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Statistical analysis of the damage to residential buildings in the Northridge earthquake
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Statistical analysis of the damage to residential buildings in the Northridge earthquake
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STATISTICAL ANALYSIS OF THE DAMAGE TO RESIDENTIAL BUILDINGS IN THE NORTHRIDGE EARTHQUAKE by Sanjeev Tankha 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 May 1995 Copyright 1995 Sanjeev Tankha UNIVERSITY O F SOUTHERN CALIFORNIA THE SCHOOL OF AACHJTECTunC UNtVEHSITY FAMK LOS ANGELES. CAUFOWMA IT O S M I I This thesis, written by ■SAHJ tHV 77KNKA4A under the direction o f his* Thesis Committee, and approved by all its members, has been pre sented to and accepted by the Dean of The School of Architecture, in partial fulfillm ent of the require ments for the degree o f . e o v L o tH fc '...................... ............................ }* * * “ ■ ■■ Date . ........... THESIS COMMITTEE C h a ir C. ACKNOWLEDGEMENTS I wish to express my gratitude to Professor G. G. Schierle, my advisor, for his invaluable guidance during the development of this thesis. The work would never have originated and could not have been completed without his interest and ideas 1 am grateful to Mr Dimitri Vergun and Professor Marc Schiller for their expert advice and invaluable suggestions for improvement Sincere thanks to Mr. Karl Deppe and Karen Penera of the Department of Building Safety, Los Angeles, and Mr. Richard Quacquarini of the Assessors office, Los Angeles for their help in getting valuable information I am also indebted to Mr James Chin and Mr Chandan Saikia for patient explanations on seismology Finally, my love and admiration goes to my wife, Sharmila, for her understanding and support, and to my parents and brother for their love and encouragement during the course of this work HYPOTHESIS AND ABSTRACT Documentation of performance o f lowrise residential buildings systems, in earthquakes, will provide data for proposing future guidelines in the selection and intigration of structural systems in the design process. This will help in the mitigation of seismic hazards. The extensive damage in the Northridge earthquake of January 17th. 1994 raised many questions about building safety in the Los Angeles Basin. The department of Building and Safety , Los Angeles, provided information on all privately owned damaged buildings that were inspected after the quake. A review of this data provided information on damage patterns which gave clues to the probable causes of the extensive damage to low rise residential buildings. Two forms of review were carried out with this data, in order to analyse and come up with strategies for damage mitigation in future earthquakes, a) Statistical analysis of number and type of buildings damaged. b) Mapping o f damaged buildings to corelate seismological and geological characteristics with building performance during the Jan 17th 1995 Northridge earthquake. TABLE OF CONTENTS Acknowledgements ii Abstract and Hypothesis iii Methodology, Summary and Conclusions vi List of Tables, Figures and Maps ix Chapter 1 Earthquakes— Causes and Effects 1 L I) Plate Theory/ Causes of Earthquakes 2 Faults 3 SeismicWaves 6 Qantifying Earthquakes 7 Seismic zones and Risk Maps 10 1.2) History of Seismic Activity in California 12 Geological and Geographic data 14 1.3) Northridge Earthquake Action. 19 Base data of Northridge seismic activity from preliminary UCB/EERC-94/01 reports from Berkeley and SCEC 20 C hapter 2 Building Response 24 2.1) Ground Motion and Building Response 25 Dynamic Loads 27 Lateral Design Elements 28 Configuration 29 Time Period 31 UBC Design Guidelines 33 C hapter 3 Damage Analysis of Residential Buildings 35 3.1) Base Data of Privately Owned Damaged Buildings from the Department of Building and Safety of LA city 36 Base data of Existing City Building Stock from the LA City Planning Department and Assessors offices. 40 3.2) Analysis Data Set and Relationship Matrix. 42 3.3) Comparisons of Damage and Initial Inferences from Graphs 47 Damage% Blocks and Dwelling Type 49 Year Built and Dwelling Type 49 Construction Type and Dwelling Type 52 IV Building Tag and Dwelling Type 54 Height of Building and Dwelling Type 56 3.4) Comparisons of Damage and Initial Inferences from Maps 59 Mapping of all Damaged Buildings 60 Mapping of Dwelling Types 62 Mapping of Year of Construction of 5+ Dwelling Type 67 C hapter 4 Findings, Conclusions and Proposals 71 4.1) Initial Inferences.derived from Statistical Analysis 72 4.2) Conclusions of Damage Patterns 76 4.3) Proposals for Mitigating Damage in the Future 79 4.4) Other Proposals in the Field and Further Research 81 Appendix A 83 List of References 92 v SUMMARY. METHODOLOGY AND CONCLUSIONS There is a continuing search for appropriate technology to reduce the loss of life and property caused by earthquakes. After each major earthquake numerous questions are brought up about the state of existing building technology The Northridge earthquake caused major damage to low rise residential buildings despite a very detailed and supposedly effective seismic building code. Damage was expected since the building codes are meant to save lives more than prevent damage. The extent of damage was not expected. The primary objective of this thesis is to bring to light the failures in low rise building technology in the city of Los Angeles and provide answers for the same Methodology The study consisted of four components 1. Collection of base data: Data sets of all buildings damaged in the Northridge earthquake were acquired from city records, along with data of the existing building stock in LA City Seismological data of the earthquake action on 17th Jan as well as geological data of the LA basin were put together in order to compare the various parameters that play a part in causing building collapse. 2. Comparisons: A matrix o f relationships such as % of damage, height, year built, construction type, type of inspection tag, was developed from the damaged building data set. This was graphed and mapped in correlation with geological data o f LA City and seismological data of the Northridge earthquake. 3. Analysis: The results showed definite damage patterns that corraborated the premise that both site and type of building factors play major parts in the amount and type of damage experienced. 4. Conclusions and proposals. The parameters observed and the damage patterns from certain correlations of parameters brought to light flaws in existing building technology and construction supervision. Such conclusions compelled to put forward proposals to mitigate similar damage in the future Conclusions The statistical analysis showed patterns of damage which led to conclusions o f the state of the building technology existing existing at the time o f the Northridge earthquake a) Failure of wood frame construction for targe buildings This was mainly due to construction quality of this building type The reasons for this are investigated in the later chapters b) The quality of construction has reduced over time in LA coty. All buildings types had a greater percentage of damage for construction after 1975. c) Location and type o f buildings played a major factor in amount and type o f damage experienced. The time periods of the sites and buildings seem to have played a major factor in damage patterns. d) Large 2-4 floor wood frame buildings had maximum damage. e) The distance from the epicenter did not necessarily mean that damage was less or more. It was more dependant on the geological and siesmological action. 0 Changes in seismic codes did have some affect but not the desired level for all building types and revisions need to be made for some building types g) A lot more work needs to be done by experts in the field of predicting earthquakes so that proper planning can be done while developing regions. Proposals and detailed findingss are discussed in the report and go a long way to explain many questions about damage in earthquakes as well as bringing out new unanswered questions that need to be addressed There are proposals for further research to answer some of these questions and also a mention of proposals by other agencies and individuals that relate to the issues brought out by this thesis. LIST OF FIGURES Chapter 1 1.1 World Map of Seismic Zones and Major Plate Boundaries 2 1.2-1.5 Types of Common Faults 3 1.6-1.7 Graben and Horst 4 1.8 Focus, Epicenter and Earthquake Induced Ground Motions 5 1.9-1.12 Common Earthquake Waves 6 1.13 Magnitude in a Graphical representation 7 1.14 The Modified MercaJli Index 9 1.15-1.16 Seismic Risk Maps of the U.S. 10 1.17 Iso-seismal Maps of Long Beach and Whittier-Narrows Quakes 1 1 1.18 The San Andreas Fault 12 1.19 Earthquake probability on the San Andreas Fault 13 1.20 Map of Known Faults in Southern California 14 1.21 Population Density in the LA Metropolitan Area IS 1.22 Map o f LA County 16 1.23 Base map of Soil Types in LA City 18 1.24 Geological Model of the Northridge Earthquake 19 1.25 Contour Map of Maximum Horizontal Acceleration 21 1.26 Table of Seismic Data of the Northridge Earthquake 22 1.27 Base Map of Soil types and Seismic Data of LA City 23 ix Chapter 2 2.1 Directions of Movement of a Building under Seismic Load 25 2.2 Modes of Vibration of a Simple Structure 26 2.3 Different kinds of Loads 27 2.4 Common Lateral Bracing Systems 28 2.5 Common Configuration Problems 30 2.6 Structures and Periods of Vibration 32 Chapter 3 3.1 Sample Data of Damaged Buildings 37 3.2 Matrix of relationships under Study 43 3.3 Distribution of Damage% Block and Type of Tag 45 3.4 Graphs of Damage% of Various Dwelling Types 48 3.5 Graphs of Number and % of Damaged Buildings in Year Built 51 3.6 Number and % of Buildings Damaged of each Construction Type 53 3.7 Graphs of Number/ % of Tagged Buildings 55 3.8 Number/ % in Height 57 3.9 Number of Buildings Damaged of Each Dwelling Type in Height 58 Table 3.1 Table of Buildings Built in Specific Time Zones 40 Table 3.2 Distribution of Damaged Buildings in Damage% Blocks 44 Table 3.3 Number of Buildings Built and Damaged 46 Table 3.4 Percentage of Buildings Damaged of Various Dwelling Types 47 Table 3.5 Number of Buildings Damaged in Each Time Range 49 Table 3.6 Percentage of Buildings Damaged in Each Time Range 49 Table 3.7 Number and % Damage and Construction Type 54 Table 3.8 Number and %Damage and Tag Given 54 Table 3.9 Number of Buildings Damaged in Height 56 Map 1 All Buildings Damaged over 25% 61 Map 2 All Single Family Units Damaged over 25% 64 Map 3 All 2-4 Family Dwelling Type Units Damaged over 25% 65 Map 4 All 5+ Dwelling Type Units Damaged over 25% 66 Map 5 All 5+ Dwelling Type Units Damaged Built from 1901 to 1934 68 Map 6 All 5+ Dwelling Type Units Damaged Built from 1935 to 1974 69 Map 7 All 5+ Dwelling Type Units Damaged Built from 1975 to 1994 70 Chapter 4 4.1 Trend o f Increasing Damage 74 Appendix A A.l Strong Motion Stations around Los Angeles 84 A.2 Horizontal Ground Displacements 87 A.3 Vertical Ground Displacements 88 A.4 Contours of Horizontal Accelarations on Rock/Soil sites :s 89 xj Fig A.5 Contours of Horizontal Accelarations on Rock Sites 90 Fig A.6 Contours of Horizonta; Accelarations on Soil Sites 91 Table A .l Station Code-Reference Table 85 Table A.2 Data from selected Stations 86 xii CHAPTER 1 1 EARTHQUAKES - CAUSES AND EFFECTS Plate Theory Historically there have been many theories suggested for the cause of earthquakes. The Plate theory put forward in the 20th century has only recently helped our understanding of this phenomenon. The theory explains the formation of the continents and oceans by stating that the earth’s upper crust is in constant motion. It is broken up into plates or sections that make up the earth's surface. The six major and the six minor plates are either moving into, away from or sliding by each other. The "Convergence’ most commonly happens in the form of one plate sliding under another plate. The plates may also move laterally or rotate as they push against one another. ‘Divergence’ of plates causes spreading, leaving spaces for molten rock from the undersurface to rise. These motions are thought to be the cause of earthquakes, volcanoes and other geological phenomena. * • * . E picenters of Earthquakes Spreading Plate 8oundary il ***** Consuming P late B oundary £ Fig 1.1 World Map showing seismic zones and major plate boundaries. Source: U.S. Geological Survey 2 Faults Earthquakes generally occur at the plate boundaries. Faults are common in the regions where two or more plates meet. Faults are... “ ...planes or surfaces in earth materials along which failure has occurred and materials on both sides have moved relative to one another in response to the accumulation of stress" (Banks, p.2) Faults may or may not be visible on the surface. Those that are visible are seen as fractures, trenches or pressure ridges. Fig 1.2 Normal Fault Fig 1.3 Thrust or Reverse Fault Fig 1.5 Normal & Slip Fault Combination Fig 1.4 Lateral or Strike Slip Fault Fault slippage and movements are classified by the direction of movement. The common types of faults found are the Normal fault where the two sides are pulling away from each other creating tension; the Thrust or Reverse fault where the