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Lateral force design (LFD) software for wind and seismic loads per IBC 2003 and ASCE 7-02

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Lateral force design (LFD) software for wind and seismic loads per IBC 2003 and ASCE 7-02

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Content LATERAL FORCE DESIGN (LFD) SOFTWARE FOR WIND AND SEISMIC LOADS PER IBC 2003 AND ASCE 7-02 by Xin Wang A Thesis Presented to the FACULTY OF THE SCHOOL OF ARCHITECTURE UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree MASTER OF BUILDING SCIENCE December 2004 Copyright 2004 Xin Wang Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 1424236 INFORMATION TO USERS The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleed-through, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. UMI UMI Microform 1424236 Copyright 2005 by ProQuest Information and Learning Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. ProQuest Information and Learning Company 300 North Zeeb Road P.O. Box 1346 Ann Arbor, Ml 48106-1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS First of all, I would like to give many thanks to Prof. G. Goetz Schierle, my thesis chair. His valuable suggestions have encouraged me to choose this thesis topic at the beginning. His precious advice is a great help for me to overcome the problems occurring in the course of completing this thesis. His guidance has made it possible for me to finish this thesis. Also, I would like to thank Prof. Jeff Guh and Prof. Douglas Noble, my thesis committee. The valuable suggestions from Prof. Jeff Guh are a great help for me to study building codes and write this thesis. Prof. Douglas Noble has given me precious advice in designing the software and patiently helped me review the thesis from time to time. Without their help, it is impossible to complete this thesis. I would like to thank Prof. Marc Schiler. Although not in my thesis committee, he has given me many valuable suggestions to help me clarify my thesis topic, write this thesis and design the software. I also would like to thank Prof Karen Kensek for her precious advice of designing the software. Many thanks should also be given to my classmates, Alice Ormsbee, Evie Giovannopoulou, Kavita Rodrigues, Jessica Mack and Rosa Neidl-Comejo. They have given me many valuable suggestions and encouragement. Finally, I would like to thank my husband, Yi An. He has supported me financially and patiently waited for me to complete my study. ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS ACKNOWLEDGMENTS................................................................................................ii LIST OF TABLES........................................................................................................... vi LIST OF FIGURES.......................................................................................................viii ABSTRACT......................................................................................................................xi INTRODUCTION.............................................................................................................1 CHAPTER 1 ......................................................................................................................4 INTRODUCTION............................................................................................................ 4 1.1. Why Should Architects Know about Wind and Seismic Design....................... 4 1.2. Cause of Earthquakes and Effects on Buildings.................................................8 1.2.1. Cause of Earthquakes.................................................................................... 8 1.2.2. Effects of Earthquakes on Buildings.......................................................... 13 1.3. Effects of Wind Forces on Buildings.................................................................16 CHAPTER 2 ....................................................................................................................21 BUILDING STRUCTURAL DESIGN TO RESIST LATERAL FORCES..............21 2.1. Amount and Distribution of Seismic and Wind Loads on Buildings.............21 2.1.1. Amount and Distribution of Seismic Loads on Buildings....................... 21 2.1.2. Amount and Distribution of Wind Loads on Buildings........................... 23 2.2. Structural Design to Resist Lateral Forces........................................................24 2.3. Introduction to Lateral Force Resisting Systems............................................. 27 2.3.1. Shear Wall Systems.................................................................................... 28 2.3.2. Cantilever Systems...................................................................................... 34 2.3.3. Braced Frame Systems................................................................................36 2.3.4. Moment Resisting Frame Systems............................................................. 39 2.3.5. Dual Systems...............................................................................................41 CHAPTER 3 ....................................................................................................................43 SEISMIC LOADS PER IBC 2003 AND ASCE 7-02................................................. 43 3.1. Introduction......................................................................................................... 43 3.2. Seismic Loads per IBC 2003 and ASCE 7-02 for the LFD Software Design 45 3.2.1. Applicability................................................................................................45 3.2.2. Base Shear V ................................................................................................47 3.2.2.1. Equations...............................................................................................47 3.2.2.2. Explanations of Factors in Equations.................................................48 iii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.2.2.2.1. Site Class....................................................................................... 49 3.2.2.2.2. S, Ss ...............................................................................................49 3.2.2.2.3. Sm sSm i..........................................................................................50 3.2.2.2.4. SDSjSD i ...........................................................................................52 3.2.2.2.5. Occupancy Category.....................................................................52 3.2.2.2.6. Seismic Use Group (SUG)........................................................... 54 3 2 2 2 .1 . Occupancy Importance Factor 1...................................................54 3.2.2.2.8. Seismic Design Category............................................................. 55 3.2.2.2.9. Response Modification Coefficient R......................................... 56 3.2.2.2.10. Building Configuration.............................................................. 57 3.2.2.2.11. Building Fundamental Period T .................................................59 3.2.3. Seismic Force Per Level.............................................................................60 3.2.4. Seismic Shear Per Level.............................................................................61 3.2.5. Overturning Moment Per Level................................................................. 61 CHAPTER 4 ....................................................................................................................63 WIND LOADS PER IBC 2003 AND ASCE 7-02.......................................................63 4.1. Introduction......................................................................................................... 63 4.2. Wind Loads per IBC 2003 and ASCE 7-02 for the LFD Software Design... 64 4.2.1. Applicability................................................................................................64 4.2.2. Wind Pressure per Level.............................................................................66 4.2.2.1. Equations...............................................................................................67 4.2.2.2. Explanations of Factors in Equations................................................ 68 4.2.2.2.1. Basic Wind Speed V .................................................................... 68 4.2.2.2.2. Importance Factor 1.......................................................................69 4.2.2.2.3. Exposure Category........................................................................70 4.2.2.2.4. Velocity Pressure Exposure Coefficient Kz, Kh..........................71 4.2.2.2.5. Wind Directionality Factor Kd.....................................................72 4.2.2.2.6. Topographic Factor Kzt................................................................ 72 4.2.22.7. Gust Effect Factor G .................................................................... 72 4.2.2.2.8. External Pressure Coefficients Cp............................................... 73 4.2.3. Wind Force per Level................................................................................. 73 4.2.4. Wind Shear per Level................................................................................. 74 4.2.5. Wind Overturning Moment per Level.......................................................74 CHAPTER 5 ....................................................................................................................75 A NEW SOFTWARE TOOL FOR WIND AND SEISMIC LOADS........................ 75 5.1. Software Scope...................................................................................................75 5.2. Software Structure..............................................................................................77 5.2.1. Software Preparation................................................................................... 77 5.2.2. Software Introduction and Tutorial............................................................79 5.2.2. Software Input..............................................................................................83 5.2.2.1. Building Info........................................................................................84 iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.2.2.2. Dimension.............................................................................................87 5.2.2.3. Wind Info..............................................................................................90 5.2.2.4. Seismic Info..........................................................................................91 5.2.3. Software Output...........................................................................................93 5.2.3.1. Wind Output.........................................................................................94 5.2.3.4. Seismic Output..................................................................................... 95 5.2.3.4. Shear Wall Output................................................................................96 5.2.4. Software Menus...........................................................................................99 5.3. An Example Structure....................................................................................... 99 CHAPTER 6 ..................................................................................................................108 EVALUATIONS AND FINDINGS............................................................................ 108 6.1. Strengths of the Software................................................................................ 108 6.2. Weakness of the Software...............................................................................109 CHAPTER 7 .................................................................................................................. 112 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK...........................112 7.1. Conclusions...................................................................................................... 112 7.2. Suggestions for Future Work........................................................................ 113 BIBLIOGRAPHY..........................................................................................................115 v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF TABLES Table 1. Allowable shear (pound per foot) for wood structural panel shear walls with framing of Douglas-Fir-Larch or Southern Pine for wind and seismic loading (ICC 2002, p.469)...................................................................................... 33 Table 2. Allowable shear (pound per foot) for concrete shear wall (Amrhein 1994)........................................................................................................................33 Table 3. Allowable shear (pound per foot) for masonry shear wall (Amrhein 1994)........................................................................................................................ 34 Table 4. Permitted procedures of Table 9.5.2.5.1 of ASCE 7-02 (SEI2003, p. 140).......................................................................................................................46 Table 5. Site Class definitions (IBC Table 1615.1.1)...................................................49 Table 6. Values of Fa as a function of site class and mapped spectral response acceleration SS at short periods of Table 1615.1.2(1) of IBC 2003................... 51 Table 7. Values of Fv as a function of site class and mapped spectral response acceleration SI at 1-second period of Table 1615.1.2 (2) of IBC 2003..............51 Table 8. Building Occupancy Category from Table 1604.5 of IBC 2003..................53 Table 9. Seismic Use Group of Table 9.1.3 of ASCE 7-0........................................... 54 Table 10. Occupancy Importance Factor of Table 9.1.4 of ASCE 7-02..................... 55 Table 11. Seismic Design Category based on SDS of Table 1616.3 (1) of IBC 2003.......................................................................................................................... 55 Table 12. Seismic Design Category based on SD1 of Table 1616.3 (2) of IBC 2003................................................................................................... 56 Table 13. Design coefficients and factors for basic seismic-force resisting systems from Table 1617.6.2 of IBC 2003........................................................... 57 Table 14. Vertical structural irregularities from Table 9.5.2.3.3 of ASCE 7-02....... 58 vi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 15. Values of approximate period parameters Ct and x of Table 9.5.5.3.2 of ASCE 7-02.......................................................................................................... 59 Table 16. Building Occupancy Category from Table 1604.5 of IBC 2003................69 Table 17. Importance Factor I (Wind Loads) of Table 6-1 of ASCE 7-02.................70 Table 18. Terrain exposure constants of Table 6-2 of ASCE 7-02............................. 71 Table 19. Wind directionality factor Kd from Table 6-4 of ASCE 7-02.................... 72 Table 20. External pressure coefficients Cp from Figure 6-6 of ASCE 7-02.............73 vii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES Fig. 1.1 An example of building collapse caused by earthquake (Bachmann 2002, p. 17)................................................................................................................. 4 Fig. 1.2 Global view of the Tectonic Plates on the Earth’s crust (USGS, no date). ..10 Fig. 1.3 Richter Scale magnitude based on seismic amplitude and distance (Anon.)......................................................................................................................12 Fig. 1.4 Period is the time (in seconds) for one cycle of oscillation............................14 Fig. 1.5 Deformations of rigid and flexible buildings during earthquakes..................15 Fig. 2.1 Wind pressure distribution on buildings (Pittack and Bias, 2000)................24 Fig. 2.2 Typical wood shear wall................................................................................... 30 Fig. 2.3 Typical reinforced brick masonry shear w all..................................................31 Fig. 2.4 Concrete shear wall resistance to lateral forces (Ambrose and Vergun 1995, p. 106)............................................................................................................ 31 Fig. 2.5 Cantilever column under gravity and lateral loads (Schierle 2001)..............35 Fig. 2.6 Structural behaviors of cantilevers under lateral loads (Schierle 2001)....... 35 Fig. 2.7 Development of frame systems by utilizing bracing elements (Ambrose and Vergun 1995, p.74)...........................................................................................37 Fig. 2.8 Common types of braced frames..................................................................... 38 Fig. 2.9 Deformation of moment-resisting frame with inflection points of zero stress......................................................................................................................... 41 Fig. 2.10 An example of dual systems...........................................................................42 Fig. 3.1 Examples of building plan irregularities.........................................................58 Fig. 5.1 A sample of input module of Code Search Excel Spreadsheet developed by Struware (no date)..............................................................................................76 viii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 5.2 Software structure and flow sequence............................................................. 78 Fig. 5.3 A sample of the LFD open page................................................................ 80 Fig. 5.4 A sample of the LFD introduction page................................................... 80 Fig. 5.5 A sample of the main menu of the LFD tutorial.......................................81 Fig. 5.6 A sample of the program scope in the LFD tutorial.......................82 Fig. 5.7 A sample of how to use the program in the LFD tutorial...............................82 Fig. 5.8 A sample of the code theory in the LFD tutorial.............................................83 Fig. 5.9 A sample of Occupancy Category in the Building Info tab........................... 85 Fig. 5.10 A sample of Lateral Force Resisting System in the Building Info tab 86 Fig. 5.11 A sample of inputting the shear stress of shear walls in the Building Info tab.....................................................................................................................86 Fig. 5.12 A sample of Number of Levels in the Building Info tab.............................87 Fig. 5.13 A sample of the last step in the Building Info tab.......................................88 Fig. 5.14 A sample of the Dimension tab.................................................................... 88 Fig. 5.15 A sample of the building summary............................................................... 89 Fig. 5.16 A sample of Fundamental Period in the Wind Info tab...............................91 Fig. 5.17 A sample of the pop-up form of the building Fundamental Period T .......92 Fig. 5.18 A sample of SI and SS in the Seismic Info tab........................................... 93 Fig. 5.19 A sample of the Wind Output tab.................................................................94 Fig. 5.20 A sample of the pop-up form to display the computation theory of the selected data in the wind output table................................................................... 95 Fig. 5.21 A sample of the graphic output of wind overturning moment....................96 ix Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 5.22 A sample of the Seismic Output tab.............................................................. 97 Fig. 5.23 A sample of Shear wall output.......................................................................98 Fig. 5.24 A sample of Shear wall graphic output.........................................................98 Fig. 5.25 A sample of inputting Wind Speed in the Wind Info tab...........................102 Fig. 5.26 A sample of selecting Exposure Category in the Wind Info tab............... 102 Fig. 5.27 A sample of selecting Seismic Site Class in the Seismic Info tab 103 Fig. 5.28 A sample of the wind output for the sample building................................ 103 Fig. 5.29 A sample of a graphic wind output for the sample building......................104 Fig. 5.30 A sample of the computation theory of the windward pressure on the selected level..........................................................................................................105 Fig. 5.31 A sample of the seismic output for the sample building............................106 Fig. 5.32 A sample of the graphic output of the seismic shear force.........................106 Fig. 5.33 A sample of the shear wall output for the design building.........................107 x Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT The hypothesis of this thesis is that it may be possible to design a software tool to teach students of architecture the building code theory of determining wind and seismic loads. The design software is intended to facilitate teaching of design for wind and seismic forces in architectural schools. The determination of wind and seismic loads is the first part of wind and seismic design. The methods to develop those forces are defined in building codes and have evolved from simple to very complex. Building code theory is not typically taught in architectural schools. This thesis presents the software, LATERAL FORCE DESIGN (LFD), developed for students of architecture. The LFD software explains the main concepts and procedures of determining wind and seismic loads based on International Building Code (IBC) 2003 and American Society of Civil Engineers (ASCE) 7-02. The wind and seismic loads generated by the software can be used to replace hand calculations and in other structural analysis softwares that do not generate those loads. Windows-based C# is chosen as the computer programming language. Key words: seismic design, wind load, lateral forces, seismic software, wind design software xi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION Buildings need to be designed to resist wind and seismic forces in addition to gravity loads. Extensive research on wind and seismic design has been conducted since the middle of the 20th century because wind and seismic forces now cause much more damage than before in terms of the losses of lives and property with the growth of cities and population. Building codes are regularly upgraded based on the findings of research regarding wind and seismic design. Buildings are designed by architects and engineers. Architects are principally responsible for building configuration. Engineers help architects design for natural forces. The success of building resistance to wind and seismic forces is closely related with building configuration. The knowledge of wind and seismic design may inspire architects to integrate lateral force design requirements and coordinate with structural engineers. Issues related to wind and seismic design cover a broad range of topics. The determination of wind and seismic loads on buildings is the first part of wind and seismic design. The methods defined in building codes have evolved from simple to very complex. Based on building codes, quite a few software packages have been developed to determine wind and seismic loads. Those software packages are designed for professional structural engineers and require that users be familiar with building codes. They involve many concepts that are not normally known to students of 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. architecture because building code theory is not typically taught in architectural schools. Because architecture students do not know building code theory, the existing software tools are not appropriate. The purpose of this thesis is to develop a software tool to teach students of architecture building code theory regarding wind and seismic loads. People can learn knowledge in many ways. Traditionally, people learn by taking classes or communicating with other people. Since computers have become popular, they have been widely used as teaching media in classrooms and at home, which has been proven quite effective. This thesis presents a software as a teaching tool to help students of architecture study the main concepts and procedures of determining wind and seismic loads based on International Building Code (IBC) 2003 and American Society of Civil Engineers (ASCE) 7-02. Building codes, such as Uniform Building Code (UBC) or IBC, are published by professional organizations. They are known as “ model” codes because they are adopted by local jurisdictions and adjusted for local conditions. To keep abreast with the most up-to-date building codes is very important. IBC 2003 is a recently published “model” code so the design software in this thesis shall be based on it. ASCE 7-02, “Minimum Design Loads for Buildings and Other Structures”, is published by the American Society of Civil Engineers. Because most provisions in IBC 2003 governing wind and seismic loads are based on or referenced directly from ASCE 7-02, the theory in ASCE 7-02 is included as basis of the thesis. 