load design of the ongoing Timber Tower Research Project Gravity Report to see if the proposed structure and dimensions are adequate to support vertical loads and floor loading (Physical Testing Report, SOM, 2017). 3.2.4 Lateral Loads at TTRP The system utilized a shear wall core made from mass timber for lateral resilience. They sat on the floor plates of CLT. They are also extended to the perimeter on all sides of the slab to increase the lateral resilience. Nevertheless, the building did not utilize an expressed lateral framing system at the perimeters (Figure 3-6). Figure 3-6: The east-west and north-south building sections of Timber Tower Research Project (Timber Tower Research Project Final Report, SOM, 2013). 35 The scope was, indeed, to relocate the building to a highly seismic zone, Los Angeles, and utilize and implement expressed lateral design for the Timber Tower Research Project. 3.3 Research Methodology Overview SOM’s Timber Tower Research Project was taken as a basis for the implementation of lateral design with mass timber. The 3D models are not available for use, thus the structural system is studied separately, and the reports from the SOM database based on structural materials and attributes of TTRP building was reviewed. The 3D model had been designed again, with the information obtained from these reports along with 2D drawings and plans. These were adopted and applied in Autodesk Revit Structures. Simultaneously, upon the building plan, the lateral framing systems had been developed. The modeling of the building and the selection of the right bracing type and placements were two separate parts of the methodology but was conducted concurrently. Whereas the lateral design methodologies used for wind loads and seismic loads were taken as a guideline for the selection process of braced frames (Figure 3-7a and Figure 3-7b) a project-specific approach was developed to come up with the braced frame layouts. . Figure 3-7a: Design methodology flow chart for a braced frame (zipper braced frame) (Reinhorn, 2007). 36 Figure 3-7b: The proposed methodology 37 3.4 Bracing Type Selection: Development of Layouts The scenarios consisted of different bracing types, as well as position and placement. To better define the selection process, the main defining factors on bracing typologies were set. These are location of braced frames, type of braced frames and placement of braced frames (Figure 3-8). 38 Figure 3-8: The selection process of braced frame topologies and scenarios are set under the parameters above, and the process followed an elimination method. 3.4.1 Location of Braced Frames Two types under the classification of the location of the braced frames are whether they are located inside the building (Figure 3-9). It should be noted that the braced cores can also be combined with framed tube exteriors or other types of frames on the perimeter of the structure (such as the First Interstate Bank of Los Angeles by I.M. Pei). When placed on the 39 perimeter of the building, braced superstructures express the architectural form, they allow architecturally unique structures. In addition, for existing buildings and seismic retrofit projects and research, it is more viable to focus on braced frames that can be planned to be located on the perimeters, compared to the ones that are located at the core, which would be much more challenging to handle and implement. As an existing research project, Timber Tower Research Project by SOM is being used, the main aim is to have a minimal intervention to the core and gravity load design of the SOM research. Figure 3-9: The location of the braced frames also consisted of ones in the core, the type was eliminated as it is not part of the research scope. 3.4.2 Types of Braced Frames The main types of braced frames that were in consideration as a part of topology development are as follows: Single diagonal bracing, chevron shape bracings, cross (X) shape, and eccentric braced frames of each type. Case Study Bracing Types: Five common types of bracing systems are Single Diagonal Bracing, A Bracing, V Bracing, K Bracing, Eccentric Bracing (further detailed), Bell Truss (Figure 3-10). Building Facade Corner Perimeters Center Bays Intermediate Bays Core 40 Figure 3-10: The selection of braced frame types was conducted on the basis upon material characteristics and limitations of wood and mass timber. Eccentricity in the braced frames increase the amount of connections and thus will be a costly alternative to concentrically braced frames. Timber braced frames with a little amount of prior research and having multiple joints and weak links created additional challenges to a new topic. Having eccentricity in the joints was cumbersome with timber connections so it was eliminated. The types of braced frames are based on the parameter of risk factors, cost, compatibility with timber and number of connections and failure mode. The single diagonal bracing is the most common types of bracing which have less structural weight and lesser average lateral displacement (Schierle, 2018). The more complex the shape and number of joints are, the more the frame becomes costlier and more prone to corner failure (Schierle, 2018). Therefore, X bracings were not to be considered as an alternative. Double diagonal/cross bracings also provide the level of extra stiffness that earthquake-proof designs does not require. For stricter architectural constraints, it was also considered disadvantageous and they required more joints and material additionally. They required a higher amount of connections, they were more costly and require more detailing and they are more prone to creating non-uniform behavior. The A-V bracing was also considered as a possible design alternative. Although they required more connections than simple diagonal bracing, the alternative had a change to be more cost-effective for the easier fabrication and effective for shorter brace length. K braces, on the other hand, was ignored as an alternative, as they are not convenient for major earthquakes. The danger with K braces included possible column failures, which could trigger a general collapse. Therefore, simple diagonal bracing and A-V bracings were selected as more viable alternatives because of their more uniform load distribution, lesser structural weight, less possible lateral displacement and possible ease of fabrication for timber elements. The selections were determined in the assumption they were more useful in seismic regions in theoretical and practical knowledge. 3.4.3 Placement of Braced Frames In the placement of braced frames, a diversification method was being used. A limited number of sets of placements, along with the factors received from the types of braced frames and location of the braced frames was utilized. The initial designs would form a basis of future and finalized detailed design and designed in consideration with the possibility of being subject to change. The placement alternatives covered the different types of braced frames mentioned beforehand and determined to be the most appropriate. All of them were to be located on the exterior of the building as the research scope is limited to expressed lateral load resisting frames. The number of scenarios (layouts) was kept to 4 alternatives to streamline the design process. Concentric Braced Frames Single Diagonal Bracing A / V Bracing K bracing X bracing Eccentric Braced Frames 41 The main principles used in the determination of the initial alternatives were done on the basis of the following: 1. All of the 4 alternatives were formed with simple diagonal bracing or A/V bracing options which were determined to be the most feasible options. The alternatives were the variations of the selected types. 2. The plan symmetry and uniform distribution of the loads and all 4 sides of the building were selected as another criterion. The main consideration in all of these alternatives was to make the layouts based on uniform distribution of loads, and ductility. All alternatives aimed to follow symmetry in plan and in load distribution. Continuous bays. 3. Obtaining gravity loads for the building and establishing the base shear for Downtown Los Angeles region (Lateral Design Graph: Rough estimation supported by hand calculations). 4. Obtaining gravity loads for the building and establishing the base shear for Downtown Los Angeles region (Lateral Design Graph: Rough estimation supported by hand calculations). 5. Other practical considerations and code guidelines (Eurocode 8) influenced the layout selection and elimination of some of the alternatives for seismic zones (such as K bracing.) 6. Continuous bay placement was a criterion. 7. Variation in the placement in bays; corner bays, intermittent bays was tried to be kept. 8. The amount of braced frame had to be kept minimal due to the fact that it will add too much rigidity to the building after some point. Minimizing the material helped the other design considerations to be better results such as the cost, the weight, and a number of connections needed. 3.5 Design Development Of Bracing Layouts And Modeling Per the factors considered above, a limited number of design layouts for bracing for the Timber Tower Research Project by SOM were determined. The Timber Tower Research Project, which is based upon Dewitt Chestnut Apartments in Chicago, had not made changes in the plan layout or the general shape attributes of the building, except for redesigning the high rise in mass timber as the main structural material. Thus, the bracing layouts that were developed made use of both of these plans. The building that the layouts were based upon is a 42 story building in Chicago that was relocated to DTLA for lateral design simulation. The building is a regular shaped building with simple rectangular geometry and reaches 120 m(395 feet) height (Figure 3-11a and 3-11b). The building's significance also comes from being the first building using Fazlur Khan’s framed tube structure. The structural system of Dewitt Chestnut Apartments and Timber Tower Research project and its gravity load modeling (remodeling) were covered before the modeling and implementation of braced frames. Manufacturing constraints to limit the non-practical ones are discussed later in Chapter 3: Methodology. There are a variety of methods on the application of the constraints in terms of manufacture, but for some material qualities, in accordance with the standards and local and regional building codes. The manufactural constraints were not taken into consideration as an initial parameter, but mostly lateral displacements and design parameters. Standards were reviewed and taken into consideration based on the availability in the tools and software. The operability of the model and manufacturability of members are demonstrated in Chapter 4 and 5. The MEP design constraints were also not the main focus of the design process as well. The cost was a consideration on the development of the project, but cost savings due to repetitive patterns and the like were disregarded, although the fabrication automation thus the cost savings could be done in future work. The general approach in braced frame selection was made for picking cost-effective connections. These parameters are discussed later as a design variable of the fabrication process for further research. 42 Figure 3-11a and 3-11b: Dewitt Chestnut Apartments in Chicago and Timber Tower Research Project, the initial building was completed in 1966 and Timber Tower Research Project was held in between 2013-2017 (Timber Tower Research Project Final Report, SOM, 2013). The floor plan dimensions were set to 80’-0” by 124’-6”, respectively consisting of 3 equal distance bays and 6 bays (Figure 3-12). The Timber Tower Research project has built up timber columns whereas the old Dewitt Chestnut Apartment consists of a perimeter framed tube and interior gravity columns. The gravity load systems were further explored later in Chapter 3. 43 Figure 3-12: Four different bracing systems based on their type and location were designed and studied by placing them in different axis. 3.5.1 General Considerations and Principles One of the main design objectives was to limit sway to a limited range, and the guideline that was followed was ASCE-7-05 seismic design recommendations and by Prof. Goetz Schierle as the maximum sway of the buildings were limited between H (Total Height)/500 or Height/400 to Height/200 in respective units. a. The amount of braced frame had to be kept minimal due to the fact that it will add too much rigidity to the building after some point. Minimizing the material would also help the other design considerations to be better results such as the cost, the weight, and the number of connections needed. b. Other considerations in all of these alternatives were to make the layouts based on uniform distribution of loads, and ductility. c. All alternatives aimed to follow symmetry in plan and in load distribution and had continuous bays. d. All the alternatives were created with Simple Diagonal Bracing or V bracing options which seemed to be the best. All 4 scenarios were the variations of these braced frame types. e. 4th of it on the other side explored more of the architectural and aesthetic potential, while for the other alternatives their aesthetic potential was not immediately explicitly thought of. The layouts are then developed to be implemented on gravity load model to be simulated. 3.6 Bracing Layout Number 1 Layout 1 was designed by placing the braced frames in the intermediate bays of the minor axis and corners bays along the major axis. The bracings were distributed in different bays but continuous on all levels. The total weight of this alternative was expected to be higher than the following alternatives of Braced Frame 2, 3 and 4. Thus the magnitude of the lateral displacement along the total height of the building was expected to be less for this alternative, along with increased uniform stiffness (Figure 3-13). 44 Figure 3-13: Braced Frame Layout 1 3.7 Bracing Layout Number 2 The Bracing Layout 2 was designed with regards to changing the placement of the braced frames in the perimeter of the building, then Layout Number 1. The same type of diagonally braced frames was used in this alternative. The main difference of adding two bays of braced frames on both sides of the perimeter would intentionally result in safer building performance and drift, at the same time possibly increasing stiffness and in addition to that, total mass and the cost of the building (Figure 3-14). Figure 3-14: Braced Frame Layout 2 3.8 Bracing Layout Number 3 The bracing Layout Number 3 utilized a mix of braced frame types. The main feature of the alternative was the use of V bracing, in contrast to Layout 1 and Layout 2 which only utilized simple diagonal bracing. The increase of connection points may result in an increase in costs or risks of failure in the nodes, but shorter bracing elements might have a positive effect in terms of shorter material dimensions and fabrication costs, in addition, the performance of the building is worth testing as it utilized a different type (Figure 3-15). 45 Figure 3-15: Braced Frame Layout 3 3.9 Bracing Layout Number 4 The main aim of the Braced Frame Scenario 4 was to try to create more architecturally interesting and aesthetically pleasable braced frame typology without compromising from the effectiveness of the model. The highly used approaches were used in this model as well, to effectively brace the building: Equal distribution of forces on all 4 sides of the building (equal resistance) (Figure 3-16). Figure 3-16: Braced Frame Layout 4 46 This model utilized the same type of bracing that was studied in Case Study 1 and Case Study 2, which are single diagonal, or D bracing. The X bracing was not further studied as opposed to D and V bracing, the connections and material would make it costly compared to the other two bracing types. In the bracing frame 4 except for the final parameters such as keeping the weight of the timber to a minimum and cost. The architectural requirements and considerations were determined and analyzed over the Braced Frame Scenarios in the following chapters. 3.10 Gravity Load Modelling The gravity load modeling of the building consisted of obtaining the existing data from the sources of SOM and existing building and re-modeling it in initially in Revit Structures. The modeling process included an adaptation process that is discussed later in Chapter 4. The gravity load model was also linked to the test in Robot Structural Analysis before implementing the braced frame layouts (Figure 3-17). Figure 3-17: Revit and Robot Structural Analysis were used for remodeling. The gravity load remodeling was conducted in Revit structures from the foundation (drilled piers foundation) to floor slabs with mass timber with taking data from the Timber Tower Research Project. The material limitations and selections were explored in Revit Structures. The material libraries and connection options were tried out and then changed step by step towards correct assemblies at this stage (Figure 3-18). 47 Figure 3-18: Revit Structural model looks familiar with Revit Architectural model if the analytical option was not turned on. In addition, although the Gravity Load Design was made before implementation of braced frame alternatives, a shoebox model and frame connections were tried out at this stage. These were later removed from the model. To compare different bracing layouts and the regular building plan that would be subjected to gravity loads and lateral loads for Downtown Los Angeles, where the building is relocated. The 4 different typologies of the building with 4 different bracing layout and configurations were designed and analyzed in Revit Structures and transferred to Autodesk Robot Structural Analysis (Figure 3-19). Cross Laminated Timber was used for floor plate and walls. Timber floor panels, timber framing within the core, solid timber shear walls at the core, reinforced concrete wall joints, and reinforced concrete link beams, built up timber columns, reinforced concrete spandrel beam were used. Figure 3-19: The modeling in Revit structures: The Timber Tower Research Project model and floor plans were adapted at this stage. The structural form and gravity load adopted for the study to make it ready for the braced frame structure. Any inconsistencies would be checked at the end again. 