two faces push against one another creating compression; the Lateral Slip fault where movement is sideways and a combination of normal and slip or reverse an slip faults. Most faults along the California coast, including the San Andreas fault show lateral slip characteristic movement with the fault plane being almost vertical. Fig 1.6 Graken Fig 1.7 Horst Geologists state an active fault as one that has had movement in the last 10,000 years. The location of nuclear plants is restricted to areas where a faults have not had moved for over 50,000 years. The presence of an active fault gives an indication of probable earthquake activity, giving reason for designing for seismic safety. The slippage of faults cause earthquakes. The origin is generally deep below the surface and is known as the Hypocenter or Focus of the earthquake. The slippage causes energy 4 to be released in the form of stress waves. The Epicenter of an earthquake is the point on the surface directly above where the energy was first released. The stress waves cause ground motion which induce motion in buildings. EPI CENTRAL DISTANCE SOIL LAYERS f : I ' m i ROCK EPICENTER I-++H h + 4 -H BASE ROCK AT THE SITE OF BUILDING + lGROUND MOTION Xf .9 ir OUTW ARD M O V IN G W A V E FRONT / >FOCUS OF EQ MAGNITUDE M L 2 A S s ACCELEROGRAM Fig 1.8 Focus, Epicenter and earthquake induced ground motions. Source: Guevera(1989) in Lagorio.1990, p.4l. 5 Seismic Waves There are four typed of waves that are created by the release of energy in an earthquake (Fig. 1.9 - 1.12) The P wave arrives first and has the form of a sound wave, alternatively pushing and pulling at the ground as it spreads. The S wave or secondary wave which arrives after the P wave shears the rock at right angles to the direction of movement. The Love wave is similar to the S wave but has a horizontal component and no vertical component. The Raleigh wave has both vertical and horizontal movements in the direction of the wave and travels slower than the love wave. Fig 1.9 P Wave Fig 1.10 S Wave Fig 1 11 Love Wave Fig 1.12 Raleigh Wave Fig 1.9-1 12 Four types o f earthquake wa\>es caused by fault rupture. Source: Arnold,!982, p.16 6 Quantification of Earthquakes The quantification of an earthquake is done by Magnitude and Intensity at a given location. The size of an earthquake is measured as Magnitude or the total amount of energy released. This is measured at the source of the earthquake. The common scale used is the Richter scale, developed by Charles Richter of the California institute of technology in 1935. The Richter Scale is expressed in Arabic numerals. The scale is logarithmic and with each unit increase the energy released increases 32 times. . O L 2 3 Fig 1 3 Circle 1 shows magnitude 1 on the Richter scale. Circle 2 has an area 32 times greater for magnitude 2 and Circle 3 has an area 1024 times greater for magnitude 3 on the Richter scale. The measurement is done by a seismograph. A seismograph records movement of a point in space over time. Measurements are taken in three axes, two horizontal and one vertical. The ink trace of the point, usually a pendulum in a box attached to the ground, records the time history of ground motion at a given location. In recording earthquakes three measurements are of most interest Acceleration, Velocity of the waves and the displacement of a point. Velocity is the rate of ground motion measured in in/sec or cm/sec 7 Displacement is the distance a point is moved from its initial position at rest. Acceleration is the rate of change of velocity. Velocity if multiplied by the mass of the building, gives inertial force that the building has to resist. It is measured in ‘G’-force (approximately 32 ft./sec/sec or 980 cm/sec/sec or 1G). Intensity of an earthquake describes the observed/experienced force at places based on observation during or immediately following earthquakes. The effects of an earthquake are different at different sites depending on the distance from the epicenter, the ground condition etc. so there are many values of intensity. The Modified Mercaili Index is the most commonly used intensity scale. Intensity on the MM scale is shown in Roman numerals. The most updated version of this index was put forward by Richter in 1956. Figure 1.14 shows the Intensities for the scale as written by Richter. 8 MODIFIED HERCALLI U R E H S m SCALE (1956 v e rs io n ) I. Not felt. Marginal and long-perlO d effects of large earthquakes. II. Felt by persons at rest, on upper floors, or favorably placed. III. Felt indoors. Hanging objects suing. Vibration like passing of light trucks. Du ration estimated. May not be recognised as an earthquake. IV. Hanging objects suing. Vibration like passing of heavy trucks; or sensation of a Jolt like a heavy ball striking the vails. Standing cars rock. Windows, dishes, doors rattle. Classes clink. Crockery clashes. In the upper range of IV, wood en walls and frames creak. v. Felt outdoors; direction estimated. Sleepers wakened. Liquids disturbed, sane spilled. Shall unstable objects displaced or upset. Doors swing, close, open. Shutters, pictures move. Pendulum clocks stop, start, change rate. VI. Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Win dows, dishes, glassware broken. Knicknacks, books, etc. off shelves. Pictures off vails. Furniture moved or overturned. Weak plaster and masonry D cracked, aonll bells ring (church, school). Trees, bushes shaken visibly, or heard to rus tle. VII. Difficult to stand. Noticed by drivers. Hanging objects quiver. Furniture bro ken. Damage to masonry D, including cracks. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices, also unbraced parapets and architectural ornaments. Some cracks in masonry C. Haves on ponds, water turbid vith mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged^ VIII. Steering of cars affected. Damage to masonry C; partial collapse. Some damage to masonry 8; none to masonry A. Fall of stucco and same masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame bouses moved on foundations if not bolted dawn; loose panel walls thrown out. Decayed piling broken off. Branches broken from trees. Changes in flow or temperature of springs or wells. Cracks in vet pound and on steep slopes. IX. General panic. Masonry D destroyed; masonry C heavily damaged, sometimes with com plete collapse; masonry B seriously damaged. General damage to foundations. Frame structures, if not bolted, shifted off foundations. Frames racked. Serious damage to reservoirs. Underground pipes broken. Conspicuous cracks in ground. In alluviated areas sand and mud ejected, earthquake fountains, sand craters. X. Host masonry and frame structures destroyed vith their foundations. Some veil- built wooden structures and bridges destroyed. Serious is mage to dams, dikes, em bankments- Large landslides. Hater thrown on banks of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly. XI. Rails beat greatly. Underground pipelines completely out of service. XII. Damage nearly total. Large rock masses displaced. Lines of sight and level dis torted. Objects thrown into the air. MASONRY A. Good workmanship, mortar, and design; reinforced especially laterally, and bound together by using steel, concrete, etc.; designed to resist lateral forces. MASONRY b. Good workmanship and mortar; reinforced, but not designed in detail to reist lateral forces. MASONRY c. Ordinary trorkmanship and mortar; no ntroc weaknesses like failing to tie at corners, but neither reinforced nor designed against horizontal forces. MASONRY D. Weak materials, such as adobe; poor mortar; low standards of workmanship; veak horizontally. Fig 1.14 Modified Mercalli Scale gives the intensity o f an earthquake at any given point. (Arnold, 1982, p. 32) 9 Seismic Zones The fault maps of earthquake prone regions present us with the problem of designing earthquake safe structures. The probability and severity of earthquakes in some regions makes a case for designing seismic safe buildings in these regions. The UBC (ICBO, 1993), along with detailed seismic design requirements gives a seismic zone map of the US based on this criteria. The Applied Technology Council have also developed a risk map. Fig 1.15 U , S. Seismic Risk Map based on effective peak velocity related accelaration, (Applied Technology Council, 1978) Fig 1.16 U.S. Seismic Risk Map, (ICBO, 1993) 10 Seismic risk maps give a sense of the level of earthquake activity in a given region. The Isoseismal maps shown in figures 1.17-1.18 give an indication of degree any particular region was affected around the epicenter. This information can help in planning strategies for future development. Fig 1.17 Isoseismal map o f the Long Beach and Whittier Earthquakes (Lagorio, 1990, p. 32) The presence of seismic risk maps and the MMI make seismic design effective to a certain degree but do not give the probability of recurring earthquakes in any given region. The complexity of earthquakes is still not fully understood and that makes these tools not very precise. The MMI is outdated and the zone map is too general. The findings and recommendations concerning these two factors, in seismic design, are addressed in the proposal section of this thesis. BURBANK 11 History of Seismic Activity in California The movement of plates along fault lines results in the build up of strain, the release of which causes an earthquake. California has an abundance of faults that make earthquakes a frequent occuring. The beautiful landscape of hills and valleys make California a pretty place to live, but the irony is that they have been formed by past plate movements. California faults are a part of the Pacific fault region. This region is one of the most active in the world and spans the coasts of Japan, New Zealand and the west coasts of North and South America. The major faults in the North American west coast region being the San Andreas fault (running in a southeast-northwest direction) and the Garlock fault (running in southwest-northeast direction). Cap* M endocino 0 100 ?0Q 300 «ILOM£T€A$ ------- Fig 1.18 The San Andreas Fault (Spangle, PEPPER, 1987, p.24) The San Andreas is the greater one and runs for over 650 miles through southern California and the coast of central California. It has been the cause of two of the greatest quakes in California in 1857 and 1906. Many recent earthquakes have been attributed to this fault, such as the Dec. 4th. 1948, 6.5 magnitude, Desert Hot Spring quake and the May 18th. 1940, 7.1 magnitude. Imperial Valley quake. Geologists and scientists claim that the probability of an earthquake of 8+ magnitude from the San Andreas fault is 2-5% annually.. Fig 1.19 shows an earthquake probability map on selected segments of the San Andreas fault. 6 0 ° 4 0 * 7 , 20 •/, f a u l t s OLEHA H 8 i HAYWARD SOUTH SF 2 0 0 KILOMETERS Fig 1.19 Earthquake probability on the San Andreas fault (U.S. Geological Survey Open File Report 83-63.) L.A.Region Historically, in the L. A. region, there has been a moderate to large quake every 20 or so years for the past 150 years and the trend, it is believed, will continue in the future. The chance of an earthquake of 6+ magnitude within LA city itself is also 2 to 5% every year (Spangle, PEPPER, p. 1). Earthquakes in California, though, are not restricted to the major faults. The presence of a mosaic of faults under the surface, both known and unknown, is a constant threat. The surface under California can be compared to a mosaic of shattered glass. Figure 2.7 shows the location of some of these faults in southern California. ° V o ' > < ■ * MTU j i r Fig 1.20 Map o f known faults in Southern California (U.S.G.S. and U S National Oceanic & Atmospheric Administration, 1971, p. 7) 14 Considering the concentration of people in the Los Angeles Metropolitan area, the concern for the prevention of loss of life and damage to property is most important. The population of this region is over 1 1 million people with some areas having a concentration of people. Los Angeles county consists of a number of cities with the city of Los Angeles being the largest and most populated. A map of the population density and the LA cities shows this trend clearly. An earthquake in the city can be devastating for lif and property as was proved by the Northridge earthquake. Studies have been done and research on predicting earthquakes continues. Even if we can make accurate predictions, of seismic activity, the whole region is a seismically unsafe and special care should taken to design buildings for seismic loads. its o Highly Urbanized [ ~~| Mt./Desen Fig 1.21 Population Density o f LA Metropolitan Area (SCAG 82 Growth Forecast/Policy, pg.40) 15 Figure 1.22 shows Los Angeles City shaded with its surrounding cities. This thesis concentrates on data taken from Los Angeles city only. The Northridge Earthquake had its epicenter in one of the most populated regions of the city, at the cross streets of Reseda and Devonshire, in the San Fernando Valley. Newhall Santa Clarita/ South Antelope Valley Burbank " , Los Angeles Glendale Monrovia Santa M onica . -C'.'-. - ■ ■ ■ * ' Inglewood. s r ; ; - - - . ; Pasadena ’ . • w ■ r~ f ■ '_________ * • .__.-if' .... ■^•Ea23 ^ "A lham bra?''-^-'- " ' rT s .'’ -‘ !2'> ^ - T . —A 3 M5Storey ^ ^ ; ---— i.'-JdoptebeBo^".: % ■ . 