2 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Depending on building configuration, various methods have been developed for wind and seismic analysis and some of these methods are quite sophisticated. Because of the level of complexity of the code, only part of the theory is used in the design software. The software is designed for regular-shaped buildings because irregular-shaped buildings require dynamic analyses or model tests. However, the design software may be used for approximate design of buildings with minor irregularities. For wind loads, the Analytical Procedure of Section 6.5 of ASCE 7-02 is used in the software. For seismic loads, the Equivalent Lateral Force Procedure of Section 9.5.5 of ASCE 7-02 is used in the software. The software generates the following: wind base shear, distribution per level of wind pressures, forces, shear, and overturning moment on main wind-force resisting systems of enclosed and partially enclosed rigid buildings of all heights, seismic base shear, distribution per level of seismic force, shear force, and overturning moment, overall length per level of common types of wood, concrete and masonry shear walls in two orthogonal directions if shear wall systems are used as lateral-force resisting systems. Windows-based C# is chosen as the computer programming language. This thesis includes two parts. Part one is the research study of structural design for wind and seismic forces. Part two is the software, LATERAL FORCE DESIGN (LFD). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 1 INTRODUCTION 1.1. Why Should Architects Know about Wind and Seismic Design Wind and seismic forces are destructive natural forces on buildings. The article published by the Wind Effects Committee (2004) indicates that the losses caused by wind account for more than 50 percent of the damage caused by the 40 worst disasters from 1970 to 2002, followed by the losses caused by earthquakes and humans respectively. Building damage can range from minor fracture to total collapse (Fig. 1.1). Buildings need to be designed to resist wind and seismic forces. Fig. 1.1 An example of building collapse caused by earthquake (Bachmann 2002, p.17). 4 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Wind design becomes crucial with the increasing number of lightweight and high-rise buildings in high-wind prone regions. Hurricanes or tornadoes tend to lift up lightweight roofs or even entire buildings from their foundations. With the growth of cities, high-rise buildings become one of the solutions to accommodate growing populations. Wind pressures increase significantly with increasing building height and wind forces increase the tendency toward overturning tall buildings. Therefore, wind design is especially important for high-rise buildings. Over one million earthquakes can be detected by seismic detection devices around the world each year. Building collapses are quite common during major earthquakes. Theoretically, earthquakes can occur anywhere on the Earth at some time (Ambrose and Vergun 1995, p. 19) but they are most prevailing around tectonic plates, notably around the Pacific Rim. Seismic forces have to be analyzed in building structural design. Architects have the leading roles in building design and determine building configuration, size and shape of the building (Gibbs 2003, p. 1). They more or less determine building structural systems, the layout of structural elements and distribution of building mass. Thus, they set up limitations for structural engineers designing for natural forces. Architects are more concerned about building functions and aesthetic design in addition to effective structures. But when wind or seismic design becomes a major concern, occupants’ safety must be assured. In his article “Conceptual design to resist earthquakes”, Tony Gibbs (2003, p. 3) reaches the conclusion that “the 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. importance of configuration is well recognized in modem standards which penalize unfavorable configuration through the application of higher factors to the seismic loads or through demands for more sophisticated analyses”. Schierle (2003, p. 10) recommends that “Design guidelines should recommend seismic joints or strengthening intersections of wings of H, L, T, U and similar configurations, to prevent diaphragm failure.” Extensive research has been conducted in the last 50 years on design for wind and seismic loads. Although it is very difficult to precisely predict building performance during earthquakes or wind forces because of their dynamic features, theories on wind and seismic design have been established and revised continuously. These theories have been helpful in establishing the basis for building codes. Wind and seismic forces are called lateral forces in building design. Although they exert forces on buildings in all directions, lateral loads (horizontal loads) are usually dominant. Various lateral-force resisting structural systems have been developed to resist lateral forces, such as shear walls, cantilevers, moment frames, and braced frames. Shear wall systems utilize wall elements that are reinforced to resist lateral forces. Braced-frame systems utilize diagonal bracing elements to resist lateral forces. Shear-wall or braced-frame systems make buildings stiffer and have less deformation under wind or seismic forces. Cantilever systems resist lateral forces through the bending of cantilever columns. Moment frame systems utilize the deformation of columns and beams to resist lateral forces. 6 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Cantilever and moment frame systems provide more flexible interior space than shear walls or braced frame systems but they have more deflection subject to wind and seismic forces, which may cause great damage to non-structural elements such as glazing or surface finishes. According to the building codes, ductile cantilevers and moment frames are subject to smaller seismic forces. The continuity of the load path for transferring lateral loads to the ground is very important. Openings, such as windows or stairways on walls or floors, break this continuity and create weak sections for transferring the forces. Those openings have to be reinforced to avoid being damaged. Building configuration has a great impact on the efficiency of building resistance to wind and seismic forces. Building configuration is classified as regular or irregular as defined in the building codes. Buildings with H, L, T, or U plan geometry have plan irregularities because of their re-entrant corners where both projections of the structure beyond a re-entrant comer are greater than 15% of the plan dimension of the structure in the given direction (SEI2003, p. 137). Subject to wind or seismic forces, such plan irregularities can create torsion and stress concentration at the intersections of building wings (Professional Publications no date, p. 10). During an earthquake, each wing vibrates differently and may hammer each other. Separation joints or reinforcement at the comers shall be designed to avoid such damage. Building materials and construction have a great influence on building responses to wind or seismic forces. Heavy buildings are preferred for wind design 7 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. because their weight is an advantage to resist wind uplift and overturning forces. Lightweight buildings are favored in seismic design because they induce less seismic forces. Wind and seismic design mostly consider building configuration, construction and materials, each of which is mainly designed by architects. The knowledge of wind and seismic design helps architects understand the requirements of structural engineers. 1.2. Cause of Earthquakes and Effects on Buildings Earthquakes occur constantly in the Earth’s crust. The history of the scientific study of earthquakes is not long and there were few records of earthquakes before the 18th century. With the development of research on seismic design, the cause of earthquakes is well understood today. The following discussions include the cause of earthquakes, the effects of earthquakes on buildings and building responses to earthquakes. 1.2.1. Cause of Earthquakes An earthquake is the vibration of the ground that follows energy release from fault lines in the Earth’s crust. This energy can be generated by volcano eruptions, man-made explosions or cracks in fault lines separating tectonic plates of the Earth’s crust (Pakiser and Shedlock, no date). Earthquakes caused by the latter are the main 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. concern in building seismic design because they are the most common and can cause greater damage. The earth is composed of various layers, including the core, mantle and crust. The core consists of a mixture of solid and liquid materials that are very hot and produce great energy. The mantle consisting of liquid materials lies between the core and the Earth’s crust. The Earth’s crust mainly consists of solid rocks that are relatively brittle. The Earth crust consists of several large irregular-shaped plates (Fig. 1.2) according to the theory of Plate Tectonics. Those gigantic plates are pushed by the energy produced in the Earth’s interior. The moving rate of those plates can be several centimeters each year, which is hardly perceptible. There are three moving patterns of those plates, moving away from, toward or sliding past one another. The deep fractures formed along plate boundaries are called faults. Based on the moving patterns of the adjacent plates, there are three types of faults. One is called normal fault resulting from plates moving away from one another. The second is called thrust fault resulting from plates moving toward one another. The third is called strike-slip fault when plates move laterally past one another. Because the two sides of the faults are not smooth, stress in rocks are built up while plates move relative to one another. When the rock’s stress limit is reached, the rocks will break and plates will split suddenly. This sudden movement of the plates releases great amount of energy that causes earthquakes. Earthquakes occur 9 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. most frequently along the plate boundaries where rocks are more brittle than within the plate. The origin of the location where the energy is released is called the hypocenter and the location directly above the hypocenter on the ground surface is called the epicenter. r* PH'tfW.? WSTfiAtrn HAH ' Fig. 1.2 Global view of the Tectonic Plates on the Earth’s crust (USGS, no date). The energy released takes the form of seismic waves that travel through the Earth in all directions. The speed of seismic waves varies depending on the type of wave and the materials that they travel through. The denser the materials are, the faster is the speed. There are primarily three types of seismic waves, P (pressure) 10 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. waves, S (shear) waves and surface waves. P waves travel at the fastest speed of 26,000 mph and can travel through liquid, solid and gaseous materials. S waves only travel in solid materials. Surface waves originate at the epicenter. S and surface waves cause the earth to shake in three dimensions. When those waves reach the Earth’s surface, the ground is vibrated by those waves. Waves have two primary characteristics that usually determine the magnitude of earthquakes. One is the wave amplitude, the measurement of the wave size. The other is the period that is the time interval between the arrival of two successive peaks or valleys of seismic waves (ATC/SEAOC Joint Venture no date, p.2). The amplitudes and periods of seismic waves can be measured by seismographs located around the world. The magnitude of earthquakes can be calculated based on those measurements. The Richter scale (Fig. 1.3) is popularly used for measuring the magnitude of earthquakes. This scale is logarithmic and the magnitude of earthquake energy increases 33 times for each scale increment. Based on this scale, magnitude 3.5 is the smallest earthquake that can be felt by human beings. Magnitudes 6 or more are considered major earthquakes. There are three primary factors that affect the amount of ground motion. One is the distance from the hypocenter to the specific location. The second is the amount of energy released from the hypocenter. The last factor is the soil condition of the site location. Ground with loose soil suffers much greater shaking than the 11 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. one with stiff and dense soil. The soil condition of a specific site has to be investigated as part of seismic design. Fig. 1.3 Richter Scale magnitude based on seismic amplitude and distance (Anon.). The effect of an earthquake is called the intensity. The scale currently used in the United States is the Modified Mercalli (MM) Intensity scale (MCEER, no date), which was developed by American seismologists Harry Wood and Frank Neumann. 12 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. This scale includes 12 increasing levels of intensity. The intensity of the earthquake can be assigned based on the observed results of people experiencing the earthquake. In earthquake prone areas, major buildings install accelerographs, an instrument for measuring the acceleration of the ground and the building. Based on the recorded data of those instruments, the maximum ground motion can be estimated for certain geographical areas, which forms the basis of seismic design. 1.2.2. Effects of Earthquakes on Buildings During an earthquake, the ground movement is transferred from the building foundation to the rest of the building. Theoretically, buildings follow Newton’s Second Law of Dynamics and generally the seismic force is: F = M A Where F = seismic force M = Mass of building (dead load + 25% live load of warehouses) A = Acceleration of ground The acceleration is defined by how quickly the ground shaking changes its speed. This equation implies that the lighter the building is, the less the seismic force will be in a given earthquake. During an earthquake, the building tends to vibrate around one particular frequency that is known as its fundamental or natural frequency. Fundamental frequency is the number of times per second that the building vibrates back and 13 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. forth. Building period (Fig. 1.4) refers to the time it takes for the building to make one complete vibration and is the inverse of frequency. As a rule of thumb, the fundamental period of a building can be estimated as 1/10 second per story. Buildings are fixed at base but the levels above the ground will move horizontally due to seismic forces (Fig. 1.5). As the building becomes more flexible, and has a higher period, its corresponding acceleration decreases and less seismic force will be induced. Period „ j one cycle) i Fig. 1.4 Period is the time (in seconds) for one cycle of oscillation. The fundamental frequency or period of the building is determined by building structural systems and height. Although the ground shakes in a complex way, a certain frequency usually predominates the ground shaking. When this predominant frequency of the ground shaking matches or is close to the building fundamental frequency, the building and the ground will shake in resonance. This phenomenon amplifies seismic drift, which may result in greater damage or even collapse. The Northridge Earthquake primarily caused collapse of 2-story homes with periods of about 0.2 seconds, similar to the predominant earthquake period (Schierle 2003, p.8). 14 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. For a building with large dimensions or complex configuration such as L, H, or T shape, separation joints should be designed to allow different portions of the building to vibrate independently. Good seismic design favors symmetrical buildings because irregular-shaped buildings usually have the problem that the center of the building mass does not match with the center of building resistance, causing torsion or a twisting effect. Dynamic seismic design methods or model tests have to be applied to such buildings. Equivalet lateral force Equivalet lateral force ground movement due to earthquakes ground movement due to earthquakes rigid buildings with less drift flexible buildings with more drift Fig. 1.5 Deformations of rigid and flexible buildings during earthquakes. Building stiffness greatly affects building response during an earthquake. Stiffness is the property of an object to resist drift induced by lateral force. Stiff buildings have less drift than flexible ones for a given lateral force. The stiffness of the building is defined by its structural systems and materials. Buildings with concrete or masonry shear walls are stiffer and have shorter periods, which is an advantage to resist wind loads. But stiff structures are subject to greater seismic forces. So stiff buildings have to be designed to resist more seismic forces. 15 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The absorption of seismic forces can be achieved by the deformation of building structural elements in certain range without breakage, which is called ductility. Ductility is a desirable feature in seismic design and greatly depends on materials, structural systems and details. The ratio of strength to weight can be used to judge material ductility. Steel is a veiy ductile material because it can deform a lot under a load in its elastic range without breakage. Concrete or masonry are brittle and break suddenly when the elastic limit is reached. During a seismic event, the vibration of the ground or buildings decays and will stop at a certain point. This phenomenon is called damping. Damping is induced due to internal friction and the absorption of energy by building elements. Damping is a desirable feature in seismic design because more damping results in shorter time in which buildings will vibrate. To increase the damping of the building especially high-rise buildings, new technologies such as base isolators (MCEER, no date) are used on building structures to reduce vibration during an earthquake. The effects of earthquakes depend on building material, construction, configuration and structural systems. The effective seismic force resisting systems have to be determined by considering those design issues. 1.3. Effects of Wind Forces on Buildings Wind is moving air over the Earth’s surface. This air movement is caused by pressure difference because air tends to move from high-pressure to low-pressure 16 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. areas. The causes of pressure difference can be primarily attributed to the Earth’s rotation and the Sun’s radiation. The Earth is surrounded with a thin layer of air called the atmosphere. While the Earth rotates, it drags the air near the Earth’s surface to rotate because of the roughness of the Earth’s surface. The air at higher level has less drag effect from the Earth. The difference in the air speed causes air to mix and form turbulence. On the other hand, the Sun gives out a great amount of energy and a portion of this energy reaches the Earth. The Earth’s surface absorbs this energy and warms up the surrounding air. When the air is heated up, it becomes less dense than cooler air, and this results in the warmer air rising up. The Earth’s surface is heated unequally by the Sun, resulting in temperature differences. The level of the air turbulence and temperature difference combined with other factors causes wind with different characteristics. Wind exists in various conditions. Hurricanes and tornadoes are winds with speeds of usually more than 75 miles per hour. They occur with some frequency in coastal areas when different temperatures and humidity meet to form ocean storms. Their highest velocity hasn’t been detected because detection instruments usually have been destroyed before they reach the highest speed. Small materials such as roof shingles or pebbles can be easily lifted up and become air-borne missiles or debris. Glazed openings are very susceptible to such debris. Sustained local wind is wind that occurs at great elevations above sea level (Ambrose and Vergun 1995, p.8). They are much milder than hurricanes or 17 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. tornadoes but, because of their enduring features, they pose major problems especially for high-rise buildings. IBC 2003 specifies design wind speed for different regions in the United States. For special regions such as hurricane regions or mountainous terrain, local climatic data have to be examined. Wind exerts forces on the surfaces that are encountered in its path (Ambrose and Vergun 1995, p.7-18). If wind encounters buildings with streamlines or rounded shapes, wind will flow around them. If buildings are bulky or boxy, wind exerts direct pressure on their windward surfaces. This pressure, considered normal to the building surface in wind design, is called positive pressure because the wind is acting toward the surface. Wind that doesn’t encounter windward surfaces flows around buildings and a drag force from wind is exerted on building side walls. Building roofs are also subject to wind force. This force may act toward or away from the roof surface depending on the sloping angle of the roof. On the leeward surfaces of the building, wind exerts pressures acting outward from the building surfaces, called suction. This pressure is considered negative. Wind has great influences on buildings. When wind changes directions constantly, it may set buildings to vibrate. As discussed before, every building has its fundamental frequency. When building frequency matches or get close to wind frequency, it amplifies building movement. The building vibration causes the movements of each building element. Some loose parts may break and injure the continuity of the load path for transferring wind loads. 