48 3.11 Robot Structural Simulation Process for the Baseline Model and the Braced Frame Layouts The modeling of case studies was initiated with modeling and testing the gravity load model in Revit Structures and Robot Structural Analysis. After that, the modeling and implementation of braced frames are conducted. Below is a summary of the Braced Frame Scenario 1 and taking the analytical model from Revit and preparing it for Robot simulation and structural analysis process (Figure 3-20). Figure 3-20: (a) From the existing model, the analytical model was created for the Gravity Load Model. Revit Structural Model was transferred to Autodesk Robot Structural Analysis as a baseline model and without braced frames. The error estimation was conducted (Figure 3-21). Figure 3-21: (b) There is a direct link between Revit to Robot Structural Analysis, the given building in the figure is without the braced frames to see the errors that would be obtained from the building model itself. 49 It is possible to make load assignments in Revit Structures, but interoperability and data transfer between two software programs were unknown, in addition Robot Structural Analysis is specifically designed for load case simulations thus it was conducted in Robot (Figure 3-22). Figure 3-22: (c) The initial simulations can be done by assigning default loads to the structure. The initial simulation was conducted within the default loads in Robot Structural Analysis first. The aim was to receive the errors that purely comes from the building form and information from the Revit Structural model, in the overall structure and sections of the structure (Figure 3-23). Figure 3-23: (d) The secondary load case simulations were conducted again with the default load cases coming from Robot Structural Analysis, but after braced frames were implemented to the building for the first time. The secondary load case simulations were conducted again with the default load cases coming from Robot Structural Analysis, but after braced frames were implemented to the building for the first time. This allows not to receive the 50 actual results but to conduct error mitigation that was done while designing and placing braced frames in the Revit model (Figure 3-24). Figure 3-24: (e) The simulations were conducted again with the default load cases coming from Robot Structural Analysis after the shoebox model was tested. The errors that were in need to be fixed again mention the instabilities at the braced frames that were mitigated by re- modeling (Figure 3-25 and Figure 3-26). Figure 3-25 and Figure 3-26: (f) Error mitigation was conducted before creating the actual braced frame and loading scenerios. Errors occured in the model imported from Revit Structral Analysis. Generally, it is common to receive errors and the errors/failure modes are categorized in the software as the number of Error Types. Although it is crucial that most of the errors should be mitigated, it is possible to get Type 1 type of errors in some cases, although the structure was modeled to mitigate the errors. Nevertheless, a remodeling process was conducted (Figure 3-27 and Figure 3-28). 51 Figure 3-27 and 3-28: (g) The releases could be defined in the structure, however for the methodology, the model was taken back to Revit and bar releases were conducted in Revit structures. The software required that errors be modified in the Robot software program interface. Such as modifying slabs and other special considerations for some elements, which is possible to remodel both in Robot or Revit. The results such as shear and bending moment diagrams were obtained at this stage. The actual all the load cases and combinations were assigned later afterward (Figure 3-29). While Multiframe and newer software programs make use of base shear and diagrammatic forces, Robot allows simulation options according to the existing building codes. Figure 3-29: (h). The results such as shear and bending moment diagrams were obtained at this stage in the model. The actual all the load cases and combinations were assigned later afterward. Summary of the steps between gravity load modeling and linking the model to Robot Structural Analysis Professional and preparing it for simulation results were demonstrated as a workflow (Figure 3-30). 52 Figure 3-30: (i) The workflow chart. 3.11.1 Creating Load Case Simulations in Robot Structural Analysis The different loading conditions were applied featuring different types and magnitude of loads after a simple model had been tested. Simulation: The records used for the analyses were chosen to satisfy the seismic zonal parameters for Los Angeles, CA, United States. (Capacity design principals had been applied to the scenarios.) The geometry definition in its final version was imported from Revit Structures. The loads were applied to the Timber Tower at a later stage, but initially, the load definitions were conducted in Robot (Figure 3-31). Figure 3-31: The default load cases were removed in order to apply project specific loads to the model in Robot. Creating Load Cases in Robot Structures: Initially, the dead load was applied (DL1). When the Load Table was checked (loads-load table) self-weight of the structure was automatically added. Simultaneously, the model on the Robot structural model with the braced frames implemented had been started to be analyzed. Within the different scenarios, load cases were created in the software. The default Load Case Values were taken out of the model. The automatic load cases were taken out from the building in order to apply project specific loads to the model in Robot. Dead Load, Live Load, Wind and Seismic Loads are subjected to the structure one by one, before the seismic loads (Figure 3-32). . For the scenarios, ASCE 2012 standard had been used. Basic Seismic Load Case: Equivalent Lateral Force method was applied to the structure. 53 Figure 3-32: The loads were defined, the dead and live load were created with default values that come with the building model, but seismic and horizontal loads require other data such as site classification, code and case requirements. Basic Seismic Load Case: Equivalent Lateral Force method was applied in the initial seismic load case. Although the equivalent Lateral load force is applied for initial -more preliminary- seismic load tests, it is convenient to apply them to most of the more advanced seismic analysis types (Figure 3-33). Figure 3-33: The records used for the analyses were chosen to satisfy the seismic zonal parameters for Los Angeles, CA, the United States with capacity design principles. For initial work and the initial analysis of step-by-step modeling the solver sets were usually left to Automatic. Different parts of the building structure were added over time to make sure the validity and ease of error estimation during the modeling process. Mass Conversion was not made at this stage. 3.12 Summary Based on the structural system overview and the gravity load model, the methodology followed its own guideline in prototype braced frame scenarios, which then resulted in limiting them in 4 alternatives (Figure 3-34). 54 Figure 3-34: Braced Frame Scenario 1, 2, 3 and 4 respectively as designed in Revit structures and linked to Robot for simulation. The selected brace frames were then implemented in the re-modeled gravity load model, which was already linked to Robot Structural Analysis. The results and design considerations are discussed in Chapter 4, along with detailed analysis and modeling and simulation adaptations. CHAPTER 4: RESULTS, EVALUATION AND DESIGN IMPROVEMENT Chapter 4 demonstrates the workflow used in obtaining results and evaluation of design; the results data and calculation data are shown, as well as design objectives that were later shown in comparison charts, followed by comparisons of architectural parameters, and concluding with a selection of an initial layout. The interoperability and results and possible invalid results reasons were also discussed. Chapter 4 demonstrates the introduction to results and discussion, data, calculation notes and results for 4 layouts, comparison charts for 4 layouts, comparing parameters and architectural objectives, evaluation of results and workflow, discussion of interoperability, limitations, assumptions, and conclusion. 4.1 Introduction The demonstration was made of the various data and results obtained from Braced Frame Scenarios. The obtained results from the simulations were evaluated both in terms of calculation notes and design objectives received from the outcomes of the 3D model. Following that, the comparison charts were created to demonstrate a holistic 55 information chart that is intended to be a guideline to compare the prototypes. The main design objectives that focus on displacement were evaluated initially, and the validity of the results and possible errors are discussed. Design objectives are evaluated between the case studies. Then an in-depth evaluation of architectural parameters and their effects on the building are discussed, to better evaluate the proposed “best” result from the configurations. After the results obtained from simulations are discussed, and the braced frame alternatives are compared according to other objectives such as their carbon content, cost, and other design parameters, one of the Braced Frame Layouts was selected for further design and validation of its results from Robot. The selected layout was not a finalized model but rather was used as a starting point to validate results through manual calculation and capacity design and then re-design the model more accurately. The validation was done through manual hand calculations and Enercalc for individual braced frame capacity design. The selected model was then joined with further design to improve the model. The model was then improved and corrected and made ready to re-simulate through Robot or similar structural analysis software programs. Afterward, the connection design strategies are explored, and prototypes are proposed. In addition, the workflow through Chapter 3 and 4 was evaluated, and limitations are demonstrated that happened through the modeling, simulation, and result in obtainment processes (Figure 4-1). Figure 4-1: The project workflow used to conduct the selection and improvement of the design process. 56 4.2 Data The results were obtained from Robot Structural Analysis after the initial simulations were conducted. Both the simplified and detailed results were obtained through Robot, demonstrating the defined properties at the modeling stage, number of nodes and finite element nodes. The analysis results showed what kind of seismic analysis mode was chosen (such as dynamic seismic analysis), for the simulations where equivalent lateral force analysis was used. Although due to code requirements and unique behavior modes it would be more accurate to use modal analysis for tall buildings, the scope of the simulation was limited to code based equivalent lateral force analysis for all 4 braced frame scenarios. The load cases in Robot Structural Analysis were used to obtain results on what is happening to the structural model after the loading (conditions) were exerted to the building. The data obtained from the software were obtained from the “calculation notes” that were generated as the result of equivalent lateral force analysis. The calculation notes were seismic simulation outputs were generated by Robot during the seismic cases. The calculation notes include the loading properties generated for building for the seismic case and can be obtained from the Analysis tab (Figure 4-2a and 4-2b). 57 Figure 4-2a and 4-2b: The initial results for seismic analysis are obtained from Robot Calculation notes. All calculation notes described below are obtained from Robot Structural Analysis interface unless mentioned otherwise. The results are just given as outputs and were discussed and evaluated later on in comparisons of braced frame layouts and also further evaluated through verification through hand calculation and design iteration. 4.2.1 Braced Frame Scenario 1 Calculation Results The analysis was conducted in the X direction and Y direction. Although the calculation notes did not display the objective function or modes of failure, it demonstrated effective results in terms of obtaining the force distribution in each story, how much period the building gets, the total base shear and total seismic weight. These were useful parameters to compare different building system alternatives with regards to facilitate obtaining relative displacement factors. The analysis mode was conducted as static and linear; where the increase in force and the responses were in a linear relationship. Case 1: DL1, Analysis type: Static - Linear Case 2: LL1, Analysis type: Static - Linear Case 3: WIND1, Analysis type: Static - Linear Case 4: ASCE 7-10 / IBC 2012 Direction_X Except for the building model and the form, the other data that was used as an input to obtain the calculation results were the ground conditions and other zone related requirements that affect the period of the building and seismic risk. The following data was based on Downtown Los Angeles Seismic Zone parameters retrieved from USGS: Data: Soil type D, S1, 0.10 SS, 0.250. Spectrum parameters: Fa =1.000 Fv = 1.500 SMS = 1.800 SM1 =0.900 SDS =1.200 SD1 =0.600 To = 0.100 TS = 0.500 TL = 4.000 I = 1.000 R = 4.000 Fundamental period: Approximated method T = 1.7 (s) Other structures Ct = 0.02 (0.048) x = 0.75 Structure range: Top story: ROOF Bottom story: Level 2 Effective height Hn = 395.50(ft) Fa represents the site amplification factor (for a specific period, 0.2 seconds). Fv represents the site amplification factor (for a specific period, 1 second). SMS and SM1 represent site-modified spectral acceleration value. SDS and SD1 represent the numeric seismic design values at different spectral acceleration values (0.2 and 1 SA). T values represent the transition periods in seconds (i.e.; TL: long term). The I factor is the importance factor of the model. R factor is the response modification factor. Base shear (The following chapters will validate base shear by V=CsxW.) k = 1.638 Cs = 0.338 Cs max = 0.338 (The result ranges demonstrated in Appendix section.) Cs min = 0.053 Effective seismic weight W = 16278.64(kip) Shear force V = 5501.4 (kip) (The calculation results for W and V needed be verified.) The base shear in a seismic event is the expected or occurred lateral force at the bottom of the structure. It is an important indication of how severe the acting forces on the building will be, and the expected damage to the building can be better estimated by the effects of the base shear. The base shear is dependent on the total weight of the building, soil types, closeness to a fault and seismic zones, stiffness of the building and so on. If the structure has a 58 lower base shear, this means lower seismic forces. Most of the time, there is a direct proportion or correlation between roof displacement and base shear. The explanations and glossary can be found in Appendix A. 4.2.2 Braced Frame Scenario 2 Calculation Results The following results obtained by equivalent lateral force analysis after the load was exerted to the building in X and Y direction. Results such as force distribution in each story, the period of the building, total base shear and seismic load were listed below. The cases represented the loading conditions. Case 1: DL1, Analysis type: Static - Linear Case 2: LL1, Analysis type: Static - Linear Case 3: WIND1, Analysis type: Static - Linear Case 4: ASCE 7-10 / IBC 2012 Direction X The cases mentioned above are the loading conditions that is exerted on the building by Robot Structural Analysis. In addition, the same data was used as an input to obtain the calculation results is the soil conditions and other zone related requirements that affect the period of the building and seismic risk. The following data was again based on Downtown Los Angeles Seismic Zone parameters: Data: Soil type: D, S1:0.100, SS: 0.250 Spectrum parameters: Fa =1.6, Fv= 2.4, SMS = 0.40, SM1= 0.24 SDS =0.27, SD1 = 0.16, To = 0.12, TS = 0.60, TL =2.00, I =1.00, R = 4.000. Fa represents the site amplification factor (for a specific period, 0.2 seconds). Fv represents the site amplification factor (for a specific period, 1 second). SMS and SM1 represent site-modified spectral acceleration value. SDS and SD1 represent the numeric seismic design values at different spectral acceleration values (0.2 and 1 SA). T values represent the transition periods in seconds (i.e.; TL: long term). The I factor is the importance factor of the model. R factor is the response modification factor. The fundamental natural period of T for the Braced Frame 2 nd Scenario was approximately 1.775 seconds (0.1 sec/story). The fundamental natural period of T for the Braced Frame 2 nd Scenario was approximately 1.775 seconds. The value was within an expected range. The period of the building may range in values as low as 0.1 seconds to up to 7 seconds. The period depends on many factors including the height of the building (high-rise structures tend to get higher periods). In addition to that, the structural material and configuration, ductility and stiffness, soil conditions, amplification, the existence of damping all change the fundamental period of the building. The simulated building was more flexible than a baseline model which is made of non-wood structural materials. Both the Braced Frame 1 st Scenario and Second Scenario resulted in the same fundamental period value 0.1%. It was assumed the result being the same results from the software taking into consideration the dimension as the primary factor. Base shear includes the following values k = 1.64 (k=1 for T<0.5 sec, k=2 for T≥2.5 sec, interpolate @ 0.5-2.5) Cs = 0.09 (should be about 0.03, the reasons of change will be discussed) Cs max = 0.09 Cs min = 0.012 Effective seismic weight W = 16278 (kip) (The data will be validated by hand calculations). (Base shear: V = CS W (Cs = seismic factor, W = DL + 25% of storage LL) V = CSW was used to calculate base shear Robot Structural Analysis using the equivalent lateral force procedure The shear force, where Cs is the seismic response coefficient. 