'w j -B e lL £ * n k a $ ‘ . ^outK 'G ate - ^Lynwopd / "ix ~ ' Compton: Bdl flower '+■, • ? Lakewood " Carson - ’ - • • ’ : 1 0 M iles 4 ? Fig 1.22 Los Angeles County (Mapinfo Corporation data files, 1994). 16 Los Angeles Geological Data Building response in an earthquake is partly governed by the soil conditions of the site. Soil conditions at a location determine the frequency of seismic waves, effecting the response of buildings. Similar buildings will have different responses on varying soil conditions. One of the directions of the research in the thesis was correlating soil type and building damage patterns in the Northridge earthquake. Within L.A. city we see varying soil conditions. There are three basic soil types across the city limits: Hard Rock, Halocene sediments (sediments deposited over the last 200 years) that are not properly compacted soils and Pliestocene sediments or marine deposits. Hard rock in the Santa Monica Mountains; Halocene sediments in the Hollywood region and along the 10 freeway; and Pliestocene or Marine deposits that make up the valley and other areas. The presence of rivers and lakes also give loose unconsolidated deposits along the banks. To these soil conditions the added phenomena of Liquefaction has a lot to do with earthquake damage. There is potential for Liquefaction in areas underlain with granular sediments where the ground water table is 33 feet (10 meters) or less (Spangle, PEPPER, p. 16). The potential for damage due to liquefaction always has to be kept in mind when investigating a site. Fig 2.9 shows a map of LA city with the soil types and potential liquefaction areas. The information is taken from the Northridge Earthquake preliminary report UCB/ EERC-94/01 and mapped using Mapinfo. 17 ★ Epicenter j | Rock F-! Haiocene Sediments □ Pliestocene alluvial/marine deposits B P o ten tial L iquefaction Fig 1.23 Map o f Los Angeles City with Soil types and Potential Liquefaction Areas, Rivers, Lakes and Major Freeways. Areas outside the city not shaded 18 Northridge Earthquake Data The Northridge Earthquake of Jan. 17th.. 1994 struck at 4:31 a.m. with the epicenter at 34.2° N and 118.53° W. The focal depth of the rupture was 18.4 km. under the surface. The magnitude, as determined by Caltech, was 6.7 on the Richter scale. The earthquake occurred on a previously unmapped fault. The fault is south dipping and adjacent to the north dipping San Fernando fault. The San Fernando fault of Feb.9th. 1971 had magnitude of 6.6 and was part of the same lateral fault system. The Whittier-Narrows quake of 1987 was also part of this system. Though that also was also on a north dipping fault. Figure 1.24 shows an perspective of the various faults in the region. Fig. 1.24 Geologic model o f the Northridge Earthquake showing other faults. (William, Patrick and Holland, Preston , Lawrence Berkeley Laboratory.) 19 The location of the fault area directly beneath a heavily populated area of Los Angeles resulted in greater damage including 61 deaths. There were very strong ground motions and large inertial forces experienced. This caused many inadequately designed buildings to collapse. Damage was also incurred far from the epicenter in places such as Hollywood and Santa Monica. This is possible due to their respective soil characteristics and a possible basin affect in certain areas. Much of the geology of the bed rock in the area ts still not known and only speculation can be made in a lot areas. Some salient features of the Northridge Earthquake were: a) Very high ground acceleration near the epicenter as well as certain locations away from the center. This could be due to local soil conditions at the locations, b) A strong possibility of liquefaction was prevalent throughout the Los Angeles basin (see figure 1.23) but this did not seem to have as much an impact as anticipated. This could be primarily due to the fact the water table in most areas was below 30’ due to the prevailing drought of the last few years. c) Deep structural basins may have affected distribution of the ground motion in many areas. Observed Ground Motions Los Angeles area has many strong motion stations that record the aspects of ground motions in earthquakes. These stations record horizontal and vertical accelerations, horizontal and vertical displacements and the time periods of the earthquakes forces. 20 These stations recorded maximum horizontal acceleration of more than 0.0 Ig at a distance of more than 2S0 km. from the epicenter. The largest ground acceleration was recorded as 1 82g at Tarzana-Cedar Hill nursery. This was on a small hill of alluvium on bed rock. The odd situation is that nearby homes did not feel such strong accelerations. This is probably due to the special soil conditions at the nursery site. This situation shows that each site could have had varying types of ground motion based on the local conditions. The UBC though describes soil factors in this region as S 1 or S2 only. The maximum horizontal accelaration recorded at rock site was 0 49g The closest site on soil, to the rupture plane, was 12 km northeast at the Jensen Filtration Plant grounds. The recording at this site was 0.98g. Figure 1.25 shows maximum horizontal acceleration contours along the recording stations. Fig. 1 25 Contour map o f Max. Horizontal Accelerations at Rock and Soil sites. Source: EERC report no. UCB/EERC-94/08, Junel994, p.25 21 The values of horizontal and vertical acceleration show the dependence of soil conditions and location of the sensors, whereas, the values for horizontal and vertical displacements show a different trend. The vertical displacements are greater nearer the epicenter and get lesser as we go radially out while the horizontal displacements get larger further away from the epicenter. This is due to the seismic waves distributing radially from the epicenter, (fig. 1.8, chapter 1, slant distance of the waves) A lot of damage directly above the epicenter was probably due to high vertical displacement. It was seen that that many structures (residential) had unusual vertical shaking and tended to bounce off their foundation. They were also probably not designed properly for vertical seismic loads and missed adequate hold-downs. Figure 1.27 gives a table of motion experienced in the Northridge earthquake. Station Name Site Condition Epicentre! Distance (km) Predominent Period (sec) Duration (sec) Sylmar (E-W) Alluvium IS 0.35 14 Arleta (E-W) Deep Alluvium 9 0 40 16.5 Tanana (E-W) Alluvium (10m?) over siltstone 7 0.35 20.5 LA Storage (N-S) Alluvium (130m?) over sandstone shale 23 0.24 15 LA Pico (N-S) Alluvium 31 0.40 13 points Hacd Vacd Patiod Vafap Hdfep 1 0.42 0.00 0.00 0.12 0.48 2 0.00 0.00 0.00 0.08 0.48 syimar-countv hosp. 0.91 0.60 0.35 2.12 1.20 van nuyt hot si boss 0.47 0.30 0.00 1.20 0.48 pocoima-fire station 0.44 0.19 0.18 0.28 0.48 LA hollywood gtoraga 0-41 0.19 0.22 0 .00 0.00 7 0.40 0 .00 0.21 0 .00 0.00 LA offica bkJg, 0.32 0.13 0.36 0 .20 0.40 burbank comm, btdff. 0.35 0.15 0.00 0 .20 0.40 arista nordhoff firs 0.35 0.59 0.40 0.32 r o.ea' tarzana-cadar hill 1.82 1.18 0.35 0.08 0.88 12 0.42 0.00 0 .00 0 .00 0 .00 13 0.00 0.00 0 .00 0.08 0.08 14 0.10 0.00 0.00 0.12 0.24 santa monica 0.93 0.25 0.21 0.28 0.40 Fig 1.27 Table o f seismic data recordings at various sites during the Northridge earthquake. Source: EERC, report no. UCB/EERC-94/08, June1994. 22 A mapping of seismic data along with the soil condition can be seen in figure 1.28. In the analysis in chapter 3 of this report, damage patterns are compared to this data and correlation made between site, soil and building damage. V Pacific Ocean i S L I ( t C ity lint 0 a d h M il** * Epicenter ■ Rock ( | Halocene ted merits ' | Pliestocene alluvial/ marine deposits ' ▲ single family 76-100% damagt ‘ A single family 51-75% damage ! A single family 25-50% damage Fig 1.28 Graphic representation o f Acceleration, displacement and time periods in the Northridge earthquake along with soil condition in Los Angeles city. 23 CH A PTER2 24 BUILDING RESPONSE Ground Motion and Building Movement During an earthquake the ground motion is directly transferred to buildings. The various kinds of seismic waves get translated into the structure, and the characteristics of ground motion, acceleration, velocity and amplitude, have different effects on the structure. The ground shaking subjects buildings to horizontal, vertical and rotational forces. One part of a building changes direction before another causing various parts of the building to move in different directions at the same time. This causes different displacements in different parts of the building(fig 2.1). HORIZONTAL MOTION ROCKING MOTION VERTICAL M O TIO N Fig 2.1 Directions o f Movement o f a building under seismic loads (McCue. 1978, p. 5) 25 In an earthquake the horizontal forces are usually greater than vertical ones. The foundation being fixed to the ground has a tendency to shift wit the ground. The superstructure, though, is free to move laterally. This movement can produce an oscillation that takes place at the buildings natural period. In tall buildings opposite floors can move in opposing directions causing big distortion. In short buildings this may not take place. The frequency of oscillation or time period depends on the mass of the building also resulting in the amount of inertial force the building has to resist. One building can have various modes of vibration as also a similar type of building can have different vibration modes depending on stiffness of the structure(fig 2.2). A structure with ductility somewhere in between a flexible flagpole and a stack of bricks is most efficient in seismic conditions. o j- - - - - - <(3D C C 3D D [G D 0C / / / / / V . 2H E E Fig 2.2 Different modes o f vibration o f a simple structure based on stiffness properties o f the structure. (Botsai-Architects and Earthquakes, 1975) 26 Dynamic Loads Vertical forces are generally perceived as static forces but lateral forces induce dynamic loads which are the foremost concern in seismic design principles. The horizontal forces are distributed through the building via the floors and roofs. These elements then transfer the loads to the vertical structural elements. Seismic design takes into consideration that the lateral loads get transferred down without a break in the load path When there is a break the structure has greater potential for damage and collapse r " T — i --------ii 2 f 1 t G ♦ 4 —k . n----------- ♦ - - - - > T ^ F - - - t I ^ u - - - - - - - - - P— - - - - 2 F r f c - - - - - - - - - - — ti 1 T 4 Fig 2.3 a) Structure subject to Vertical load, b) Earthquake lateral and vertical loads c) Combination o f gravity and Seismic Loads. (Lagorio, 1990, p. 33) 21 Lateral Design Elements Lateral forces are restricted by vertical bracing systems. Most structures have rectangular components. A pin connected rectangle has no lateral stability, but a bracing applied to the system can make it resist the lateral forces. The bracing produces triangulation which is an inherently stable form. Since triangles are not very popular architectural elements a number of systems have been developed such as Shear Walls, Braced Frames and Moment resisting frames (Figure 2.4). NORMAL POSITION (DOTTEO LIN E) FRAME SHEAR WALL GROUNO MOVEMENT DIAGONAL OR "X"BRACING Fig 2 4a-c Lateral bracing systems commonly used in the structural design o f seismic resistant buildings. (Lagorio, 1990, p. 35) 2 8 Building Cofiguration A building has to be flexible in order to be able to absorb energy but shift enough to resist excessive deformation. The ability to deform and come back to its original shape is the desired flexibility. The building should also be sufficiently rigid so as not to deform too much. The combinations of stiff and flexible elements in any building can create or mitigate seismic hazards. High variable behavior arising out of different architectural elements and forms can lead to extensive damage. This issue must be handled by architects in the design stage. The basic plan form can be a very critical factor in earthquakes. Regular configurations are good for seismic design and are also easy and cost effective to engineer. Irregular configurations make problems complex. Four common configuration problems are Soft stories, Discontinuous Shear Walls, Variations in Perimeter Strength and Stiffness and Re-entrant comers. The various forms and shapes of problem configuration are shown in the following chart from Christopher Arnold’s paper on configuration problems. The four basic types of problems listed are: Buildings with irregular configuration; Buildings with abrupt changes in lateral resistance; Buildings with abrupt changes in lateral stiffness andUnusual or novel structural features. In the last category Mr. Arnold has included fabric structures as having a problem configuration. This building form should not be included as fabric structures have very low mass and in calculation of horizontal forces wind forces govern over seismic forces. These forces are atleast 30 times greater than seismic forces. ‘IRREGULAR s t r u c t u r e s OR FRAMING SYSTEMS” (SEAOC) A- BULOMGS W T T H HRCGULAR CONFWURAJlON B. BULDMGS WITH ABRUPT CHANGES M LATERAL RESS1ANCE Intsrropban of bnm s C. BULDMGS WITH ABRUPT CHANGES 14 LATERAL STIFFNESS 4 A fan»t r ta n p t in A n •* i R s, ✓ r i C L UNUSUAL OR NOVEL STRUCTURAL FEATURES S ta tti an hrilridti Fig 2.5 Common configuration problems o f various types o f buildings (Arnold, Christopher 1982) 30 Time Period The time period of a building and the time period of an earthquake have great significance in seismic design. “When the natural period of vibration of any body ( or building system ) coincides with the natural period of rhythmic impulses applied by a dynamic force ( or earthquake ground shaking ) a synchronized resonance between the two results” (Lagorio, 1990, p.45). Successive cycles make the structure deform a greater amount till the point when the structure gets one final ‘push’ and collapses. This can happen without the maximum force of the earthquake having been felt. Earthquake time period The time period of an earthquake depends on factors such as depth of the origin and the soil conditions of the site and the magnitude of the earthquake. The distance of the earthquake may not always be a factor. The Mexico earthquake originated 200 miles from the maximum damage areas. The time period of that earthquake was 1.5 to 2 seconds and the resulting damage was primarily to tall structures with the same building period. Soil types can also greatly alter the time period of an earthquake. Soft or unconsolidated soils slow the period and cause slow continuos shaking. Such soil sites tend to also increase the duration of shaking causing the structures to resist the dynamic pulses for a longer time. This can be very detrimental to any building system. 