18 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. If buildings have a high percentage of openness and wind can easily get into the building, internal wind pressure has to be considered. Parking structures are good examples. When wind gets into the building, it pushes walls out and can double the wind pressure on those walls. Although building overall pressure will not be affected by internal wind pressure because wind pressures in opposite directions can cancel out, each building element, such as wall or roof, has to be designed to resist the sum of external and internal pressures. The wind effects combine to exert pressure on buildings and tend to slide, turn over or lift up buildings. Horizontal wind forces may easily slide lightweight buildings such as mobile homes off their foundations. The dead weight of the building and the connections of each building element generate frictions that resist these horizontal-sliding forces. Because of overturning moment acting on buildings by wind forces, wind also can topple buildings. This type of damage has been found commonly in lightweight houses as a result of hurricanes or tornadoes. When wind flows over the building, it creates a strong lifting force or suction, especially on roofs. Buildings are composed of various elements that are connected together. Those connectors are usually the weakest links to resist wind uplift forces. If they are broken, the load path is discontinued and damage can occur. This effect is not immediately seen during construction. Proper connection methods should be followed as specified in building codes. Wind speed is also influenced by building surroundings. In cities with a high density of buildings or trees in forests, wind pressure is less because wind velocity is 19 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. reduced due to the surrounding buildings or trees. In more open areas, such as suburbs or coastal areas, wind has a higher speed and a higher pressure has to be applied on buildings. Wind effects are defined by building surroundings, configuration, construction and structural systems. Wind design becomes crucial in high-wind prone areas and all those considerations have to be kept in mind. 20 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 2 BUILDING STRUCTURAL DESIGN TO RESIST LATERAL FORCES 2.1. Amount and Distribution of Seismic and Wind Loads on Buildings Wind and seismic forces are lateral forces that exert forces primarily horizontally on buildings. Most buildings have their structural elements oriented in two perpendicular directions, known as their primary orthogonal axes. Wind and seismic forces are designed to be applied along each orthogonal direction that buildings have to resist. Forces that act in the direction parallel to the planes of walls or frames are called in-plane forces. Forces that act perpendicular to the planes of walls or frames are called out-plane forces. The following section will discuss the amount and distribution of seismic and wind loads on buildings. This study helps understand how these loads are generated and distributed in buildings in order to design effective structural systems to resist those forces. 2.1.1. Amount and Distribution of Seismic Loads on Buildings Seismic loads are generated by dead weight and acceleration of the building. Thus, seismic loads depend on the dead load of the structure and all elements permanently attached to it, as well as 25 percent of the live load in warehouses. Seismic force is dynamic and it is very difficult to precisely simulate dynamic loads during an earthquake. To simplify the analysis process, a static lateral force 21 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. procedure is used to analyze base shear (the seismic force at the building base) for regular-shaped buildings as substitute for dynamic seismic forces. The seismic force at each level is defined by base shear and the portion of the total gravity dead loads of the building assigned to that floor and its height above ground. Higher floors assume greater seismic forces since acceleration increases with height. Since all seismic forces are resisted at the foundation, seismic shear at each level consists of the seismic force of each level plus the sum of all seismic forces above. Base shear is the sum of all seismic forces at ground level. The methods of determining base shear and its distribution at each level will be discussed in Chapter 3. The overturning moment due to seismic forces at each level is resisted by the dead weight of the building. But this overturning moment needs to be examined in seismic design. Seismic forces also cause buildings to move horizontally. The lateral movement and deflection of the building is called drift. Building codes specify the maximum drift and story-drift at each level as 0.5% of the respective height, i.e. height/200. Seismic design for irregular-shaped buildings requires dynamic analyses. The distribution of seismic loads is influenced by building configuration and stiffness of structural elements. The irregularity of building configuration is defined in IBC 2003 and determines the seismic design methods of ASCE 7-02. 22 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.1.2. Amount and Distribution of Wind Loads on Buildings IBC 2003 specifies that wind shall be assumed to come from any horizontal direction and wind pressures shall be assumed to act normal to the surface considered (ICC 2002, p.283). Wind forces in form of pressures are applied normal to building surfaces. Wind loads vary with building height, applied surfaces, and surroundings. Steady wind causes static load on stiff buildings, but dynamic load on flexible buildings. Gusty wind causes dynamic load. For regular-shaped buildings, a static wind load analysis method is used to simplify the process. The distribution of wind pressures on building surfaces is specified in IBC 2003 (Fig. 2.1). The wind pressures on windward walls increase from the lower levels to the upper levels. Building side walls are subject to suctions and wind pressures are distributed uniformly along the side walls. Flat roofs are subject to wind suction. The leeward walls facing away from the wind are subject to wind suctions, assumed to be distributed uniformly over the leeward walls. For irregular-shaped buildings, wind tunnel tests are usually done to analyze distribution of wind pressures. Wind forces at each level are the product of wind pressure and tributary surface area at that level. Since all wind forces are resisted at the foundation, wind shear at each level consists of the wind force of that level plus the sum of all wind forces above. Wind base shear is the sum of all wind forces at ground level. The overturning moment due to wind forces at each level is resisted by the weight of the building. The methods of their determination are discussed in Chapter 4. 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. , leeward roof windward wall f -I - side wall Fig. 2.1 Wind pressure distribution on buildings (Pittack and Bias, 2000) 2.2. Structural Design to Resist Lateral Forces Because both wind and seismic loads are applied horizontally to buildings, the manners of building structural resistance to wind and seismic loads are quite similar. In most cases, building configuration and structural systems provide certain resistance to lateral forces. Lateral-force resisting systems are developed by considering and utilizing these basic structural elements. The main idea of lateral force resisting systems is to transfer wind and seismic loads through various structural elements to the ground. It requires a complete load path for transferring lateral loads. The components in the load path consist of multiple structural elements and connections between them. They range from elements like large frames to small connectors, such as nails. 24 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Most lateral-force resisting systems are based on some combinations of horizontal and vertical structural elements. In lateral-force resisting systems, horizontal structural elements such as rooves and floors are called diaphragms. Roof or floor diaphragms are connected with and supported by vertical structural elements such as walls or frames. The basic functions of horizontal diaphragms are to collect lateral loads induced at their level and transmit those loads to the vertical structural elements immediately below (Ambrose and Vergun 1995, p.98). Because horizontal diaphragms vary in many ways such as material, geometry or span-to-depth ratio, they demonstrate different properties subject to gravity or lateral loads. Horizontal diaphragms span between vertical structural elements and will deform under lateral loads. Based on the level of deformation, diaphragms can be classified as either flexible or rigid. Concrete and some steel diaphragms are considered rigid. Wood- frame diaphragms can be flexible or rigid depending on construction details and factors such as spanning distance or span-to-depth ratio. Precast concrete diaphragms must be attached in proper manners to transfer load. Poured-in-place concrete diaphragms are the stiffest diaphragms and are considered rigid. The classification into rigid and flexible diaphragms determines the method that lateral loads are distributed to vertical structural elements of the lateral-force resisting systems. Flexible diaphragms resist load in proportion to the tributary area supported. Rigid diaphragms distribute lateral loads based on their relative stiffness. 25 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Vertical structural elements of lateral-force resisting systems are used to transfer lateral loads from the horizontal diaphragms above to the level below. These vertical structural elements vary in many ways such as materials or construction. Vertical resisting systems include shear walls, cantilevers, moment- resisting frames and braced frames. Building foundations in the load path work to transfer the accumulated lateral loads to the ground. Foundations are embedded in the soil and may take various forms according to the design loads and soil conditions. The resistance of the building foundation to lateral loads is achieved by the soil pressure and friction between its surfaces and the soil around them. Besides primary horizontal and vertical diaphragms in the load path to transfer lateral loads, secondary structural elements such as chords and collectors may be found along the boundaries of horizontal diaphragms (Ambrose and Vergun 1995, p. 113-115). Such structural elements are called either chord or collectors based on the axis along which lateral forces are applied to horizontal diaphragms. Perpendicular to the direction of lateral forces, chords limit the horizontal displacement of diaphragms induced by lateral forces. Parallel to the direction of lateral forces, collectors are located along the edges of diaphragms. They transfer lateral loads from horizontal diaphragms to shear walls or frames when they are not continuous along the edge of horizontal diaphragms because of openings such as windows or doors. In such cases, collectors are required to span over the openings to transfer lateral loads. Headers and twin top plates in 26 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. wood-frame buildings and beams in steel, concrete or masonry construction are used for such purposes. Anchorage elements attach various structural elements together. The types of anchorage elements depend on materials and design requirements. In wood-frame buildings, nails, bolts, and metal connectors are commonly used to join structural elements. Nail size and spacing have great influences on the shear capacity of wood panels specified in building codes. In steel construction, bolts or welds are commonly used. Various connection methods have been developed based on design requirements and their capacities are closely related with construction details. Studies of the failure of the residential buildings after hurricanes revealed that the breakage of anchorages between structural elements have caused the most damage. When buildings are subject to wind and seismic loads, entire structural systems work to resist and transfer lateral loads. A complete load path has to be maintained to transfer lateral loads to the ground. 2.3. Introduction to Lateral Force Resisting Systems Lateral-force resisting systems are designed to resist and transfer lateral loads to the ground. Various types of lateral-force resisting systems have been developed. The type selection is often determined by the architectural design of the building. Because most buildings have horizontal diaphragms transfer lateral loads to vertical structural elements, the types of lateral-force resisting systems mainly 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. depend on the vertical elements. There are four basic broad categories of lateral- force resisting systems; shear wall systems, cantilevers, braced-frame systems, and moment-resisting frame systems. Dual systems combining them are also used. Each type of lateral-force resisting system involves many considerations such as its structural behaviors, materials, construction, geometric forms and so on. The following paragraphs discuss five common types of lateral-force resisting systems regarding their structural behaviors for resisting lateral forces. 2.3.1. Shear Wall Systems Shear walls are commonly used in bearing wall systems to provide lateral- force resistance. Shear walls are common in apartment buildings, using party walls as shear walls, but are not common in office buildings that require flexibility. Bearing wall systems are structural systems utilizing walls to provide the support for all or major portions of vertical loads (SEI2003, p. 102). To determine whether walls are shear walls depends on many factors such as wall location, materials and construction details. The purposes of shear walls are to resist and transfer lateral loads from the diaphragms immediately above to the story below or the building foundation. When lateral loads are applied to shear walls, the predominant forces generated in shear walls are shear stress in which the fibers within the wall try to slide past one another. The shear forces are transferred from the upper to the lower portion of the wall. 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Shear walls cantilever above the ground and lateral forces tend to push the walls over and bending forces are generated in shear walls. This push-over tendency is called overturning moment. This overturning moment increases from the top to the bottom of the building. The boundaries of shear walls are usually reinforced to resist bending stress in addition to causing overturning, lateral loads tend to slide shear walls off their supports so sliding resistance is generated at the base of shear walls. Shear walls can be constructed from a variety of materials that depend on the building construction type. The typical wood-framed shear walls (Fig. 2.2) consist of top plates, stud and sill plates sheathed with wood structural panels. Because wood shear walls are pretty light, they require anchor bolts to resist sliding forces and hold-downs to resist overturning moment. The foundations of wood-framed shear walls are commonly constructed of concrete and the connections between the bottom plates of the shear walls and their foundation become crucial for the success of the transfer of lateral loads. Significant damage of wood-framed buildings after 1994 Northridge earthquake has been found resulting from these connections. Reinforced masonry shear walls (Fig. 2.3) should be used where the code does not permit combustible materials and/or where the height of the structure exceeds the code limit for wood (3 stories or 4 stories with fire sprinklers) (CBTC, no date). Reinforcing steel bars are arranged in some manner in those walls to provide lateral- force resistance. Apartment buildings use party walls as shear walls. Office building 29 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. shear walls are usually walls that enclose stairways or elevators because they run the entire height of the building. Top plates Studs Nails Wood sheathing panel Sill plates - Footing - Anchor bolt Hold-down Fig. 2.2 Typical wood shear wall Concrete shear walls (Fig. 2.4) are constructed with concrete and reinforcing steel bars. Reinforcing steel bars are arranged in some manner to provide lateral- force resistance. Concrete is poured in the form and surrounds the rebars to allow the walls to have certain strength and stiffness. The design of shear walls has to consider the strength and stiffness required to resist the magnitude of lateral loads. The strength of wood-frame shear walls varies depending on factors such as wood panel thickness, fastener sizes and spacing. The allowable stress for concrete and masonry shear walls is defined by strength and amount of steel reinforcing and the strength of concrete or masonry. 30 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reinforcing steel bar spacing Max. 4' in seismic regions Mortar Brick masonry Fig. 2.3 Typical reinforced brick masonry shear wall Tension Compression Friction between the foundation and soil Lateral Forces Shear Wall Shear Wall Foundation Fig. 2.4 Concrete shear wall resistance to lateral forces (Ambrose and Vergun 1995, p. 106) 31 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A shear wall is a cantilever with the base fixed on the ground. When lateral forces are transmitted from the horizontal diaphragms to shear walls, the top of shear walls can move. The level of this movement, or deformation, depends on the wall stiffness. The stiffness of shear walls is defined by many factors, such as material, amount of steel reinforcing, and height-to-width ratio. This thesis presents a software that will be described in detail in a later chapter. In the design software, overall length per level of shear walls in wood, concrete, or masonry in each orthogonal direction can be generated if shear wall systems are used as lateral-force resisting systems. Default values of shear stress in pound per foot (plf) will be used if users do not specify shear stress. For wood shear wall, common types of plywood (Table 1) from Table 2306.4.1 of IBC are used. Because shear stress of concrete or masonry shear walls varies depending on many factors, default values (Table 2, 3) are taken from the LDG software (Schierle, 1996). 32 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 1. Allowable shear (pound per foot) for wood structural panel shear walls with framing of Douglas-Fir-Larch or Southern Pine for wind and seismic loading (ICC 2002, p.469). FRAMING OF DOUGLAS-FIR-LARCH OR SOUTHERN PINE FOR WIND OR SEISMIC LOADING PANEL GRADE MINIMUM NOMINAL PANEL THICKNESS (INCH) MINIMUM FASTERNER PENETRATIO NIN FRAMING (INCH) NAIL (COMMON OR GALVANIZED BOX) OR STAPLE SIZEk FASTERNER SPACING AT PANEL EDGES (INCHES) 6 4 3 2e STRUCTURAL 1 SHEATHING 5/16 1-1/4 6d 200 300 390 510 3/8 1-3/8 8d 230d 360d 460d 610d Note: d. Shears are permitted to be increased to values shown for 15/32 inch sheathing with same nailing provided (a) studs are spaced a maximum of 16 inches on center or (b) if panels are applied with long dimension across studs. e. Framing at adjoining panel edge shall be 3 inches nominal or wider, and nails shall be staggered where nails are spaced 2 inches on center, k. Staples shall have a minimum crown width of 7/16 inch. Table 2. Allowable shear (pound per foot) for concrete shear wall (Amrhein 1994) ALLOWABLE SHEAR (POUND PER FOOT) FOR CONCRETE SHEAR WALL THICKNESS (inches) WALL CAPACITY (psi) ALLOWABLE SHEAR (Plf) 12 300 43,200 12 250 36,000 10 250 30,000 10 200 24,000 8 200 19,200 8 150 14,400 8 100 9,600 33 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 3. Allowable shear (pound per foot) for masonry shear wall (Amrhein 1994) ALLOWABLE SHEAR (POUND PER FOOT) FOR REINFORCED MASONRY SHEAR WALLS THICKNESS (inches) WALL CAPACITY (psi) ALLOWABLE SHEAR (Plf) 16 75 14,400 12 75 10,800 8 75 7,200 12 35 5,040 8 35 3,360 12 15 2,160 8 15 1,440 2.3.2. Cantilever Systems A cantilever system is a lateral-force resisting system in which lateral forces are resisted entirely by columns acting as cantilevers from the foundation (SEI2003, p.97). This system resists gravity load in compression and lateral loads through the bending of cantilever columns (Fig. 2.5). This system has much more deflection compared with shear wall systems. Deflection is good for absorbing energy generated by wind and seismic forces. Cantilever columns usually consist of single or multiple towers (Fig. 2.6) rising from the foundation. In wood pole houses, wood pole cantilevers from the foundation that is typically about 4 or 5- foot deep. The bending moment induced by lateral loads increases from the top to the bottom of cantilever columns. Towers can be hollow or solid columns or slender walls. Because those towers usually have gigantic dimensions from the consideration of structural design, the interior space of 34 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. those towers is commonly utilized as service rooms or circulation areas such as stairway or elevators. Single tower cantilever Twin tower cantilever Single tower cantilever with suspended floors Fig. 2.6 Structural behaviors of cantilevers under lateral loads (Schierle 2001) The floors can be suspended from single tower or pin-connected between two towers. Pin connections between tower and floor avoid transferring bending moment from floors to the tower. One benefit of this system is that it provides open space without being interrupted by columns such as those found in frame systems and it allows flexible arrangements of furniture or partitions. Gravity load ±y f? A Lateral load Compression Bending resistance from the foundation Fig. 2.5 Cantilever column under gravity and lateral loads (Schierle 2001) 35 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Because the bending moment of the cantilevers is considerable, this system requires large and deep foundations to transfer and resist bending moment to the ground. When multiple towers are used, their foundations are usually tied together to increase the overall size of the foundations and higher stability can be achieved. The common materials for cantilevered columns are concrete or masonry. Their heavy weight is good for resisting overturning moment. But they are brittle and reinforcing steel bars are placed in concrete and masonry construction to resist tensile stress. Cast-in-place concrete with reinforcing bars has been commonly used in this system for high-rise buildings. This practice usually requires longer construction time and involves more labor than steel frame buildings. 2.3.3. Braced Frame Systems A braced frame system is essentially a vertical truss or its equivalent that is provided in a bearing wall, building frame or dual system to resist lateral forces (SEI 2003, p. 99). This system actually has been utilized for centuries and diagonal- bracing elements can be found in the wood-framed residential or church buildings since early times. The braced-frame system has been developed for building frame systems to resist lateral loads. Building frame systems may be complete space frames providing support for gravity and lateral loads. When it is subject to lateral loads, the bottom of the frame system is fixed at the ground but its top will be allowed to move horizontally. Diagonal bracing elements inserted in the rectangular bays of building 36 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. frames is one of the solutions to constrain the movement of the frame members, which has proven very efficient. A braced frame system can be braced in various ways. The simplest braced frame is to insert a single diagonal member in one bay of a building frame to brace against lateral loads. Tension forces will be generated in the bracing member for lateral loads coming from one direction and compression forces will be generated when lateral loads comes from the opposite direction. Compression forces may cause buckling of the slender bracing member so tension members are more desirable. X-bracing may be developed to avoid compression members (Fig. 2.7). However, X-bracing restricts the opportunity for wall openings. There are two broad categories of braced frames, concentric and eccentric (Fig. 2.8). In concentrically braced frames, all members are connected at each joint with all bars meeting at one point and subjected primarily to axial forces. lateral forces crisscrossed instability of frame stability achieved system due to lateral by using single bracing to avoid Fig. 2.7 Development of frame systems by utilizing bracing elements (Ambrose and Vergun 1995, p. 74). 