59 Shear force V = 1467 (kip) is the result obtained from Braced Frame 2 nd Scenario. The results will be validated through hand calculations or Lateral Design Graph. The results were further analyzed in the following parts of the chapter. The explanations and glossary can be found in Appendix A. 4.2.3 Braced Frame Scenario 3 Calculation Results The results obtained by equivalent lateral force analysis after the load was exerted to the building in X and Y direction. (same as the 1 st and 2 nd Braced Frame Scenarios) was again conducted. From the Calculation Notes the force distribution in each story, how much period the building gets, the total base shear and total seismic weight were again obtained, to use in comparing different building system alternatives with regard to facilitate estimating relative displacement factors. The analysis types were static and linear, where the increase in force and the responses were in a linear relationship. Case 1: DL1, Analysis type: Static - Linear Case 2: LL1, Analysis type: Static - Linear Case 3: WIND1, Analysis type: Static - Linear Case 4: ASCE 7-10 / IBC 2012 Direction_X In addition to the building model and the form that changed, the other data that was input to obtain the calculation results is the soil conditions and other zone related requirements that affect the period of the building and seismic risk stayed the same as the rest of the Scenarios. The data is based on Downtown Los Angeles Seismic Zone parameters and is the same as the calculation notes of Braced Frame Scenario 1 and 2. Data: Soil type = D, S1 = 0.100, Ss = 0.250 Base shear k = 1.638 Cs = 0.090 (The result and normal ranges can be found in Appendix A). Cs max = 0.090 Cs min = 0.012 Effective seismic weight W = 16251 (kip) (The data needs to be verified.) Shear force V = 1464 (kip) (The data will be discussed and verified.) The explanations and glossary can be found in Appendix A. 4.2.4 Braced Frame Scenario 4 Calculation Results The analysis types were static linear, where the increase in force and the responses are in a linear relationship. Case 1: DL1, Analysis type: Static – Linear Case 2: LL1, Analysis type: Static - Linear Case 3: WIND1, Analysis type: Static - Linear Case 4: ASCE 7-10 / IBC 2012 Direction_X Except for the building model and the form, the other data that is input to obtain the calculation results is the soil conditions and other zone related requirements that affect the period of the building and seismic risk. Data: Soil type D, S1 = 0.10, Ss = 0.25 Spectrum parameters: Fa = 1.60 Fv = 2.40 SMS = 0.40 SM1 = 0.24 SDS = 0.27 SD1 = 0.16 To = 0.12 TS = 0.60 60 TL = 2.00 I = 1 R = 4.00 (The data needs to be discussed.) Fundamental period: Approximated method T = 1.77 (s) (The calculations from Robot) Other structures Ct = 0.02 (0.0488) x = 0.75 Structure range: Top story ROOF Bottom story Level 2 Effective height H = 395.50 ft Base shear k = 1.638 Cs = 0.090 Cs max = 0.090 Cs min = 0.012 (needs verification) Effective seismic weight W = 16236 (kip) Shear force V = 1463 (kip) (The results will be checked with hand calculated verification.) The explanations and glossary can be found in Appendix A. 4.3 Comparison Charts The 4 scenarios that investigate the different braced frame lateral design configurations are listed below with their properties. The results that were looked at were displacements, total weight, and seismic weight, base shear, carbon footprint, number of connections and complexity and cost. The images on the upper left show the structural plans with placements of braced frames highlighted in black. The upper right images show the elevations obtained from the 3D models (Figure 4-3). 61 Figure 4-3: Sample layout for identity charts that shows the relevant information 4.3.1 Braced Frame Scenario 1 Chart Braced Frame Chart 1 demonstrated the design parameters as well as building geometry. The deformation tab against the seismic loading condition (input parameters mentioned before) was obtained through Robot Structural Analysis simulation and diagrams (Figure 4-4a and 4-4b). (Figure 4-4a and 4-4b): The maximum deformations were retrieved through the structural simulation and was used in the results charts interchangeably in displacement and deformation. Except for the displacements, the base shear was also used as a parameter. The fundamental periods were almost the same for all 4 building configurations. For braced frame configuration 1, the Base Shear was estimated to be V=5501.47 kips (Figure 4-5). The other architectural parameters included were connections (dependent on the number of bars), total weight of the building, total carbon embodiment, and cost. 62 Figure 4-5: The units and results of the design criteria were demonstrated in terms of standardized formulas for the building model and all 4 configurations, and they are simplified to better compare. 4.3.2 Braced Frame Scenario 2 Chart For the Braced Frame Scenario 2 the braced frame configuration 2, the Base Shear was estimated to be V=1467.02 kips and maximum deformation as 3.32 inches. The carbon and the cost directly affected by the total mass of the building as proposed by the formula, and a straightforward quantity of connection points were around 672 (Figure 4- 6). 63 (Figure 4-6): Although not shown in the chart above, Braced Frame 2 gave unreliable data with excessive deflection. The issue with the Braced Frame Scenario 2 was the out of scale maximum deflections on Ux Uy and Uz dimensions. The results will be further discussed in following chapters, but at this stage it was noted that the following data gave unreliable results (Figure 6-4a and 4b), giving an excessive amount of bar deflections on multiple planes in Robot. The results also seemed inconsistent with the rest of the data obtained from Calculation Notes. The other architectural parameters will be discussed as a part of the architectural considerations part (Figure 4-7a and Figure 4-7b). 64 Figure 4-7a and 4-7b: The unreliable results need to be verified by secondary or additional resources, through re- validation through the same software or other software programs or faculty approved source. 4.3.3 Braced Frame Scenario 3 Chart Braced Frame Chart 3 results showed shear force results as 1464.59 kips; the total displacement was estimated as 3.3 inches. The total connections and cost were estimated to be initially higher than other configurations (Figure 4-8). Figure 4-8: The connections and cost are higher compared to the Braced Frame Layout 1 and 2. Nevertheless, the results gave higher bar deflection like Braced Frame Scenario 2. Again, the data showed some unreliable results that need verification from faculty sources or re-modeling through alternative software programs (Figure 4-9a and Figure 4-9b). The results obtained from the bar graph were excessive, showing more than a thousand- inch displacements in different planes. The possible reasons were discussed later in the chapter. Based on the seemingly high cost and out of scale displacement, this alternative was eliminated. Layout 3 65 Figure 4-9a and 4-9b: The maximum bar deflections demonstrated excessive results thus needs to be eliminated or re- calculated. 4.3.4 Braced Frame Scenario 4 Chart The Braced Frame Scenario 4 resulted in a shear force of 1463.29 kips according to Robot Structural Analysis and again estimated around 3.3 of displacement in the structure (Figure 4-10). The Braced Frame Layout 4, according to the shear forces and displacement, as well as the number of connection points and estimated carbon, and having minor or no errors during simulation process, was initially seen as a valid and usable example amongst all layout results. Figure 4-10: The model was initially determined to be a valid model for further discussion. 4.4 Comparison of Results and Design Parameters The comparisons against design parameters were performed once the initial results from simulations and calculation notes were obtained. For all of the design parameters, a relativistic approach was followed. This means, instead of trying to find the values in a specific range for the design parameters, comparisons and relative values were evaluated within the Braced Frame Layouts. For all design factors, (including cost, carbon, connections) the Layout holding the minimal value was considered the best result. In addition, as deflections gave either excessive or invalid results (and was estimated to be caused by interoperability issues or bugs), they were not included in the graphs, but a comparison of base shear and approximate displacements were shown. 66 For general displacement factors, any value that is less than the ratio h/500 to h/200 is considered an acceptable range for buildings (Schierle, 2019). The h in the ratio stands for the total building height. In addition, although a base shear is a factor that is reflective of many inputs (building height, weight and so on) in general for seismic design, the smaller number is considered better. 4.4.1 Displacement The maximum displacements 4 showed a considerable difference from Layout 1 versus 2, 3, and 4 (Figure 4-10). It should be noted that the Braced Frame 2 and Braced Frame 3 are shown in red as they showed excessive results in displacement and deflection. Accordingly, the Braced Frame Layout 1 and Braced Frame 4 was found to be usable, and in addition, the Braced Frame 4 is evaluated to be better than others. These results were consistent with the shear force exerted on the building. Both the shear force and displacement were found relatively higher. The causes of differences will be discussed at a later stage. In displacement chart, the approximate values of Braced Frame Layout 2 and 3 were estimated, as the base shear is known, but they are eliminated from the chart and comparisons as they showed excessive joint displacement and shown in red in the figure (Figure 4-11a, Figure 4-11b and Figure 4-11c ). Figure 4-11a: According to the seismic load calculations and displacement ratios, the last three layouts were found better. 67 Figure 4-11b and 4-11c: The red portion showed on the right showed the unacceptable range for more stringent displacement limitation. 4.4.2 Connections and Cost The number of connections in the building models was estimated to have the highest amount of costs in lateral load resisting frames (Figure 4-12). The fabrication of frame members, the fabrication and implementation of connections as well as the total amount of labor add a significant amount of load on budget. Figure 4-12: The Braced Frame Layout 3, with its high number of bar members and connections, was observed to be the most disadvantageous alternative for this measure. 4.4.3 Carbon The embodied carbon was set as one of the main design parameters since one of the main advantages of using mass timber as a structural system is to reduce the embodied carbon of the built environment greatly. The embodied carbon of the structure includes the vertical lateral load resisting systems, and a straightforward comparison was made to determine the relative amount of embodied carbon for each of the Braced Frame Layouts, by checking the total mass of each lateral load resisting frame alternative (Figure 4-13). Note that the numbers on the left side of the graph do not reflect the real values of total embodied carbon of the alternatives, they show the simplified versions to compare 68 alternatives within each other of the standardized formula (based on total weight of framing and overall structure: Mass = 16246113.769 (lb) x CO2 per weight ) to evaluate the braced frame alternatives within each other. Figure 4-13: Embodied carbon and carbon emission wise, the best alternative is determined to be the Braced Frame Layout 4. 4.5 Discussion per Architectural Parameters Besides the main design objectives in structural façade framing, there are several other architectural parameters in terms of design efficiency and architectural considerations that also influence the efficiency of the building. These range from the ease of access, making use of daylight, the flexibility of plan and lease options, the location of openings and so on. These were defined as the following: access and leasable space, window to wall ratio, load to structure, depth, cost, fire and code compliance, and serviceability. Ground levels of the buildings are high-density circulation areas that link the outside of the building to the services inside. The braced frames were designed in a way that they are continuous until the ground floor. In general, the bracings at the ground level may be identified as a difficulty by excluding the access points to the building. In some other cases, the accommodation of the services requires the ground floor to have maximum accessibility, such as having storefronts and retail areas. In these circumstances, the ground floor enclosures and placements or braced frames may need to be distributed accordingly. For the Timber Tower and Braced Frame Layouts, these were not of primary concern. Nevertheless, for the general suitability and ease of accessibility on the ground floor, the ground level braced frame placements were found useful to evaluate. The architectural parameters mentioned above are not linked to the analytical model only, but to the properties that can be obtained from the architectural model (Figure 4-14). 69 Figure 4-14: From left to right Braced Frame Layout 1, Layout 2, Layout 3 and Layout 4 respectively. During the selection process, several other architectural considerations were conducted Braced Frame Layouts (Figure 4-15). Figure 4-15: Given the layouts of the ground floor braced frame placements, the options selected were Braced Frame Layout 1 and Braced Frame Layout 4. a. Access: The access was based upon the ground level access and which alternatives allowed a more open plan at the ground level for pedestrian access and services that would potentially require an open plan, such as retail/commercial, recreational purposes and so on. The evaluation was based upon the consideration that more open access would be more beneficial in terms of ground level usage and leasable area. Thus the alternatives that had closure in corner bays were considered more disadvantageous, and the alternatives with minimal interruption were considered better. The metric that was used to evaluate the access was based upon the total undisturbed linear footage in the floor plans of the buildings. The more undisturbed linear foot was considered better access for the buildings and also was considered advantageous for leasability of the building. The braced frame layouts only changed their configuration on the perimeter, but not the core, so access at the core criteria did not affect the selection on a significant scale (usually, the architectural parameters such as accessing the circulation shafts and other services and braced frames at the core affect each other more, than the perimeter braced frames.) 70 b. Window to Wall Ratio (Openings): The ideal window to wall ratio, and which range is more beneficial depends on various factors. To make the buildings more leasable, in general, higher window to wall ratio is better (up to the maximum allowed by law, approximately 40% in some locations), although it does not have a direct effect on the gross leasable area. Nevertheless, the lower WWR might have a positive effect on lowering the building’s operational costs. This will also help decrease the quantity and size of the HVAC equipment needed. Whereas the braced frames were not considered as a full closure like a cladding or shear wall would do, more openings and larger window lines were determined to be beneficial. Although the braced frames did not pose a requirement for full closure like shear walls, depending on the number and placement of the bracing on the layouts, it was observed Scenario 2 would be relatively disadvantageous compared to the other. Total undisturbed surface area was taken as a metric, and for the comparison, the higher openings were considered better for increased occupant comfort, more daylight and more lease options. Nevertheless, it should be noted that the number of open surfaces in façade and WWR required an extensive study that should be covered in future work. In terms of daylighting braced frames did not seem to affect the entering of daylight as much. This evaluation was based upon visual inspection only, and the daylighting might be affected by the thickness of the cross sections, as well as the access in the building may change. c. Load to Structure: Design of lateral systems and façade structures usually pose some challenges in terms of weight to the structure and the rest of the façade systems. These usually pose a bigger issue in cases of shear walls and heavier materials, unlike timber. The brace frames were used throughout and consequently, this would cause a smaller issue than other types of construction materials and methods. Again, in terms of frame depth, the criteria become a bigger issue for moment frames, shear walls, and in the utilization of concrete or reinforced concrete. Nevertheless, when 4 layout alternatives were compared, it can be said that Scenario 4 was seen to be slightly better as it has a lower total weight. The standard used to evaluate the most advantageous or disadvantageous options was based upon the weight of the structure, obtained previously from Robot. As the same baseline building was used for all 4 layouts, the difference occurred from changes in the weight of the braced frames. The results of this metric were taken from Revit models and Robot calculation notes. d. Depth: The depth is a more significant parameter in shear walls, moment frame, and other façade systems. For the braced frames, initially, there was no difference in cross sectional area or sizes of members in between different Braced Frame Layouts. Nevertheless, the braced frame members may need to get thicker after design iterations and hand calculation. This is based on the need to validate the results, and the validation of the capacity design of members (the frame members’ thickness was iterated after initial simulations). This might change the thickness and depth as well as the amount openings that might be affected, for all configuration types. The depth metric is the cross-sectional area and the dimensions of the individual braced frame members throughout the building, and the results did not differ in between. e. Cost: The cost was based on the amount of labor that is estimated to be put in the building construction. The cost was associated with detailing and the number and complexity of connection points and materials at this stage: the cost estimation was done only in terms of quantity of bars and number of connections. Precise cost estimation for the selected and improved layouts ws not yet done although it would be better for evaluating the real economic feasibility of the designs. Nevertheless, as connections add costs to the budget, a direct correlation between the number of braced frames and nodes of members was done in order to determine a general understanding of which alternative would be more, or less costly in terms of this aspect. This evaluation assumed there are no great differences in the cost between varying total lengths of braced frames (as the alternatives were not always put in the same bays with the same dimensions.) The metric that was used to compare costs was the number of members and connections. The cost for individual members is a part of the discussion and future work. f. Fire Ratings and Code Compliance: As much as the fire ratings and code compliance is generally an important issue to be considered for architectural design decisions, this was not considered to be a parameter that affected the design iterations and changed greatly in between. Nevertheless, code compliance was an important design consideration that needs to be covered. The evaluation of this parameter is usually based upon how the buildings and building parts comply with the codes and tests and measures are taken to evaluate the fire ratings and safety against fire. This was not done in this case. 71 g. Serviceability: The objective of implementation of braced frames was to limit drift not only in a range of structural safety but also serviceability as well, which varies greatly by architectural decisions given. Servicability is an outcome of the sway of the buildings, and it is linked to displacement and shear forces exerted on the building. This evaluation eliminated the alternatives 2 and 3, as they gave excessive and unreliable results. The evaluation of this consideration was based upon the results from Robot. Iterations in designs and re-checking the seismic outcomes (or physical testing) might have given different results for this parameter. 