31 Building Period All buildings have a natural period of vibration. Low rise buildings have a shorter period and high rise buildings have a longer period. Different structures have different modes of vibration. 0.08 sec 0.1 sec 0.5 sec 1-2 sec 7 sec Fig 2.6 Different Structures have a different period o f vibration The Period of a building can be altered, if the natural period of a site is known, by introducing a structural system to make it stiffer or more flexible. In the recent Kobe Earthquake even though a lot of tall buildings were built on reclaimed land, which tends to have a time period similar to tall buildings, there was little damage as the foundations had deep piles supported on solid ground thus altering the building response. Seismic risk maps presently do not take into account the localized site factor but the client and designer must take care to find out the site characteristics and take proper decisions when 32 choosing a structural system. Predictions of earthquakes are also striving to understand the periods of an earthquake. Knowledge of which can greatly help in seismic design. This thesis strives to put in light some aspects of this phenomena. Though soil periods are independant of building type the type of building on a site can affect the kind on damage experienced. UBC Philosophy Making a building totally damage proof in an earthquake is an ideal goal and difficult to achieve. Any moderate to large earthquake will cause some damage. Loss of life, though, is unacceptable. Building codes are written with the premise that the minimum standards recommended will protect life and reduce damage. The total collapse of any structure must be prevented in order to save lives. To do this the structure must be able to withstand periodic shaking an be able to absorb the energy of the seismic waves, no matter what the material of construction. With this in mind the UBC proposes a guideline, in Chapter 23. for seismic design. It takes into account all aspects of lateral loads in putting forward a logical approach to seismic design. The formula used to calculate the lateral load is written as: V = W ZIC/Rw V is the base shear or total equivalent static lateral force. 33 w is the total dead load of the building. Z is the zone factor developed from the probability map (fig 1.14). The zone factor depends on the seismic zone.(Table 23-1, UBC, 1991) I is the importance factor. It is 1 for all buildings except for schools , hospitals, police and fire stations for which it is 1.25. Rw is the structural systems factor. (Table23-0.UBC, 1991) C is the numerical co-efficient relating to the horizontal force factor, where C=1.25S/T0666.(max 2.75). Where S is the site/soil characteristic, T is the time period of the structure based on height. (T-(C,/h)7 5 ) and C,= After each major earthquake a review and update of the codes is always called for. There have been several major changes made in the building codes over time. In 1934, after the Long Beach earthquake of 1933, for the first time it was mandatory for all buildings to be designed to resist lateral loads. A 13 story height limit was introduced, which was removed in 1959. After the San Fernando earthquake standards were updated in 1974. In 1978 a further change was made taking into account ductility for concrete and masonry construction. The Northridge damage will result in further revisions of the 1991 UBC. The pros and cons of a building code and the performance of low rise residential buildings, that were built with the code guidelines, is one of the aspects of this thesis. 34 CHAPTER 3 35 DAMAGE ANALYSIS OF RESIDENTIAL BUILDINGS Base Data of Damaged Buildings The City of Los Angeles, department of Building and Safety has compiled an ‘Earthquake Damage Assessment File’ of all privately owned damaged buildings from the January 17th. Northridge earthquake. This file records all buildings that were inspected for damage by city inspectors. The information was brought back and compiled into a data file with other information about the buildings. The total number of buildings inspected for damage were approximately 140,000. The data file had information about these buildings in the following categories: a) Address— The location of the building with Street name, number and direction. b) Use— The building use, weather residential , commercial, institutional etc.. c) Number of Units— For residential buildings only. d) Occupancy— Number of people inhabiting the building. e) Tag— Type of posting, weather Red, Yellow or Green. f) Floors— Height of building in Number of floors. g) Construction— Type of construction as stipulated in the UBC, as type I to V or URM. h) Size— Size of the structure. j) Zipcode— The zipcode at the location, k) Year— The year of construction of the building. 36 Figure 3.1 shows a sample of the data for residential buildings as laid out in a table format using Mapinfo. 5TRE STR fcTKEETNAME m tlu s E b e |j n i P a m a I c o s t Po s t e d E l o I c o n s Eeze I o f c o I year 2801 AV 1 0 0 10000 KEEN 1 V BOX 42 rao it 12 3631 AV 1 0 0 15000 20 X 20 >0011 20 __0 24 28 6 26 25 0 13 21 __9 _ 21 16 27 6 14 24 24 31 2 2 26 26 24 12 26 68 68 6 29 62 59 0 67 67 67 70 25 25 25 39 8 21 26 77 1521 IV ST S O 100000 I0X 25 >0017 363S AV S O 100000 10 X 45 >0018 4907 W ST 100 120000 LfRM 16 X 26 >0016 3715 IV ST 1 0 0 20000 SOX 25 90018 4210 IV B8TH ST 8 0 200000 SOX 50 >0018 4716 V ST 90 1 0 0 0 0 0 15 X 30 >0016 1738 W E58TH ST 100 160000 I5X 30 >0062 1 8 0 1 IV BSTH ST 80 160000 (OX 40 >0062 1 1 1 1 IV 69TH P L 1 0 0 100000 SOX 25 >0037 1230 IV 19TH PL 100 80000 10 X 40 90037 1800 IV I1ST ST 90 8000 RED 10 X 20 >0062 1847 IV 11ST ST 90 8000 RED 15 X IS >0062 186 (7 T H PL R 8 0 30000 RED SOX 30 >0011 365 W S7TH PL 90 200000 RED SOX 25 >0037 1475 IV SI ST ST 85 100000 RED SOX 30 90062 229 S4TH ST 1 0 0 70000 RED MX 30 90011 729 56TH ST 100 250000 RED 18 X 33 90011 356 S4TH ST R 85 120000 RED SIX 30 90003 2016 S4TH ST R 90 20000 RED 10 X 20 >0047 134 S2ND PL 85 25000 RED 10 X 20 >0003 1743 y v (3RD ST 100 12000 RED 18 X 20 >0047 IPX 20 900*4 1218 W NTH PL 93 10000 RED 1 V 418 IV AVE 42 8 0 15000 RED 1 V 10 X 20 >0065 731 AVE 50 8 0 25000 RED 1 L T R M 15 X 20 >0042 7352 ALABAMA AV 90 10000 RED 15 X 20 >1303 13752 IALDERGROVE ST 90 200000 RED (OX 40 >1342 13802 ERGROVE ST 90 220000 RED (OX 40 >1342 1429 ANDRO ST 1 0 0 240000 RED SOX 40 >0026 2905 A V 1 0 0 240000 RED 15 X 35 >0016 3818 .TAMESA DR 1 0 0 300000 RED 10 X 35 >1604 7015 AV 85 136000 RED S O X 40 >1335 18100 CT 100 1200000 YELLOW 160 X 80 11701 iREW AV to t 150000 YELLOW SOX 50 >1344 11707 iREW Un d r e w av 80 120000 YELLOW (5 X 40 >1344 11711 I angi ARM AV 8 0 150000 YELLOW SOX 50 91344 2703 GELO DR 100 200000 RED (OX 50 >0077 18609 18609 4INTA [ a r m in t a ST 100 20000 RED ST 100 500 RED BOX 15 91335 IX 8 >1335 18609 A R M IN T A ARM1NT> ST 100 1000 RED 10 X 30 >1335 19156 ST 90 12000 CSREEN 10 X 20 >1335 5260 AUCKLAND S AVALON AV 80 120000 RED 1 V I S O X 45 91601 4109 B L 90 50000 RED BOX 20 >0011 3923 AVE DEL SOL 80 100000 RED (OX 50 >1604 9834 BABBITT AV R 90 180000 RED 2 V (OX 50 >1325 Fig.3.1 Random sample o f Base Data file o f damaged buildings in the Jan. 17th. Northridge Earthquake Source: Dept, o f Building and Safety, Los Angeles City. Nov. 1994. 37 All the categories were reviewed for relevance to the thesis study and several adjustments were made to the original data set. The following points were reviewed in the data set in order to get rid of inconsistencies for the analysis. a) Damage percentage— This percentage was defined as the ratio of the cost of repair of the damage incurred to the building cost. The cost of building was taken from the city assessors file and relates to the appreciated value of the original cost as per the city standard This does not match the true market value of the building The cost of damage was put down as assessed by the inspector This value is put down as the judgment of the inspector and could be subject to error. Since the process was the same for all records the data was taken to be uniform and comparisons could be made Records with 0% damage were ignored in the final data set since these buildings did not have any significance for the study. b) Construction— The construction type mentioned in the records was too general and did not give information on the actual building materials and structural system used For example a type V building could be wood frame construction or a mix of steel frame and shear wall construction. This prevented from analyzing each building system separately (one of the objectives of the thesis) and a more general analysis of construction type had to be done c) Year Built— There were many records that had the year built as ‘O'. This meant that the building was built in before or in 1900 or that the information was unavailable. This prompted the data set to be taken from 1901 onwards 38 d) Number of Units— The number of units, for residential buildings, gave the scope for dividing the buildings damaged into single family, 2-4 family units and 5+ family units. This division was consistent with the city assessors files on building stock in Los Angeles. These two sets of data gave a very important comparison which was very useful in analysis, the percentage of building damaged as a ratio of existing buildings. The data was constantly updated, by Karen Penera (department of Building and safety) under the supervision of Karl Deppe, with more buildings being added and corrections being made to remove inconsistencies. The data set used was updated as of November 1994. With all these changes a final data set of residential buildings was formed with the total number of buildings damaged as 48,000 approximately. 39 Base Data of Eiisting Building Stock The damaged building data set by itself can only give number and type of buildings damaged. In order to compare percentages it was necessary to get information on the existing building stock in the city of Los Angeles. The Los Angeles City Assessors office has a record of all existing buildings in the city Information was extracted from their files as to the number of existing buildings for the categories investigated. The categories most relevant to the research were 1) the year the buildings were built and 2) the building type as number of units. Table 3 .1 shows a listing of the number of existing buildings according to these categories. The total number of buildings built was maximum during the 1935 to 1974 phase and then slowed down in the 70’s and 80’s. We see that 5+ story buildings construction more or less remained constant This was significant in the study in comparing damage percentages for these two time ranges. 1 family units 2-4 family units 5+ family units 1901 to 1934 108,112 20,726 8,687 1935 to 1974 352,000 46,485 13,754 1975 to 1995 200,010 12,700 13,716 Table 3.1 Number o f existing residential buildings in Los Angeles City as in the file s o f the City Assessors data file s on December 1994. 40 The two categories shown in figure 2.7 above, are described as: 1) Category of Year Built— The research looked at three major time ranges based on major code changes affecting seismic design. The three time ranges being 1901 to 1934, 1935 to 1974 and 1974 to 1995. These corresponding code changes were in 1933 when the seismic code was introduced and in 1974 when major revisions were adopted. 