37 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. X-bracing chevron bracing V-bracing chevron bracing (concentric) (concentric) (concentric) (eccentric) Fig. 2.8 Common types of braced frames An eccentrically braced frame is a diagonally braced frame in which at least one end of each brace frames into a beam a short distance from a beam-column joint or from another diagonal brace (SEI 2003, p.99). Because bracing elements do not come to one joint, they induce both shear and bending moment in the beam to which they are connected. The axial brace force and beam bending moment resist lateral loads. Eccentrically braced frames provide greater stiffness than moment frames but less stiffness than braced frames. The piece between braces and column are called link beam. The link beam length defines the relative stiffness. An eccentrically braced frame leaves more open space that can be used for openings such as windows or doors. Braced frames have been commonly used in high-rise buildings to constrain horizontal drift. The Bank of China building designed by I.M.Pei and Thomcrown Chapel by Fay Jones are outstanding examples with braced frames as their architectural expression. The joints of bracing members are very crucial in transferring lateral loads and have great influence on the performance of braced frame systems. The typical joint construction consists of bracing members bolted to gusset plates. Those joints have 38 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. to be designed to resist any undesirable failure due to loosening or rotting because the failure of those joints result in the total failure of building frame systems. The most common material for bracing elements is steel. Fatigue may be caused in the bracing members that are elongated or compressed by the vibration of lateral forces. Fatigue causes bracing members to lose their ductility. If bracing members are designed for axial loading, they must be designed to avoid loadings other than those required for their bracing functions. Bracing members sometimes may be in conflict with architectural elements such as windows or doors. The design of bracing elements has to take that into consideration. 2.3.4. Moment Resisting Frame Systems A moment resisting frame system is a structural frame in which members and joints are capable of resisting forces by flexure as well as along the axis of the members (SEI2003, p.99). Columns and beams comprise the most typical building frame systems. To classify whether a frame system is a moment-resisting frame is determined by the joints between columns and beams. In moment-resisting frame system, joints are designed as rigid joints such that the angle formed by columns and beams remains a right angle and the joints can transfer bending moments generated by structural members that have to be designed for axial loads and bending moment. The forces and bending moment are increased greatly while subject to both gravity and lateral 39 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. loads other than either load separately. In this case, structural members have to be designed for combined gravity and lateral loads. The construction details of joints are usually designed by structural engineers or manufacturers. In steel construction, structural members can be bolted or welded together. Usually the beam web is bolted to the column flange while beam flanges are welded to column flanges, with stiffener plates welded between column flanges aligned with beam flanges to resist the bending stress of beam flanges. In site-cast concrete construction, moment joints are designed with column rebars extending though beams and beam rebar extending through columns. Ductile design, required by the UBC since 1976, requires less steel and more concrete than balanced design to avoid brittle concrete failure in favor of ductile steel behavior. The resistance of moment-resisting frame systems to lateral forces is achieved by the deformation of structural members (Fig. 2.9). This requires the construction material should have high ductility. Steel is veiy ductile and a desirable material for moment-resisting frame systems. Concrete is also used but concrete is brittle and easily develops fractures under bending moment. Nonetheless, concrete frames may be designed by ductile theory described above. Moment resisting frame systems are subdivided into three types in IBC 2003: ordinary moment frames, intermediary moment frames and special moment frames. The differences of each type lie in the level of ductility under lateral loads. Ordinary frames are the least ductile and special moment frames are most ductile. Their construction details have to be examined and followed as specified in building codes. 40 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. gravity loads I II II I M I lateral loads deformation o f moment-resisting frame under gravity loads deformation o f moment-resisting fr ame under lateral loads Fig. 2.9 Deformation of moment-resisting frame with inflection points of zero stress Compared with shear wall and braced frame systems, moment-resisting frame systems have more deflection that is good for energy absorption and results in lower seismic forces. On the other hand, higher deflection poses problems for non- structural elements. Moment frames are most common for office buildings that require planning flexibility. 2.3.5. Dual Systems If single type of lateral-force resisting systems can not satisfy building resistance to lateral forces, dual systems combine shear wall, braced-frame or moment-resisting frame to provide structural resistance to lateral forces. A dual frame system (Fig. 2.10) is a structural system with an essentially complete space frame providing support for vertical loads. Lateral-force resistance is provided by moment-resisting frames and shears walls or braced frames (SEI 2003, p.99). 41 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Frame systems Bracing element Roof diaphragm Chord/ collector Floor diaphragm Shear wall Foundations Fig. 2.10 An example of dual systems For a dual system, the moment frame shall be capable of resisting at least 25% of the design seismic forces. The total seismic forces will be distributed in proportion to the rigidity of vertical lateral-force resisting elements, which requires considerable computation and design details. This system makes use of the advantages of various types of lateral-force resisting systems and achieves a stiffer structural system with using space frame systems. For example, combining moment frames and braced frames to reduce lateral drift that is small at the base or braced frames and small on top of moment frames. 42 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 3 SEISMIC LOADS PER IBC 2003 AND ASCE 7-02 3.1. Introduction The procedures of determining minimum seismic loads are defined in building codes. Building codes typically take life safety as major concern, which means buildings are mainly designed to protect the public from injury or death. So building codes specify the minimum design loads (Ambrose and Vergun 1995, p.26). Beyond this scope, higher design loads may be used by structural engineers when additional concerns need to be met. The International Building Code (IBC) 2003, published by International Code Council, is one of the most popular “model” building codes. A “model” building code is a code that is adopted by local jurisdictions and forms the basis of local building codes. Although it has not been adopted by all states because it takes some time for new “model” codes to get locally adopted, IBC 2003 will be gradually put into use in the future. Every structure shall as a minimum be designed and constructed to resist the effects of earthquake motions (ICC 2002, p.302). Sections from 1613 to 1623 of IBC 2003 govern seismic design. The content is quite comprehensive and covers various criteria of seismic design ranging from general concepts to construction details such as the anchorage for various types of shear walls. 43 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. IBC 2003 specifies three methods for determining seismic loads: Simplified Analysis Procedure, Equivalent Lateral Force Procedure and Dynamic Analysis Procedure. The application of each method is determined by building configuration and seismic design category. The Simplified Analysis Procedure is basically used for low-rise (less than 60 ft) light-framed constructions and its application is specified in Section 1616.6.1 of IBC 2003. The Dynamic Analysis Procedure requires constructing a mathematical model of the structure to represent the distribution of building mass and stiffness, which is quite complex. The Equivalent Lateral Force Procedure can be used for regular-shaped buildings and its application is specified in Section 1617.4 of IBC 2003. By considering the level of complexity of each method and the scope of the software that is designed for students of architecture, the Equivalent Lateral Force Procedure is used for designing the LFD software and the methods are given in Section 9.5.5 of ASCE 7 (ICC 2003, p.331). ASCE, the American Society of Civil Engineers, published ASCE 7-02 for determining wind and seismic loads that are also defined in IBC 2003. IBC 2003 shall be followed if any conflict occurs. For the concepts explained both in IBC 2003 and ASCE 7-02, the ones in IBC 2003 are cited in the following sections. 44 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.2. Seismic Loads per IBC 2003 and ASCE 7-02 for the LFD Software Design The LFD software is designed to teach students of architecture the main concepts and procedures of seismic and wind loads based on IBC 2003 and ASCE 7- 02. For seismic loads, the software computes: base shear, distribution per level of seismic force, shear force and overturning moment. Other seismic design analyses, such as drift or redundancy, involve very complex structural analyses and are beyond the scope of the LFD software. The following sections discuss the main theory of Equivalent Lateral Force Procedure that serves as the scope of the design software. 3.2.1. Applicability Equivalent Lateral Force Procedure is one of 6 alternative analysis procedures specified in ASCE 7-02. The application of each procedure (Table 4) is determined by Seismic Design Category and building structural characteristics. Table 4 indicates the application of the Equivalent Lateral Force Procedure of ASCE 7-02. Several concepts need to be explained. The concepts include Seismic Use Group (SUG), Seismic Design Category (SDC), fundamental building period T, plan irregularity and vertical irregularity. Their explanations are given in the following section. 45 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 4. Permitted procedures of Table 9.5.2.5.1 of ASCE 7-02 (SEI 2003, p. 140) Seismic Design Category Structural Characteristics Index Force Analysis Section 9.5.3 Simplified Analysis Section 9.5.4 Equivalent Lateral Force Analysis Section 9.5.5 Used in LFD software Modal Response Spectrum Analysis Section 9.5.6 Linear Response History Analysis Section 9.5.7 Nonlinear Response History Analysis Section 9.5.8 A All structures P P P P P P B, C SUG-1 buildings of light-framed construction not exceeding three stories in height NP P P P P P Other SUG-1 buildings not exceeding two stories in height NP P P P P P All other structures NP NP P P P P D, E, F SUG-1 buildings of light-framed construction not exceeding three stories in height NP P P P P Other SUG-1 buildings not exceeding two stories in height NP P P P P P Regular structures with T < 3.5 Ts and all structures of light-frame construction NP NP P P P P Irregular structures with T<3.5 Ts and having only plan irregularities type 2,3,4, or 5 of Table 9.5.2.3.2 or vertical irregularities type 4 or 5 of Table 9.5.23.3 NP NP P P P P All other structures NP NP NP P P P Notes: P - indicates permitted, NP - indicates not permitted, SUG - indicates Seismic Use Group, T - indicates fundamental building period. 46 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.2.2. Base Shear V Base shear is the total design shear at the base of the building. The general concepts are discussed in Section 2.1.1 of Chapter 2. The equations of determining seismic base shear and the concepts involved are discussed in the following sections. Seismic force shall be designed for the directions that most critical load effects will be induced. To simplify this process, seismic forces are assumed to be applied along each orthogonal direction of the building. 3.2.2.I. Equations The seismic base shear V of the building in a given direction shall be determined as described in the following equation: V = CSW (Eq. 9.5.5.2-1 of ASCE 7-02) V: seismic base shear, in kips Cs: seismic response coefficient W: the effective seismic weight of the structure, including the total dead load and other loads listed below: 1. In areas used for storage, a minimum of 25% of the floor live load 2. Where an allowance for partition load is included in the floor load design, the actual partition weight or a minimum weight of 10 psf of floor area, whichever is greater. 3. Total operating weight of permanent equipment. 4. 20% of flat roof snow load where flat roof snow load exceeds 30 psf. 47 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The determination of Cs can be found in the following equations excluding Seismic Design Category E and F: Cs= SD S / (R/I) (Eq. 9.5.5.2.1-1 of ASCE 7-02) The value of Cs should also be in the range specified as follows: Upper bound: Cs = SD 1 / T (R/I) (Eq. 9.5.52.1-2 of ASCE 7-02) Lower bound: Cs = 0.044SdsI (Eq. 9.5.5.2.1-3 of ASCE 7-02) The determination of Cs for Seismic Design Category E and F is specified as follows: Cs = 0.5 Si/ (R/I) (Eq. 9.5.S.2.1-4 of ASCE 7-02) Sds: Design spectral response acceleration at short periods, in g-sec., see Section 3.2.2.2.4. R: Response modification factor, see Section 3.2.2.2.9. I: Occupancy importance factor, see Section 5.222.1. SdF Design spectral response acceleration at a period of 1.0 second, in g-sec., see Section 3.2.2.2.4. T: Fundamental period of the structure, in sec., see Section 3.2.2.2.11. Si: Mapped maximum considered earthquake spectral response acceleration at a period of 1.0 second, in g-sec., see Section 3.2.2.2.2. 3.2.2.2. Explanations of Factors in Equations Many factors are involved in determining seismic base shear of the building as described above. Their explanations are given in this section. Other concepts for determining those factors are also discussed. 48 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.2.2.2.I. Site Class The soil condition of the building site has great influence on the ground motions during an earthquake. The building site shall be classified for determining seismic base shear of the building. The classification is determined by site properties in terms of soil shear wave, standard penetration resistance and soil undrained shear strength. Those data are provided in soil investigation reports. When soil properties are not known, Site Class D shall be used. The full discussion of Site Class is included in Section 1615.1.1 of IBC 2003 and 9.4.1.2.1 of ASCE 7-02. Table 5. Site Class definitions (IBC Table 1615.1.1) Site Class Soil Profile Name A Hard Rock B Rock C Very dense soil and soft rock D Stiff soil profile (used if site class is not available) E Soft soil profile F Soil vulnerable to potential failure or collapse under seismic loads. Site-specific procedure is required. 3.2.2.2.2. Si,Ss Si and Ss represent the maximum considered earthquake ground motions in particular geographic regions. Sr. Mapped maximum spectral response acceleration at 1-second period Ss: Mapped maximum spectral response acceleration at a period of 0.2 seconds 49 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The values of Si and Ss for Site Class B for the USA are listed in Figure 1615 (1) through (10) of IBC 2003. They are marked on contour lines and shown as certain percentage of “g” due to the Earth’s gravity. For sites located between contour lines, straight-line interpolation or the value of the higher contour shall be used. The full discussion of Si and Ss can be found in Section 1615.1 of IBC 2003 and 9.4.1.2 of ASCE 7-02. The site-specific procedures for site Class F, where soil is vulnerable to potential failure or collapse under seismic loads is not considered in the LFD software. 3.2.2.2.3. Sms,Sm i The values of Si and Ss listed in Figures 1615 (1) through (10) of IBC 2003 are for Site Class B. Their values for other site class need to be adjusted using the site coefficients of IBC Table 1615.1.2(1) or 1615.1.2(2). Smi is the value of Si adjusted for site class effects and the maximum considered earthquake spectral response acceleration at 1 second. Sms is the value of Ss adjusted for site class effects and the maximum considered earthquake spectral response acceleration at short period of 0.2 seconds. They are determined as follows: SM s= FaSs (Eq. 9.4.1.2.4-1 of ASCE 7-02) SM i = FvSi (Eq. 9.4.1.2.4-2 of ASCE 7-02) Sms: Maximum spectral response acceleration at 0.2-second period, in g-sec Fa: Acceleration-based site coefficient at 0.2-second period, see Table 6. 50 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ss: Mapped maximum spectral response acceleration at 0.2-second period, see Section 3.2.2.2.2. Smi: Maximum considered earthquake spectral response acceleration at a period of 1.0 second, in g-sec. Fv: Velocity-based site coefficient at a period of 1.0 second, see Table 7 Si: Mapped maximum spectral response acceleration at 1-second period. Table 6. Values of Fa as a function of site class and mapped spectral response acceleration SS at short periods of Table 1615.1.2(1) of IBC 2003 Site Class Mapped Spectral Response Acceleration at Short Periods Ss £ 0.25 Ss = 0.5 Ss = 0.75 Ss =1.00 Ss 2:1.25 A 0.8 0.8 0.8 0.8 0.8 B 1.0 1.0 1.0 1.0 1.0 C 1.2 1.2 1.1 1.0 1.0 D 1.6 1.4 1.2 1.1 1.0 E 2.5 1.7 1.2 0.9 0.9 F Note b Note b Note b Note b Note b Note: Use straight-line interpolation for intermediate values of Ss b. Site-specific geotechnical investigation and dynamic site response analysis shall be performed. Table 7. Values of Fv as a function of site class and mapped spectral response acceleration SI at 1-second period of Table 1615.1.2 (2) of IBC 2003. Site Class Mapped Spectral Response Acceleration at 1-Second Period Si £ 0.1 Si = 0.2 Si = 0.3 Si =0.4 Si > 0.5 A 0.8 0.8 0.8 0.8 0.8 B 1.0 1.0 1.0 1.0 1.0 C 1.7 1.6 1.5 1.4 1.3 D 2.4 2.0 1.8 1.6 1.5 E 3.5 3.2 2.8 2.4 2.4 F Note b Note b Note b Note b Note b Note: Use straight-line interpolation for intermediate values of Si b. Site-specific geotechnical investigation and dynamic site response analysis shall be Performed 51 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The full discussion of Smi and Sms can be found in Section 1615.1.2 of IBC 2003 and 9.4.1.2.4 of ASCE 7-02. 3.2.2.2.4. SD S ,SD 1 S ds and S d i are design spectral response accelerations used to determine seismic base shear of the building. Their determinations are specified as follows: Sds = 2/3 SMs (Eq. 9.4.1.2.5-1 of ASCE 7-02) S di = 2/3 SM i (Eq. 9.4.1.2.5-2 of ASCE 7-02) Sds: Design spectral response acceleration at short periods, in g-sec. Sms-' Maximum considered earthquake spectral response acceleration at 0.2-second period, in g-sec., see Section 3.2.2.2.3. Sdi: Design spectral response acceleration at a period of 1.0 second, in g-sec. Smi: Maximum considered earthquake spectral response acceleration at a period of 1.0 second, in g-sec., see Section 3.2.2.2.3. The full discussion of S d i and S d s can be found in Section 1615.1.3 of IBC 2003 and 9.4.1.2.5 of ASCE 7-02. 3.2.2.2.5. Occupancy Category Occupancy category is assigned to every building for applying lateral loads and snow or ice loads. There are four categories (Table 8) ranging from Category I to IV with the increasing level of hazard to human life in the event of building failure due to lateral forces. 52 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 8. Building Occupancy Category from Table 1604.5 of IBC 2003 Nature of Occupancy Category Buildings and other structures that represent a low hazard to human life in the event of failure including but not limited to. Agricultural facilities, certain temporary facilities, minor storage facilities I Buildings and other structures except those listed in Categories I, III and IV II Buildings and other structures that represent a substantial hazard to human life in the event of failure including but not limited to: • Buildings and other structures where more than 300 people congregate in one area • Buildings and other structures with day care facilities with capacity greater than 150 • Buildings and other structures with elementary school, secondary school or day care facilities greater than 250 • Buildings and other structures with a capacity greater than 500 for colleges or adult education facilities • Health care facilities with a capacity of 50 or more resident patients but not having surgery or emergency treatment facilities • Jail and detention facilities • Any other occupancy with a capacity greater than 5,000 III Buildings and other designed as essential facilities including, but not limited to: • hospitals and other health care facilities having surgery or emergency treatment facilities • Fire, rescue and police stations and emergency vehicle garages • Designated earthquake, hurricane or other emergency shelters • Designated emergency preparedness, communication, and operation centers and other facilities required for emergency response • Structures containing highly toxic materials • Aviation control towers, air traffic control centers and emergency aircraft hangars • Building and other structures having critical national defense functions • Water treatment facilities required to maintain water pressure for fire suppression IV Category I: buildings with a low hazard to human life such as agricultural facilities Category II: buildings excluded in other categories Category III: buildings representing a substantial hazard to human life in the event of failure such as elementary schools with more than 250 people. 53 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Category IV: essential facilities such as hospitals, police or fire stations For a full discussion of Occupancy Category see Section 1.5 of ASCE. 3.2.2.2.6. Seismic Use Group (SUG) A seismic Use Group (Table 9) is assigned to each building for seismic design and determined by building occupancy category. For a building with two or more occupancies not included in the same seismic use group, the highest Seismic Use Group shall be assigned. The full discussion of Seismic Use Group can be found in Section 1616.2 of IBC 2003 and 9.1.3 of ASCE 7-02. 