4.6 Evaluation of Workflow, Validation, and Selection Among the initial calculation results and design objectives, Braced Frame Layout 4 was determined as the best option. The Braced Frame Layout 4 was planned to be used as the prototype for the initial selection, but not a finalized design as the results was planned to be supported through secondary validation and further design. So, it should be noted that the selection was used as a baseline to iterate upon with more accurate results. One of the reasons for this was the requirement of further design and analysis and validating the finalized, improved design through secondary processes. This type of workflow can be used as a guideline on its own, the design would not be finalized without further study or detailed design ideally. The secondary reason for iterative workflow (or a cyclic design workflow) was to discuss the validity and accuracy of results. As some simulations in Braced Frame Layout alternatives gave unreliable or inapplicable results (such as in Braced Frame Layout 2 and 3). The workflow and possible errors and non-interoperability of the software programs being used will be discussed. In addition, the hand calculations and use of ENERCALC will be discussed for validation and further analysis. 4.6.1 Interoperability Autodesk Revit Structures and Autodesk Robot Structural Analysis Professional have a direct link in each software that allows Revit model to be exported to Robot; the Robot model can be exported to Revit as well via direct integration or intermediate files (if multiple computers are being used for the same model). Through the Structural Analysis link, the models can be updated through importing to Robot or Revit. From Robot to Revit it also allows the user to get the results of the updated model. The level of information the Robot model obtained depended on the Revit and Revit Structural models and what type of data is transferred. The Robot models solely worked on analytical models. These represent the engineering systems of building models rather than the visual and architectural representation of the buildings. The architectural representations of building elements can be misleading, as analytical models are visually more straightforward descriptions of 3D models (Figure 4-16 and 4-17). Figure 4-16: In Revit, the two different views (3D model on the left) showed the architectural representation of the braced frame scenario 1 and the analytical model (the model on the right) of the same scenario. 72 Figure 4-17: Robot Analytical Model showed the analytical model that is obtained from the Revit 3D model In Figure 6-12. Revit is an object-based modeling program comprises of different architectural, electrical, mechanical and structural objects whereas Robot obtains uses and outputs analytical data only. The analytical model that was in Robot, which is solely the representation of engineering data, took this information from the “objects” and family types in Revit and Revit Structures. For this reason, the export process on what type of analytical properties is transferred during two software programs was crucial. It was possible and easier, to see the what is transferred between Revit objects and Revit Analytical Model/Revit Structures model object, as the models can be transferred through a direct link with no need for changes or updates during export. In addition to that, the load cases (i.e. dead and live loads) could start to be created in Revit Analytical models before sending to Robot. In addition, as long as the object had analytical representation in Revit (Structures), getting analytical properties could be done just by enabling those properties. As the braced frame scenarios were initially designed in Revit Structures and not as a 3D architectural model, this was not a problem with the data transfer. As explained previously in Chapter 3, Methodology, the main data transfer of the models occurred between Revit Structures and Robot Structural Analysis for the Braced Frame Scenarios. During the transfer of the Revit objects (that make up the structural model) to Robot, what amount of data is transferred played a crucial role in determining the level of accuracy and expected results in Robot. How to do the translation between Revit objects and Robot elements were described in Autodesk webinars, and the webinars demonstrated their directionality as well The directionality whether and how the software programs receive the model data and family types). Some objects could be exported only to Robot, where the objects in Revit turn out to be analytical nodes or conditions in Robot, while some can be imported back to Revit as a different set of objects (“Interoperability Webinar,” www.knowledge.autodesk.com, 2019) (Figure 4-18). 73 Figure 4-18: Specific component exchange between Revit and Robot (“Interoperability Webinar,” www.knowledge.autodesk.com, 2019). There are structural and material limitations for the glulam members in Revit, and the comparison between Revit to Robot was made to see the differences in the contained data in Revit and Robot for the same model and same structural members in following parts of the chapter. The braced frames designed in Revit Structures translated to Robot Structural Analysis to structural elements as bars which the braced frames are nested under and they are the family types that contain the material properties and data exchange. The information transfer to Robot contained several main elements that the precedent design contained in Revit. Robot contained the analytical geometry and the dimensional input of the glulam beams in Revit, both in length (feet or m) and the location in the Cartesian coordinate. The properties tab had section data, not only the dimensional section but also the gamma angle (rotation angle) of the bars and elements (Figure 4-19). The cross-sectional properties included in the Properties tab consisted of Basic, Dimensional and Mechanical properties for the braced frame. The cross-sectional properties included the section types used in the same project and could be changed. The Section chart included the interchangeable elements that already existed in the Braced Frame Scenarios (Figure 6-15). The basic sectional properties obtained data from the bar member allowed the calculation of bar members and properties. The Ax showed the area and the Iy and Iz showed the moments of inertia (Figure 4-19). 74 Figure 4-19: The sectional properties in Robot Structural Analysis Professional 2019. The Dimensions in Sectional Properties in the software consisted of depth and the width of the section (which the glulam member was classified under in Revit and was represented in Robot in bf and d). Vy, Vpy, Vz, and Vpz represented the values used to calculate normal stresses based on Y and Z axes. The values represented the distance of fibers with furthest distance to each other in given axes (Figure 4-20) (“Section Properties”, help.autodesk.com, 2019). Figure 4-20: The dimensional values are used for normal stress calculations, the values from Braced Frame Scenario -1 and representation of values in the cross-sectional image (“Section Properties”, help.autodesk.com, 2019). The Mechanical Properties under the Sectional Properties tab included the Ay, Az, Ix, Wx, Wy and Wz (Figure). The Ay and Az represented reduced sectional areas in y and z directions respectively and are used by Robot to determine 75 shear deformations. The Wx demonstrated the section modulus, Wy and Wz demonstrated the reduced shear areas of the area (“Section Properties”, help.autodesk.com, 2019). The I (Ix) represented the moment of inertia of the cross section (Figure 4-21). The material properties were associated with glulam members (Softwood, Lumber). The E represented the Youngs modulus, G Shear modulus, NI Poisson’s ratio, Ro unit weight and Re resistance, LX thermal expansion coefficient. As it could be obtained from Material Properties, the strength and stiffness properties which were significantly higher in glulam and laminated timber than conventional timber. Figure 4-21: The stiffness properties on Material Properties tab provided the data on the load bearing capacity of glulam braced frames. The releases (the member ends) defined the structural releases. The list field showed the possible types of releases that could be assigned to the braced frames or other types of nodes. The release type for the Braced Frame Scenarios had been shown as pinned connections as modeled in Revit and in “xxxxff” boundary conditions. The different type of end supports get a different degree of freedom thus there is a change in the fixed degrees and free degrees in different types of supports. The free degree of freedom was represented in “f” and the fixed degree in “x” in Robot Release properties. The frame members in Braced Frame Scenarios were Revit-xxxxff Pinned (Figure 4-22). The system in Robot Structural Analysis worked in the form of varying degrees of freedom whereas the coordinate system differed for other software programs such as STAAD Pro (Taher, 2016). Figure 4-22: The offsets were not applicable for the braced frame alternatives thus was translated to Robot properties as not applicable. The elastic ground was only applicable to some structure types and was not applicable to the braced frame scenarios. Again, the brackets (bracket beginning and end) were not applicable in Methodology (Chapter 3) thus did not contain data in Robot bar properties (Figure 4-23a and 4-23b). 76 Figure 4-23a and Figure 4-24b: The bracket properties not applicable in Revit and Robot. 4.6.2 Limitations and Assumptions The modal analysis is a dynamic analysis mode. It shall be performed to have more valid tall building analysis for future work. Tall buildings, in reality, do not show a static behavior under seismic loads. The real behavior is considered a dynamic behavior. The amplified effect of seismic forces is different which might be higher than estimated design codes, which may be enough for short buildings but not for tall buildings. Tall buildings show longer periods which may eventually cause different modes of drift and effects to the structure. These might cause bigger shakings eventually at tall buildings (“How Seismic Waves Affect Different Size Buildings” USGS, 2019). In addition, the modal response spectrum analysis is a more common approach to check the performance of the structure in terms of serviceability (“TBI Guidelines for Performance-Based Seismic Design of Tall Buildings”, 2010). Modal response spectrum analysis the performance is checked and or redesigned. There were also limitations in materiality that is sourced from the software programs being used, which will be discussed later in the future work and conclusion. 4.7 Summary In Chapter 4, the data obtained from Braced Frame Scenarios was shown, with the main design objectives that focus on displacement, base shear, and deformation. The uncertain and erroneous results were also pointed out at this stage. For the results that were questionable in terms of displacement, a comparison in base shear was done. The comparison charts were created to list and evaluate the results and assess the best results within these parameters, cost, and carbon. In addition, parameters and comparisons of architectural requirements were conducted. 77 An initial design was selected as the most valuable result in these parameters. An important aspect is that the result and selection of the Braced Frame alternative were not certain nor final, but this was taken as a baseline for further study and improved design. CHAPTER 5: DESIGN ITERATION, VALIDATION AND MEMBER CONNECTION RECOMMENDATIONS The chapter includes verification through hand calculation and also utilizing it together with Enercalc software, to validate the cross-sectional properties and stress design capacity of used braced frame members. The individual frame members are re-iterated and improved without changing the general layout and configuration of Braced Frame 4. The connection strategies are then provided to conclude the discussion. The chapter makes an introduction to the work being conducted, a summary of hand calculations, ENERCALC calculations and error estimation in the selected layout and structural models, design iterations after capacity design and finalized design. The chapter concludes by recommendations on the core and connections of mass timber members and in the building. 5.1 Introduction Initially, hand calculations were done to determine the loads on the structure and the seismic weight and the base shear. After that, the loading conditions per story and to individual frame members were determined, and Enercalc was used to evaluate the allowable stresses in members. The slenderness of the existing frame members was problematic, therefore they were improved at this stage simultaneously. The Enercalc results showed the members cross sectional sizes should be changed, and additional bracing might be needed. The results are then put back to the Revit Analytical Model, and the changes were made in the model shown in plans and in 3D views. The finalized model was made ready, to test, if needed in Robot Structural Analysis or other structural analysis software programs for simulation. After that, connection designs were discussed, and general strategies for the project were recommended to be applied in the future. 5.2 Hand Calculations Hand calculations were used on further analysis on capacity design of timber braced frame members which were tested as a timber column in ENERCALC. First, the total amount of loads exerted to the building was calculated The SOM Timber Tower Research Project handbooks and design data was taken as a guideline to evaluate the total loading conditions, including the dead loads and the live loads. The floor plan and assemblies and occupancy conditions of Timber Tower project were taken as a reference. After a straightforward calculation was done to estimate the total loads per floor area (1016.26 kips), the total weight of the structure was also determined (42682.92 kips). The total base shear was estimated by determining the Cs values initially. Cs= Sds/ (R/I) Sds was determined by Seismic Design Maps, and for Downtown Los Angeles it was estimated as 1.567 in numeric seismic design value at 0.2 sec SA. Although the R values are in between 1.5 to 2 for timber structures, for CLT the R value ranges up to 4.5. The importance factor was determined to be equal to 1. Thus, the Cs value is estimated to be 0.35, and the base shear at the bottom was estimated to be 14939.02 kips, whereas the top story shear was estimated to be 355 kips. Please see Appendix B for the detail of hand calculations. 5.3 Frame Analysis in ENERCALC ENERCALC is a software program and an established structural engineering library that allows the engineers and architects to perform stress design and evaluate the capacity of frames and individual members in their buildings within the range of building codes (“Engineers Depend On Structural Engineering Library,” www.enercalc.com, 2019). Although it is used mostly for simulations of low and mid-rise structures, there are materials and members available that are used in tall building design. For a better demonstration of the process, the calculations in ENERCALC will be explained in stages: 78 Step 1. The base shear (along with interim shear and top shear at the height of the building) were obtained by hand calculations (See Appendix B). Step 2. The base shear and top shear forces are exerted on the plan, and the loading to individual braced frame members was estimated through load distribution on the plan scale. The loading on the frame was estimated, and after the diagonal glulam members loading conditions were determined on a geometric scale, the glulam braced frame members were translated to Enercalc as timber columns. The timber column design allowed two different analysis methods in design stresses. ASD vs LRFD Analysis Method are two methods used, the ASD being the allowable stress design and the LRFD being the load and resistance factor design. (Figure 5-1). Figure 5-1: Design Stress Section at Enercalc interface. Step 3. The wood column section properties include making a selection from glulam columns, offering a variety of species and lamination types, showing the reference values for each of the listed glulam members and allowable stresses (Figure 5-2). The Western Species and DF (Douglas Fir) from California was used. The L levels are attributes for the lamination grades. The L1C1 with close density and knot size (occupying less than ¼ of the cross section) was selected. This also contained information on the slope of grain (being less than 1:12) for the complete member piece. (“Standard Specifications for Structural Glued Laminated Timber of Softwood Species," American Institute of Timber Construction n.d). 