2) Number of Units--The number of living units were divided into three categories. Single family units, 2-4 family units that were either single houses or semi-detached buildings and 5+ units that were mainly condominium complexes. If a building was demolished or destroyed and cleared away the record was removed from the city assessors file and remained only as a parcel of land. Many of the damaged buildings may have been removed from the data file that were tom down. This was more prominent with commercial and institutional buildings such as the Kaiser center. Residential buildings were more or less undisturbed and not demolished as private owners seem reluctant to spend the money to demolish heavily damaged buildings. Another source of data for existing building data was the LUPAMS file. This is the Land Use Planning and Maintenance software of the City of LA planning department This file though is not up to date as the funding is very low and they do not have the manpower to keep the information present. Earlier, outside agencies used to handle the data for the department. Older data, though was available and did match the information acquired from the assessors office. 41 Analysis of the Data Set for Residential Building! The Northridge Earthquake caused extensive damage to residential buildings in Los Angeles. The amount and type o f damage varied greatly depending on the location of the buildings. The city of Los Angeles* department of Building and Safety compiled a listing o f alt privately owned damaged buildings. The various categories of information in this data set are shown in figure 3.2. Using these categories a more detailed break up of the information was necessary for an in depth analysis of damage patterns. The first step was to extract all residential buildings from the data set. This gave a table of 48,885 residential buildings. A more detailed breakup was done and a matrix of relationships to study was formed. (Table 3.2) Y ear Built category was divided into three time zones. These were decided according to the code changes made over time concerning seismic design. These code changes had a major impact on the way buildings were built and therefor had a direct relevance to damage patterns. Percentage Damage blocks were taken as 25-50%, 51-75% and 76-100% in conjunction with the way the city divides its data for analysis. This would help the City in using and matching the information from this study. Additionally, it was observed that buildings damaged from 1-24% were mainly green tagged, not seeming to have structural damage, did not have too much relevance to the study. 42 Construction type was categorized according to the UBC, 1991. Type I to IV had very few low rise residential buildings. The major bulk were o f type V construction with a few Un-reinforced Masonry structures that had been retrofitted. Height of the buildings was an important classification. Low rise construction is limited to S stories and the break up was done as 1 floor, 2-4 floors and 5+ floors. Tag given to the building on inspection after the quake was either green, red or yellow. Red would mean major structural damage and green was safe to enter A majority of green tagged buildings were in the 1 -25% damage block. This was a little ambiguous as a 25% damaged building or a 100% damaged building could have a similar tag but very different type of damage. Year Built %Damage Construct. Height Tag Res. Type 1901-1934 1935-1974 1975-1994 25%-50% 51%-75% 76%-100% Type I-IV Type V URM’s 1 Floor 2-4 Floors 5+ Floors Green Yellow Red Single Fam. 2-4 Families 5+ Families Fig. 3.2 M atrix o f relationships under study fo r residential building damage in the Northridge Earthquake. 43 Residence type was one o f the most useful classifications of the data. These were divided into single family, 2-4 family and 5+ family type o f residences. This information shows the building type. A single family would generally be a detached 1-2 floor structure, whereas, a 5+ family type dwelling would probably be a condominium complex of 1-4 floors. This classification was also in tandem with the data in the city Assessors files. This helped a lot while making comparisons with the existing building stock in Los Angeles city Family type was used in most comparisons as the base category. The percentage value is the ratio of number of buildings in each block to the number of buildings damaged in each type. This was the first damage distribution done in the analysis. The data was obtained from the city of Los Angeles dept, o f Building and Safety. Damage % Construction I-IV V URM Height 1 2-4 5+ R Tag Y G Total Number 1 to 10% no. 44,144 % 11 - 20% no. 56 2265 26 1323 1045 3 155 838 1399 2,392 % 4 94 1 55 44 .1 7 35 58 21 - 30% no 7 672 14 302 395 - 153 346 201 700 % 1 96 3 43 56 - 22 49 29 31 - 40% no. 6 303 12 136 185 - 123 149 50 322 % 2 95 4 42 58 - 38 46 16 41 -50% no. 17 298 10 151 173 - 159 141 27 327 % 5 91 3 46 53 - 49 43 8 51 - 60% no. 3 91 6 44 55 - 60 33 7 100 % 3 91 6 44 55 - 60 33 7 61 -70% no. 1 58 1 22 37 1 38 19 3 60 % 2 96 2 37 62 1 63 32 5 71 - 80% no. 3 153 10 65 101 - 118 42 6 166 % 2 92 6 39 61 - 71 25 4 81 - 90% no. 1 89 2 59 33 - 79 10 3 92 % 1 97 2 64 36 - 86 11 3 91 - 100% no. 5 203 11 124 94 - 195 11 13 219 % 2 93 5 57 43 - 89 5 6 Table 3.2 D istribution o f damaged buildings in %damage blocks. 44 A graph of the data in the table above shows that the number of green tagged buildings were mainly distributed in the 1 to 20% blocks and the red tagged buildings were distributed from 20% and above. This shows that the lower damaged buildings were not seriously damaged and probably had only superficial or cosmetic damage. This was a factor in removing this block of data from the analysis. The remainder from 25% to 100% damaged buildings numbered approximately 5000 80 o o n .c C D 0) _c TJ C D CD CD C D I. 60 - 50 - 40 - 30 - 20- 10 - 0-f 10 20 30 40 50 60 70 80 90 100 Damage % Blocks Fig 3.3 Distribution o f damage % and the tag type. 45 Comparisons and Initial Inferences from Graphs From the matrix of relationships being investigated, the dwelling type was taken as the common factor under which all further relationships were examined. With this redistribution of categories data matching was carried out by graphing and mapping. Definite patterns of damage were observed and preliminary inferences were made from these. The base data of the number of buildings was reorganized and matched with the number of buildings damaged. Figure 3.3 shows a table of the number of buildings built and damaged in each time zone. What is interesting here is the additional information of the number of total dwelling units that were damaged. This gives a clearer picture of the impact of the damage on housing. Over time we see that Los Angeles has had a slowing down of building activity. This could be due to lack of open land leading to the construction of larger condominium complexes instead of single family homes. This trend seems to have affected the kind of damage incurred. Number of Buildings Built Number of Buildings Damaged Total Number of Dwelling Units Damaged Single Family Units 520,274 1,332 1,332 2 to 4 Dwelling Units 70,422 211 624 5 and more Dwelling Units 47,425 463 12,184 Table 3 3 Table showing the number o f buildings built compared to the number o f buildings damaged and the total number o f dwelling units damaged. 46 Damage Percentage Blocks and Dwelling Type A Damage% block breakup of 25%— 50%, 51%— 75% and 76%— 100% was done with the dwelling types. The maximum number of buildings damaged were between 25% to 50%. The maximum number of buildings damaged were single family units with 5+ dwelling type second most. The ratio of number of buildings damaged to the total number of existing buildings gives a true picture of the impact of the damage. This value is called percentage of buildings damaged and will be referred to thus for this report.. Table 3 4 gives the numbers in percentage of buildings damaged of each dwelling type. 25% to 50% damage block 51% to 75% damage block 76% to 100% damage block single family unit 0.18% 0.21% 0.67% 2— 4 family units 0.03% 0.03% 0.12% 5+ family units 0.05% 0.06% 0.19% Table 3 .4 Percentage o f buildings damaged o f each dwelling type. Figures 3.4 a and b, on the next page, show clearly the damage pattern in three dimensional graphs. The true picture is seen when we start taking the percentages of buildings damaged We now see that in actual fact 5+ dwelling types have the greatest damage. This was significant in that larger buildings seem to have had greater damage overall. 47 Number of Buildings Damaged single fwiKfy 2-4 familias DwaMing Typa 5 fam ilies* Percentage of Total Existing Buildings & 3 •O / 5(>\\\\\\\ •Ingto family 2-4 lamily Dwelling Type S fam ily+ Figure 3.4 a and b Damage in number o f buildings and as a percentage o f total existing buildings. Classification in percentage blocks. 48 Year of Construction and Dwelling Type The next step was to see when these buildings were built. The year of construction was of significance since the damage pattern would give a good understanding of the effectiveness o f the building technology in each time zone. As mentioned earlier, the time zones were divided according to building code changes. A direction for investigation can be inferred from the pattern of damage seen in this matching. Table 3.5 shows the values for the number o f buildings built and the number of buildings damaged that were built in these time zones. Table 3 .6 shows the values of percentage of buildings damaged in each time zone. These values show a clear damage pattern when we look at the three dimensional graphs generated. 1901 to 1934 1935 to 1974 1975 to 1994 Built Damaged Built Damaged Built Damaged single family unit 108,122 174 20,736 18 8,687 94 2— 4 units 352,000 682 46,485 34 13,754 249 5+ units 200,010 476 12,700 159 13,716 120 Table 3.5 Number o f buildings built and the number o f buildings dam aged in each time zone as defined by building cock changes. 1901 to 1934 1935 to 1974 1975 to 1994 single family unit 0.074 0.136 0.028 2— 4 units 0.179 0 051 0.023 5+ units 0.124 0.531 0.192 Table 3 6 A percentage o f buildings damaged in each time zone. The graphs in figures 3.5 a and b show the disturbing trend that buildings built after 1974 have the greatest damage. Also the greatest damage was incurred by larger buildings of 5+ units. These two factors taken together give a direction to analyze the cause of the 49 damage. The fact was that the earthquake had its epicenter in the San Fernando valley where most o f the buildings built were relatively new construction. The odd thing is that the valley has mostly single family houses but more condominium type buildings with 5+ units were damaged. The down side is that these buildings were supposed to have stricter design codes that do not seem to have worked very well. The reason for that, as discovered from inspection of various engineers and inspectors, was poor quality of construction. An independent study done has proven that this has been a major problem in the construction of type V buildings.(Schierle,1993) In addition to this fact it was larger buildings that seem to have been harder hit. it is possible that the present codes do not adequately take into account all the complexities of a building of this scale. The mass, configuration and construction type may not be adequately accounted for in the design process. This point is discussed in greater detail in the proposals generated out of this study. 50 Number of Damaged Buildings 1001-1834 S+ family Num ber of Existing Buildings 1901-1004 aioglatenily 2-*tarrWy 5+ ta n w ty Ha »id anr » Typa Percentage of Existing Buildings Raatdanca Typa Fig. 3.5 a and b Graphs showing number and percentage o f buildings dam aged by year built. Construction Type The predominant construction for residential buildings is type V. It is difficult to draw from the data what exact structural system was used for an individual building. A tour of the sites showed, though, that a majority of the buildings damaged were wood framed type V construction. There were almost no type I to type IV since a low rise building in these construction types is extremely rare. Un-reinforced masonry buildings built earlier in the century and later retrofitted had some damage This was mainly in the older Hollywood area of Los Angeles. The kind of damage in each type of construction also varied. URM buildings had a lot of comers of buildings shattered as well as external skins of masonry falling down. Masonry chimneys falling down was very common. Type V buildings had damage of a very different nature. Smaller single family houses had the structure slip off their foundations and in some cases failure due to inadequate shear walls. Larger complexes had more serious damage in the failure of sofi stories. Parking garages, being very common in such complexes, did not have their support systems designed adequately for lateral loads A major cause for such damage was also due lack of construction quality control. It was noticed in some complexes that plywood used was not upto standards specified or that nail spacing used was not adhered to during construction (Schier)e,1993) A larger issue of construction by developers and supervision by the designers comes into the picture at this point. These are discussed in chapter 4. Figure 3.6 a and b shows the number and percentage of damaged buildings of each construction type. 52 Number of Buildings Damaged Dwelkng Typa Percentage of Total Existing Buildings wngt* family 2-4 familiea 5 fafniliea+ Dwafling Typa Fig. 3.6 a and b Number o f buildings damaged o f each construction type and the percentage o f total existing buildings. We see that as a percentage the larger dwelling types had the maximum damage and the other types had similar damage. This could be a factor due to type o f building system and configuration o f the various dwelling types. 