3.2.2.2J. Occupancy Importance Factor I Occupancy Importance Factor I (Table 10) shall be assigned to each building according to its seismic use group and used for determining seismic base shear. Its value is specified in the following Table. For a full discussion of Occupancy Importance Factor I, see Section 9.1.4 of ASCE 7-02. Table 9. Seismic Use Group of Table 9.1.3 of ASCE 7-0 Seismic Use Group I n in Occupancy Category I X n X 111 X IV X 54 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 10. Occupancy Importance Factor of Table 9.1.4 of ASCE 7-02 Seismic Use Group Occupancy Importance Factor I I 1 II 1.25 in 1.50 3.2.2.2.8. Seismic Design Category Seismic Design Categoiy determines the methods to define seismic base shear of ASCE 7-02. It is assigned to each building and determined by seismic use group and the design spectral response acceleration S d s or S d i. Each building shall be assigned to the most severe seismic design category as specified in either of Tables 11 or 12. For a full discussion of Seismic Design Category see Section 1616.3 of IBC 2003 and 9.4.2.1 of ASCE 7-02. Table 11 is used in the LFD software. Table 11. Seismic Design Category based on SDS of Table 1616.3 (1) of IBC 2003 Value of SD S Seismic Use Group I n in SDS<0.167g A A A 0.167g< SDS<0.33g B B C 0.33g<SD S<0.50g C C D Q.5Qg < Sd s Da Da Da Note: 8 Seismic Use Group I and II structures located on sites with mapped maximum considered earthquake spectral response acceleration at 1-second period, S1 ; equal to or greater than 0.75g shall be assigned to Seismic Design E and Seismic Use Group III structures located on such sites shall be assigned to Seismic Design Category F. 55 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 12. Seismic Design Category based on SDI of Table 1616.3 (2) of IBC 2003 Value of Sdi Seismic Use Group I II HI Sdi <Q.Q67g A A A 0.067g<SD1<0.133g B B C 0.133g < Sdi <0.20g C C D 0.20g < SD 1 D“ Da Da Note: a Seismic Use Group I and II structures located on sites with mapped maximum considered earthquake spectral response acceleration at 1-second period, Si, equal to or greater than 0.75g shall be assigned to Seismic Design E and Seismic Use Group III structures located on such sites shall be assigned to Seismic Design Category F. 3.2.2.2.9. Response Modification Coefficient R Response Modification Coefficient R is assigned to a lateral-force resisting system defined in ASCE 7-02. This coefficient represents a relative rating of the ability of a structural system to resist seismic loads without collapse. R is the denominator of the design seismic base shear of the building. The larger value of R results in the lower design base shear. Various types of lateral-force resisting systems are included in Table 1617.6.2 of IBC 2003. Common types are used in the LFD software. R value can be specified by users for types of lateral-force resisting systems not included in the design software. For a full discussion of Response Modification Coefficient R, see Section 1617.6 of IBC 2003 and 9.5.2.2 of ASCE 7-02. 56 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 13. Design coefficients and factors for basic seismic-force resisting systems from Table 1617.6.2 of IBC 2003 Basic Seismic-Force Resisting System Response Modification Coefficient R System Limitations and Building Height Limitations (Feet) by Seismic Design Category A or B C D E F 1. Bearing Wall Systems Ordinary steel braced frames in light- frame construction 4 NL NL 65 65 65 Ordinary reinforced concrete shear walls 4.5 NL NL NP NP NP Special reinforced masonry shear walls 5 NL NL 160 160 100 Light-frame walls with shear panels - wood/steel panels 6.5 NL NL 65 65 65 2. Building Frame Systems Composite Eccentrically braced frames 8 NL NL 160 160 100 Ordinary composite braced frames 3 NL NL NP NP NP Ordinary reinforced masonry shear walls 3 NL 160 NP NP NP 3. Moment-resisting Frame systems Special steel moment frames 8 NL NL NL NL NL Ordinary steel moment frames 3.5 NL NL NP NP NP Ordinary reinforced concrete moment frames 3 NL NP NP NP NP Note: NL - indicated not limited, NP - indicated not permitted 3.2.2.2.10. Building Configuration Building configuration shall be classified as regular or irregular for determining seismic base shear. The classification needs to consider diaphragm flexibility, plan irregularities and vertical irregularities. Each of these has several categories and their descriptions are defined in Section 9.5.2.3 of ASCE 7-02. 57 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Most categories require advanced structural analyses such as stiffness of vertical structural elements and torsional effects of structural systems. Those analyses are not usually taught to architecture students. Based on the input building information, the LFD software can compute two types of vertical structural irregularities (Table 14) that are defined in IBC 2003 and ASCE 7-02. Buildings with L, T, U or I shapes (Fig. 3.1) have plan irregularities. The concepts that building configuration greatly influences seismic design are taught to students of architecture. The full discussion of Seismic Design Category can be found in Section 1616.5 of IBC 2003 and 9.5.2.3 of ASCE 7-02. Table 14. Vertical structural irregularities from Table 9.5.2.3.3 of ASCE 7-02 Irregularity Type and Description 2 Weight (Mass) Irregularity Mass irregularity shall be considered to exist where the effective mass of any story is more than 150% of the effective mass of an adjacent of any story is more than 150% of the effective mass of an adjacent story. A roof that is lighter than the floor below need not be considered. 3 Vertical Geometric Irregularity Vertical geometric irregularity shall be considered to exist where the horizontal dimension of the lateral-force resisting system in any story in more than 130% of that in an adjacent story. L shape T shape U shape I shape Fig. 3.1 Examples of building plan irregularities 58 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.2.2.2.11. Building Fundamental Period T Buildings are set in motion by seismic forces. Building fundamental period is the time in seconds for the building to complete one cycle of oscillation. The building period depends on building height and structural systems. The fundamental period can be roughly estimated as 1/10 second per story. The approximate method of determining fundamental period Ta defined in ASCE 7-02 is described as follows: Ta= Cthn x (Eq. 9.5.5.3.2-1 of ASCE 7-02) Ta: approximate fundamental period in seconds hn: Building height above the base in feet Ct ,x: coefficients (see table 15) Table 15. Values of approximate period parameters Ct and x of Table 9.5.5.3.2 of ASCE 7-02 Structure Type c, X Moment resisting frame systems of steel in which the frames resist 100% of the required seismic force and are not enclosed or adjoined by more rigid components that will prevent the frames from deflecting when subjected to seismic forces 0.028 0.8 Moment resisting frame systems of reinforced concrete in which the frame resists 100% of the required seismic force and are not enclosed or adjoined by more rigid components that will prevent the frame from deflecting when subjected to seismic forces 0.016 0.9 Eccentrically braced steel frames 0.03 0.75 All other structural systems 0.02 0.75 59 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The full discussion of building fundamental period T can be found in Section 9.5.5.3 of ASCE 7-02. 3.2.3. Seismic Force Per Level For buildings with multiple stories, the design seismic base shear shall be distributed at each level. Its distribution is based on the weight assigned to each level and the height of that level above the ground. The seismic force Fx induced at any level is specified as follows: Fx = CvxV (Eq. 9.5.5.4-1 of ASCE 7-02) Fx : Seismic force at any level, in kips V: Base shear, in kips, see Section 3.2.2 C vx: Distribution factor, determined as follows: Cn-= (Eq.9.5.5.4-2 of ASCE 7-02) 2 > ,A ‘ i = \ W i, wx: the portion of the total gravity dead load W located or assigned to Level i or x hi, hx: the height from the base to Level i or x, in ft. k: an exponent related to the structure period as follows: 1, structures with periods of T<0.5 sec (approximately 5-stoiy building) 2, structures with periods of T>2.5 sec, (approximately 25-story building) 2 (used in the design software), or value by linear interpolation between 1 and 2, for structures having a period of 0.5 sec < T < 2.5 sec 60 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The full discussion of seismic force at each level can be found in Section 9.5.5.4 of ASCE 7-02. 3.2.4. Seismic Shear Per Level Seismic shear at each level equals the sum of seismic force at the level plus all seismic forces above. Ground level shear equals the base shear. More discussions can be found in Section 2.1.1 of Chapter 2. The shear force Vx at any level shall be determined as follows: Vx = % F i (Eq. 9.5.5.5 of ASCE 7-02) i ~ x Vx: Seismic story shear in kips E: Seismic force at Level i, in kips, see Section 3.2.3 The full discussion of seismic shear at each level can be found in Section 9.5.5.5 of ASCE 7-02. 3.2.5. Overturning Moment Per Level Seismic forces generate overturning moment that is usually resisted by building weight. The overturning moment at each level is the sum of all seismic forces above that level multiplied by their distance to the level considered. The overturning moment can be determined as follows: M x = Y 4Fi(hl - h x) (Eq. 9.5.5.6 of ASCE 7-02) i= x Mx: overturning moment at any level x, in kip-fit 61 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. F;: Seismic force at Level i, in kips, see Section 3.2.3 hi, hx: Height from the base to Level i or x, in ft. The full discussion of overturning moment can be found in Section 9.5.5.6 of ASCE 7-02. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 4 WIND LOADS PER IBC 2003 AND ASCE 7-02 4.1. Introduction Buildings, structures and parts shall be designed to withstand the minimum wind loads (ICC 2002, p.283). Several methods are given in IBC 2003. The Simplified wind load method is fully described in Section 1609.6 of IBC 2003. This method is designed for regular-shaped buildings with mean roof height less than 60 feet or not exceeding the least horizontal dimension of the building, whichever is less. Because of its limitation, this method is not used in the LFD software. IBC 2003 also states that wind loads on every building or structure shall be determined in accordance with Section 6 of ASCE 7 (ICC 2002, p.283). Three methods of determining wind loads are defined in ASCE 7-02. Method 1 is the Simplified Procedure that is quite similar to the simplified wind load method of IBC 2003. Method 2 is the Analytical Procedure. This procedure is designed for regular shaped buildings. This procedure classifies buildings according to their rigidity and openness. The methods of wind design are assigned to analyze wind loads on buildings according to those classifications. Method 3 is a wind tunnel procedure. This procedure is used for buildings with highly unusual shapes and unusual response characteristics due to wind forces. Method 2 is used in the LFD software. 63 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Considering the complexity of these methods and the scope of the software for architecture students, wind loads on main wind-force resisting systems of enclosed and partially enclosed rigid buildings of all heights are computed. The concepts and principals of this method are discussed in the following sections. 4.2. Wind Loads per IBC 2003 and ASCE 7-02 for the LFD Software Design The software computes: Wind base shear (k) Wind pressure per level (psf) Wind force per level (k) Wind shear per level (k) Wind overturning moment per level (k’) The following sections discuss the theory of wind desi gn that serves the scope of the design software. For the concepts that are described both in IBC 2003 and ASCE 7-02, the ones in IBC 2003 are cited. 4.2.1. Applicability The methods of determining wind loads in the design software are used for main wind-force resisting systems of enclosed and partially enclosed rigid buildings of all heights. Some concepts shall be explained, namely main wind-force resisting systems, rigidity, and openness. 64 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Main wind-force resisting systems refer to the assemblage of structural elements that provide support and stability for the overall structure, consisting of basic lateral-force resisting systems. Wind load on components, such as windows, is not included in the software. Buildings are classified as rigid or flexible, determined by building fundamental period. Rigid buildings are buildings with fundamental frequency greater than or equal to 1 Hz or fundamental period less than or equal to 1 second (SEI2003, p.24). The determination of fundamental period is discussed in Section 3.2.2.2.11 of Chapter 3. Based on the openness of the building, buildings are classified as enclosed, partially enclosed or open buildings. Different methods of wind design are used according to this classification. Their determinations are defined as follows. A. Open buildings have each wall at least 80% open. This rare condition is not included in the software. The condition is expressed for each wall as follows: Ao > 0.8 Ag (Eq. 16-31 of IBC 2003) • ) Aq = total area of openings in a wall that receives positive external pressure, in ft Ag = the gross area of that wall in which A« is identified, in ft B. Partially Enclosed Buildings Partially enclosed buildings comply with both of the following conditions. Condition 1 is that the total area of openings in a wall that receives positive external pressure exceeds the sum of the areas of openings in the balance of the building envelope (walls or roof) by more than 10%. Condition 2 is that the total area of 65 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. openings in a wall that receives positive external pressure exceed 4 ft2 or 1% of the area of that wall, whichever is smaller and the percentage of openings in the balance of the building envelope doesn’ t exceed 20%. The conditions are expressed by the following equations: Ao > 1.10 Aoi (Eq. 16-32 of IBC 2003) 3.2. Ao > 4 ft2 or > 0.01 Ag, whichever is smaller, and A ™ / Ag i < 0.20 (Eq. 16-33 of IBC 2003) where Ao, Ag are as defined for open building Ao i = the sum of the areas of openings in the building envelope (walls and roof) not including Ao in ft2 A^ = the sum of the gross surface areas of the building envelope (walls and roof) not • y including Ag in ft C. Enclosed Buildings Enclosed buildings are buildings that do not comply with the requirements for open or partially enclosed buildings. 4.2.2. Wind Pressure per Level Wind pressure is the wind force per square foot of building surface. Wind load is applied along each orthogonal direction of the building and exerts forces normal to the building surface. The program computes wind pressures for: Windward wall Leeward wall (facing downwind) 66 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Combined pressure (windward + leeward) The theory of determining wind pressures is discussed in the following sections. 4.2.2.I. Equations The method of determining wind pressure on various building surfaces is defined as follows: p = qGCp - qi(GCpi) (Eq. 6-17 of ASCE 7-02) qi(GCpi) determines internal wind pressure. In the design software, internal wind pressure is ignored because wind pressures on the entire building are considered. Thus the software wind pressures is expressed as follows: p= qGCp Factors: p: Design wind pressure, in psf (min. 10 psf) G: 0.85, gust effect factor, see Section 4.2.2.2.7. Cp: External pressure coefficient, see Section 4.2.2.2.8. q: Velocity pressure, in p sf, qz for windward walls evaluated at height z above ground; qh for leeward walls and roofs evaluated at mean roof height. qz, qh are determined as follows: qz = 0.00256Kz Kz tKdV2 I (Eq. 6-15 of ASCE 7-02) qh = 0.00256Kh KztKdV2 I (Eq. 6-15 of ASCE 7-02) V: Basic wind speed for Exposure C, miles/hour, see Section 4.2.2.2.1. 67 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I: Importance factor, see Section 4.2.2.2.2. K < j: 0.85, wind directionality factor, see Section 4.2.2.2.5. Kz: Velocity pressure exposure coefficient, see Section 4.2.2.2.4. Kh: Velocity pressure exposure coefficient, see Section 4.2.2.2.4. K zt: Topographic factor, see Section 4.2.2.2.6. 4.2.2.2. Explanations of Factors in Equations The factors are described in this section. 4.2.2.2.I. Basic Wind Speed V Wind speed is measured regularly for a given geographic area. The basic wind speed for the United States, used in the determination of velocity pressure, is given in Figure 1609 of IBC 2003. The basic wind speed in the Figure is the 3-second gust wind speed at 33 feet above ground for Exposure Category C. Mountainous terrain, gorges and special regions shall be examined for unusual wind conditions. The basic wind speed shall be increased where records or experience indicate that the wind speeds are higher than those reflected in the Figure (SEI2003, p.28). The basic wind speed for the special wind regions including Hawaii, Puerto Rico, Guam, Virgin Islands and American Samoa shall be determined by the local jurisdiction. The full discussion o f the basic wind speed can be found in Section 1609.3 of IBC 2003 and 6.5.4 of ASCE 7-02. 68 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 42.2.2.2. Importance Factor I Importance factor I (Table 16) is assigned to each building for determining velocity pressure. This factor is determined according to occupancy category described in Section 3.2.2.2.5 of Chapter 3. Table 16. Building Occupancy Category from Table 1604.5 of IBC 2003 Nature of Occupancy Category Buildings and other structures that represent a low hazard to human life in the event of failure including but not limited to : Agricultural facilities, certain temporary facilities, minor storage facilities I Buildings and other structures except those listed in Categories I, III and IV II Buildings and other structures that represent a substantial hazard to human life in the event of failure including but not limited to: • Buildings and other structures where more than 300 people congregate in one area • Buildings and other structures with day care facilities with capacity greater than 150 • Buildings and other structures with elementary school, secondary school or day care facilities greater than 250 • Buildings and other structures with a capacity greater than 500 for colleges or adult education facilities • Health care facilities with a capacity of 50 or more resident patients but not having surgery or emergency treatment facilities • Jail and detention facilities • Any other occupancy with a capacity greater than 5,000 III Buildings and other designed as essential facilities including, but not limited to: • hospitals and other health care facilities having surgery or emergency treatment facilities • Fire, rescue and police stations and emergency vehicle garages • Designated earthquake, hurricane or other emergency shelters • Designated emergency preparedness, communication, and operation centers and other facilities required for emergency response • Structures containing highly toxic materials • Aviation control towers, air traffic control centers and emergency aircraft hangars • Building and other structures having critical national defense functions • Water treatment facilities required to maintain water pressure for fire suppression IV Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table 17. Importance Factor I (Wind Loads) of Table 6-1 of ASCE 7-02 Occupancy Category Non-Hurricane Prone Regions and Hurricane Prone Regions with V = 85-100 mph and Alaska Hurricane Prone Regions with V > 100 mph I 0.87 0.77 H 1.00 1.00 HI 1.15 1.15 IV 1.15 1.15 Note: 1. Hurricane prone regions include the U.S. Atlantic Ocean and Gulf of Mexico coasts and Hawaii, Puerto Rico, Guam, Virgin Islands and American Samoa. The full discussion of importance factor for wind loads can be found in Section 1604.5 of IBC 2003 and 6.5.5 of ASCE 7-02. 4.2.2.2.3. Exposure Category The roughness of the ground surface such as buildings or trees has great influence on wind pressure. An Exposure Category is assigned to justify this effect. There are three exposure categories defined as follows: Exposure B: urban, suburban and wooded areas Exposure C: open terrain with scattered obstructions 2002,p.290). Exposure D: areas exposed to wind flowing over open water For a site located in the transition zone between exposure categories, the category resulting in the largest wind forces shall be used. The full discussion of Exposure category can be found in Section 1609.4 of IBC 2003 and 6.5.6 of ASCE 7-02. 70 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.2.2.2A. Velocity Pressure Exposure Coefficient Kz, Kh The basic wind speed shown in Figure 1609 of IBC 2003 is the wind speed measured at 33 ft above the ground for exposure category C. It is used to determine velocity pressure. The velocity pressure at other heights and for other exposure categories shall be obtained by adjusting the values from the Figure. Velocity pressure exposure coefficients Kz or Kh are used for this. Kz: Coefficients for windward walls Kh: Coefficients for leeward walls and roof Two methods can be used for their determination. One is to get their values directly from Table 6-3 of ASCE 7-02. The other method is to use the formula of Table 6-3 of ASCE 7-02. This is the method that is used in the LFD software. The formula are described as follows: For 15 ft. < z < z g Kz= 2.01 (z/zg)2/a ; For z < 15 ft. Kz= 2.01 (15/zg)2/a a and zg are determined in Table 17. Table 18. Terrain exposure constants of Table 6-2 of ASCE 7-02 Exposure a z* (ft) 2m in (ft) B 7.0 1200 30 C 9.5 900 15 D 11.5 700 7 Note: zm m = minimum height used to ensure that the equivalent height z is greater of 0.6h or zm in . For buildings with h < z ,™ ,,, z shall be taken as zm in 71 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.2.2.2.5. Wind Directionality Factor IQ The wind directionality factor IQ is used to determine velocity wind pressure. According to ASCE 7-02,0.85 shall be assigned to this factor. The full discussion of wind directionality factor IQ can be found in Section 6.5.4 of ASCE 7-02. Table 19. Wind directionality factor Kd from Table 6-4 of ASCE 7-02 Structure Type Directionality Factor Kd Main Wind Force Resisting System 0.85 4.2.2.2.6. Topographic Factor IQ, If a building is located on the upper half of an isolated hill or escarpment, the building can expect to experience higher wind speeds than if it is situated on level ground. Topographic factor Kz t is used to adjust this fact. If there are no abrupt changes in the topography and the structure is on the ground level, Kzt= 1 . For buildings located at isolated hills, ridges and escarpments constituting abrupt changes in the topography, topographic factor shall be determined according to Section 6.5.7 of ASCE 7-02. The value of 1 is used in the LFD software. The full discussion of topographic factor Kz t can be found in Section 6.5.7 of ASCE 7-02. 4.2.2.2.7. Gust Effect Factor G Gust effect factor G is used to determine wind pressure. For rigid structures, G shall be taken as 0.85 or calculated according to equation 6-4 of ASCE 7-02. The 72 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. value of 0.85 is used in the LFD software. The full discussion of gust effect factor can be found in Section 6.5.8 of ASCE 7-02. 4.2.2.2.8. External Pressure Coefficients Cp The External pressure coefficient Cp is used to determine wind pressure. This factor accounts for the variations of wind pressures on various building surfaces. The value of Cp (Table 19) for windward walls, leeward walls, and roofs in various shapes such as gabled or monoslope is given in Figure 6-6 to 6-8 of ASCE 7-02. In the LFD software, only a flat roof is allowed. The full discussion of external pressure coefficients Cp can be found in Section 6.5.11.2 of ASCE 7-02. Table 20. External pressure coefficients Cp from Figure 6-6 of ASCE 7-02 Wall Pressure Coefficients, C„ Surface L/B cD Use with Windward Wall All values 0.8 qz 0-1 -0.5 2 -0.3 q h Leeward Wall > 4 -0.2 Flat Roof All values -1.3 q h Note: 1. Plus and minus signs signify pressures acting toward and away from the surfaces respectively. 2. Linear interpolation is permitted for values of L/B. 3. B: horizontal dimension of building normal to wind direction. L. horizontal dimension of building parallel to wind direction. qz, qh: velocity pressure evaluated at respective height. 