79 Figure 5-2: The software library showed the types of engineered wood products and varieties of glulam types, and the species used throughout the study. Step 4. The ASD analysis method is selected, and as the used glulam laminations were >=4 in the earlier stages, similar selection was applied to the wood grade and laminations (Figure 5-3). (Figure 5-3): Once the selection was made, the values of glulam column were automatically applied, such as the bending values, and E modulus. Step 5. The following calculations included the testing against the top story shear and base shear as the loading conditions would be highly different. Initial loading was applied on the column with loading lower than actual top shear. After that, the columns were tested against the top shear, estimated to be 355 kips and with the base shear at the bottom was estimated to be 14939.022 kips (Figure 5-4 and Figure 5-5). 80 (Figure 5-4): The sample loading was done with the existing cross-sectional properties retrieved from Revit. Although the loading conditions were not unsuccessful, it was realized the slenderness ratio is excessive for the members at this stage. 81 Figure 5-5: It should be noted that the design combination results and detailed results could not be retrieved, as the error shows that the slenderness exceeded for the braced frame members. Step 6. At this stage, the slenderness ratio of frame members was checked, as ENERCALC gave errors on how the slenderness exceeded in calculations. The Slenderness Ratio of Frame Members: For structural wood columns in compression, the bar elements should be sized in such a way that their slenderness ratio does not exceed 50. The slenderness ratio for the structural column is le/d, where le represents the total height of the column, and d the depth, or the width depending on the dimensions. The larger of the ratios (Width/Height or Depth/Height) is taken into consideration to determine the value below 50. The slenderness ratio larger than the given value would result in decreased compressive strength in columns and bracing and eventually failure related to buckling. Although glulam and mass timber can bear higher compression loads, it was initially observed that the members were too slender for the Braced Frame Layouts. This could be observed through the empirical analysis of the model and seemingly out-of-scale members. In addition, when the dimensions of the initial members are checked, the slenderness ratio was determined to be excessively big. Due to different bay dimensions, different lengths of glulam bars were used from the same cross-sectional properties in Revit Structures (Figure 5-6). 82 Figure 5-6: The glulam member on the left, occupying the wider bays is 3.125 by 12 glulam bar with almost 24 feet (288 inches) length (East and West elevation of the façade). The one shown on the right is occupying the narrower center bays (North and South elevation of the façade) of the building. The length of the smaller bar is in between 15 to 16 feet (192 inches). Given the cross-sectional depths, the slenderness ratio for the bars are, respectively For the frame member on the left: l1/d1: 92.1 For the frame member on the right: l2/d2: 61.4 The numbers obtained from the Braced Frame Layout 4 model showed that the members are excessively slender (l/d ratio way above 50). The slenderness ratio is excessive with 3.125 x 12 cross sections, so the ones used in the project cannot be used (Figure 5-5). The glulam members dimensioned in ENERCALC utilize 12 ¼ inches depth members, thus keeping the slenderness ratio of the members in a permissible range. It should be noted that the availability of cross-sectional properties of structural glulam members obtained from the American Institute of Timber Construction was higher in terms of dimensions, as the section properties of western species included bigger widths and depths than the ones obtainable from ENERCALC Wood Column Library (Figure 5-7). The capacity design of timber frames was limited in cross sectional dimensions than the American Institute of Timber Construction database provided; in actual use, bigger cross-sectional sizes and larger capacity design could have been possible without additional bracing. 83 Figure 5-7: American Institute of Timber Construction database provided a larger range of cross sectional members. Step 7. The overall column height given in ENERCALC was used to calculate not the slenderness ratio but the analysis of bending stress ratios. There were some limitations on not being able to increase the overall column height to 24 feet. The reason for the limitation was that the demo/student version of ENERCALC was utilized for analysis and that full access to the program features was not available. The automatically given value was almost the same as the braced frame members on North and South façade with the shorter length, thus the analysis in ENERCALC was more valid for the mentioned bar elements. The capacity of the longer bar elements of the model would arguably be more, but to keep the consistency, the cross-sectional features throughout the building was decided to be kept as same as the ones which are 15 feet length. Step 8. When the loading conditions for the base of the building and corresponding framing conditions are checked (approximately 3734 kips), the capacity design calculations were run in ENERCALC, and it was estimated that within the limits of cross sections available, it was unable to sustain the loads in braced frames at the base (Figure 5-8). 84 Figure 5-8: Failure mode occurred even though with the biggest cross sections available were used. Step 9. In the following step, the limit state of the capacity design within the largest cross-sectional glulam members were investigated. With this, it was aimed to see the largest amount of load that can be taken at the ground level frame members if the cross-sectional dimensions had been increased (Figure 5-9). Figure 5-9: The largest loading the cross-sectional member at the base could take was estimated to be around 1440 pounds force, which was observed almost one-third of the actual loading to the frames. Step 10. The initial design loading conditions and hand calculations that distribute the shear force to tributary areas, and neglected the utilization of a shear core to have a lateral load resisting system entirely with expressed braced timber 85 framing. Nevertheless, the high amount of lateral forces exerting on the building and the secondary investigation showed there is a need for a lateral load resisting wall or framing at the core as well once more (Figure 5-10). Figure 5-10: The initial layout of the shear core and implementation of Braced Frame 4. Step 11. During this further analysis, the initial attempt was to eliminate the need of the shear core as well, but there is a need for additional perimeter bracing or shear or braced core. Because of this, additional bracing was added and ENERCALC calculation was re-done. Step 12. In the secondary iteration, the improvement was done upon decreasing the total occupied size of the shear core or adding a braced frame core as well. During this design change, the core area size decreased to a 45x45 feet, which allowed two bays on both four sides of the core to be braced with glulam frames from the same sizes at the exterior of the building (Figure 5-11a and Figure 5-11b). 86 Figure 5-11a and 5-11b: The design changes proposed two different alternatives that are expected to be successful in the capacity design of the braced frames in the secondary calculation. 45x45 feet shear core or 45x45 braced frame core. To keep the scope and have a straightforward approach to load distribution, the later, braced frame core was selected. The core was determined to be changed into a 45x45 braced frame core. Step 13. As a result from the secondary calculations for top shear, shear force in intermediate stories, and base shear, the following 3 cross section types were found successful in stress design and selected (Figure 5-12). . 87 Figure 5-12: The section types were re-tested for improved (decreased) design loads after increasing the total number of members. Step 14. The top shear loading on members is approximately 30 kips, although the 5.125 x 12 cross sectional members pass the test, the slenderness ratio is too high for the existing building model, thus 6.75 x 7.5 could be used (Figure 5- 13). Figure 5-13: The slenderness ratio was determined to be excessively high for the braced frame members at the top, even though the selected cross-sectional glulam column was successful in ENERCALC. Step 15. The seismic shear at the middle of the building and it’s loading on braced frame members resulted in (around 650 kips) cross-sectional member selection: 12.25x 33. 88 Step 16. The seismic shear at the base of the building and it’s loading on braced frame members resulted in (around 1260 kips) cross-sectional member selection: 12.25x 55. Step 17. The 3 different cross section types needed throughout the building were determined as above. These 3 types would be used on 1/3 rd of the building height each. 5.4 Design Iteration As mentioned early in the chapter, the selection of the Braced Frame Layout 4 (and the other possible configurations) was subject to change based on the thickness and cross-sectional properties of the individual members. One reason for the change was to allow further studies. An iterative process was used that included validations, design, and calculation. The further design process not only changed the cross-sectional properties but also re-validated the need for the extra lateral support at the core. Design development is an ongoing and complex stage, thus the iterations mentioned above need to be re-validated and designed and calculated more throughout with other forms of failure modes and details, such as connection design, and in the future shall be tested via different means and tools such as SAP, ETABS or RFEM and with different loading conditions. The following section demonstrated general strategies that can be applied to the structure in terms of connections based on glulam and mass timber member properties. The main changes that were made were the changes in cross-sectional sizes of the members, and utilizing different members along the building. The first 1/3 rd of the building (from ground up) utilized thickest bar members utilizing 12.25x55 thick bars. The middle 1/3 rd of the building utilized 12.25,33 thick bar members in bays. The smallest members were used at the top with 6.75 x 7.5 thick dimensions (Figure 5-14). <Page intentionally left blank.> 89 Figure 5-14: The improved design of Braced Frame Layout 4 showing the iterated cross sections of perimeter braced framing. In addition to this, it was realized through ENERCALC that the stresses on the individual braced frame members were getting excessively high especially if the shear core from the buildings was going to be eliminated. Due to this, an alternative core was developed (which was included in the hand calculations and ENERCALC calculations) that utilized a braced frame core by adding 2 diagonal braces on each side. Each member is around 12.25x 33 thick and can be supported by reinforced concrete link beams (Figure 5-15a and Figure 5-15b). 90 Figure 5-15a: Recommended system for the core, showing the timber frames inside the building core. 91 Figure 5-15b: 3D view, exploded view of 3D view, and analytical model. 92 The following section included general strategies that were not covered in design iteration. They include general strategies for member properties and connection strategies and recommendations for connection design. 5.5 Sectional Member Properties and Connection There are general strategies that can be applied to the structure in terms of connections based on glulam and mass timber member properties. Among different type of mass timber products, glulam members were used throughout the modeling and simulation process for braced frames for the unique characteristics of the material type. It is a material commonly used for columns and beams and found useful to resist horizontal forces and is adequate to be used for diagonal bracing members, where stiffness to a certain extend is needed for the braced frame scenarios to keep the sway restricted. The western species of glulam member described the commonly extracted species that are dominant in the Western United States (Figure 5-16). Glulam types may include spruce, pine, and some other timber species, but the most common and leading species in the West Coast is the Douglas Fir. Douglas Fir is easy and cheap to obtain, in addition, in North America it is one of the strongest species, providing required stiffness for structural members (Howe et al. 2006). Figure 5-16: Mass timber member types that exist as Revit families. The laminations that are recommended for general usage makes use of 1 inch and 2-inch-thick laminations (AITC 113- 2010, 2010). Glulam can be fabricated in a variety of dimensions. The design and sizing and properties was limited by the modeling limitations in Autodesk Revit. The glulam members can show a variety according to their lamination orientation. Horizontal glulam is usually the most commonly used type in construction (Figure 5-17). Figure 5-17: The horizontal lamination is the most common type of lamination of glulam structural members, however, reversed layers (center figure) and vertical arrangement is also possible. The family properties for glulam member in Autodesk Revit did not currently accommodate the information based on the lamination direction and assembly of glulam members. The assumption was based under the knowledge that most commonly used lamination type is horizontal lamination, and Revit material properties follow similar material properties. This and similar issues will be discussed further in Chapter 6, Limitations and Future Work. 93 5.6 Connection Methods Throughout the schematic design and initial simulations, timber braced frame connections were a significant part of the case studies as they highly affect the behavior of the individual elements and overall structure. The seismic behavior of the frames varied greatly between different types of connections. One of the biggest interacting factors on the behavior of the selected lateral systems is being dependent on the ends of the braces. The connections and how they behave also depend on the strength and stiffness of the frames. In the events of an extreme load in lateral direction and failure of timber members, most of the cases there is a failure in connections and how they are put together. The connectors are the interim building elements, or components that connect, attach, bind two (or more) larger building components together. They can connect the same structural building materials (such as steel to steel) or different elements/ hybrid ones (concrete to steel, wood to steel and so on). The alternatives of how wood to wood and wood to steel connections is a significant part of the discussion for timber braced frames. The connections do not only play a role in transferring gravity loads but also lateral loads such as wind and seismic and transfers the loads to the foundations. Wood can have weak tension and shear capabilities based on the grain direction on the load transfer path. This needs to be avoided, which causes tension and shear strength weakness. The use of multiple connections and uniformly distributed load paths allow the loads and stress to not concentrate on one point. Some recommendations for the implementation and maintenance of connections and connectors are as follows: • The size of fasteners and connectors should be consistent with the size of the mass timber components (or any structural material component, per se). Usually, the smaller size of fasteners the better. • Connections and fasteners for the wood members should be installed with consideration of mechanical and physical properties of wood. The removal of moisture from wood members and shrinkage is an important design factor to decrease the risks that will occur from it. • Using multiple fasteners over singular fasteners enables better load paths. Although there is a variety of timber construction types and building systems (general timber frame construction, light frame wood construction, heavy frame timber construction, and mass timber construction), the connection types that are used are similar. Even so, different timber building systems and mass timber building materials may need different approaches and considerations to connection design based on the type of timber product and the utilization in the building (dimensions, span of the component, location in the building and so on). Just like any other timber frame or mass timber construction material, there are behavioral risks of glulam that is inherited from properties of timber. 5.7 Recommendations for Connections Some key principles at connection detail design of glulam elements include structural and architectural design concerns. The structural principles can focus around the mechanical properties of glulam, the failure modes in specific incidents, shrinkage and swelling and changes in moisture content and so on. The recommendations are given so that they can contribute to the future work of connection design and analysis, as discussed in Chapter 6. 1. Eccentricity is risky and should be avoided. Not only the eccentric joints have been unsuitable through the schematic design in Chapter 3, but also in connections eccentricity increases risks of internal moments and stresses that are on the weaker grain side of the wood trusses. Out of plane behavior should be avoided. (Technical Note: Glulam Connection Details, “www.law.resource.org,” n.d). The used braced frame types in the initial design stage consisted of concentrically braced frames, and no eccentricity occurs in neither ends of bracing members, thus the design criteria are already fulfilled. 94 2. When the glulam is in direct contact with concrete or masonry building components, it creates a high risk of moisture transfer and decay, especially when in open contact or in poorly designed or incorrect connections. From a practical standpoint, direct contact is not preferred (Glulam Connection Details, APA). Nevertheless, the scenarios at Chapter 3 utilized glulam braced frame in connection with mass timber built-up columns. The application of them did not pose the risks as they are not in direct contact concrete/masonry building components. The Timber Tower Research Project is a concrete jointed timber frame, but the concrete in the floor plates was not in direct contact with the implemented braced frames. The only part to be careful was the spandrel beam to braced frame contact, which can be altered by several preventive measures. The Timber Tower and the braced frames do not create a risk factor for eccentric joints nor decay occurring from concrete or masonry. If so, a drainage hole or additional connection shall be placed, or extra treatment to the members -glulam members- should be made. 3. The end grains are risk posing parts at both glulam braced frames and other members. The end grains of the connections tend to absorb fluids and moisture faster and more prone to environmental factors effects. This is also the reason for the end grains of wood members being in darker color, or more prone to moist or cracking. The tendency to absorb moisture can result in checks, cracks and local failures that can result in bigger risks in structural integrity (Nigohos, 2019). Treatment and seal of end grains, protection by covers, like seal or gutters, minimize contact areas of these ends, box covers, metal covers. Sealant applications also help glulam structural members to be protected against crack and checking. If it is going to be concealed proper measures should be taken. Contact faces should be decreased, and a minimized number of contact faces is an additional strategy that can be followed. 4. Trapped moisture can lead to excessive moisture absorption in the building enclosure and may result in partial decay of glulam members (Barber, 2018). The more contact faces and number of connection points there is, the more risk of moisture entrapment there will be. One of the strategies that can be applied is to provide drainage and connection details that allow the drainage in between the glulam members (Barber, 2018). 5.8 Summary The chapter is a continuation of Chapter 4 where the Braced Frame Scenario 4 was picked to validate and design in detail. Manual calculations were done, and capacity design of the braced frames was calculated, ENERCALC material library was used to test the capacity designs of the frame members. Finally, the needed iterations on cross- sectional properties to due stress design were determined, and the design was developed with further iterations of calculations. The cross-sectional sizes, in addition to the change in shear core and the need for adding more braced frames were determined. The results show that the existing cross-sectional properties and dimensions of the member should be iterated based on the calculation results from stress design. In addition to that, whether it is a shear wall or framed tube system, additional lateral load resisting system should be existent for Braced Frame Layout 4. Due to this, an alternative braced frame core was recommended, which was included in ENERCALC calculations. The process continued by providing general strategies for connection design and detailing. 95 6. CONCLUSION, SCOPE AND FUTURE WORK This chapter includes discussion, current limitations, future work, and conclusion. 6.1 Introduction Possibilities of mass timber were explored as a main structural material in the lateral systems of high-rise buildings with specific requirements for lateral design and assessed the performance of braced frames in high rise wood construction. This included not only a seismic evaluation but also architectural parameters and constraints (Fig. 6-1). The design objectives were selected so that the designer can apply similar practical limitations and determine the best design solution from a multi-objective perspective. Four case studies were examined. Figure 6-1: The methodology that was followed showing key steps throughout the study Mass timber has the potential to mitigate the problems of construction in the future, as it is a highly sustainable material and highly beneficial in terms of embodied carbon. It is observed that there are only a few, but promising examples, of high-rise buildings made from mass timber or hybrid timber systems. These existing examples demonstrate high-level use of mass timber, but still, incorporate conventional materials for lateral load resisting frames in general. Timber has the potential to be used for both gravity load design, and earthquake resistance while 96 also can responding to practical construction constraints. It was stated that the seismic behavior of timber was better than conventional materials used, such as concrete, taking evidence from the data from past earthquakes (Rainer and Karacabeyli, 2000). Mass timber can it can be used in lateral load resistance against wind and earthquake motions, as there are examples started to be seen in high rise construction with mass timber. Braced frame systems can be used in mass timber systems as a response to seismic forces. A baseline building was chosen, the “Timber Tower Research Project” by Skidmore, Owings & Merrill. The site of the building was changed to Los Angeles and braced with timber frames against earthquake loads. The data collected from SOM’s research project on a mass timber office building in terms of shape and layout attributes, material and building systems selection, and other building data was used to both determine the possible layouts and remodel the building in Revit. SOM’s Timber Tower Research Project was re-modeled in Autodesk Revit Structures and linked to Autodesk Robot Structural Analysis for the baseline scenario simulation. Then four layout alternatives were modeled (Fig. XX) They were linked to Autodesk Robot Structural Analysis software program to test and evaluate against design considerations that included displacement, base shear. The weight embodied carbon (footprint based on the amount of material) of the structure, and cost based on the quantity of connections were compared based on the results from simulation and model data. Identity charts and comparisons were made on displacement (based on shear force and seismic factors), embodied carbon (based on weight) and the number of connections (affecting cost). The later design considerations included access, window to wall ratio, leaseability, weight to structure, labor and cost. The design considerations like code compliance, fire safety were also included as a design consideration but was pointed out that they were unconsidered at this point. Braced Frame Layout 2 and Braced Frame Layout 3 were eliminated as they gave invalid simulation results in Robot. Braced Frame 1 and 4 were selected as valid designs, and based on the design considerations described above, and to move on with a single design layout, Braced Frame 4 was selected for the detailed design. The frame analysis and results of individual stresses were not used from Robot Structural Analysis but were calculated through manual calculations and ENERCALC. In addition, as the results from an initial simulation with minor emphasis on cross-sectional members required a need for validation to compare base shear, acting forces on stories and individual frame members. The validation by hand calculations and Enercalc was found useful to correct the errors in the members and cross-sectional properties. A cyclic process, where initial results are taken from Robot but then validated and detailed to be taken back to Robot or another structural simulation software, was found useful to eliminate errors in design selection in the initial process, and also to refine and improve the configurations in general. Following an iterative process helped optimizing cross-sectional properties and dimensions to have a more accurate typology in design. The secondary process utilizing hand calculations and Enercalc showed the real sizes needed for the building to withstand existing and possible exerting forces. The results (along with design improvements) showed that, except for invalid results from the software simulations, mass timber braced frame layouts performed well during the simulated earthquake (simulation and analysis depends on the seismic code that can be obtained from Robot, and it defined the earthquake impact and output parameters). In addition, a straightforward approach on utilizing multiple objectives can be taken for mass timber buildings, and both the research itself and the architects in the sector would benefit from that. The results illustrate an efficient lateral design topology that can be achieved with respect to strength, carbon footprint, cost, and governing factors on material quantity. Mass timber in high rise structures can be a practical solution to the potential insufficiencies in existing and future buildings with regards to sustainability, lateral resilience and practical constraints. An evidence-based design methodology can help choose efficient topologies that respond to multiple architectural and structural constraints. 6.2 Current Limitations There are several problems that need to be addressed. These include problems with the existing workflow, the mass timber construction and material properties in BIM and Revit, interoperability of software programs, and code restrictions and updates. 97 6.2.1 Problems of the Existing Workflow The problems and limitations with the existing workflow need to be addressed before passing to the limitations of software. • The process during the simulation was done in a more simplified manner with code supported equivalent lateral force analysis. The seismic simulation could have been done with modal analysis with more detailed inputs for tall building response behaviors. • There were erroneous results from two of the alternatives created in Robot as they gave excessive displacements or invalid results based on deflection and stresses. These specific errors need to be fixed. • There are minor differences in the total shear force between the results in between each other and in between hand calculation and Robot results. This issue may be linked to how Robot translated the material properties in the simulation. These errors need to be resolved. For the other architecture design objectives (access and lease options, window to wall ratio, load to structure, cost, fire and code, depth, servicability) require additional research, which will extend beyond making a relative comparison in between braced frame alternatives solely. • The Level Access: The criterion was based upon on the total undisturbed linear footage on the building alternatives. In addition, the core access was not considered a major issue in the existing scenarios, nevertheless, the iterated design showed that there is a need for shear core or framed core. The effect of this on the access to the core of the building and relevant services should be investigated through a prototype architectural design. The lease options should also be considered and compared through detailed consultation with experienced architects and project managers. • The Window to Wall Ratio: The criteria was based on the WWR ratios. The WWR ratio has multiple effects on the building that affects the leasibility of the building and leasable areas, the total energy consumption on the building, amount and quantity of HVAC equipment, occupant comfort. A Pareto front analysis should be made to optimize the WWR ratio (and making certain that it followed specific jurisdiction requirements). In addition, in the existing methodology, the individual WWR ratios were not calculated accurately, but rather a basic approach was taken on comparing the alternatives only. This should be fixed and extensive analysis should be done for WWR and daylight analysis as well. • Load to Structure: The load of the configurations to the structure was based upon on the total weight of the buildings obtained from Robot, and the lesser load was considered more feasible amongst the options. The load to the structure should be validated by taking into consideration frame members but also connectors and secondary elements as well. • Cost: The connections parameter was considered as a part of design comparisons and selection, as the number of connections was directly linked to the number of connectors being used, which increases the cost by increasing material and labor. Cost estimation requires extensive research both from the review of products to labor hours estimation, and possible empirical research and on-site factor considerations. In addition, having a professional cost estimator's expertise would be useful. • Depth: The depth of the members was not calculated extensively in the existing methodology, in addition to that, there was no estimation of the exact difference in cross-sectional members and dimensions. In reality, not only the selected design (Braced Frame 4) would change, there would be changes in other braced frame layouts as well. An in-depth study should be done for the exact dimensioning of the members and their implementation to the building. A more accurate dimensioning would show the exact depth of members needed and more valid comparison between alternatives can be made. 98 • Fire and Code Compliance: This was considered as a part of the design considerations, as for timber and mass timber, fire safety is an essential topic. The future work should include IBC code compliance and fire rating analysis for the gravity and lateral load design, both in the perimeter and at the core. • Serviceability: The serviceability is both a structural and architectural design criterion and directly linked to the outputs from Robot (on displacement and base shear). Thus, the alternatives giving excessive or invalid results from that were eliminated. Simulations should be conducted in alternative software programs. In addition, a scale model of the buildings should be tested in shake tables and observations should be noted for each of the building alternatives. 6.2.2 Material Properties for Software Use The utilization of mass timber lateral load resisting frames in high rise building construction needs a higher level of emphasis on wood structures and mass timber building materials in software features. Currently, the Revit family of libraries is limited. There need to be better material libraries and properties for wood to allow the flexibility and detail of variations in CLT (cross laminated timber), glulam, and other variants of mass timber (DLT, LVL, NLT and so on). The availability of additional information is currently mainly limited to private companies and architectural/engineering firms and their material libraries. Generally, their features are limited to the render and visual properties of mass timber. Techniques of construction and analytical data they contain is non-existent or unknown. Most of these are also not designed to work together with Robot and other existing structural analysis or simulation software programs. When they exist, these material properties tend to be limited to the project-specific materials and do not contain extensive family library with mass timber members. 6.2.3 Standardized Libraries Mass timber product libraries are highly dependent on the forestry and location, and those available for Revit should be localized and standardized for the United States and Canada as well. The need for a more extensive material library for mass timber does not only come from the differences in the naming of structural materials in Europe or in the U.S. but also the differences in material properties and availability of different forest products. The USA Standards and European norms in CLT and other types of timber products show differences. The lumber species differ between Europe and U.S and Canada and even through different regions of the U.S. Due to the timber being a natural product, it is usual that the variants between the species across the state affect the material properties, but besides the species, the manufacturing process also creates many variants between products of fabricators in Europe or the U.S. These include grading, sizing and dimensioning, drying and related requirements, lamination methods and use of adhesives, different fire and strength requirements, service classes and visual character of the products. Nevertheless, even though the mass timber product libraries may vary according to geographical location and fabricator, it would be advantageous to have specific Revit families for the wood construction industry. The number, accuracy, and variety of Revit mass timber families library should be increased and standardized. With standardized libraries, the design process in Revit could be streamlined. The information should follow international and national standards, and continuous improvement and updates should be done. The properties included in Revit analytical models should be more consistent with the Robot and other simulation tools. They should make it easier for the user to see the mechanical, physical and structural properties of the members, and to adapt the design accordingly. These decisions in properties should be tracked to ensure the viability of the model. This will help create a more integrated design process. An online library with mass timber families (with necessary data in members) will help decrease errors and increase efficiency in structural design, simulation, and fabrication. This can also extend to connection designs and connectors library for Revit and BIM. The properties should include standardized properties. These properties should include grading, nominal and standard sizes like lengths, width and depth, lamination characteristics and adhesives, mechanical strength properties and mechanical properties such as bending, compression, shear and tension and similar factors as it changes due to the nature of wood. The transparency between Revit and Robot Structural Analysis tools can be supported by gaining the 99 same data from the Revit model as well (Figure 6-2). Being able to obtain the quality data will reduce errors and general assumptions and create more accurate results in the design process. Figure 6-2: The data from the glulam braced frame members can be obtained from Robot Structural Analysis but cannot be obtained from Revit. 