53 Building Tag The seriousness of building damage can also be gauged from the type of tag that was awarded. A red tag would mean serious damage, probably structural, whereas a yellow or a green would mean lesser degree of damage. In the graphs in figures 3 .7 a and b the maximum number of buildings were red tagged (over 25% damaged only) for all categories. When compared to the percentage of existing buildings damaged the 5+ dwelling type had the greatest percentage of buildings red tagged This is a very serious trend and not desirable in any situation. It is impossible to have absolutely no damage in any earthquake, but the damage should not be very serious. In the case of the damage experienced in the Northridge earthquake the amount of serious damage was more than the amount of less serious damage. This fact questions the state of the existing building technology. On the strength of these graphs it can be said that certain building systems should be investigated to see the exact nature of the failures and modifications made accordingly. single family 2-4 families 5+ families Type I-IV 11/0.002% 2 / .003% 28 / 0.059% Type V 1287/0.25% 197/0.28% 407/0.86% U.R.M’j. 27 / 0.005% 12/0.02% 25/0.05 Table 3 .7 Number o f buildings damaged in each construction type, fo r all buildings damaged over 25%. single family 2-4 families 5+ families Red tagged 552/0.106% 112/0.159% 234 / 0.493% Yellow tagged 510/0.098% 76/0.11% 187/0.394% Green tagged 263/0.051% 23 10.033% 38 / 0.08% Table 3 .8 Number o f buildings damaged based on type o f tag given, fo r all buildings damaged over 25%. 54 Number of 0 im aged Buildings •ingtt (amity 2-4 famtty OweKngTyp* Percentage of Total Existing Buildings u n g i * ( a m i t y 2-4 family Dwelling Typ* Stamiliaa+ Fig. 3.7 a and b Graphs showing the number and percentage o f total existing buildings damaged according to the tag given by inspectors. 55 Height of Buildings In figures 3.8 a,b and c we see a further damage pattern based on tagged buildings. The data is further broken up into height of the buildings by number of floors. It is seen that one and two floor buildings were damaged more. Two to four storied buildings are damaged more than any other floor height. Of these two floor buildings were damaged the maximum in all types of buildings. One of the reasons could be that the time period experienced in the major damage areas (0 1-0 2 secs). The larger buildings of the same height had the maximum damage meaning that the mass and configuration of these buildings had a factor in play. The construction quality was probably a major factor single family 2-4 families 5+ families red tagged 1 FI. 344 29 2 2 FI. 200 78 121 3 FI. 6 5 89 4 FI. 19 5 FI. 3 yellow tagged 1 FI. 277 19 2 2 FI. 224 55 123 3 FI. 8 2 51 4 FI. 8 5 FI. 2 green tagged I FI. 176 2 FI. 81 22 17 3 FI. 3 1 19 4 FI. 2 5 FI. Table 3 9 Table o f number o f buildings and type o f tag given after the earthquake by inspectors. 56 Damage in Haight of Floor* Raaidantial Building* tangle twraty tX vaM m g Type Damage as % of Total Existing buildings R esidential B uidings /I Dwelling Type Fig. 3.8a and b Number and percentage o f total existing buildings damaged as ised categorised by the number o f stories. 57 Singla Family Raaidancaa R a d T a g g e d Yallaw T a g g e d Q iaen T a g g e d B u d d in g T a g 1 F lo o r B u d d m g e 2 F lo o r B u d d in g * £ — / 3 F lo o r B u ild in g * 2 to 4 Family R asidancas B u ild in g T a g 5-f Family R aaid an cas Buildings B u ild in g T a g Fig. 3.9 a,b and c Number o f buildings damaged in each category o f building type, distributed by height and type o f tag given by inspectors. Maximum number o f buildings were red tagged fo r all heights and dwelling types. This is not a desirable trend 58 Data Comparison and Initial Inferences from M aw Mapping of data helps greatly in seeing damage patterns and relating them to the geological and seismic data. Facts that are seen through graphs are confirmed and further inferences are made via mapping. Various maps were done in the analysis using Mapinfo software. A base map was prepared with the soil types and the seismic data from the Northridge earthquake was added to it. This is shown in Figure 1.28 in chapter 1 The data of the building damage was overlaid on this base map to enable comparison of various factors. Categorization of the data was the same as used in the graphs. The dwelling types were further divided into damage percentage blocks and mapped. This was done to see seriousness of damage o f the various building types. Liquefaction shown in the base map is not shown in further maps since the region defined only shows potential liquefaction areas, and not where liquefaction actually occurred. In the Northridge earthquake liquefaction was not a factor. The area where it did occur there was no building damage. The presence o f potential liquefaction areas does give an indication of certain soil t ypes though. The presence of high clay content soils and loose sand are common in these areas and may have played a part in extending the time of ground shaking resulting in greater damage. The exact behaviour of htese soil types needs to be investigated. 59 The data of damaged buildings mapped is from the City of Los Angeles only. Though there was damage in other areas, such as Santa Monica, these areas are not part of the study. M ap 1 All Buildings Damaged above 25%. The map shows definite patterns of damage across the city There are concentrations of damage in some areas which need to be explained It is expected that will be damage around the epicenter, but in addition there are three pockets of damage at varying distances from the epicenter. The damage around Studio City, Hollywood Blvd and Adams Blvd. point to the fact that other factors may have played a part in causing damage. This distribution was studied in greater detail by individually mapping the three dwelling types. 60 # Pacific O cean i * i A tingle family 76-100% damage A tingle family 51-75% damage A single family 25-50% damage # 2-4 family 76-100% damage # 2-4 family 51-75% damage O 2-4 family 25-50% damage ■ 5+ families 76-100% damage B 5+ families 51-75% damage D 5+ families 25-50% damage Map 1 Graphical representation o f all buildings damaged over 25 % in the Northridge Earthquake. The division is by dwelling type to match the data in die graphs. 61 Map 2 ,3 and 4 Damage Distribution of Different Dwelling Types. The data was divided to see the location of the various building types. Map 2 Single Family Dwellings: There were a maximum number of single family units damaged compared to other dwelling types. The damage was spread fairly evenly in all the regions described earlier. The interesting situation is that there was a lot of damage in the Santa Monica mountains as well as the Hollywood hills region The soil in both these regions is rock, resulting in the time period in the areas being very short and similar to that of one story structures. It is also common for buildings in these areas to have stilts and other unusual configurations. These factors combined together may have resulted in most of the damage. The area South of the Epicenter had greater vertical displacement than horizontal displacement. This vertical movement of the ground caused many houses, that did not have adequate hold downs, to shift off their foundations. Shear wall failure and masonry chimneys was also a common occurrence. In a few areas further north, such as Simi Valley, there was soil subsidence due to development on unstable farmland In looking at the year of construction of the damaged buildings it is seen that most of the serious damage was to structures built before 1974. Newer structures did not seem to have very serious damage except in the rock areas. This brings us to conclude that the design and construction of the wood framed houses built after the code changes in 1975 was reasonably good for single family houses. Map 3 2 to 4 Family dwellings: Almost no damage of this building type around the epicenter. Two major pockets of damage were around Hollywood Blvd. and Adams Blvd. along with some damage around the Studio city area. The investigation showed that most of the buildings built in the Hollywood area were Un-reinforced masonry buildings that 62 had comer shattering. These were built pre 1934 as were the buildings damaged around Adams Blvd.. The damage near the river in studio city consisted of newer buildings built after 1934. These were mostly type V construction and 2-3 stories in height. The height of buildings in this region affected the damage a lot probably due to the time period of the soil matching that of the 2 storied buildings. The data of the time period at this location is unavailable but based on the damage pattern we can say it was around 0.2 to 0.3 seconds. Map 4 5+ Dwelling Type: The inferences from the graphs show that these type of buildings had the most serious damage. The damage concentrations were almost the same as in the other categories The damage around studio city as well as Hollywood areas was concentrated in a region just below the mountains. Soil which had soft sedimentary deposits not properly consolidated. Also this kind of interface between two soil types seems to have been the cause of multiple rippling of the shock waves creating greater displacements. Most of the buildings in these regions were 2-3 stories high. Questions about configuration and construction quality have already been voiced earlier. The intensity of damage of these structures and a search for answers gave reason to further break up the distribution into year of construction. 63 s t V * . ,V ^k ' p Pacific O cean ■ R ock [ 1 Pliestocane alluvial/ marina deposits U A single family 76-100% damage A single family 51-75% damage A single family 25-50% damage Map 2 A ll single fa m ily residences damaged in the Northridge earthquake. The damage was fa irly widespread but different regions had different causes fo r the damage. 64 A Pacific O cean l i i Halocene sedim ents | | Ptiestocene alluvial/ marine deposits # 2-4 family 76-100% damage # 2-4 family 51-75% damage O 2-4 family 25-50% damage Map 3 Damage o f a ll 2 to 4 fa m ily dwellings. This type o f buildings is not very common these days and therefore had little damage. O f the buildings that were damaged the buildings were mostly in the older regions o f the city and mainly 2 to 3 floors in height. 65 % * I I 1 c * y ■ e w id e rte s i n n °__* /Ts m u m \Jy * Epicenter ■ R «= k [~ ~ ~ | Halocene sedim ents | | Ptiestocene alluvial/ m arine deposits ■ 5+ families 76-100% damage £! 5+ families 51-75% damage G 5+ families 25-50% damage Map 4 A ll 5+ Dwellings type buildings damaged in the Northhdge Earthquake. 66 Map 5, 6 and 7 5+ Dwelling Type and Year o f Construction Investigating the damage according to year of construction we find an interesting trend. A smaller number of pre 1934 buildings were damaged These were concentrated around the Hollywood region. The construction o f ‘34--’74 was around the LA. river as was the case with the post ‘75 damaged buildings. Earlier in the graphs we saw the % o f total existing buildings damaged was greatest for post 1975 construction. In certain cases the site may have played a major part in causing damage but a more even distribution of these buildings built after 1975 points to other reasons as well It is an obvious conclusion that the building standards set forth for these structures are not performing as well as they are supposed to. 16 People were killed in a building of this type. This is not acceptable and a review o f the quality control and standards needs to be carried out 67 A € Pacific O cean - 1 B o u n t f i n w 5 _ _ ? (H K M il* * X j-7 * Epicenter ■ Rock I I Hatocenc sediments | | PUestocene alluvial/ marina deposits ■ 5+ families 76-100% damage 6 5+ fam ies 51-75% damage □ 5+ families 25-50% damage Map 5 Building damage o f 5+ Dwelling type. A ll buildings were constructed before the 1934 code change that brought seismic design guidelines fo r the first time. 68 % % V Pacific O cean . . i i i nut ****** CHy • o u n d v iH 5___? £ t \ m um xjy * Epicenter ^ Rock I I Halocene sediments [~] Pliestocene alluvial/ m arine deposits ■ 5+ families 76-100% damage 19 5+ families 51-75% damage E D 5* famies 25-50% damage Map 6 Damage to 5+ Dwelling type buildings built between 1935 and 1974. M ost o f these buildings were 2 to 3 flo o rs high. 69 X O, v Pacific O cean «» » _ L M * t City B o u n ttr tt* n u \ m © Epicenter ■ Rock F I Halocene sedim ents | | Pliestocene alluvial/ marine deposits ■ 5+ families 76-100% damage Si 5+ families 51-75% damage ID 5+ families 25-50% damage Map 7 Damage to 5+ Dwelling type buildings built after 1975. Soil type, height o f the buildings, configuration problems as well as construction quality seem to have a played a part in the damage patterns. 70 CHAPTER 4 FINDINGS, c o n c l u s i o n s a n d p r o p o s a l s Findings The damaged building data analysis gave an insight into damage patterns of buildings in the Northridge earthquake. The major findings of the study are: a) Type of Dwellings Damaged: A very large number of single family units were damaged, as compared to 2 to 4 and 5+ dwelling types When these numbers are converted to the percentage of total existing buildings of each type, the greatest damage is seen in the 5+ dwelling type. The numbers are of the existing building stock in LA city. The number o f housing units damaged was also the maximum in the 5+ dwelling type (Table 3 .3, chapter 3). Even though the area around the epicenter consists o f mainly single family units the damage to the housing stock of LA was greater due to damage to larger buildings away from the epicenter. Large 5+ dwelling type buildings accounted for almost 10 times greater damage to living units. The seriousness of this fact must be addressed in future development. b) Magnitude and Type of Damage: There were a lot more buildings damaged from 1 to 24%. Most of these buildings were green tagged and had only superficial damage. The analysis of data of buildings damaged above 25% revealed that an increasing number of buildings were red tagged as the percentage of damage increased (Fig 3 .3) This shows that the kind of damage experienced in these structures was structural failure. It is impossible to prevent damage in any earthquake but the kind of damage can be restricted to less serious non-structural damage. The trend should be that red and yellow tagged 72 buildings reduce instead o f the dramatic increase we see here. If this can be achieved by good design and construction the efforts o f the designers will have been rewarded. c) Construction Type: A majority of residential buildings in LA city are o f Type V construction. Most o f these are wood framed buildings It was thus natural to see that this kind of construction had the greatest damaged. Within this construction type we see a further breakup of the kind o f buildings damaged. Mostly 2 to 3 storied buildings were damaged. The masonry buildings that were damaged were also of similar height This was explored further to see the probable causes for the pattern. d) Height of Buildings: The maximum damage was to 2 to 3 floor structures. The numbers showed maximum damage to larger buildings of the same height than smaller ones. The maximum damage occurring to 5+ dwelling types. Damage was also common in 2 to 3 floor masonry construction. The location and the type of seismic waves may have been a factor in this trend e) Year of Construction: Construction activity on Los Angeles has slowed down over the past 20 years. In the post war period there was a lot of development. In this time the seismic codes did exist but were not all that stringent. The codes were upgraded after the San Fernando earthquake. The surprise was that despite the up-gradation in seismic codes there was more damage to buildings constructed after 197S than to buildings built earlier, found when comparing the percentage o f damage between buildings constructed and buildings damaged in each time range. This trend is absolutely unacceptable by any standard. Even if we take all three time zones we see the trend is the same. Construction quality for seismic design has been going down even as seismic codes have been getting tougher. So is the answer in stricter 73 codes or something else? It was observed at various sites that serious negligence of quality in construction material and building supervision had been the cause for shear wall and other failures. 3.5 2.5 C I 1 1 1 0.5 * 3 1935-1974 1975-1994 1901-1934 Year Built -» single family — 2-4 families — 5 + families Fig 4 1 Trend o f increasing damage in the three time zones. The construction quality was getting worse while seismic codes were getting more strict. f) Location: A lot of the serious damage to larger buildings was concentrated in areas of un-consolidated sedimentary soil The area around studio city along the LA river had a lot of damage. The time period of the site,coinciding with the time period of the buildings may have played a part in the damage patterns. The mapping of the data shows this definite trend. Single floor structure damage was spread around the epicenter where vertical displacements were high and caused foundation failure in addition to chimneys and boundary walls collapsing. There was also single floor failure around the rock areas where the time period of the quake was shorter and the building configuration are generally odd due to the sloping sites. 74 The combination of all these factors leads us to believe that the damage was not caused by any one factor. The action of the earthquake was complex and multidimensional and the interaction of the type of site and type of building had a lot to do with the damage patterns. The study helped unravel some of the questions and the conclusion arrived at are discussed next. 75 Conclusions Los Angeles has a history of seismic activity and has been making code changes over time to mitigate building damage caused by earthquakes A major change was made in 1974 after the San Fernando earthquake. Building construction in Los Angeles over the past century has been very active and the valley area near the epicenter is a relatively recent development The codes did have some mitigating affect on damage but failed in many areas too. This does not necessarily mean that the codes are bad but that seismic activity is much more complex than generally realized. The codes worked well for single family structures but performed very poorly for large condominium complexes. Building designers have to be aware that codes are only a minimum standard set down from past experience. 1 Construction Supervision: As stringent and complex the codes may get the final response of the building to lateral loads will only be as good as the quality of construction. The actual construction of the building has many areas that are not taken into account Presently, the supervision of the construction is not perfect as also the checking of quality of construction. A well designed building constructedpoorly defeats the purpose of any standards laid down. Large complexes that suffered serious damage were generally built by developers who are more concerned with profits and tend to only cater to minimum standards for inspection . In a litigious environment as we have in LA the designer will not take responsibility for inspection to see if his/her designs have been carried out properly. This is one of the main reasons that single family houses perform better since the owners have a greater interest in the construction process. Developers however take quality as more o f a code rquirement than a safety issue. This is not to say that private houses are safe. The level of awareness of the individual to matters of seismic safety has a lot to do with the final outcome. 2. Construction Quality: A more serious problem identified at a few sites was the quality o f materials and construction. Poor design that barely meets standards does not in any way help the building resist lateral loads. Missing anchor bolts, soft stories, discontinious shear walls and other such design flaws commonly lead to building collapse, as seen in the Northridge meadows apartment complex where 16 people died. The overlooking of these basic requirements in design increase the vulnerability of the building. 3. Configuration Issues: Issues of configuration are rarely discussed in the design of the buildings and the awareness of this factor seems to be very low. Sofi story collapses were common in the Northridge earthquake. This was probably due to the ignorance of the designers of such issues or the neglect of the issue as peripheral and unimportant. There has been research done on this issue but the awareness level among designers is low. The case needs to be stressed urgently as a matter to be looked into by all designers. 4. Site and Soil: The location of the site and the soil characteristics are rarely looked into. The soil factor in the code does not seem to be an adequate design factor. The local site 77 factor can vary greatly. A simple design solution of compacting the soil before construction or using piles for larger buildings can solve a lot o f forseen problems due to soft and unstable soils. A few extra dollars early on can prevent damage later Studies are being done to relate various earthquakes to see if the prediction of seismic activity at any particular site can be done. This will give a more detailed site factor. It is not necessary that a building designed for site A will perform well in site B. The time period of an earthquake may not depend on the distance of the earthquake but more on the geological condition at the site. In some sites due to the presence of rock faces abutting soft soils the waves actually bounced back and forth amplifying in a ripple effect. 5. Effectiveness o f Codes: Building practice in Los Angeles had a lot to do with the kind of damage experienced. It is shocking to see that the buildings built after each code change had greater damage than buildings built before. This factor needs to be addressed urgently if we are to mitigate further damage. The building industry, builders, designers and owners have to be more aware of good building practice in seismic regions. Proposals need to be put forward to see this carried out. The cause for mostly 5+ dwelling type buildings being damaged has to addressed and changes made to see this does not happen in the future. 78 Proposals This thesis puts forward proposals for mitigating seismic damage as seen in the Northridge earthquake. The proposals are arrived at from observation o f damage patterns seen in the statistical analysis. Structural System for Large Lowrise Residential Buildings: This factor has by far the greatest impact on the amount of damage experienced in the Northridge earthquake. It was seen that wood framed buildings of the 5+ dwelling type had serious quality problems. It is either that this construction does not perform too well in seismic condition or that that the buildings were just built badly. Either way the buildings did not perform If the performance o f these structures cannot be assured by the present set of inspection and building processes then it is not fair to the people living in these structures to have to live in sub*standard buildings, exposing them to seismic hazards and possible loss of life. Wood framed construction should be limited to smaller buildings where good supervision can be ensured. With the amount of detail required and the possibility o f human error in construction , quality control can be a problem for type V buildings Concrete moment resisting frames or steel moment resisting frames can probably ensure better quality control for larger buildings as the process is better supervised and the quality is better However steel construction is also under close scrutiny due to the failure of welded joints The code could also be changed to accommodate the possibility of errors in construction. This can be done by raising the minimum standards to a higher level of competency. The 79 contractors and construction laborers could be tested for a minimum level o f knowledge required for a particular type of construction. This can go a long way in ensuring good seismically safe construction. Site Factor: A more careful localized site study should be done before starting any construction and a structural system should be designed by the engineer to mitigate any damage that might be caused by local amplification due to the site The issues relating to site have been discussed in this thesis. All these should be taken into account before arriving at a structural system. The clients should insist on the designer to cater for specific site conditions. Development Planning and development of Los Angeles should incorporate seismic risk and development should be limited to areas where seismic risk is minimal Site factors, height limits, building density and other such issues should be taken into account Proper planning will reduce the effect of serious damage to the housing stock and displaced people as well as loss to life and property. Awareness: The largest push by professionals in the field needs to be to increase awareness of all the above mentioned factors. Everyone involved in the building process nedds to aware of the shortcomings of the present system and must strive to overcome them. This will go long way in correcting mistakes made earlier and lead to safer cities in th efuture. 80 Further Research and Other Proposals The findings of this thesis are based on statistical analysis of the damage in the Northridge earthquake. The major finding was that large condominium complexes with wood framed construction performed very badly. There were numerous factors that made this happen. The performance of this building type needs to be tested via models and simulation for the exact performance. Research in the development of safe building systems for this type of construction needs to be done. This will lead to further the knowledge of the performance of building systems in various conditions. The proposal of examining the code requirements for certain building types needs to be addressed. This needs to be done via publishing the findings of this thesis and other such research. Legislation is needed to carry out some of these proposals. The Seismic Safety Commission in its preliminary report to the Governors office of California also has proposed this. The Commission has also proposed that the state building code be amended to assign more responsibility to the building designers for the seismic safety of buildings (LA Times, Feb.4th. 1995). The other recommendations of the commission take into account retrofitting old buildings and also limiting construction near active faults This is in tandem with this thesis’ proposal to consider local site factors more carefully and have planning development take these into account. Another proposal put forward by Prof. James Ambrose (USC School of Architecture) is that the insurance industry play a greater role in seismic safety. The building standard 81 adopted by a builder for seismic safety should be rewarded by the insurer by lower insurance rates. This will ensure that buildings get better inspection and better seismic design building performance. The main issue discussed here is the quality o f construction. This has been a major cause for extensive damage and major upheaval is needed to see that is not done in the future. The study has hopefully brought out some of the chinks in the armor. Awareness is a big step forward in mitigating seismic damage. 82 APPENDIX A 83 AREA 3 . AREA 2 ■■ i o < '* > • L O S A N G E L E S Jftwtww C O O Fig A. 1 CSMIP strong motion stations in the vicinity o f Los Angeles City. Stations are identified by a three digit code cross referenced to station names in Table A. 1 . (CSMIP,No.87-05) 84 Cod* S t a t i o n H i m Cfid* S t a t i o n Naae 025 P « la S p rin g s - A ir p o rt 299 P a la S p rin g s - D e s e rt H o s p ita l 0 3 ! Lockwood V a lle y - P lu s Ranoh 303 L os A n g e la s - H ollyw ood S to ra g e B ld g . FT 0*7 V ascuez Rocks P ark 305 Leona V a lle y #1 051 Cuddy l'»l la y - Tubb* Ranch 306 Leona V a lle y #2 055 Leon* V e lle y #5 - R i t t e r Ranch 307 Leona V a lle y A 3 07* T am o - F ir* S t a t i o n 308 Leona V a lle y A* 082 Saw m ill M ountain - C a lta c h S a t s a le S t a . 309 Leona V a lle y A 6 061 Boron 310 A n te lo p e B u tte s 08* H t. A ble - K ern Co. Highway M a io t. S t* . 311 Long Beach - C3ULB E n g in e e rin g B ld g . 1 087 A rle t* - N o rd h o ff Av* F ir * S t a t i o n 312 R iv e r s id e - R iv e r s id e Co. Admin. B ld g . 088 FacoUaa - K agel Canyon 313 Lake Mathews - Main Dam 092 Roaanond - A ir p o rt 321 H e s p e ria 093 M ojave - LAEW P S to ra g e Sh*d 322 Sherm an O aks - U nion Bank B ldg. ..108 W heeler R ldga - T a jo n H i l l s 011 F ie ld 323 Long Beach - H arb o r A daio. B ldg. 122 F e a th ir r y — Park- - P ark M a ln t. B id*. 326 Lake Mathews - O lka 1 123 R iv e r s id e - A ir p o r t 328 P u d d ia g s to n * R e s e r v o ir - P u d d ln g sto c a Daa 1*7 P o in t Mugu - N aval A ir S t a t i o n 329 I r v in * - UCI E n g in e e rin g B ldg. 1*8 P o in t Mugu - L aguna Peak 330 P a lo a a r M ountain - P a lo a a r O b s e rv a to ry 1*9 D e s e rt Hot S p r in g s - P la rs o o B lvd F i r * S ta . 331 H eaet - S t e t s o n Av* F ir * S ta tio n 157 Los A ngelas - B ald w in H i l l s , 332 Los A n g eles - C e o tu ry C ity B u llo c k S to r e 159 San Padro • 2 5 th S t . F ir * S t a t i o n 333 O c e a n sid e B 16a N avport Baach - I r v i n * Av* F i r * 'S t a t i o n 353 V en tu ra - H a ll o f J u s t i c e 165 S a n ta C a ta lin a I s l a n d - A ir p o rt 368 Downey * C ounty M a ia t. B ld g . 168 F u e rta La C ru z - U3F3 S to ra g e B ld g . 370 B urbank - C a l i f o r n i a F e d . S a v in g s B ld g . 172 T e a e c u la - CDF F ir * S t a t i o n 385 B urbank - P a c i f i c Manor 196 Inglew ood - U nion O il Y ard 386 Van Nuys - H o lid a y In n 197 H u n tin g to n B each - L aka S t . F ir * S t a t i o n 389 C e n tu ry C ity - LACC N o rth 198 M u rrie ta Hot S p rin g s - C o llin s Ranch 390 C e n tu ry C ity - LACC S o u th 199 W in c h e ste r * Bergman Ranch 395 Long Beach - H arb o r A d a in . B ld g . FF 200 W in c h e s te r - H idden V a lle y F a re s 396 K a llb u - P o io t D io* S c h o o l 201 W in c h e s te r - Fag* B ro s . Ranch 399 H t. W ilso n - C a lte c h S e in a ic S t a t i o n 202 San J a c i n t o - V a lle y C e a e te ry *00 Los A n g e le s - 0 O regon P ark 204 San J a c i n t o - Soboba *01 San M arino - S o u th w e ste rn A cadeay 206 S i l e n t V a lle y - P oppet F l a t 402 A lta d e n e - E ato n Canyon P ark 207 P a c o lo a Dais *03 Los A ngela* - l l 6 t h S t . S chool 210 C ogsw ell R e s e r v o ir - C o g sw ell Daa 404 Rancho P a lo s V erdes - 308*0 H aw thorne B lv d . 231 Los A n g eles - UCLA H a th -S c le n c * B ld g . *05 R o llin g K i l l s E s t a t e s - Rancho V is ta S ch o o l 232 P a la d a l* - H o lid a y Inn *06 Los A n g eles - V in c e n t T hoaaa B rid g e 236 Los A n geles - H ollyw ood S to ra g e B ld g . *36 T a rz a n a - C edar H i l l N u rse ry 237 Mojave - Oak C reek Canyon *61 A lh a a b ra - Frem ont S c h o o l 2*1 Long Beach - R e c re a tio n P ark 663 Los A n g e le s - S e a r s W arehouse 2*2 Long Beach - Rancho L os C e r r ito * <64 N o rth H ollyw ood - S b e r s to n - U n lv e r s a l H o te l 2*7 Big D a lto n R e s e r v o ir - B ig O a lto o Daa *66 E tlw a ad a - SCE Power P la n t f3 251 Wood Ranch R e s e r v o ir - Daa and D ik es *68 Lds A n g elas - C3ULA A d a in . B u ild in g 266 H eaet - C ity L ib r a r y *69 Lake Hugbea A* 267 H eaet - V a lle y H o s p ita l *7* L a n c a s te r - A ir p o rt C o n tr o l Tower 269 A c tls - H V Y 1* /B * ck u s Road *75 L a n c a s te r - A ir p o r t FF 270 F a ir a o n t R e s e r v o ir - F a irm o n t Daa *81 R e d lan d s - R e d la n d s F ed . S a v in g s B ld g . 271 Lake Hughes | l *95 R e d la n d s - I n t e r s t a t e Van L in a s W arehouse 272 Lake Hughes #9 *97 Rancho C ucaaonga - Law A J u s t i c e C e n te r 27* Rosaaond - Godd* Ranch 511 Pomona - F i r s t F e d e ra l S a v in g s B ld g . 276 P iru 51* S y le a r - O liv e View M e d ic a l C e n te r 277 C a s ta lc - H a s le y Canyon 515 San B e rn a rd in o - V a n lr to w e rs 278 C a s ta lc - O ld R idge Rout* 516 San B e rn a rd in o - S u n w est O f f ic e B ld g . 279 N evhall - LA C ounty F ir * S t a t i o n 517 L a n a a s te r - M ed ical O f f i c e B ld g . 280 Laka F lr u - S a n t* F e l i c i a Daa 521 P a la d a l* - H o lid a y In n FF 281 P o rt K uenea* - N aval L ab. 522 S an B e rn a rd in o - 2nd t A rrow head 28 2 C a m a rillo - F ir * D ep t. S u p p ly B ld g . 523 L ake Hughe* MB 283 M oorpark - V e n tu ra C ounty F i r e D ep t. 525 Poaona - * th A L o c u st FF 285 San B e rn a rd in o - C5USB L ib r a r y 526 L a n c a s te r - M e d ic a l O f f ic e B ld g . FF 287 S ac B e rn a rd in o - H ilto n In n 533 Long B each - C ity H a ll Table A. 1 Station code reference table. (CSMIP,No.87-05) 85 S ta b a i C oordsnm s Epsoentral Maximum A ic e lender) No Stan on Maine N Lai W Long D isinter Fire-Acid B s c Struct. 24386 Van Nuys - 34221 116 471 6 km — 2 2 * * 0 59g H 7 slory Hotel 24436 Tarzana - Cedar Hill Nursery 14 160 1 18.534 7 km 1 82g H 1 ISg V ‘ -■ — 24067 Arieta - Nordhoff Ave. F u r Station 34.236 116 439 9 km 0 35g H 0 59g V — — 24322 Sherman O aki - 34.154 116 465 10 km ... 0 4 6 * H 1J 0.18g V . 0 90g H 13-ttory Com m ercial Bldg. J 24514 Sylmar 6s4ory County Hospital 34.326 116444 15 km 0.9 Ig H 0.60g V 0 82g H 0.34g V 2 3 lg H 24066 Pacotma - K acel Canyon F uc Sta. 674 34 2 66 118 375 17 km 0 4 4 g H 0 I9g V ' — 24207 Pacoime Dam 34 334 116.396 18 km ... M * > \.y 0 43g V >2 3g H > l.7g V 24464 North Hollywood - 20-tlory Hotel 34 136 118 359 19 km — 0.33g H y . 0.15g V " 0.66g H 24231 _ L ot A ngrier ■ 34 069 118 442 19 km ... 0 29g H . „ ‘ 0.77g H 7-ttocy U niventry Bldg. 0.25g V 24315L Century City ■ 34 064 116417 20 km 0 2 7 g H — — LACC North 0 .15g V 24643 L ot Angeles - 19-story Office Bldg 34 059 118.416 21 km . . . 0 32g H 0 I3g V 0.6Sg H 2 4 3 |5 B u tt e d - 34 1ST 118 311 21 km — 0.3GR H 0 I3g V '-1 0 79g H 10-story Residential Bldg 24370 Burtiank - 34 165 118 308 22 km 0 35g H - 0 49g H 6-story Com m ercial Bldg. 0 I5g V ' 24670 Lot Angeles - IHV405 Interchange Bridge 34 031 118.433 23 km — — 1 OOg H 1 83g V 24303^ Lot A ngeles - 34 090 116.339 23 km 0 4lg H „ — — Hollywood S to n g e B ldg Flue Field o. 19* V ' 24236 Los Angeles - Hollywood Slorage B ldg 34090 118 336 23 km 04lg H , 0 .19g V*- 0.29g H 0.1 Ig V 1 61g H . 24536 Santa M onica • City Hall G rounds 34 011 118 490 24 km 0.93g H 0.25g V — 2 4 JJ1 Wood Ranch Dam 3 4 2 4 0 116.820 26 km - 0.39g H 0 ISg V 24157 LA - Baldwin Hills 34 009 116 361 28 km 0 24g H — — 0 10* V 24612 Lot Angeles - Pico end Sen tous 34 043 1 18 271 31 km 0 W i H 0 U tf V ... — 24602 L ot Angeles - 32-nory Office Bldg 34.051 118 259 32 km ... 0 15g H 0 11g V 0.41 g H 24611 L ot Angeles - Temple and Hope 34 059 118 246 32 km 0 19k H 0 lO f V — — 24655 L ot A n g ela - 6-ttory Parking Structure 34 021 116.269 32 km * * ’ ■ 0 29f H 0 22g V 1 2 lg H 0 5 2 g V 24629 Lot A n g ela - 54-story Office Bldg 34 046 118 260 32 km 0. )4g H O OSg V 0.l9 g H 24652 Los A n g ela - 6-story Office Building 34 021 116.287 32 km ... 0 24g H OOSg V 0.59g H 0 l8g V Table A.2 Data from selected stations fo r the 17th. January 1994 Northridge Earthquake. (UCB/EERC-94-08) 8 6 < 3> < 3 > Fig A. 2 Approximate horizontal ground displacements (UCB/EERC-94-08) 87 5 U * f * C £ P H O J C C n O H O f j m o i a u r c r / m r mipiurc p u m :ER-, < 3 > <3> w - Note Scale of displacement vectors is t cm - 5 cm. Fig A . 3 Approximate vertical ground displacements. (UCB/EERC-94-08) 8 8 SU tT A C F M OJECTUH V tfnoX kU lfFA U f D U FTU K FU N E © S P O - K B C fc ilgbon C M ilow Srt* d u v lc a lw prowtod dr ftwngri^ogonc) atcapl (or USGS t u o m S i»s were ly»«edy d w eled by M ftalfpetofy; how w r, USC nation* war* (1*7*) n wt*h « n fo # mandate «#» V( » M X ) m% u r w flggiqnaiafl M *rortr I u d te a iM td i ' S or t on 3 3 1 on Sot d ol ol Fig A. 4 Contours o f maximum horizontal accelarations based on recordings at rock and soil sites. (UCB/EERC-94-08) 89 G> . \ JUVACE MMJtCWH Of AfftCM ttTTFAULT lurruK fu m o 4 ira w*» prQwd<d by * * own*n'«9anrr «KO*pt lor USGS lO koni u dBCum d < r ly d u v f tf d by « u r te i pdogy: how sysr USC n i o r t war* d u s t e d t e u d i M toal i m l r t e hm V i v > aoo mn » d sw g rm d u *roc*r m u Fig A . 5 Contours of maximum horizontal accelaration based on recordings at soil sites (UCB/EERC-94-08) - © — 7 0»..■ ' < o... ipt to* USG5 U USC M toni by it* own#tfe9*ncy Fig A. 6 Contours o f maximum horizontal accelarations based on recordings at rock sites (UCB/EERC 94-08) 91 BIBLIOGRAPHY Arnold, C. and Reitherm an, R. (1982) Building Configuration and Siesmic Design. Wiley, New York. AIA/ACSA (1994) Buildings at Risk, Council on Architectural Research, NHRP, Washington, DC. Botsai et al (1975) Architects and Earthquakes. N ational Science Foundation and the AIA Research Corporation, Washington, DC. Bolt, B.A., Horn, W .L., et al. (1977) Geological Hazards, Springer-Verlag, New York. CSM IP Reports (1994) Report Nos. 94-6B, 94-08, 94-09, 94-10, California Strong Motion Instrumentation Program (CSMIP), Sacremento. CSM IP Report (1987) Report NO. OSMS 87-OS, M otion Records from Whitier Earthquake, Sacremento. Dept, of City Planning, L.A., (1990) Socio Demographic Studies, City Of Los Angeles. Dept, of City Planning, L.A., (1993) Population Estim ate and Housing Inventory fo r City o f Los Angeles, Los Angeles, CA. Iacopi, R., (1964) Earthquake Country, Lane Book Company, San Francisco. Lagorio, H. J., (1990) An Architects Guide to Non Structural Seismic Hazards, Wiley, New York. LUPAMS, (1993) Report on Land Use Planning in LA, Department of City Planning, Los Angeles. McCue, SkafT, Boyce (1978) Architectural Design o f Building Components fo r Earthquakes, National Science Foundation andM B T Associates. Naiem, F., (1989) The Seismic Design Handbook, Van Norstrand Reinhold, New York Schierie, G.G., (1994) Architects and Earthquakes, Seminar Report, USC School of Architecture, Los Angeles. Schierie, G.G., (1993) Quality Control in Seismic Resistant Construction, National Science Foundation Report, Washington, DC. 92 Spangle, W ., (1987) Pre Earthquake Planning fo r Post Earthquake Reconstruction (PEPPER) Southern California Earthquake Preparedness Project (SCEPP), Sacremento. UCB/EERC (1994) 94/01 Preliminary Report on Northridge Earthquake, U.C. Berkeley, San Francisco, CA. 93 INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. 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Creator
Tankha, Sanjeev
(author)
Core Title
Statistical analysis of the damage to residential buildings in the Northridge earthquake
School
School of Architecture
Degree
Master of Building Science
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Building Science
Degree Conferral Date
1995-05
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University of Southern California
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architecture,engineering, civil,OAI-PMH Harvest
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English
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Schierle, Goetz G. (
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), Ambrose, James (
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), Schiler, Marc (
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committee member
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