4.2.3. Wind Force per Level Wind force at each level is computed as product of wind pressure and tributary areas exposed to wind (assuming normal pressure). The combined wind pressures of windward walls and leeward walls at orthogonal directions are used. The tributary 73 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. areas are computed as product of overall length of the building dimension normal to wind direction and the height (half the story height above and below the level considered). 4.2.4. Wind Shear per Level Horizontal shear induced at each level equals the sum of wind forces from the levels above to the design level. This value equals wind base shear at the ground level. The horizontal shear force Vx at any level shall be determined as follows: Vx = £ Fi i ~ x Vx: Wind shear at any story x (k) Fji Wind force at Level I (k), determined in 4.2.3. 4.2.5. Wind Overturning Moment per Level Wind forces are applied to the building at some distance above the ground. They generate overturning moment that is usually resisted by building dead weight. The overturning moment at each level is the sum of all wind forces above that level multiplied by their distance to the level considered. The overturning moment can be determined as follows: i ~ x Mx: Overturning moment at any level x, in kip-fit Fj: Wind force at level 1 (k), determined in 4.2.3 hi, hx: the height (in ft.) from the base to Level i or x 74 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 5 A NEW SOFTWARE TOOL FOR WIND AND SEISMIC LOADS 5.1. Software Scope Since building codes have evolved from simple to very complex, computers are broadly used by designers in practice. Based on building codes, quite a few software packages have been developed to help structural engineers design for wind and seismic forces. However, they are not easy to use for students of architecture. One example existing software is Code Search Excel Spreadsheet (Fig. 5.1) developed by Struware (no date). This software can calculate lateral loads based on several building codes including IBC (2000 or 2003) and ASCE 7 (1998 or 2002). It utilizes Excel spreadsheets and allows users to input various factors based on building codes. Users have to be familiar with building codes so that correct inputs can be made. Another example existing software is Wind Loads on Structures 2002 designed by Standards Design Group (no date). This software can generate wind loads based on ASCE 7-02 and ASCE 7-98. It allows users to input factors to generate wind loads but also requires users to know building codes for each input. There are more software packages that can be found for determining wind and seismic loads based on building codes. They are designed for users who are familiar with building codes but they are not easy to use for students of architecture because building code theory has not been consistently taught in architectural schools. 75 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. &£jE*e ficlt Wewfrisert Rymat XooteQata iVfidow fcjelp D OS S3 ^ ® 3 i A 8 1 $ ) ? Times New Roman group « = fN D E X (G ro u p s,ln d ex _ g ro u p .l) A R fi T P E ! F i I D ; n H fJ : W 1 0 ! 1 9 * ' 2 0 ! 2 1 ' L i « 2 3 i 24 25 ; 2 6 27 1 2 8 2 9 ■ 3 0 : 3 t : 32 : 3 .3 i 3 4 i 35 : 3.7 i 3 8 '3 9 • 4 0 ! 4 1 I 4 2 I 4 3 ■ M i ► R e a d y R f i I. C o d e : L!: „r Oo;l‘ .' II. Occupancy: Occupancy Group = f" " -■ ' | 1 U . Type of Construction* Fire Rating' R oof ® 0 ) hr F lo o r - i> .M r IV. Live Loads: R o o f a n g l e ( 8 ) 0 / i; R o o f Q to 2 G O s f Z O p sf 200 to 600 s f 2 4 - 0 OZArea over 600 sf. 12 psf F l« w !” S ta ir s & E x itw a y s i < ■ B a lc o n y 1 " i M echanical P a r t i t i o n s W ind L o a d s : ASCE 7 - 98 Importance Factor ) Wind speed i trip ;.! Directionality (K<0 t 0 O ': Mean R o o f Ht (h) ' I * - T Parapethl above grd * L.T-vU Exposure ’ £ Enclosure Classif. ■ t |> Tnfermloressitr* + /-0 18 rt- | User Coda IIV. Essential Facilities - see code JASCE7 Load Comb nations Used | b - demo only ▼j [eictosed Building j r j Pifcle rooms - u;:' Galeeny and Decks - typtcal T Nons 'T Partitions ▼ ~3 Tnferml oressitr* + /-0 18 I , N \ Tide \C o d e x G &Enct /M a p / NM/FR$<60 / C&C<90 / MWfRS adh / C&C>60 / Ottier Wind / Sesnvc /S n o w / Roof | < M vnwU snow/sei ou die “ tab! I D e fa u lt’ w i n LoadProvj- values m lh< fo r th e r n p r L f Fig. 5.1 A sample of input module of Code Search Excel Spreadsheet developed by Struware (no date) To teach students of architecture the building code theory for determining wind and seismic loads, this thesis presents a new software tool called Lateral Force Design (LFD). LFD is based on IBC 2003 and ASCE 7-02. To help students gain a better understanding of building code theory, LFD can compute the loads including wind base shear, distributions per level of wind pressure, force, shear and overturning moment on main wind-force resisting systems of enclosed and partially enclosed rigid buildings of all heights, seismic base shear, distributions per level of 76 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. seismic force, shear and overturning moment and the overall length per level of shear walls if shear wall systems are used as lateral force resisting systems. There are two main goals for the LFD. First, LFD is a teaching tool. It should lead users through the whole application and provide the descriptions for all the input, output and intermediate entities. LFD is also a computing tool. It should allow users to input relevant parameters for determining wind and seismic loads on common structures, and then give outputs in an easy-to-understand way. LFD is not meant to have industrial strength but it is more for teaching purposes, thus it should streamline some complex formula and conditions into simple ways for users to understand computing procedures. 5.2. Software Structure Based on the software scope discussed above, LFD consists of 4 components, help, input, computation and output. Fig. 5.2 shows the software structure and arrows indicate the data flow sequence. The following sections discuss the software preparation, each component and its sample structures. 5.2.1. Software Preparation LFD is designed to run in Windows because Windows is more popularly used than other operating systems. Window-based C# is chosen as the computer programming language. 77 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LFD is quite complex with a broad scope. Many factors are involved in determining wind and seismic loads, and so the “divide and conquer” strategy is used in designing the whole application. In computer terms, this is called “an object oriented programming” paradigm. Basically what it does is to separate the functionalities of an application into small parts so that each part can do a particular job well, then connects them together whenever it is needed. In this way, most parts can be reused in different other parts of the system. LFD - Introduction LFD - Tutorial LFD - Main screenl LFD Input Output — | Wind info — I Building info Seismic info Fundamental eriod } Seismic force Seismic shear r Occupancy " " SI, Ss ) category Dimension i~ 1 Site Class "Wind speed LFRS ' Seismic R value ) , Exposure \ -. category / o v e r tu r n / Wind overturn moment . moment , Number of levels Seismic output — r —I Wind output [ - (Wind pressure) < Wind force C Wind shear ) Note: Red arrows show LFD flow sequence. Fig. 5.2 Software structure and flow sequence LFD is implemented in C#, which is one programming language in the Microsoft .Net Framework programming language family. To run .Net applications, 78 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the .Net framework is required. The framework is installed on Windows XP by default. To run in other operating systems such as Windows 2000, it is required to install the .Net framework first. The .Net framework can be downloaded from the website http://www.microsoft.com/downloads/details.aspx?FamilvID==d7158dee- a83f~4e21 -b05a-009d06457787&displavlang=en for free. The file size is about 20 MB. The best screen resolution for running the LFD software is 1024 * 768 pixels. Those descriptions are included in the Readme file in the LFD software package. The software, Lateral Force Design, does not need complicated installation. To run it, just copy and paste the whole folder to a location on the hard disk, then double click the executable, LFD.exe, in Windows Explorer to start the software. 5.2.2. Software Introduction and Tutorial After users start LFD, the LFD open page (Fig. 5.3) appears and displays the information such as the software scope, author and advisors. The user can click continue button to proceed. The next page is the LFD introduction page (Fig. 5.4). It gives users two options. Users can click the design button to display the software design screen to input factors for generating wind or seismic loads. The design screen will be discussed in the following sections. Users can click the tutorial button to display the LFD tutorial. The tutorial main menu (Fig. 5.5) has 4 buttons for users to select. Users can click each button to display the corresponding content. 79 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ‘;H mO to p y n fc h t A LATERAL FORCE DESIGN ClicM here to continue ^ latefa! Fote* Design (LFD) is a computer pregram to teach students of architecture building code theory of wind and seism ic design. This program is written by; XlnWang Under directions of: Prof. 0. Qoetz Schierle Prof. Douglas Noble Prof. Jeff Ouh M aster of Btiildng S c ie n ce Prog!am S chool of A rchitecture U niversity of Southern California Copyright 2 0M Fig. 5 .3 A sample of the LFD open page A LATERAL FORCE DESIGN T h e fig u tr above eh aw s a coB ajrjeil huilclitigtn 1994 • H rrthnttg* ■ Click here fa start Tutorial Click here to start D esign Buildings shaft be designed to resist wind and seismic forces. This program is designed to teach students ot architecture the main concepts and procedures of determining wind and setemfe loads based on International BttHdmg Code (IBC; 2003 and American Society Civil Engineers (ASCE) 7-02. For the user who is not familiar with this program. It's recommended to click the tutorial button. You can get to know the Information such as how to use this program and building code theory used in this program. For the user who is familiar with this program, it is recommended to click the design button to generate wfnd and seism ic leads on the design building. GOOD LUCK! Fig. 5.4 A sample of the LFD introduction page Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. t v F B - ly tp r-ja l M a tn i& e n u ‘ LATERAL FORCE DESIGN L ateral F orce D esign ttFD ) Is d esig n e d te te a c h s tu d e n ts of » r c h ile a u r e 'tJ w m am p ro c e d u re s a n d c o n c e p ts of d eterm ining w ind a n d seism ic lo ad s b a s e d on In tern atio n al Building C ode <fBC) 2003 an d A m erican S o ciety o f CivH E n g in eers (A S C E ) 7 * 62, If you have finished reading through LFP Tutorial, please click design button In LFD Introduction screen to start computing wind or seismic loads on the design building. Click each button below to select. i Program S co p e Use Program Code Theory Quit Tutorial Fig. 5.5 A sample of the main menu of the LFD tutorial Program scope (Fig. 5.6) displays the software scope. Users can click the tutorial main menu button to return to the tutorial main menu. If users click the button of Use program, it gives users step-by-step instructions about inputting, outputting and software menus. Users can click back and next buttons to look through the content or click the tutorial main menu button to return to the tutorial main menu. Fig. 5.7 shows a sample structure. The Code theory button (Fig. 5.8) displays the computation theory used in LFD. Users can click the glossary button to display unit abbreviation, key terms of wind and seismic loads from ASCE 7-02. The code theory of wind, seismic loads and shear walls can be displayed by clicking the corresponding button. The Quit tutorial button allows users to quit the LFD tutorial. 81 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LATERAL FORGE DESIGN Lateral Force Design (LFD) is an interactive program designed to teach students of architecture the main concepts and procedures of determining wind and seismic loads based on International Building Code (IBC) 2003 and American Society of Civil Engineers (ASCE) 7-0 2 . LFD can generate wind and seismic loads on buildings with regular shapes. It can be used for preliminary wind or seismic design. For wind loads, Analyticai Procedure of Section 6.5 of ASCE 7-02 is used. The wind loads that the software can generate include: wind base shear, distribution per level of wind pressure, force, shear and overturning moment on main wind-force resisting systems of enclosed and partially enclosed rigid buildings of all heights. For seismic toads, Equivalent Lateral Force Analysis of Section S.5.5 of ASCE 7-02 is used, The seismic loads that the program can generate include: seismic base shear, distribution per level of seismic force, shear and overturning moment. if shear wait systems are used as buitding teteral-torce resisting systems, LFD can generate overall lengths per level of concrete, masonry or wood shear walls in both orthogonal directions. T u to ria l M a m M e n u Fig. 5.6 A sample of the program scope in the LFD tutorial LFD Input Instruction LFD is designed to teafh students of atrlutectun? the main concepts anti procedures of determmmg wind or scism k loads based o r international Building Code (XBO 2003 and American Society o f Civil Engineers (ASCF) ?-02. L FD is an untevactive program that allows the user to input the building information to. generate wind or sevsmir toads on the design buitding. There are 4 input tabs in LFT) as ihiistiated in the graph to the right. They are Building Info. Dimension, Wind Info and Seismic Info. Each tab collects th e building information to generate wind or seismic leads, LFD can compute wind or seismic loads on regular-shaped buddings. Foi the first lim e user, it is recommended to u se a simple budding as a design example. Click r# x t b atlo u to proceed w ith L FD input m strncrioji, Turorhu Main fd&mj Fig. 5.7 A sample of how to use the program in the LFD tutorial 82 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. £ |! lF D - B tfJ ld in g C o d e T h e o ry LFD Glossary L F D glossary includes the explanations ofunit abbreviations u sed in L F D an d tern; definitions &c-m ASCE 7 02 1 F iu t A b b r e v ia tio n s U n it A lib te v i.U io n D e s c r ip tio n f t f o o t q - s e c q - s e c o n d k i p U l o - p o u n d k - f t k i l o - p o u n d x f o c i m p h m i l e p e r h o u r p l f p o u n d p e r lin e a r f o o t p s f p o u n d p e r s q u a ts f o o t * e c s e c o n d s f s q u a r e f o o t ' 2 T eun Definitions o f M Vuul Loads from S'ecQon 6 2 of AS.C3* “-02 Basic Wind Speed. V 3-secon<l gust sp e e d at 33 & a b o v e the ground in E x posure C ; Biuhhng and Orhet Snu< nu e. Flexible. slender buildings and other sm icm res that have a fundam ental natural 1 frequency less than I H z ; Btuldmg oi Other S tim h u e, Regular shaped: a bialdaig o r other structure having no unusual geom etrical irregularity ' ui spatial form T utorial M ain M en u ' G lo ssary ' W ind t S sK m ic S h e ^ rW a l5 Fig. 5.8 A sample of the code theory in the LFD tutorial 5.2.2. Software Input If users click the design button in the LFD introduction page, the LFD design screen appears. Input is accomplished through the User Interface (UI). All the UI related functionalities of the software are implemented in Windows Forms, the standard Windows application interface. Color systems are used for the LFD text to make the Ul more interactive and also help users navigate in LFD. All input entities and next-step instructions are in red. This color is to tell users what are required inputs and how users proceed in LFD. The black color is used for the instructions of each input. The blue color is 83 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. used for next and back buttons. It is the navigation color. The green color is used in the wind and seismic output forms. This color indicates what are inputs or computed results. The LFD input collects the information for generating wind or seismic loads on the design building. It is divided into 4 parts including Building Info, Dimension, Wind Info and Seismic Info. Each part of input is represented with a tab page. The following sections discuss each input and its sample structures. 5.2.2.I. Building Info The Building Info tab collects the building information for generating wind or seismic loads. The top portion of this tab provides the general input instructions such as how to input and proceed. Each input is to the left, and its explanations are in the textbox to the right. There are 3 entries that are given step by step. Each entry has its default value. In order, they are occupancy category, lateral-force resisting systems and number of levels. The Occupancy Category (Fig. 5.9) allows users to select which occupancy category the building belongs to. The general instructions to the right let the user know what the Occupancy Category choices mean. If users select one of the categories to the left, the textbox to the right displays the definition of the selected occupancy category based on IBC 2003 and ASCE 7-02. In this way, the building code theory is taught to users while they input their data. Users can proceed by 84 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. clicking the next button or go back by clicking the back button whenever any change needs to be made. Fie lnpii? luturis1 Print Hejp ll B tttU lbnj lirfo Otlnenstoi) , W ind lido S eism ic info W iitd Output Saisu iic Output Occupancy Category aiegary .n teo o ry U Cateqory tit O n e g a fy !V Tn generate wind or seism ic loads on the building. LFD requires inputting the building information. There are 3 step inputs in this tab. For each step, make a selection or use default value to the left, The general instructions of each input ran be found to the right. Click each .selection to display its explanations. For the first-time user, it's recomm ended to follow each step by clicking next burton to proceed or back button to go back to th e previous step. For advanced users, input menu lists each input. W henever the input is changed, p lease click next button to update the input O e a e ia l h t s tjit'ir o n s o i O* • t ip a m y C a te g o ry B ased on the nature of occupancy, buildings arc classified for the p u rpose? o f applying w ind an d earthquake provisions o f SBC 2 0 0 3 and A S C E 1 -02 CvtUfKVuy t ategoty ranges fr«>tn Category I to Category IV reSechng die increasing level o f hazard to human life in the event U building .'tm tu u lfailure The higher applicable category dial! be assigned to buddings wifi multiple uses M a k e v i s e li^ n tm u i ' O iu ip a iu v < \ > t e thftt the de vigil brultlmg belongs to « > th e let? Click h ere to display gen eral input tnstnictiotis Fig. 5.9 A sample of Occupancy Category in the Building Info tab. Lateral Force Resisting Systems (Fig 5.10) allows users to select which lateral force-resisting systems the building has. Three common types of lateral force resisting systems and its subcategories are provided. Users can make a selection and get its input instructions in the textbox to the right. If shear wall systems are used as lateral force resisting systems, the allowable shear stress of shear walls can be specified after the entry of lateral force resisting systems (Fig. 5.11). 85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. B tiild im j In to D im e n s io n W b td h ito S e is m ic In fo W tin l O u ip H t S v m trtc O u tp u t To gwverat*? wind or seismic loads on the building, LFD requires inputting the building information. There arc % stop inputs in tliis tab, Far oath stop, make a selection or use default value to the loft. The general instructions of each i»i[nit ran bo found to the right. (Tick each selection to display its exphnnrtion*. For the fust-tim e user, it's lecaram ended to follow each step by t licking next button to pioceed 01 bark button to gn back tn the previous step. Koi advanced nsers, input menu lists each input W lnm ew i flip input is th.uigvd please click next button to update the input. Lateral Force Resisting Systems Categoty '« S f o a r Vtfarl S y stem s fik sced f r a m e S y stem s Sfom ent R esistin g fr a m e S y stem s Sub-Category W a ll S y s te m s ^O'dcaiy ~*nfo»ced Coroete ?he« Waifs .STieiu WaU System s, S h ear w all system ? are com m only u sed m b ean p a '.vail system s w here b>.-aiuig wall'- ptcv u ic support lot ,VM!vi portions >;1 vei tic al loads It shear wall system s are used a? I..FKS, - 1 types of d irat wall system s are pro v id ed in the program . W o o d shear walls are c suvmoaly u sed si the buddings •-vuh n o m ore than 3 stones or 4 stones with lire sprinklers T h e definition of each ty p e can b e found in Section 9 2 1 o f A S C E 7 -0 2 M ake s e l f h o n u» r itv g m v to t h e W it Click h»t«> to display geneiol input matmctiom Fig. 5.10 A sample of Lateral Force Resisting System in the Building Info tab i.Laicr.al fa rc e Dsatfiii fUTfl ; . Main % r.eeh Rie Inpot tjtwvst ftint H efcl PI Building Into Dimension Wind Info Seism ic Info W h n l Output Seismic Output To generate wind oi seismic loads an the building, LFD requires inputting the building information. There arc 3 step inputs in this tab. For each step, m ake a selection or use default value to the left. The general instructions of each input can be found to the tight. Click each selection to display its explanations. For the first-time user, it's lecom m ended to follow each step by clicking next button to proceed or back burton to go bark to the previous step. For advanced users, input menu lists each input. W henever die input is changed, please click next button to update the input. mmm Shear W aff Systems S h o rn W all Type ( Allowable Shear jimmt value iter© S tress p i t C onciete ( S‘h e a i Wall L F D can generate the overall length p t f level ot shc«u walls tn each orthogonal duectiorj o f the building for resisting } w m d vi seismic forces ; The overall Iejurtii p c i levei -;«f sheai walls in X Y : ■djrcctKin — v/trid or seismic shc«o p e t level in X or Y 1 Aivcfi»n /allo w ab le ih ear c-f shear w jH ? = R R - ols*ai wjJI Fig. 5.11 A sample of inputting the shear stress of shear walls in the Building Info tab Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Number of Levels (Fig. 5.12) allows users to input the number of levels of the design building. The last step of Building Info (Fig. 5.13) tells users that they have finished the required inputs and go to Dimension tab to continue inputting the building information. i BtiiiiHmj tnfe Dimension Wind tufa Seismic firfo Wind Output Seismic Output To generate wind or seism ic loads o« the building, LFD require* inputting the building; information. There are 3 step inputs in this tab. For each step, m ake a selection or use default value to the left. The general iitstrurtioro of each input For the first time user. it's recomm ended to follow each step by clicking; nevt button to proceed or back button to go back to the previous step. For advanced users, input menu lists each input. W henever the input is changed, p lease click next button to updale the input. Number of Levels : F k v»v uspnt die uumb**i of levels of the Umkbiig m tin* input bo.v P m the leti 1 hen » b>k nevf hurt on r < > Fig. 5.12 A sample of Number of Levels in the Building Info tab S.2.2.2. Dimension The Dimension tab (Fig. 5.14) is used to collect the building information including building plan, height and deadload at each level. All inputs and their instructions are to the right. The textbox to the right displays the general input instructions for the Dimension tab. LFD provides the instructions for each input by clicking question marks. The grid lines to the left allow users to draw the building plan at each level. 