6.2.4 Plug-Ins In addition to the limited number of available add-ons and product libraries for Revit, the existing ones are also mostly limited to architectural projects in Revit. There is little or no information on how the information is translated to the analytical models in Revit Structures and Robot Structural Analysis (or other structural software programs compatible with Revit model imports). The issue that exists with using the more specialized add-ons is whether they can be used for analytical models for structural analysis and the lack of information on the mechanical properties and the amount of information transferred to the analytical model. The increase of the plug-ins of multiple firms in the U.S. will help the BIM sector and architects to utilize more accurate product information to make more precise models. The add-ons can be created by the program users without extracting external library data through coding languages for Revit such as Revit API (Application Programming Interface). For instance, it is possible to access add-ons for CLT wall, floor and roof libraries from Autodesk developers like AGACAD, and it allows the user to design the building model in CLT framing (“Wood Framing CLT | AGACAD TOOLS4BIM” 2019) (Figure 6-3). Figure 6-3: The add-ons for CLT framing and walls 100 The adaptation of tools in BIM programs for the U.S. mass timber industry would be useful as well as customized plug-ins for Revit and other software programs for mass timber types and structural components. Other useful plug- ins would include the following: calculate embodied carbon, determine the approximate cost of the structure, create simple connections automatically, and optimizing the size of the structural elements., 6.2.5 Revit and Robot Interoperability Issues and Robot Errors Autodesk claims that the Revit to Robot link is an error-free and convenient export method that allows the user to use the analytical models created in Revit and use them in Robot for load simulation and design (“Revit - Robot Integration,” 11.01.2019, www.knowledge.autodesk.com). However, there are discrepancies between software programs. The issues of interoperability may have created results that are inaccurate or needs secondary validation through other resources. The possible reasons of failure in several modes in Robot are the following: • Inconsistency of library data. As discussed earlier as well, there are changes in the availability of different properties for user to manipulate in mass timber members. This should be resolved. • Crashing. The crashing and bugs should be mitigated between the Revit model to the Robot structural model in displaced members and joints and overlaps that can not be overseen in Revit Structures. For example, observations were made that although the analytical model in Revit was designed without errors in frame members, and they were placed correctly and concentrically, some errors occurred in several members in Robot as some errors occur of members overlapping or isolating nodes. The software program should be fixed by the manufacturer so that these problems do not occur. 6.2.6 Code Updates Code compliance is a practical concern, and software needs to be kept updated. Several code change proposals have been made for the utilization of wood in multi-story buildings that exceed the traditional standards. At the end of 2018, the International Code Council positively voted on the changes in code and created three new construction types for timber buildings (“AWC: Tall Mass Timber Code Changes Get Final Approval," www.awc.org, 2019). Type IV-A, IV-B, and IV-C is the new types designated for mass timber buildings that allow maximum 18, 12 and 9 stories respectively. Each comes with an additional requirement for structural safety and serviceability. These types of construction of mass timber were determined that it will be added in the International Building Code for 2021. The upcoming IBC with new Construction Types is expected to be released in late 2020 (“AWC: Tall Mass Timber Code Changes Get Final Approval”, 2019). The existing 2018 IBC Code has determined the minimum dimensions that should be used in buildings, and the upcoming types of construction will be required to follow these dimensions (“2021 IBC, 2018 Group A, Tall Timber Proposals Review Guide," www.awc.org/pdf/tmt/TMT- ProposalsReviewGuide-180308.pdf, 2019). The dimensions of the glulam members also need to be updated to comply with the IBC 2018 or newer codes. This includes the adaptation according to the requirements and availability of fabricated materials in California and in proximity to Los Angeles as well. The code compliance has been a significant determinant and limitation in the design of timber and mass timber structures. Both from the architects, structural engineers and fabricators point of view, validation of compliance and requirements is a significant discussion in mass timber building construction. A building should comply with the code requirements of IBC and local building codes. The most current editions (2018 and in the future 2021 editions) is expected to allow more flexibility in high rise timber construction, so the IBC code can be taken as a baseline. In addition, a “code update” or changes would be useful. 6.3 Future Work Future work could include the validation of current results and braced frame typologies (not only through Robot Structural Simulation but also other software tools and physical testing), fabrication of building models to scale, and fabrication of connections and members (scaled or full-size models), physical testing, creating guidelines for future research and practice for other people and industry, and studying the same models through consideration of other design features and possible issues occurring from timber properties and high rise loadings. 101 6.3.1 Validation There are abundant software programs that can be used for structural and seismic simulation. SAP, ETABS, Multiframe, Dlubal programs, and multiple others allow the user to estimate the displacement, drift, shear forces and whole building analysis as well as individual members, bar and frame analysis. Validation of results should be done using one or several of these software programs. Dlubal (Structural Engineering Software for Analysis and Design) has several tools for structural analysis and design of timber structures and allows to import data into Revit models. Revit to Dlubal software programs such as RFEM and RSTAB allow data exchange to and from Revit. RFEM is used for structural analysis for different structural materials and structural systems. RFEM creates FEA model for structural analysis, and it takes object-based models as Revit thus allows to obtain and export data. The irregularities between physical and analytical models are reported to be fixed as the program regulates problems of disconnection and non-intersecting lines and creating an upgraded structural model (“Dlubal Interfaces with Autodesk Revit | Dlubal Software”, 2019). The future work should include the validation of the Braced Frame Scenarios in RFEM or RSTAB, initially in RFEM, with the existing Revit models. The RFEM software creates an add-on in Revit as Dlubal link (Figure 6-4). The story drift of the building should be simulated and then compared with results from Robot Structural Analysis. Figure 6-4: Revit to RFEM interface 102 RFEM allows to obtain the story drift of the building that is subjected to seismic loads and works similar to Robot. In RFEM, user-defined or standardized methods can be used for mass combination cases. The examples on Dlubal Technical Articles refer to ASCE 7-16 Section 2.3.6 for creating the combination (“Determination of Story Drift According to ASCE 7-16 Under Seismic Loads | Dlubal Software” 2019). The drift is calculated after the determination of global deformations to see the displacements in stories (story drift: Δ). Max. the difference in displacement can be displayed in the building model nodes, and comparisons are made in single stories. 6.3.2 Fabrication and Physical Testing Physical testing of the members and a scale model of the building should be done in order to observe the performance of the building under simulated (test environment) lateral forces. Once the testing is done, the results will demonstrate the more accurate performance of mass timber braced frames and their connections. Initially, a scaled model of the building model (or models) should be created, as well as the braced frame configurations implemented to them, or later on as a secondary model. This way the differences of behavior can be tested and compared with the baseline model and with each other. Other physical parameters such as weight and amount of complexity can be tested and observed through the construction of scaled models. Shake tables can be used for the physical testing of models. The individual frame members can be fabricated, and connections can be tested at this scale. This would require a larger shake table which would be more technologically advanced and updated (which can perform and test the shaking in multiple dimensions), and can be found in earthquake engineering and simulation labs. Even full-scale mock-ups can test tested on some shake tables. 6.3.3 Connections Although metal connections are often used in mass timber construction, wood could also be used instead. The next steps would be designing connections on a software tool and analysis of them through behavior analysis through the finite element method, and fabrication of the connections and connectors. As done in the selection of braced frame layout methods, a diversification method can be used initially to evaluate the practicality of different type of both connections and connectors for timber frame construction, including the steel connectors and the ones used for heavy timber braced frame systems. The existing research and possible damage evaluation will be used for potential elimination. Existing research on connections and testing would be a useful resource to develop new possible connections, as they may include the data of the behavior of connections and connectors under different loading conditions. Afterward, modeling of different types of connections will be realized with possible subcategories (i.e: joint type as the main category and subcategories involving iterations of the same joint Type). The detailed design of the models could be followed by verification, both in terms of structural analysis and fabricating the alternatives for physical testing analysis. The required changes would be made according to expected failure types. If possible, they will be implemented to on scale or mock-up models of the building and braced frames. Although most of the software programs under BIM offer structural connection design, they are mostly utilized for concrete or steel (i.e.: Tekla). These programs can be adapted or updated to comply with the needs of the mass timber industry and connection design. 6.3.4 Create Guidelines A set of guidelines for structural design and fabrication methodology would help designers handle this process more easily. The methodology followed its a basic guideline in prototype braced frame scenarios. Although every structure is unique and needs to fulfill its own requirements, a guideline that will be created upon the existing study would be beneficial to have a standard in the workflow and design methodology for mass timber buildings and mass timber lateral design. The guideline can both focus on general design and construction methods and tools, how to fulfill the design objectives without much compromise in other objectives (i.e: cost vs embodied carbon, or structural weight and load resistance), connection methods, and assembly guidelines. A comprehensive guideline can help 103 architects increase their knowledge of structural and architectural aspects of mass timber construction, and facilitate the process for mass timber buildings and their lateral designs in the future. 6.3.5 Other Features Other works that need to be done are as follows: • Wind and seismic resistance are important parameters in lateral design of the building. In addition, consideration of high winds instead of seismicity would greatly affect the layouts and designs of braced frame topologies. The alternative study will be conducted for wind forces and in a different location dominated by extreme wind forces, outside Los Angeles. • Thermal, moisture, deterioration and crack problems could be analysed and addressed in a detailed level to create solutions based on design objectives on improvement on these criteria. • Structural borne acoustic problems could be studied. • Detailed estimation of embodied carbon of the structures, detailed cost analysis and improvement of total weight and cost of the structure and braced frame members would improve the methodology. 6.4 Conclusion Different vertical lateral load resisting systems for tall mass timber buildings were explored. A project-specific method was developed to determine, through an elimination method and structural guidelines, the frame typologies and configurations that was estimated to be the most successful and feasible. SOM’s Timber Tower building, an existing timber high-rise research project, was used as the baseline building. To keep the comparisons feasible, a limited number (4) of braced frame layouts were considered. Timber is not only a sustainable material but also found to be a sufficient and effective material that could replace steel braced frames and moment frames. The findings showed a feasible lateral design in a high-rise timber building can be accomplished without compromising from safety during earthquakes. Nevertheless, a superimposed approach was needed to be developed to create an efficient and better solution. The end result was created by utilizing both Revit and Robot software programs, as well as hand calculations and utilizing frame analysis programs. Another result of the methodology showed that the initial design taken from Robot was needed to be iterated as the frame and individual stresses were not obtained from the software, and following the capacity design, the cross-sectional dimensions increased highly in the design configuration. In addition, although the shear core area could have been smaller, or designed with core bracing, there is still a need to supplement the high rise building with a lateral load resisting core, for this building. The making of detailed connection design and analysis and testing the building (to scale) would increase the accuracy of the results. A prototype workflow was developed for architects to follow to utilize mass timber and allowing a straightforward structural design workflow and design procedures that can be applied to future projects. Both in the sense of materials and methods, it touches a field with little existing work done. It aims to impact architects and structural engineers by showing how one could use mass timber while following code in lateral design, increase collaboration between architecture and structural engineering, and challenge existing design methods and materials to push forward the use of mass timber. Consequently, integrating timber bracing systems to high rise buildings on the perimeter and at the core created an efficient design to be used for future applications, and it is a sustainable and innovative replacement for both in terms of other types of conventional materials and other forms of lateral systems. 104 REFERENCES Abrahamsen Rune B. and Malo, Kjell Arne. “Structural Design and Assembly of “Treet” – A 14-Storey Timber Residential Building in Norway.” World Conference on Timber Engineering, 2014. Ambrose, James and Vergun, Dimitry. Simplified Building Design for Wind and Earthquake Forces. John Wiley & Sons, 1995 . American Institute of Timber Construction. 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TBI Guidelines Working Group. Guidelines for Performance-Based Seismic Design of Tall Buildings, PEER Report 210/05. Berkeley, California: Pacific Earthquake Research Center, University of California, 2010. "The Great Fire: Chicago 1871". 2019. The University Of Chicago Magazine. https://mag.uchicago.edu/law-policy-society/great-fire-chicago-1871. “The New York Preservation Archive Project.” http://www.nypap.org/ United Nations, Department of Economic and Social Affairs, Population Division. World Population Prospects: The 2017 Revision. New York: United Nations, 2017. Torrey, Barbara Boyle. “Urbanization: An Environmental Force to Be Reckoned With.” https://www.prb.org/urbanization-an-environmental-force-to-be-reckoned-with/ Zu, Gonbo and Lam, Kit Ming. “LES and Wind Tunnel Test of Flow around Two Tall Buildings in Staggered Arrangement.” Computation 6, issue 28 (March 2018) "2017 Tall Building Year In Review". 2019. Skyscraper Center. http://www.skyscrapercenter.com/year-in-review/2017. 107 APPENDIX A The following data of Appendix A was obtained from OSHPD U.S. Seismic Design Maps and ASCE Seismic Design Requirements for Building Structures Chapter 12. For more information on seismic values and definitions, Chapter 11 of the same guideline can be referred. 108 109 110 111 APPENDIX B: Calculations DESIGN DATA Uniform No of levels 42 Length (feet) 124.5 Width (feet) 80 Story height (feet) 9.4 Dead Load DL (psf) 30 LOADS DEAD LOAD 463.14 kip DEAD LOAD 2 69.12 kip SUPERIMPOSED 169.32 kip LIVE LOAD 384.4 kip 25 LIVE LOAD 99.6 kip TOTAL LOADS per FLOOR 1016.26 kip TOTAL WEIGHT 42682.92 kip CLT R factor 4.5 Importance factor 1 Cs Sds/ (R/I) Cs 1.588/ (4/5/1)= 0.35 W 1016.26 kip V Cs x W V Cs x W =355 Top Shear 355 kip Number of Floors 42 Base Shear 14939.022 kip TOP SHEAR 112 BASE SHEAR 3734.75 kip 3734.75 kip The ENERCALC calculations were done in terms of using the top shear, base shear 88.75 kip 88.75 kip

Abstract (if available)

Abstract As cities are getting denser and larger, tall buildings are becoming prototype of large and expanding cities around the world. Among the conventional building materials used in the last century, timber is increasingly becoming more prominent with its engineered production and smaller ecological footprint. Although mass timber in high rise construction is getting more common, there still is not enough information on how an all-timber high rise will be affected by the fire, wind, or seismic forces. In addition, many designers are resorting to the use of reinforced concrete or steel lateral load resisting systems in mass timber buildings instead of using wood exclusively. ❧ Lateral system design alternatives with timber for high rise construction were developed, focusing on timber braced frame building systems. A timber braced frame exoskeleton was analyzed in different locations in the world that are dominated by lateral loads. Skidmore, Owings & Merrill’s Dewitt-Chestnut Apartments in Chicago, Dewitt Chestnut Apartments, was taken as the benchmark building. SOM’s ongoing research: “Timber Tower Research Project” had already applied the concept of converting this building to timber, which then was used to develop a new timber structural. Based on the research by SOM, further investigation on the building was made in order to design the lateral system from wood, with the assumption that the building was being relocated to Los Angeles as a region dominated by seismic forces, focusing initially on downtown Los Angeles, CA. The results showed how an efficient lateral design topology can be achieved in terms of strength, carbon footprint, cost, and material quantity. Further application of the load resisting frames and their variation in typology can be studied in different locations that are subjected to different lateral forces. The workflow aimed to be used as a guideline to design and improve mass timber lateral systems, which can serve as a more sustainable alternative to steel and concrete.

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Asset Metadata

Creator Goren, Isik (author)

Core Title Lateral design with mass timber: examination of structural wood in high-rise timber construction

School School of Architecture

Degree Master of Building Science

Degree Program Building Science

Publication Date 04/25/2019

Defense Date 03/20/2019

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag BIM,braced frames,Building Information Modelling,lateral design,mass timber building systems,multi-objectivity,OAI-PMH Harvest,seismic design,sustainable construction materials,tall timber structures,Timber Tower Research Project,topology design

Format application/pdf (imt)

Language English

Contributor Electronically uploaded by the author (provenance)

Advisor Schierle, Goetz (committee chair), Kensek, Karen (committee member), Konis, Kyle (committee member), Shahi, Santosh (committee member)

Creator Email ig.isikgoren@gmail.com,isikgore@usc.edu

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c89-144836

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Legacy Identifier etd-GorenIsik-7246.pdf

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Document Type Thesis

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Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the a...

Repository Name University of Southern California Digital Library

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