87 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fte iF fw f T b * < h W P u t * H e f t j Building h » t< > Dlmerrsisn Windkito $«t«m ktnfn Wfcnl Output Sqiemfo Output To generate wind or seism ic loads on the building* LFD requires inputting the building information There at e 3- step inputs in this tab. For each step, m ake a selection or use default value to the left. The general instructions of each input can be found to the light, ( ’lick each selection to display it* explanations. For die first tim e user, it's recomm ended to fallow each stop by clicking next button to proceed or back button to go back to the presiaus step. For advanced users, input m enu lists each input. W henever the input is changed, please click next button to update the input. Congratulations! You have finished the input in this tab. Please click Dimension tab to input building dimension and deadioad. ..J Fig. 5.13 A sample of the last step in the Building Info tab W i.s ite f a f ftfrt*i.Oe*% n (!.fW • . M ain Si;r<t!i Building M o Dimension Wind hifn Sekwk bit* Wind Output Output CUc* h e te iq d isp lay gw iftM l in p u t in ttru d io n s ’ G e n u a l In p u t lu s tiu tn o n * oil : D im e nsion T ab l i a s t s f r e q u i r e s i n p u t t i n g t h e < i e s i g n h i a l r j u i g d i n w 'r s i o n s a m i d e » r f l n » i i ’. ’ h i V ■ t h e q u e s t i o n u i a k b y S t a l e i n p u t t o d i s p l a y ' t h e i n s t r u c t i o n o f t i e s i n p u t i n t h i s t e t f b o x ’ /'A t ! <tD i n p u t s , p l e a s e - c l i c k , t h e t w o b u t t o n s a t t h e b o t t o m T h e b u t t o n o f a r i a l y a a i g : ■ b u i l d i n g c o n f i g u r a t i o n p i o w ' f e s i j i c D iinension a n d D aa d to a d Click ii«i« to iHsptuy btuttfimj iitmnt.ny f0i :< direction. vertical Click h e ie to a ititly re b u ild in g c o n liijm atio n • Fig. 5.14 A sample of the Dimension tab 88 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. After all the inputs are finished, users can click the two buttons at the bottom. If users click the button for displaying building summary, a pop-up form (Fig. 5.15) displays all the building inputs so far to help users check the correctness of each input. If users click the button for analyzing building configuration, a pop-up form appears to explain the concept of building irregularities based on ASCE 7-02 and display the output table in which lists the ratios of building dimensions and deadload at adjacent levels. ® 1 H > - Building Summary Building Summary Occupancy Category: CategotylV Lateral Force Special Steel Moment Frames Resisting Systems: Number of Levels: 4 Mean Roof Height (ft): 40 g Roof Shape: flat (only flat root is used in i FIJ) Display 30 view of rite building Table of Building Dimension and Deadload Level Height tit] A lee (sf | Oeadload (p:t| C 0 10 3750 100 3 1 10 3750 100 3 2 10 3750 100 3 3 10 3750 100 3 4 0 3750 50 1 Fig. 5.15 A sample of the building summary 89 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.2.2.3. Wind Info After users finish the inputs on the tabs of Building Info and Dimension, they can click Wind info tab to input the information for generating wind loads. The top portion of this tab provides the general input instructions such as how to input and proceed. The input is to the left, and its explanations are in the textbox to the right. There are 3 entries that are given step by step and each entry has its default value. In order, they are Fundamental Period, Wind Speed and Exposure Category. Their input instructions are to the right. After finishing each input, users can proceed by clicking next button or go back by clicking back button whenever any change needs to be made. Fundamental Period (Fig. 5.16) computes the building fundamental period based on the inputs. LFD can compute wind loads on rigid buildings that is defined in ASCE 7-02 and has the fundamental period less than 1.0 second. This computation lets the user know if the design building is rigid. The instruction is to the right. If users click the computation button, a pop-up form (Fig. 5.17) appears to display the formula and brief explanations of each factor based on ASCE 7-02. Wind Speed allows users to input the design wind speed for the building location. Exposure Category allows users to select the exposure category of the building. The last step of Wind Info tells users that they have finished the required inputs and tells them to go to Wind output tab to compute wind loads or Seismic Info to continue inputting the information for generating seismic loads. 90 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. L ateral l-cras G estgn (l.fD ) - M ain S creen Pfe* Inp^s tijf.wral H«Sp Bitll4bi«| Info Dimension I Wind Info Seismic Info Wind Output Seismic Output The- information to generate wind loads is K-rpnifd to W input in this tab. There are V step uipnts For each step, make a selection or ox* default value to the left. The instructions af each input can be found to the right. For the fust-tim e user, it's recomm ended to follow each step by clicking, next button to proceed or hack button to go back to the previous step. For advanced users, each input is listed in input menu. W henever the input is changed, p lease cKck next button to update the input. Building Fundamental Pei iod I Buildings m ay be v itia te d by w ind vi seisirui forces The tun* in i'ccon*!? for th* building to com plete one ry rle o f vibration n ta S -d budding b sid am fn tsi p eriod T , w inch u usually determ ined b y L ateral force resisting system s and budding height A s a rule o f thum b, the fundam ental perred o f a building con b e estim ated as (HO seco n d p e r story • 0 " , sec for one ttorv bwldmg • 0 .2 sec for tw o story building • 0 3 sec toi il«?c st : r ; lunklaig i c?nod in seconds i CtK* h e r ? to co m p u te tm ittfim j fu n d a m a n ta l p e rio d T Fundamental Period T Fig. 5.16 A sample of Fundamental Period in the Wind Info tab 5.2.2.4. Seismic Info The Seismic info tab collects the information for generating seismic loads on the design building. The top portion of this tab provides the general input instructions such as how to input and proceed. The input is to the left, and its explanations are in the textbox to the right. There are 3 entries that are given step by step. In order, they are Spectral Response Acceleration Si and Ss, Site Class and Response Modification Coefficient R. Their input instructions are to the right and each input has its default value. After finishing each input, users can proceed by clicking next button or go back by clicking back button whenever any change needs to be made. 91 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Spectral Response Acceleration Si and Ss (Fig. 5.18) allow users to input the values of Si and Ss for the building location. Site Class requires users to select the site class category of the building. Response Modification Coefficient R allows users to input R value of lateral force resisting systems or R value of the selected lateral force resisting systems is used. The last step of Seismic Info tells users that they have finished the required inputs and go to Seismic output tab to compute seismic loads on the design building. -T iiflifd m en tat P ei kxJ 1 Building Fundamental Period T The method of determining approximate building fundamental period T is based on Section 9.5,5.3.2 of ASCE 7-02 and described below. Lateral Force Resisting System: Fundamental period Ta is: T*=C,hax S p e c ia lS te c lM o n i^ m ! ra m e s 0.5-t seconds Fq. 9,5.5.3.21 of ASCE 7-02 Ta: 0.54 sec. Approximate building fundamental period, tin: 40 ft. Mean roof height. f " # " ] Cf; 0.028 Approximate period parameter. Table 9.S.5.3.2 of ASCE 7 02 x: 0 # Approximate period parameter. Table 9.5 5.3.? of ASCE / 02 Values of Ct and x of Table 9,53.3,2 of ASCE 7-02 ------- — ................. ................................... .............. • — .... S u u c tw e T ype Cf X Moment resisting frame sy ste m s of steel in which the fram es resist 100% of the requited seism ic force and are not en clo sed or adjoined by m ore rigid com ponents that will prevent the fram es from deflecting w hen subjected to seism ic forces 0 023 O S Moment resisting frame sy s te m s of reinforced concrete in which the fram e resist 1 00% of the required seism ic force and are not en clo sed or adjoined by more rigid com ponents that will prevent the fram e from deflecting w hen su b jected to seism ic forces 0 013 0.9 Eccentrically braced steel fram es 0 03 0 75 All other structural sy ste m s 0 02 0 75 Fig. 5.17 A sample of the pop-up form of the building Fundamental Period T 92 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. * T .a t« ra H o rc e ftestgrt (J..F0F W ain S creen I Building Info ftimfetisjou Wind itifo ; Seismic M tfo Wind Output Seism ic Output The information to generate seism ic loads is required to he input ui this tab. There ate 3-step inputs. For each step, m ake a selection or use default value to the left. The instructions of each input ran be found to the tight. For the fu'st-time n*«v, it’s refom ttwoded to follow each step by rlkkm g ue-sr button to proceed or bark button to display the previous step. For advanced users, each input is listed in input menu, W henever the input is changed, please click next button to update th e input. S p e c tra l R e s p o n s e A cceleration SC - C i .8 Q V itw h fap s of S1 • ^ iA ’ ‘i U i^vy IVUps ui S s : Specttal R espon se Acceleration S ', '>j o irl.'U feftrescnt tlie maximum •im ii'irieH e:«ttyjuakf ground rrtohons in particular g eographic areas and their ; r a liu ’ s hav*: to be- sp e c ifie d bs g e n e . .itc se ism ic load*- ; 'H ie values of b ) a n d S-, foi Il'ib? C lass B for FU-A <e? ■ listed ui Figure R C f. (I) through (TO) o f IB C 200.3 T hey are m ark ed on co n to u r lines an d show n as certain percentage o f "g" due to the E arth ’s gravity L F D doesn’i p io n d c s all Oj a n d m aps bee ause of tier considerable am ount o f the m aps T h e user can click Ihe w>w man huUrwi ;.i dip Wt tn ijjioUv ; • > . and F.. true Fig. 5.18 A sample of SI and SS in the Seismic Info tab 5.2.3. Software Output Users input through the UI layer, then the values for each entry are recorded in the application. The computing logic is based on the building code theory described in Chapter 3 and 4. The flow of data follows input -> process (either wind or seismic processing) -> output paths. The output of wind or seismic loads is displayed on separate tabs. The following sections discuss the LFD output. 93 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.2.3.I. Wind Output After users finish the inputs on the tabs of Building Info, Dimension and Wind Info, they can click the Wind Output tab to compute wind loads on the building. On the Wind Output tab (Fig.5.19), the top portion displays the instructions such as how to compute wind loads. Users can click the button of computing wind loads then wind loads in X (horizontal) or Y (vertical) direction appears in the output tables. Fte input tutw?4l Pnrt Help | Building Info D im ension W ind Info S eism ic Info Wllitf Output S eism ic Output Whirl loads on the building are computed on this tab. Click compute wind load button to display wind toads on the building. Click display Ijiiiltling tumnidiy to drsplay all the inputs. Click be*e to display Htitlding summary ; Usck hete to compute Wind I aads Wind Loads in X direction Click hero to generate Shear Wall level p _vrin d w a«jl;K fi p„6»w»£fe>hj p_combf»^pd| F 1045 -8.76 18 21 7 10.45 -87& 13 2', t* 1 1 0 6 - 8 7 6 1 . 8 & ' 1 ' 1205 -0.7C 20.01 1! 12,78 -8 76 2155 1 < 13.4 -0.75 2216 i> 14 02 -8 76 22 76 S Wind Loads in Y direction N a t e c l i c k c i - a t a o n ( b e T a b l e a b o v e t o d i s p l a y th e c o m p u t a t i o n t h e o r y F i i g h t - c i i c k n > d i e T a b l e a t i o * ? t o d i s p l a y t f - a t a g r a p h X direction is h orizontal direction. Click here to generate Shear Wall R_vtsnA'i»'dipf(] p_*ftwacdsp?r) »_comb**N»p*Ii Note click s'aia on th e Table above to display the computation theory Right dick n the ' able above to display data graph Y direction is vertical direction. Fig. 5.19 A sample of the Wind Output tab Each table lists windward pressure, leeward pressure, combined pressure, force, shear and overturning moment at each level. Users can click wind load data on the output table, then a popup form (Fig. 5.20) appears to display the computation 94 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. theory of the selected data. If users right click in the output table, users can select which wind load to be displayed graphically. A pop-up form (Fig. 5.21) appears to display the distribution per level of the selected wind load. iiSf-J f D - Wmd Pressure LFD can compute external wind loads on the rigid buildings with regular shapes. Analytical Procedure* o f Section 6.5 a f A SC E 7 02 is used as the computation theory o f wind loads. W in d w a rd P r e s s u r e p - qGCp P ftwfl: G: Cp: 10.4* 0 .8*. 0.3, 15.17 e v a lu a te d h e ig h t i: t: ft 0 .0 0 2 5 6 ' K c ' K / r K e T ^ M From f q . 6 17 o f ASCII 7 fl? d e s ig n w in d w a r d p ressu re p at e v a lu a te d h e ig h t z a b o v e th e g ro u n d . Q g u st e ffe c t factor from S e c tio n 6 .5 .8 o f ASCE / 412 e x te r n a l p r e ssu r e c o e ffic ie n t from F ig. $-6 o f ASCE 7 4 2 v e lo c ity p ressu re for W indw ard w a ils e v a lu a te d a t h eig h t i. e v a lu a te d le v e l: 3 E g. 6-15 o f ASCE /-0 2 H*: lt.85 v e lo c ity p ressu re e x p o s u r e c o e ffic ie n t from f a b le 6.3 o f ASCE 7-0? Krt: 1, to p o g r a p h ic factor from S e c tio n 6.5 .7 o f ASCE 7 07 Kd: y.85 w in d d irectio n a lity factor from l a h le 6 4 of ASCE 7 4 2 V: 85 rnph. d e s ig n w in d s p e e d from F ig. 1609 o f IBC 2003 I: 1.!!» Im p ortan ce factor from T a b le 6-1 o f ASCE 7 4 2 Fig. 5.20 A sample of the pop-up form to display the computation theory of the selected data in the wind output table 5.2.3.4. Seismic Output After users finish the inputs on the tabs of Building Info, Dimension and Seismic Info, they can click the tab of Seismic Output to compute seismic loads on the building. The design of the Seismic Output tab is quite similar to the Wind Output tab. But there is only one output table (Fig. 5.22) because seismic loads in X or Y direction are equal. The output table lists seismic force, shear and overturning 95 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. moment at each level. Users can click seismic load data on the output table, then a popup form appears to display the computation theory of the output data. If users right click in the output table, users can select which seismic load to be displayed graphically. A pop-up form appears to display the distribution per level of the selected wind loads. LFO Graph Output Level 8S4 131 9.41 2037 4I S Fig. 5.21 A sample of the graphic output of wind overturning moment 5.2.3.4. Shear Wall Output On the tabs of Wind and Seismic Output, there is a button for generating shear walls. If shear wall systems are chosen as lateral force resisting systems, users can 96 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. click this button to compute the overall length per level of shear walls that is displayed in a pop-up form (Fig. 5.23). The top portion displays the computation theory and the lower portion is the output table. If users do not specify the allowable stress of shear walls, default values are used. The overall length per level of shear walls can be displayed graphically if users click the output data (Fig. 5.24) l a t e r a l fart# D esign (IFD J 'M a f h 'f e te e n , Fite Jnjjyt Tutorid Pflnt Hefc | 8iiii4in<j ifdo Dimension W ind info Seismic Info W ind OntjmT : Seismic Output S eiw u c on the building a ie computed on tius tab. Click compute seisioir load button to display seism ic loads t the buihlmg ('lick display bnitding summary to display all the inputs Click Itete to display BuiitfiiMj Suuiimuy 5 Click here to compute Seismic toads Seismic 1 oads in X or V detection Click hem to generate shear wadis U''«t 'i&sttki-'wctll'.ipi M-Aftj 0 0 257.81 10455 73 1 u v . 25781 73?',' b 2 28.65 243 43 5442.71 ) 429? 714 84 325*4 ?? 4 57 29 17t 88 1575.52 5 7 1 fcl 114 58 429 63 5 4297 42 37 0 N & t& . oKk dats on the Tatue abovt to display the computation th&o?y Ri^ht-cack < n ibs Tab/e above to display data graph. X ditecdon horizontal direction. ¥ direction is vertical direction. Seismic l.onds in X or Y direction are equal. Fig. 5.22 A sample of the Seismic Output tab 97 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. t « ! f O Shear W all LFD calk generate the overall length per level of shear walls in X or Y direction, X I' the user doesn't specify shear stress, LFD uses default values. Shear Wall overall lenoth 1 it ner level of shear walls * shear V *+1 (ki^ a? ,hfe !fSvel immediately overall length tx per level of shear w alls* afaove/ sft(ja( qx ^ of shea( Wit|Js Type of Shear wall: C o n c r e t e User-selected allowable Shear {pH }; Dofauh. vakm- Click here to display default values of shear stress Table of overall length per level of shear walla Level ShearFotceVxfWp i m t j L3(f«i L « * "l 257 81 5 9 7 7 1 6 3.59 If 1 257 81 5 6 4 6 7 6 3.12 1 C 2 243.49 4,9? 5.9? 716 8 ‘ 3 214.34 3.98 4 7? 5 7 3 7 4 171.88 2,65 3.13 3 8 2 4 5 114.58 0.33 1.13 1 4 3 1 6 4 2 9 ? Q 0 0 0 Note click the column data of Vx or L* to show shear wall length graph Fig. 5.23 A sample of Shear wall output "W\" ...... 'S '6 4 '\' \ --------- s-sA- Fig. 5.24 A sample of Shear wall graphic output 98 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5.2.4. Software Menus LFD has the menus including file, input, tutorial, print and help. The file menu includes new project, open project, save, save as and exit. New project lets users start a new project. Open project lets users open a saved project. Save lets users save a project. Save as lets users save the project as a new name. Exit lets users quit LFD design screen. The input menu lists all the entries associated with each input tab. It allows users to easily access one particular entry. It is designed for advanced users or saved projects when only some changes are needed. The tutorial menu allows users to access the software tutorial while the software is in the design screen. The print menu tells users how to print in the software. LFD can not send data directly to a printer. The recommended method is to click the print screen button on the keyboard then paste the screen image into another software such as Microsoft Word or Adobe Photoshop to print. The help menu includes the flow chart of the software. The flow chart shows the software components and data flow sequence. It helps users navigate in the software. 5.3. An Example Structure This section uses a sample building and displays its wind and seismic loads that are computed in LFD. The input information in LFD is described as follows. 99 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 . The inputs in the Building Info tab Finish the step-by step inputs in this tab and the inputs used for the sample building are listed as follows: A. Select Occupancy Category (Fig.5.9) as Category III by clicking its radio button. B. Select Lateral Force Resisting Systems (Fig. 5.10) as Ordinary Reinforced Concrete Shear Walls by clicking the radio button of shear wall systems then making the selection in its sub-category. C. Use default values of concrete shear walls (Fig. 5.11) by selecting Concrete in the dropdown list. D. Input Number of Levels (Fig. 5.12) as 6 in the input box. 2. The inputs in the Dimension tab Finish the required inputs in the Dimension tab and the inputs used for the sample building are listed as follows: A. Select Scale (Fig. 5.14) as 5 in the dropdown list. B. Select Level (Fig. 5.14) as 0 in the dropdown list. C. Select Same till Level (Fig. 5.14) as 5 in the dropdown list. D. Draw the building plan at each level in the grid lines to the left (Fig. 5.14). E. Input Height(ft) as 10 in the input box (Fig. 5.14). F. Input Deadload (psf) as 100 in the input box (Fig. 5.14). G. Input Roof Deadload (psf) as 75 in the input box (Fig. 5.14). 100 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. H. Click the Execute button. This button has to be clicked for LFD to record the building plan, height and deadload inputs for the selected levels. 3. The inputs in the Wind Info tab Finish the step-by step inputs in this tab and the inputs used for the sample building are listed as follows: A. Input Basic Wind Speed (mph) as 85 in the input box (Fig. 5.25). B. Select Exposure category as Category C by clicking its radio button (Fig. 5.26) 4. The inputs in the Seismic Info tab Finish the step-by-step inputs in this tab and the inputs used for the sample building are listed as follows: A. Input Spectral Response Acceleration Si as 0.8 g-sec. and Ss as 1.5 g-sec in the input box (Fig. 5.18). B. Select Site Class as D by clicking its radio button (Fig. 5.27). After finishing all the inputs as described above, LFD can compute the wind and seismic loads on the sample building. Users can click the Wind Output tab or Seismic Output tab to get the computation results. The following figures show some samples of the computed results. Fig. 5.28 shows the wind loads on the sample building in X and Y directions by clicking the button of computing wind loads. Wind loads can be fully displayed by moving the sliding bar in the output table. 101 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I Fife Input Tutorial Font Hafc Buildhiy hife Cinneiis&m ; Wind Into irelsm k h rio W ind Output S eism ic Output The information to generate wind loads is required to be input in this tab. There m e 3 step inputs. For each step, m ake a selection or use default value to the left. The instructions of each input can be found to the right. For the first-tune user, it's recommended to follow each step by clicking next button to proceed or back button to go back to the previous step. For advanced users, each input is listed in input menu. W henever the input is changed, please click next button to update the input. B asic W ind S p e e d V CHck here to v iew w ind speed map B a ste Wmd Speed V. B anc w ind sp * ed V te r the bulking location bav to be specified to g enerate wind loads M ountainous lei ram, gorge ? and tv.iruc one-pi one region?. shall b e exam ined to r unusual wind s . M idrtiotu H urricane pro n e regions include the U S Atlantic O cean and O tiif o f M exico coasts w here the baste wind sp eed t? greater tli.ui 9 0 rnpb, Hawuu, P u erto Rico, G uam . Virgin Islands and A m erican Sarr.vu P lease follow ti.e steps below jo com plete this input S to p I C V k. i v w wir.d sp eed m ap button to the left to disolavthe basic wind s o c e d m a o c f Fieure Fig. 5.25 A sample of inputting Wind Speed in the Wind Info tab tlFDj Main Sem en . t n p c £ T l k -x i s I P s r i H e tp 1 Building Info Dimension Wind Into Seismic WhnJ Onipm Seismic Ouq»ut The mfoiination to generate wind loads is required to bo input in this tab. There are 3-xtop inputs. For earh step, m ake a selection or use default value to the left. The instructions of earh input can be found to the nglti. For the first-tim e user, it's recom m ended to follow each step by clicking next button to proceed or back button to go back to the previous step. Far advanced users, each input is listed in input menu. W henever the input is changed, p lease click next button to update tire input. E x p o su re C a te g o ry ! xjjftSiiie B * I xposnre C fOpfauii} t Kp«mne 0 Ctenei*1Iitslinctunic ofF xp osm e Category The rrojghr,e3s of the ground surface such as buddings or trees has great usfliieiice on wind speed txposm e category ts assigned to rhe Inal ding site to justify this effect There are three exposure categories ranging from B to ft defined m IBC 200? and ASCE 7 -0 2 M i ! , - s * * ! i " . ' t o > U t o d i v j d i y d i e b l l - i t f y p f f i i . v n m * . u i e u « ii tu to g w i N o t* the hill discussions o f expo m re category arc m Section H.U9 4 of IB C 2 0 0 1 - and o t> f of AS CF. 7-C.2. Ctick here to display g enera! input instructions ; Fig. 5.26 A sample of selecting Exposure Category in the Wind Info tab 102 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. llaterai .Fort* Besi&n (IFPl .M ein Screen PiM Itffet Tutorial Prmk Help I Building tnte IMrnensten W ind hrfo : Sefemtc Inla W ind Output S e ism ic Output 'flit* m form vitieti to g e n e ia f r seism ic lo a d s is req u ired to bp input in Hu* tali T h ere axe S -step inputs. F oi ea rh ste p , m a k e a s e le c tio n nr u s e rlefanit v a lu e to th e le ft. T h e in x h itr fia m o f e a c h input can h e found to th e tigh t. Fw th e lirst-rim e u ser, it's reco m m en d ed to fo llo w e a ch ste p b y click in g n e x t button to p r o c e e d or b ark b u tton to d isp lay (lie p rev io u s step . Fin a d v a n ced u s e r s , e a c h input is liste d in input m en u . W h e n e v e r th e input is th o n g ed , p le a s e c lick n e x t b u tto n to u p d a te th e input. Site Class ■ SiteQ aasA ■ ' S ite C lass 0 • Site Firm r • * S i t e O a ss 0 i B f i f i m h j •' sit« u.vss r • Site Class ] i T he soil condi tton o f the budding site h as g re a t influence : on setstmc load.’ and shall b e cW sified jiccoidin.-2: to the | roil p roperties | T he categories. -T site d as? range from site d a s? A to F • ' Site class 'v i? not u se d in tin.-! pro g ram becau se it lequu e • m ore ~ompie:-: analy?'-? /oil p ro p erties arc i u nknow n. Site C lass 1 / shall b e u sed T heir b n e f j descriptions car. b e fo und in the T able belo w ; , c ; 'rte Class ll'-'> * d P reble W&me Fig. 5.27 A sample of selecting Seismic Site Class in the Seismic Info tab f a t « f 4 l t ; 0 f c c b « i g n : W a i s t S c r e e n [ Building lute bhnenshm Whitt Info 'seismic Into ; Wind Output Seismic Output W ind lo a d s on th e building a te com puted on this tab. Click com pute w ind load button to display w ind lo a d s on the building. Click display building su m m ary to display all th e inpute. Click h em to display HuiMiug Sum inaty [ Click hgfft lo com pute W ind 1 ca d s | W ind l o a d s in X direction W ind 1 o.)d s in Y direction Click h ors to g en era te S h ea r W all Click h e te to gen era te Shear W all . Q,Wtr«teAK&>ll] p . !eowa«3pslt ft.cotnbrwdSorll Fxjkci L«v«f &_wmd^a<lfp5fl p„*eowa&$psf| p <:<vnb»«iSprtt UH& •87b 1921 i s i * 0 te 4b S A 15 71 ’ 0 45 ■S7£ 1S21 19 21 ; 1 1045 0 O S 157? -I O b' 9 / 6 1 9 9 ? 19.82 : i 1? U6 & A 18 A i?l>5 < 8 78 20 ;U 20.81 3 12 OS r> Jf; 1? 1 1 2 /9 9 7 6 2 \ 5S 21.59 ; 4 12 79 -b/6 1804 13 4 * 7 8 ?2 1 6 ; r. i*4 •5 26 18 fir'; '40; •8.76 2 2 ; ■ % 11.39 b 1402 0 9 2 / Wore- clsck d a ta o n th e T ab le above tc d isp la y th e c e m p u ta te m tbBfwy Right-d ic k in tiyft T sb te above t o d isp la y d a ta g rap h X direction Is horizontal direction. Wole click data on the Tabte above in display th e computation theory R i y b J - i ' l u ' k m the Table shove to display date graph Y du ectk u i is vertical direction. Fig. 5.28 A sample of the wind output for the sample building 103 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. If users right click in the output table, they can select which wind load can be displayed graphically. Fig. 5.29 shows the graphic output of the wind overturning moment of the design building. The computation theory can be accessed by clicking each data in the output table. Fig. 5.30 show the pop-up form on which displays the formula and factors used for computing wind pressure at level 0 based on ASCE 7-02. If more explanations are provided in LFD, users can click the question marks to get more information. SSl-FO - Graph Output 443.36 2716 Fig. 5.29 A sample of a graphic wind output for the sample building 104 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Ir® I f I) W ind P i c t u r e LFD can compute external wind loads on the rigid buildings with regular shapes. Analytical B ore dure of Section 6 5 of ASCE T-02 is used as the computation theory of wind loads. Windward Pressure p ■qGCp From Eq. 6-17 © f ASCE 7-02 p (psQ; 10.45 design windward pressure p at evaluated height z above the ground, 0 G; 0.33, gust effect factoi from Section 6.5.8 of ASCE 74)2 Cp; 0 J . external pressure coefficient from Fig. 6 6 of ASCE 7 02 q(psf): 15.37 velocity presstu© for windward walls evaluated at height z. evaluated height r. G f t «T 0.00256* K* 4 1 Kzt ‘ Kd * V2* I Eg. 615 of ASCE 7 02 Kz; 0.83 velocity pressure exposure eoefficiem from Table 6-3 of ASCE 74)2 Kzt 1. topograph** factorfrom 5ection6.5.7 of ASCE 74)2 0 Kd: 0.03 wind directionality factor from Table 64 of ASCE 7-02 mph, design wind speed from Rg. 1609 ot IBC 2003 1 : 115 Importance factor frontTable 64 of ASCE 7-Q? Fig. 5.30 A sample of the computation theory of the windward pressure on the selected level Fig. 5.31 shows the seismic loads on the sample building in X or Y directions by clicking the button of computing seismic loads. The output table lists the seismic force, shear force and overturning moment per level. Fig. 5.32 shows the graphic output of the seismic shear forces. Wind and seismic loads are used to design building structural components such as frames or shear walls. LFD provides one design example for shear walls systems. Based on the allowable shear stresses of shear walls, LFD can compute the overall length per level of shear walls used for resisting lateral forces in X or Y direction. The layout of shear walls mainly depends on building configurations. 105 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. H e Input Tutorial Print Hefc I B uilding Into D im ension W ind fnfrt S e ism ic btlo W ind Output [ S eism ic Output Seismic loads mi the building a te computed on this tab. Click compute seismic load button to display seismic loads t tlie budding Click display buildutu xummatv to display- all the inputs Click bete to display Building Smnrtuuy C lick h e re to co m p u te S e ism ic L ead s Solwnic Loads lit X or Y direction Click here to g e n e ta te sh eer w a tts tsv e l 'sheafFoicefktp} M 4k*t 0 v 25? 8! !ti455 n 1 ‘ 4 32 257 01 7877 S z 2SB5 243 49 5442 7i 3 42 9? 254 94 3294 2? 4 57 29 171 88 5575.52 * i 7161 11458 429 G9 $ 4 2 9 7 42.97 0 Motft click data on ihe Table above s* display the corrwsMtattontheory. Right-click m the fa b le above to display stota graph. X direction b horizontal direction. ¥ diracltorr is vertical direction. Seism ic Loads In X or Y direction are equal. Fig. 5.31 A sample of the seismic output for the sample building P^L ffi Graph Output Fig. 5.32 A sample of the graphic output of the seismic shear force 106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Fig. 5.33 shows the shear wall output by clicking the button of generating shear walls on the Seismic Output tab (Fig. 5.31). OS LFD- Shear W4 U L F D can g en era te th e overall len gth p er le v e l o f sh ea r w alls in X or Y direction. I f th e u se r d o e sn ’t sp ecify sh ea r str e ss. L F D u s e s default valu es. Shear Waff overall length Lx per level of shear walls ■ shear Vx+1 (kip) at the level immediately above'shear stress qx (pit) of shear walls Typo of Shear wall: Concrete User-selected allowable Shear (pit): l>ef«uli v.uktt** Click here to display default values of shear stress Table of overall length per level of shear walls Level ShearFofceVrfwp It (hi 359.38 359.38 340.95 304. OS 248.8 175.08 82.93 8.32 7.89 7 04 5 78 4.05 1.92 0 1.2(81 9 9 8 9 47 8 45 6 91 4 8 6 im 11.98 n 3 " 1014 8.29 5 84 2 7 S 0 I - 14 14 1 2 K 7 3 . 0 Nete: click the column data of Vx or L* to show shear wall length graph Fig. 5.33 A sample of the shear wall output for the design building This pop-up form lists the computation theory of shear walls, type of shear wall and its allowable shear used for the sample building. The output table lists the required overall length per level of shear walls based on the seismic shear at each level. The output data is 7.89 ft at level 1, which means that 8-feet of shear wall is required at level lfor resisting seismic shear forces. If 2 shear walls are used, the length of each shear wall can be 4 feet. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 6 EVALUATIONS AND FINDINGS 6.1. Strengths of the Software This software is designed as a teaching tool to help students of architecture study the main concepts and procedures of determining wind and seismic loads based on IBC 2003 and ASCE 7-02. The software can also generate wind and seismic loads based on IBC 2003 and ASCE 7-02. Generally speaking, the software reaches its goals. The software emphasizes its teaching function. It is designed to be easy to use and understandable to users. For the computing function, the software can accurately generate wind and seismic loads based on IBC 2003 and ASCE 7-02. The strengths of the software can be summarized as follows: 1 . The software has clear instructions to help the user use the application. The tutorial has the step-by-step input and output instructions. The tutorial can be launched any time while the design screen is in use. Step-by-step input and input instructions provide the information for users how to input and proceed. The input process is also a learning process. The user gets to know the main concepts of determining wind and seismic loads based on IBC 2003 and ASCE 7-02. 2. The UI of the software is interactive. The colored text and images are used to make the main concepts of building code theory understandable to users. Because the software is designed for students of architecture, the software allows 108 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. users to draw building diaphragm at each level and displays building summary. It makes the software more interesting to the user. 3. The software addresses the main concepts of building code theory. The relevant and source information is provided for the user to explore more in building codes. 4. The software output is easy to understand. The building summary helps the user check the correctness of each input. The computation theory is provided step by step that makes it easy to understand. The graphic output helps the user visualize the distribution of wind and seismic loads at each level. 5. The software is not only a teaching tool but also a design tool. The output can be used in preliminary wind and seismic design and other structural analysis software that do not generate wind or seismic loads. 6. The software is a success in terms of its performance and capacity. The building code theory is quite complex and the software can accurately generate wind and seismic loads based on IBC 2003 and ASCE 7-02. 7. The knowledge that the software provides is very helpful for the user to get to know wind and seismic design and building codes, which may benefit them as an architect in the future. 6.2. Weakness of the Software The LFD software also has some weaknesses described as follows: 109 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 . The software uses only imperial units but not metric units. Metric units are the international standard. If both units are used, the computing results will be more useful. 2. Some inputs may be hard to understand by users who have no background knowledge about lateral forces and lateral force resisting systems. Although the software provides brief instructions for all inputs, some of them involve many concepts that can not be explained fully in the software. 3. The software has the limitations in drawing building diaphragms. Although the software allows the user to draw building diaphragms, simple diaphragms with straight lines can be drawn in the software but not curvy lines. Each point of drawing lines has to be on the grid line so the buildings with the dimensions not complying with the grid lines can not be drawn accurately. 4. The software doe not provide all the geographic maps of Si and Ss The software provides users the website addresses for acquiring the maps of Si and Ss and the values of Si and Ss by entering the zipcode of the design building. If the software is designed by using a map system that includes all the maps, it will be easier for users to input the values of Si and Ss 5. Unfortunately the software does not allow the user to print the data or graphs directly to a printer. If the software could print out the results, it would be more helpful for users to study. 6. LFD allows users to specify only one type of allowable shear stress of shear walls. In reality, allowable shear stresses of shear walls may be changed at 110 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. different levels. If LFD gave users more selections, they would potentially have a better design for their shear wall systems. 7. LFD allows the user to draw building diaphragms at each level. For some building shapes such as taper-shaped buildings, the user has to input building diaphragms at each level. If LFD can provide the selections for common types of building configurations, it will be more efficient for users to input building configuration. I ll Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER 7 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK 7.1. Conclusions In general, the software, LATERAL FORCE DESIGN, reaches its goal as a teaching tool to help students of architecture study building code theory of determining wind and seismic loads. The software informs the main concepts and procedures of determining wind and seismic loads based on IBC 2003 and ASCE 7-02. It gives students the general ideas of building codes and how they define wind and seismic design. The knowledge may prepare students as architects to have better communication with structural engineers in the future. The building code theory lets students know that many factors, such as building configuration, construction and structural systems, have great influence on wind and seismic design. It may stimulate students to explore more on wind and seismic design. Compared with the books of building codes, the software is more efficient in terms of informing building code theory. The software emphasizes the main concepts and avoid the complex ones. The building code theory is informed to students while they input the factors. The UI of the software is designed to make it more interesting to the user. The software is also a design tool. The loads generated by the software can be used in preliminary wind and seismic design. 112 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The software may incorporate some advanced computation, such as wind loads on open buildings, to allow users to explore more about how those factors influence the lateral loads rather than only concepts are introduced. Building codes define wind and seismic design. The determination of wind and seismic loads is only one tiny part of wind and seismic design. The software addresses only the main concepts. More work can be continued to develop a software fully addressing building code theory for advanced users. 7.2. Suggestions for Future Work As discussed before, the software has some weaknesses. A recommendation of improving the software should be proposed. The metric unit shall be incorporated in the software. The software is based on International Building Code that is aimed to become international standard. If the software uses both imperial and metric units, the software may be used where imperial units are not used. The inputting of building diaphragms can be improved in the software. If the software can read the files from other drawing applications such as ACAD, accurate building geometry can be generated. If the software allows to send the data to printers, it will be more helpful for users to study the computation theoiy. The software may be included or make data readable in other structural analysis applications. If the data can be read directly in other structural analysis applications, users do not need to input the data. Those data can be used for 113 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. generating the deformations of the building under wind or seismic loads in those applications and students can visualize the effects of wind and seismic forces on buildings. The software only covers a tiny part of building code theory on wind and seismic design. More issues regarding wind and seismic design, such as building configuration, materials or construction that are mainly determined by architects, can be studied in the future. To take the full advantage of this application, it can be proposed that this application be used in related building science courses. On the one hand, students will benefit from working with a visual, friendly interface instead of walking through the overwhelming IBC2003 or ASCE standards. On the other hand, future students who are interested in working on improving it can get their hands on experience, and identify areas for improvement with their own interaction with the software. An open source application can be provided to those who are interested in taking it to the next level. 114 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. BIBLIOGRAPHY Ambrose, J. and Vergun, D., 1995. Simplified Building Design For Wind and Earthquake Forces. 3rd ed. New York: John Wiley & Sons, Inc. Amrhein, J., 1994. Masonry Design Handbook. Masonry Institute of America. Anon, (no date). What is Richter Magnitude [online]. Available from: http ://www. seismo. unr. edu/ftp/pub/louie/class/100/magnitude. html [Accessed 4 May 2004], Arnold, C. and Reitherman, R., 1982. Building Configuration and Seismic Design, John Wiley & Son ATC/SEAOC Joint Venture, (no date). Building Safety and Earthquakes - Part A: Earthquake Shaking and Building Response [online]. Available from: http:// www. atcouncil. org/pdfs/bp 1 a. pdf [Accessed 4 December 2003], ATC/SEAOC Joint Venture, (no date). Building Safety and Earthquakes - Part B: Earthquake Forces in Buildings [online]. Available from: http://www.atcouncil.org/pdfs/bplb.pdf [Accessed 4 December 2003]. ATC/SEAOC Joint Venture, (no date). Building Safety and Earthquakes - Part C: Earthquake Resisting Systems [online]. Available from: http://www. atcouncil. org/pdfs/bp 1 c. pdf [Accessed 4 December 2003], ATC/SEAOC Joint Venture, (no date). Building Safety and Earthquakes - Part D: the Seismic Load Path [online]. Available from: http:// www. atcouncil. org/pdfs/bp 1 d. pdf [Accessed 4 December 2003], Bachmann, H. (2003). Seismic Conceptual Design of Buildings - Basic principles for engineers, architects, building owners, and authorities [online]. Available from: http://www.bwg.admin.ch [Accessed 4 December 2003], 115 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CBTC. (no date). CBTC - Shear Wall Basics [online]. Available from: htto://www. mcvicker. com/vwall/pageQO 1 .htm# A 1 [Accessed 4 May 2003], Gibbs, T. (2003). Conceptual Design to Resist Earthquakes [online]. Available from: http://www.disaster.info.desastres.net/LIDERES/english/iamaica/presentations/Tony Gibbs ConceptualDesigntoResistEarthquakes.doc [Accessed 4 January 2004]. ICC, 2002. International Building Code 2003. Illinois: International Code Council, Inc. MCEER. (no date). How do Earthquakes Affect Buildings [online]. Available from: http://mceer.buffalo.edu/infoservice/faqs/eqaffect.asp [Accessed 4 January 2004], MCEER. (no date). How do Buildings Respond to Earthquakes [online]. Available from: http://mceer.buffalo.edu/infoservice/faqs/brespons.asp [Accessed 4 January 2004], MCEER. (no date). What are Earthquake Magnitude and Intensity [online]. Available from: http://www.mceer.buffalo.edu/infoservice/faqs/howmeasr.asp [Accessed 4 January 2004], MCEER. (no date). What are some Advanced Earthquake Resistant Techniques [online]. Available from: http://www.mceer.buffalo.edu/infoservice/faqs/asdesign.asp [Accessed 4 January 2004], Pakiser L. and Shedlock K. (no date). What Causes Earthquakes [online]. Available from: http://www.mceer.buffalo.edu/infoservice/faqs/how.asp [Accessed 4 January 2004], Pittack, L. and Bias, J. (2000). Wind Load Design Analysis: ASCE7-98 [online]. 2000+ EngineeRunner Inc. Available from: http://www. geocities. com/lpittack/cae614proi .html [Accessed 4 January 2004], 116 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Professional Publications, (no date). Lateral Forces - Earthquakes [online]. Available from: http://216.239.57. lQ4/search?q=cache:plevxwlYWsIJ:ppi2pass.com/catalog/servlet/ MvPpi fl Errata-ARES5chl4.pdf+lateraI+forces+-+earthquakes&hUen [Accessed 4 January 2004], Schierle, G. G., 2003. Northridge Earthquake Field Investigations: Statistical Analysis of Wood-frame Damage. CUREE Publication No. W-09 Schierle, G. G., 2001. Woodframe Project Case Studies. CUREE Publication No. W-04 Schierle, G. G., 1993. Quality Control in Seismic Resistant Construction. National Science Foundation Report Schierle, G. G., 2002. Northridge Earthquake: Residential Wood Structure Damage. Yokohama, Japan: Proceedings, Structural Engineers World Congress (SEWC). Schierle, G. G., 2002. Northridge Earthquake Field Investigations: Statistical Analysis of Woodframe Damage. Boston: Proceedings, 7th National Conference on Earthquake Engineering. Schierle G. G., 2002. Northridge Earthquake Field Investigations: Damage to Residential Woodframe Projects. Denver: Proceedings, ASCE Structures Congress. Schierle, G. and Vergun D., 1999. Northridge Earthquake Damage Testing. Proceedings, UCSD Workshop Schierle G. G., 1996. Quality Control in Seismic Design and Construction. New York: Journal of Performance of Constructed Facilities, American Society of Civil Engineers. Schierle G. G., 1994. Quality Control for Seismic Safety. LA Architect (cover page) Schierle G. G., 1994. Computer Aided Seismic Design. Journal of Architectural and Planning Research. SEI, 2003. SEI/ASCE 7-02 - Minimum Design Loads for Buildings and Other Structures. Virginia: the American Society of Civil Engineers. 117 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sells, C., 2004. Windows Forms Programming in C#. 1s t ed. Boston: Addison- Wesley. S. K. Ghosh Associates, no date. CodeMaster - 2000IBC. Standard Design Group, (no date). Wind Loads on Structures 2002 [online]. Available from: http:// www. standardsdesi gn. com/WLS/2002/2002. htm [Accessed 10 March 2004], Struware. (no date). Code Search Excel Spreadsheet [online]. Available from: http://www. struware. com/prodO 1 .htm [Accessed 10 March 2004], Structures & Codes Institute, no date. CodeMaster. USGS. (no date). Earthquakes, Faults, Plate Tectonics, Earth Structure [online]. Available from: httn://earthquake. uses, gov/faq/plates.html [Accessed 10 February 2004], Wind Effects Committee. (2004). The ASCE/SEI Wind Effects Booklet [online]. Available from: http://www.seiwec.net/mb/ [Accessed 4 February 2004], 118 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.

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Asset Metadata

Creator Wang, Xin (author)

Core Title Lateral force design (LFD) software for wind and seismic loads per IBC 2003 and ASCE 7-02

Degree Master of Building Science / Master in Biomedical Sciences

Degree Program Building Science

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

Tag Architecture,education, technology of,engineering, civil,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-324168

Unique identifier UC11337766

Identifier 1424236.pdf (filename),usctheses-c16-324168 (legacy record id)

Legacy Identifier 1424236.pdf

Dmrecord 324168

Document Type Thesis

Rights Wang, Xin

Type texts

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 au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA