The Lorna Prieta, California, Earthquake of 

October 17, 1989-Building Structures 

MEHMET CELEBI, Editor 

PERFORMANCE OF THE BUILT ENVIRONMENT 

THOMAS L. HOLZER, Coordinator 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1552-C 

Prepared in cooperation with the National Science Foundation 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1998 
DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

MARK SCHAFER, Acting Director 

Any use of trade, product, or firm names in this publication 

is for descriptive purposes only and does not imply endorsement 

by the U.S. Government 

Manuscript approved for publication , , November 14,1997 

Library of Congress catalog-card No. 92-32287 

For sale by the 

U. S. Geological Survey 

Information Services 

Box 25286 

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Denver, CO 80225 
CONTENTS 

Introduction------------------------------------------------------ 

By Mehmet celebi 

Performance of building structures-A summary --------- 

By Mehmet celebi 

Measured response of two tilt-up buildings ----------------- 

By Sharon L. Wood and Neil M. Hawkins 

Seismic response of a 42-story buildingr--------------------- 

By James C. Anderson and Vitelmo V. Bertero 

A summary of unreinforced masonry building damage 

patterns-Implications for improvements in 

loss-estimation methodologies ------- --------------- ---- 

By Bret Lizundia, Weimin Dong, William T. Holmes, and 

Robert Reitherman 

Housing repair and reconstruction after the earthquake --- 

By Mary C. Comerio 

Page 

Cl 
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

PERFORMANCE OF THE BUILT ENVIRONMENT 

BUILDING STRUCTURES 

INTRODUCTION 

By Mehmet celebi, 

U.S. Geological Survey 

Several approaches are used to assess the performance 

of the built environment following an earthquakepreliminary damage surveys conducted by professionals, 

detailed studies of individual structures, and statistical 

analyses of groups of structures. Reports of damage that 

are issued by many organizations immediately following 

an earthquake play a key role in directing subsequent detailed investigations. Examples of these preliminary studies for the Loma Prieta earthquake are the readily available 

excellent reports cited below: 

Astaneh, A., Bertero, V., Bolt, B., Mahin, S., Moehle, J., 

and Seed R., 1989, Preliminary report on the seismological and engineering aspects of the October 17, 

1989 Santa Cruz (Loma Prieta) earthquake: University of California, BerkeleyEarthquake Engineering 

Research Center Report 89/14, October 1989, 5 1 p. 

Benuska, L., ed., 1990, Earthquake spectra: Loma Prieta 

Earthquake Reconnaissance Report, May 1990, v. 6 

(supp.), 448 p. 

Housner, G.W., Chairman, and Thiel, C.C., ed., 1990, 

Competing against time: Report to the Governor of 

California from the Governor's Board of Inquiry on 

the 1989 Loma Prieta earthquake, 264 p. 

Lew, H.S., ed., 1990, Performance of structures during 

the Loma Prieta earthquake of October 17, 1989: U.S. 

Dept. of Commerce, National Institute of Standards 

and Technology, National Institute of Standards 

and Technology Special Publication 778 (ICSSC 

TRll), 201 p. 

Bay Area Regional Earthquake Preparedness Project 

(BAREPP) and Federal Emergency Management 

Agency (FEMA), 1990, Putting the pieces togetherthe Loma Prieta earthquake one year later, in Proceedings of a national conference, Oct. 15-18, 1990: 

253 p. 

Earthquake Engineering Research Institute, 1989, Loma 

Prieta earthquake of October 17, 1989-Preliminary 

reconnaissance report: Earthquake Engineering Research Institute Report 89-03, 51 p . Plafker, G. ,and Galloway, J., eds., 1989, Lessons learned 

from the Loma Prieta, California, earthquake of October 17, 1989: U.S. Geological Survey Circular 

1045, 48 p. 

Detailed studies of individual structures and statistical 

analyses of groups of structures may be motivated by particularly good or bad performance during an earthquake 

(see table 1). Beyond this, practicing engineers typically 

perform stress analyses to assess the performance of a 

particular structure to vibrational levels experienced during an earthquake. The levels may be determined from 

recorded or estimated ground motions; actual levels usually differ from design levels. If a structure has seismic 

instrumentation to record response data, the estimated and 

recorded response and behavior of the structure can be 

compared. Following the Loma Prieta earthquake, the two 

reports listed below played an important role in providing 

information on recorded ground and structural response: 

Maley, Re, Acosta, A., Ellis, F., Etheredge, E., Foote, L., 

Johnson, D., Porcella, R., Salsman, M., and Switzer, 

J., 1989, U.S. Geological Survey strong-motion records 

from the Northern California (Loma Prieta) earthquake 

of October 17, 1989, U.S. Geological Survey OpenFile Report 89-568, 85 p. 

Shakal, A.F., Huang, M., Reichle, M., Ventura, C., Cao, 

T., Sherburne, R., Savage, R., Darragh, R., and 

Petersen, C., 1989, CSMIP strong-motion records from 

the Santa Cruz Mountains (Loma Prieta), California, 

earthquake of 17 October 1989: California Office of 

Strong Motion Studies Report 89-06, 196 p. 

These reports are issued by two organizations 

established structural instrumentation programs: 

fornia Strong Motion Instrumentation Program 

of the California Division of Mines and Geology 

that have 

the Cali- 

(CSMIP) 

(CDMG) 

of the State of California and the National Strong Motion 

Program of the United States Geological Survey (USGS). 

The paper in this volume by celebi provides an extensive summary of studies of recorded responses for instrumented structures in the San Francisco Bay area. Such 

studies constitute an integral part of earthquake-hazardreduction programs leading to improved desigdanalyses 

procedures. In addition to the aim of studying recorded 

responses for buildings and other structures to improve 
c2 PERFORMANCE OF THE BUILT ENVIRONMEiNT 

Table 1. Papers presented in this volume categorized as detailed investigations and statistical summaries 

Author(s) Title of paper 

Detailed Investigations 

celebi Performance of building structuresA summary 

Wood Measured response of two tilt-up buildings 

Anderson and Bertero Seismic response of a six-story reinforced concrete building 

Anderson and Bertero Seismic response of a 42-story building 

Statistical Summaries 

Lizundia and others A summary of unreinforced masonry 

building damage patterns-Implications for improvements in loss-estimation methodologies 

Comerio Housing repair and reconstruction after the earthquake 

Perkins and Chuaqui Impact of the earthquake on habitability of housing units 

desigrdanalyses procedures, a second motivation for studying the Loma Prieta earthquake response data from instrumented structures is that the probability of magnitude 7 or 

larger earthquakes occurring in the San Francisco Bay 

Area from major faults, including the San Andreas and 

Hayward faults, is considered to be approximately 67 percent or higher within a 30-year period (Working Group, 

1990). Furthermore, for the tall buildings in San Francisco and vicinity, epicenters of these expected earthquakes 

may be closer than the distances to the Loma Prieta epicenter. These buildings may thus be subjected to motions 

larger and different from those recorded during the Loma 

Prieta earthquake. Therefore, studies of this type will 

help to better predict the performance of structures during future earthquakes. Furthermore, a considerable number of these tall buildings are on soft soil sites in San 

Francisco and vicinity, which provides an opportunity to 

assess their responses and design parameters under amplified motions. The paper summarizes numerous studies of 

recorded response data from instrumented structures that 

have been published to date. Also, the paper includes references to the low-amplitude (ambient) vibration testing 

of five buildings that also recorded the Loma Prieta earthquake. 

The paper by Wood and Hawkins investigate the seismic behavior of two tilt-up buildings (a two-story building in Milpitas and a one-story building in Hollister), both 

built within the 10 years prior to the earthquake. Both 

buildings are within 50 km from the epicenter and recorded similar responses despite the fact that they were 

constructed by different methods. The authors report that 

the transverse accelerations at the center of the roof of 

each building were approximately three times that at the 

base of the buildings. The significance of this study is 

that design provisions of tilt-up buildings must be improved so that the flexibility of diaphragms is decreased. 

Similar studies and changes made in the building codes 

(for example, Uniform Building Code, 1991) are included 

in the paper by celebi. 

Anderson and Bertero present studies of 6-story and 

42-story buildings that had recorded response data. They 

developed three-dimensional, linear elastic models for both 

buildings and studied their responses. In the case of the 

6-story building, under recorded Loma Prieta base motions the models confirm that limited inelastic behavior 

takes place, as was observed in the actual inspection of 

the building following the earthquake. They report that 

the 42-story building remained elastic during the earthquake. They attribute this to the fact that the designers of 

the building opted to use a site-specific design-response 

spectra range that was more conservative than the code 

minimum requirements. 

Lizundia and others studied the performance of 

unstrengthened, unreinforced masonry (UM) bearing-wall 

buildings damaged during the earthquake from a data base 

of 4,800 such buildings. The results are compared with 

those from data bases of past earthquakes and correlated 

with intensity scales. They present a loss estimation methodology using the correlated results. 

Perkins and Chuaqui studied the impact of the earthquake on 16,000 housing units in the San Francisco and 

Monterey areas that were assessed to be uninhabitabledefined as unable to be occupied due to structural problems. The data set was collected by telephone and in-person 

interviews with additional effort to quantify other characteristics of the units (such as the age and type of construction). Using models designed to provide estimates of a 

number of uninhabitable units, they produced such estimates for the San Francisco Bay area and Monterey areas 

for future earthquake scenarios. These estimates are important in directing efforts to retrofit vulnerable structures. 

Comerio also studied housing losses that occurred as 

a result of the earthquake and consequent attempts 

to provide emergency, temporary, and housing recovery 

service in the San Francisco, Oakland, Santa Cruz, and 

Watsonville areas. She makes specific recommendations 

on how Federal, state and local governments can improve 
INTRODUCTION c3 

recovery services, These recommendations are (1) REFERENCES CITED 

postearthquake housing recovery requires planning, (2) 

existing recovery programs should be streamlined to ex- Uniform Building Code, 1991, International conference of building officials: Whittier, Calif., 1050 p. (and other editions). pedite services9 and (3) programs Working Group on California Earthquake Probabilities, 1990, Probabilibe most effective if they are administered at the local ties of large earthquakes in the San Francisco Bay Region, Califorlevel. nia: U.S. Geological Survey Circular 1053, 51 p. 
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

PERFORMANCE OF THE BUILT ENVIRONMENT 

BUILDING STRUCTURES 

PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

CONTENTS 

By Mehmet celebi, 

U.S. Geological Survey 

Page 

c5 

5 

10 

11 

11 

17 

20 

20 

28 

45 

52 

5 5 

5 5 

62 

63 

67 

67 

7 1 

7 1 

7 1 

7 1 

7 1 

72 

72 

72 

74 

ABSTRACT 

The purpose of this paper is to summarize studies of 

the performance of building structures during the earthquake. The majority of studies summarized herein are of 

those buildings which were instrumented prior to the earthquake and whose responses were recorded during the earthquake. Planning for, acquiring, and studying the recorded 

responses of building structures is an important part of 

earthquake-hazard-reduction programs. Such studies help 

in forecasting performance during future events and therefore are essential for mitigation efforts. Furthermore, such 

studies facilitate confirmation and improvement of design 

and analyses methods. 

There is as great a variation in the type of buildings 

studied as there is in their performance. In this summary, 

the studies reflect, in varying detail, those issues related 

to the design andlor analyses methods. The dynamic characteristics of the buildings, if identified, have been included. The behavior of the buildings is discussed in terms 

of translational and torsional modal characteristics and 

actions such as soil-structure interaction (translational, rotational or rocking, and radiation damping), drift ratios, 

and resonation (or combination thereof) exhibited and identified from the recorded responses. Specific conclusions 

that are derived from the studies are also summarized. 

Included in the paper are summaries of some specific 

studies of performance characteristics such as pounding 

based on observations made following the earthquake. 

INTRODUCTION 

Studies of recorded responses of instrumented structures constitute an integral part of earthquake-hazard-reduction programs leading to improved designlanalyses 

procedures. The California Strong-Motion Instrumentation 

Program (CSMIP) of the California Division of Mines 

and Geology (CDMG) of the State of California and the 

National Strong Motion Program of the United States Geological Survey (USGS) have established structural instrumentation programs to measure structural responses to 

earthquakes. While these programs are prominent, other 

institutions and private owners have also instrumented 

structures throughout the continental United States, Alaska, 

Hawaii, and Puerto Rico. 

During the earthquake the response of numerous buildings and other structures throughout the San Francisco 

Bay area, Santa Cruz, and vicinity were recorded. Summaries of strong-motion records retrieved by CDMG and 

USGS from different types of structures and ground 

stations are provided by Shakal and others (1989) and 

Maley and others (1989). A summary of records from 

instrumented buildings (only) are provided in table 1, 

which lists 5 buildings instrumented by USGS and 23 by 

CSMIP. Although there are records from buildings instrumented privately by their owners, they are not included in 

the table because the data from such structures are not 

generally available. Buildings instrumented by owners 

c5 
C6 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 1.-Summary of instrumented buildings that recorded the earthquake 

-- - - - - - . -. . - .-.- - -- -. -- -- -. -- -- - -- - 

Epicentral Building description Number of Peak acceleration (g) 

dist ance(km) - - --- - - - - -. . - channels - (horizontal) 

96 Pacific Park Plaza, 24+3 FF FF(0.26 g) 

633 Christie Ave.,Emeryville ; FF=Free-Field Ground (0.22 g) 

30 stories, symmetrical, threewinged reinforced concrete (on bay 

mud) 

Hayward City Hall; 1 1-story, 

reinforced concrete framed structure 

(on consolidated alluvium) 

Great Western Bldg., 2168 Shattuck 

Ave., Berkeley; reinforced concrete 

core, truss structure at roof supports 

the suspended floors (on stiff soil) 

Chevron Bldg., 575 Market St., San 

Francisco; 4 1 -story, momentresisting steel framed structure on 

precast piles 

Transamerica Bldg.; 48-story+204 

ft tower steel framed on 9 ft 

basemat (on stiff soil) 

4-story concrete bldg., Watsonville 

(CSMIP No. 47459) 

3-story steel bldg., San Jose 

(57562) 

1-story gymnasium, West Valley 

College, Saratoga (58235) 

2-story historic commercial 

building, Gilroy (57476) 

1 -story warehouse, Hollister 

(47391) 

10-story concrete residential bldg., 

San Jose (57356) 

10-story concrete commercial bldg., 

San Jose (57355) 

3-story, steel, Santa Clara County 

Office Bldg., San Jose (57357) 

2-story masonry office bldg, 

Palo Alto (58264) 

3-story concrete school office bldg., 

Redwood City (58263) 

2-story concrete office bldg., 

Belmont (58262) 

9-story concrete government office 

bldg., San Bruno (58394) 

6-story office bldg., San Bruno 

(58490) 

4-story steel hospital bldg, So. San 

Francisco (5826 1) 

6-story, concrete UCSF bldg., 

San Francisco (58479) 

Roof wing (0.39 g) 

FF(O.10 g) 

Ground(0.07 g) 

12th floor (0.13 g) 

Basement (0.1 lg) 

13th floor (0.23 g) 

Basement (0.1 1 g) 

25th floor (0.23 g) 

Basement (0.1 1 g) 

49th floor (0.3 1 g) 

Ground (0.39 g) 

Roof (1.24 g) 

Ground (0.28 g) 

Roof (0.67 g) 

Ground (0.33 g) 

Roof (0.87 g) 

Ground (0.25 g) 

Roof (0.99 g) 

Ground (0.18 g) 

Roof (0.82 g) 

Ground (0.13 g) 

Roof (0.37 g) 

Ground (0.1 1 g) 

Roof (0.38 g) 

Ground (0.11 g) 

Roof (0.36 g) 

Ground (0.21 g) 

Roof (0.55 g) 

Ground (0.09 g) 

Roof 0.17 g) 

Ground (0.1 1 g) 

Roof (0.20 g) 

Ground (0.16 g) 

Roof (0.36 g) 

Ground (0.14 g) 

Roof (0.46 g) 

Ground (0.15 g) 

Roof (0.68 g) 

Ground (0.09 g) 

Roof (0.28 g) 

Organization 

USGS 

USGS 

USGS 

USGS 

USGS 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY C7 

Table 1 .-Continued. 

- -- 

Epicentral Building description Number of Peak acceleration (g) organizationp 

distance(km) channels (horizontal) - - 

1 8-story, steelkoncrete, commercial 

bldg., San Francisco (58480) 

47-story steel office bldg., San 

Francisco (58532) 

3-story steelkoncrete office bldg., 

San Rafael (68341) 

14-story concrete residential bldg., 

Santa Rosa (68489) 

5-story concrete commercial bldg., 

Santa Rosa (68387) 

2-story, tilt-up, industrial bldg., 

Milpitas (57502) 

6-story concrete office bldg., 

Hayward (58462) 

13-story steelkoncrete CSUH 

Admin. Bldg., Hayward (58354) 

4-story, concrete, CSUH Science 

Bldg., Hayward (58488) 

24-story, concrete, residental bldg., 

Oakland (58483) 

2-story masonrylsteel office bldg., 

Oakland (58224) 

3-story concrete Piedmont Jr. High 

School., Piedmont (58334) 

2-story steel hospital bldg., 

Berkeley (58496) 

10-story concrete commercial bldg., 

Walnut Creek (58364) 

3-story concrete commercial bldg., 

Pleasant Hill (58348) 

8-story, masonry, residential bldg., 

Concord (58492) 

3-story, concrete, City Hall, 

Richmond (58503) 

3-story, steel, office bldg., 

according to the Uniform Building Code (UBC) recommendations also are not included in this table. Figure 1 

shows the locations of some of the buildings discussed in 

this summary relative to the epicenter. 

While the primary motivation in studying recorded responses of buildings and other structures is to improve 

designlanalyses procedures, a second motivation for studying the Loma Prieta response data is that the probability 

of magnitude 7 or larger earthquakes occurring in the San 

Francisco Bay area from major faults, including the San 

Andreas and Hayward faults, is considered to be approximately 67 percent or higher within a 30-year period (Working Group, 1990). Furthermore, these earthquakes may 

originate at distances that are closer to major urban areas 

Ground (0.14 g) 

Roof (0.27 g) 

Ground (0.20 g) 

Roof (0.48 g) 

Ground (0.04 g) 

Roof (0.1 3 g) 

Ground (0.06 g) 

Roof (0.21 g) 

Ground (0.06 g) 

Roof (13 g) 

Ground (0.14 g) 

Roof (0.58 g) 

Ground (0.12 g) 

Roof (0.45 g) 

Ground (0.09 g) 

Roof (0.24 g) 

Ground (0.05 g) 

Roof (0.1 8 g) 

Ground (0.18 g) 

Roof (0.38 g) 

Ground (0.26 g) 

Roof (0.69 g) 

Ground (0.08 g) 

Roof (0.1 8 g ) 

Ground (0.12 g) 

Roof (0.30 g) 

Ground (0.10 g) 

Roof (0.25 g) 

Ground (0.13 g) 

Roof (0.24 g) 

Ground (0.06 g) 

Roof (0.24 g) 

Ground (0.12 g) 

Roof (0.24 g) 

Ground (0.12 g) 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

CSMIP 

-- - - Roof (0.29 g) - - --- -- - - . - - - . - 

than the 97-km distance of the Loma Prieta event from 

San Francisco and may generate motions larger than those 

recorded during Loma Prieta. Therefore, studies of this 

type will help to better predict the performance of structures during future earthquakes. 

A third motivation for these studies is that in the San 

Francisco Bay area there are several tall buildings on soft 

soil sites having seismic instrumentation. Records obtained 

from these buildings are particularly important to evaluate the performance of such structures in response to amplified ground motions and possible soil-structure 

interaction effects. Figure 1 shows the location of some of 

the buildings that are covered in this paper and that were 

subjected to amplified motions during the earthquake at 
PERFORMANCE OF THE BUILT ENVIRONMENT 

Â 

SCCOB 

MAIN SHOCK 

EPICENTER O 

37 O 

Figure 1 .-Location of some instrumented buildings relative to epicenter. Pacific Park Plaza (PPP), Transamerica Building (TRA), Embarcadero Building (EMB), Chevron Building (CHE), two-story building in Oakland (OAKL), California State University (Hayward) (HAYW), 

Santa Clara County Office Building (SCCOB), Yerba Buena Island (YBI). 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c9 

approximately 100 krn from the epicenter. To demonstrate at dedicated free-field stations in the vicinity or at the 

the degree of amplified motions at these sites during the ground floor or basement of the four tall buildings are 

earthquake, response spectra of ground motions recorded compared (fig. 2) to the spectrum from the station on 

RESPONSE SPECTRA -- NS(360) -- 5 % DAMPING 

PERIOD (SECONDS) 

RESPONSE SPECTRA -- EW(90) -- 5 % DAMPING 

I I I I I Ill1 I I I I Illll I I I I I Ill 

PERIOD (SECONDS) 

Figure 2.-Response spectra of free-field Emeryville site of Pacific Park Plaza (PPP), Transamerica Building (TRA), 

Embarcadero Building (EMB), and Chevron Building (CHE) compared to response spectrum of rock site Yerba Buena 

Island (YBI). 
c10 PERFORMANCE OF THE BUILT ENVIRONMENT 

Yerba Buena Island, a rock site, also located at approximately 100 km from the epicenter. While the largest peak 

acceleration at Yerba Buena Island was 0.06 g, the amplified peak accelerations at the soft soil sites of some of 

these tall buildings varied between 0.12 and 0.26 g. The 

response spectra depict the degree of amplification of peak 

acceleration, represented by zero-period accelerations as 

well as the frequency (period)-dependent spectral acceleration. Particularly between the periods 0.1 and 2 seconds, which is of engineering interest, the spectral 

amplification ratio is as high as 5 or 6. 

The purpose of this paper is to review and summarize 

studies of instrumented buildings that recorded the Loma 

Prieta earthquake. Since the earthquake, almost all of the 

recorded building response data has been made available 

by CSMIP and USGS, and a significant number of studies 

of the data set have been completed. Damage surveys or 

subsequent related studies of buildings are not within the 

scope of this paper. For further information of such studies, the readers are referred to the numerous reports published since the earthquake (see Benuska, 1990). Although 

every effort was made to include summaries of all the 

studies herein, it is likely that some were not in wide 

circulation and thus were not available. 

Also of interest to the engineering and scientific community is the low-amplitude (ambient) vibration testing 

of five of the buildings in table 1 (Marshall and others, 

199 1, 1992; Celebi and others, 199 1, 1993; Celebi, 1996). 

The results are summarized later in this paper. 

METHODS OF ANALYSES 

In studying the recorded responses of buildings and 

other structures, several methods have been used, including spectral techniques, system identification methods, and 

finite element modeling and analyses. The spectral analyses are based on Fourier amplitude spectra, autospectra Sx 

and S, cross-spectral amplitudes S and coherence functions (y) and associated phase angfes using the equation 

from Bendat and Piersol (1980): 

The procedures used in system identification analyses 

estimate a model based on observed input-output data 

(Ljung, 1987). Simply stated, the input is the basement or 

ground-floor motion and the output is the roof-level motion or one of the levels where the structural response is 

detectable. In most of the system identification analyses 

presented in this paper (for example, Celebi, 1996), the 

ARX (acronym meaning AR for autoregressive and X for 

extra input) model based on the least-squares method for 

single input-single output (Ljung, 1987) coded in commercially available system identification software was used 

(The Mathworks, 1988). 

The damping ratios are extracted by system identification analyses in accordance with the procedures outlined 

by Ghanem and Shinozuka (1995) and Shinozuka and 

Ghanem (1995). These procedures are based on the following equations: 

where 

which are readily calculable from results of system identification routines. In this case, z, is the j-th pole of S(z) 

which represents the system of linear equations defining 

the model accepted to represent observations. The poles, 

z., are the positive imaginary part represented by the equa- J tion: 

The low-amplitude (ambient) vibration data was analyzed by conventional spectral analysis techniques. System identification techniques were not applied to the 

ambient vibration data because of the unknown system 

input characteristics. Furthermore, because of the higher 

noise-to-signal ratio, system identification techniques 

do not minimize the errors simply arid solutions appear 

unreliable. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY Cll 

RESPONSE OF REINFORCED 

CONCRETE BUILDINGS 

PACIFIC PARK PLAZA (EMERYVILLE) 

The set of records from Pacific Park Plaza is possibly 

the most studied building response data during this earthquake. The building was constructed in 1983 and instrumented in 1985, is 30 stories, and is the tallest reinforced 

concrete building in northern California. A general view, 

a plan view, a three-dimensional schematic, and its instrumentation are shown in figure 3 (Celebi, 1992, 1996). 

Twenty-one channels of synchronized uniaxial accelerometers are deployed throughout this structure, with an 

additional three channels of accelerometers located at the 

north free-field outside the building. All are connected to 

central recording systems. In addition, a triaxial strongmotion accelerograph is deployed at a free-field site on 

the south side of the building (SFF or EMV'). 

The building is an equally spaced three-winged, cast- 

.in-place, ductile, moment-resistant reinforced concrete 

framed structure. The foundation is a 5-foot-thick concrete mat supported by 828 (14-inch-square) prestressed 

concrete friction piles, each 20-25 m in length, in a primarily soft-soil environment, with an average shear-wave 

velocity between 250 and 300 m/s and a depth of approximately 150 m to harder soil. The building had considerably amplified input motions but was not damaged during 

the earthquake. The east-west components of acceleration 

recorded at the roof and the ground floor of the structure 

and at the associated free-field station (SFF in fig. 3B), all 

at approximately 100 krn from the epicenter, are shown in 

figure 4A. The motion at Yerba Buena Island (YBI), the 

closest rock site, had a peak acceleration of 0.06 g and is 

shown in figure 4 to indicate the level of amplified shaking at the free-field site and at the ground floor of the 

building. The corresponding response spectra are shown 

in figure 45. For all three components of acceleration, the 

calculated response spectra of the south free-field site (SFF 

or EMV) of Pacific Park Plaza and Yerba Buena Island 

are compared in figure 4C (Celebi, 1992). These response 

spectra show that the motions at EMV were amplified by 

as much as five times when compared with YBI. This is 

also inferred by the amplitude of the peak accelerations 

(0.26 g for EMV and 0.06 g for YBI). Furthermore, the 

differences in peak acceleration at the free-field station 

(0.26 g) and that at the ground floor of the building (0.21 

'1n most studies, the site of the south free-field (SFF) is referred to as 

the Emeryville site (EMV). The data from this site is one of the mostused ground motion records from the Loma Prieta strong-motion data 

set. 

g) (fig. 4A) suggest the possibility of significant soilstructure interaction. 

The building has been studied in detail by Celebi and 

Safak (1992), Safak and Celebi (1992), Anderson and 

Bertero (1994), Anderson and others (1991), Bertero and 

others (1992), Kagawa and others (1993), Kagawa and 

Al-Khatib (1993), Aktan and others (1992), and Kambhatla 

and others (1992). The predominant response modes of 

the building and the associated frequencies (periods) [0.38 

Hz (2.63 s), 0.95 Hz (1.05 s), and 1.95 Hz (0.51 s)] 

are identified by all these investigators using different 

methods, including spectral analyses, system identification techniques, and mathematical models. These three 

modes of the building are torsionally-translationally 

coupled (Celebi, 1996). The frequencies are clearly identified in the cross-spectra (Sxy) of the orthogonal records 

obtained from the roof and ground floor (fig. 5A, B), the 

south free-field site (SFF) (fig. 5C), and the normalized 

cross-spectra of the orthogonal records (fig. 5D). A site 

Figure 3.-A, Pacific Park Plaza. B, Plan layout and three-dimensional 

schematic of Pacific Park Plaza showing dimensions and strong-motion 

instrumentation (Celebi, 1992, 1996). 
A 

SOUTH FREE-FIELD 

ROOF 

30 th Floor 

21 st Floor 

13 th Floor 

- GROUND 

Figure 3 .-Continued. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

LPE (EW) : PACIFIC PARK PLAZA AND YERBA BUENA ISLAND 

1.2 1 

r 1 ROOF (EW) : PEAK = 0.38 G 

J J n 

2 , I 

Z 0.6- 

FREE-FIELD (EW) : PEAK = 0.26 G 

-0.2 - YBI (EW) : PEAK = 0.06 G 

TIME (SEC) 

B LPE (EW): PACIFIC PARK PLAZA AND YERBA BUENA ISLAND 

c PERIOD (SEC) 

o NORTH-SOUTH v--='-l - EAST-WEST VERTICAL 

PERIOD (SECONDS) PERIOD (SECONDS) PERIOD (SECONDS) 

Figure 4.-A, Loma Prieta earthquake (LPE) east-west components 

of acceleration recorded at roof, 

ground floor, and south free-field 

(SFF) station of Pacific Park Plaza. 

Also shown is east-west component 

of acceleration at Yerba Buena Island (YBI). B, Response spectra of 

motions. C, response spectra of all 

three components of acceleration at 

south free-field (EMV) compared 

with YBI. 
C14 PERFORMANCE OF THE BUILT ENVIRONMENT 

frequency at 0.7 Hz (1.43 s) is also identified. The peak at 

0.7 Hz that appears in the cross-spectrum of the roof (fig. 

5A) appears as the dominant peak in the cross-spectra of 

the ground floor and the south free-field (SFF) (figs. 5B, 

C). When the normalized cross-spectra are calculated for 

the ground floor and free-field, the site frequency at 0.7 

Hz is distinguishable from the structural frequencies in 

the normalized cross-spectrum of the roof (fig. 5D). The 

c FREQ (HZ) 

xl~~ PPP:SFF (A350 & A260) 

FREQ (HZ) 

0.7-Hz site frequency is further confirmed by the transfer 

function (fig. 6) calculated by using Haskell's shear-wave 

propagation method (Haskell, 1953, 1960) and site characterization information by Gibbs and others (1994). The 

figure shows the transfer function and the variation of 

shear-wave velocities with depth. The depth to bedrock 

has been adopted from a map by Hensolt and Brabb (1990) 

as 150 m (-500 ft). 

D FREQ (HZ) - 

PPP:ROOF, GR.FL, & SFF 

1 

*7 I f^Ac&L@U50 VS. k60) 

I 

"0 1 2 3 4 5 

FREQ (HZ) 

Figure 5.-Cross-spectra of accelerations at (A) roof, (B) ground floor, and (C) south free-field (SIT). Normalized cross spectra (D) show site 

frequency 0.7 Hz at ground floor and SFF but not at roof. 

SITE TRANSFER FUNCTION: EMERYVILLE SITE OF PPP t 

4 - .-. 

2 - 

b 

0 0.5 1 1.5 2 15 3 3.5 4 4.5 5 

FREQ (HZ) 

Figure 6.-Site transfer function of Pacific Park Plaza indicates first peak at 0.7 Hz. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY C15 

Figure 7 shows the results of applying the system iden- top floor and by comparison of the amplitude spectra of 

tification technique. The match between the observed and these responses. The damping ratios extracted from the 

calculated response is excellent, as evidenced by corn- system identification analyses corresponding to the 0.38- 

parison of the calculated and observed responses at the Hz first-mode frequency are 11.6 percent (north-south) 

SYSTEM IDENTIFICATION-PACIFIC PARK PLAZA-LPE I 30TH. FL (350) Recorded ' 

- 

- 

0 5 10 15 20 25 30 35 40 45 

TIME (SEC) 

INPUT-GRND. FL (350) 

@60001 I I 1 

â‚¬ Recorded 

SYSTEM IDENTIFICATION-PACIFIC PARK PLAZA--LPE 

FREQ (HZ) FREQ (HZ) 

Figure 7.-System identification applied with accelerations recorded at roof and ground floor of Pacific Park Plaza. 
C16 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 2.-Summary of dynamic characteristics for Pacific Park Plaza 

[- -, not available] 

1990 AMBIENT TESTS (from Celebi, Phan, and Marshall, 1993) 

N-S 0.48 - - - - - - 0.6 --- - - - 

E-W 0.48 - - - - - - 3.4 --- --- 

1989 (LPE) STRONG-MOTION TESTS (from Celebi, Phan, and Marshall, 1993) 

N-S 0.38 0.95 1.95 11.6 - - - --- 

E-W 0.38 0.95 1.95 15.5 --- - - - 

1985 FORCED VIBRATION TESTS (from Stephen and others, 1985) 

N-S 0.590 1.660 3.09 1.7 1.3 2.9 

E- W 0.595 1.675 3.12 1.8 1.9 3.2 

Torsion 0.565 1.700 3.16 1.5 1.32 1.7 

1985 AMBIENT VIBRATION TESTS (from Stephen and others, 1985) 

N-S 0.586 1.685 3.149 2.6 1.8 0.8 

E-W 0.586 1.685 3.125 2.6 1.2 0.4 

Torsion 0.586 1.709 3.125 3.8 1.4 1 .O 

MODAL ANALYSES [rigid (R ) and flexible (F) foundation] (from Stephen and others, 1985) 

N-S 

E- W 

Torsion 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY C17 

and 15.5 percent (east-west) (Celebi, 1996). Such unusually high damping ratios attributed to a conventionally 

designed/constructed building require explanation. The 

building with its large mat foundation in a relatively soft 

geotechnical environment is capable of energy dissipation 

in the soil due to radiation (or foundation) or material 

damping. The subject of radiation damping for this building has been discussed in detail by Celebi (1996). 

The dynamic characteristics determined from Loma 

Prieta response records of Pacific Park Plaza as well as 

those determined from low-amplitude tests prior to 

(Stephen and others, 1985) and after the earthquake 

(Marshall and others, 1992; Celebi and others, 1993) are 

summarized in table 2. These low-amplitude tests will be 

discussed later in this paper; however, it is important to 

note that there are significant differences in the dynamic 

characteristics of the building that were derived from the 

strong (Loma Prieta) shaking data and from the low-amplitude data. Also, it is noted in table 2 that although 

flexibility of the foundation was considered in the 1985 

analyses, the structural frequency remained the same as 

the frequency determined with fixed base assumption. The 

differences in the frequencies for strong- and low-amplitude motions are attributed to soil-structure interaction 

(SSI), as studied from the records and mathematical modeling (celebi and Safak, 1992; Safak and Celebi, 1992; 

Kagawa and others, 1993; Kagawa and Al-Khatib, 1993; 

Aktan and others, 1992; Kambhatla and others, 1992). A 

study of the building for dynamic-pile-group interaction 

by (Kagawa and Al-Khatib, 1993; Kagawa and others, 

1993) indicates that there is significant interaction. The 

study shows that computed responses of the building using state-of-the-art techniques for dynamic-pile-group interaction compares well with the recorded responses. 

Anderson and others (1991) and Anderson and Bertero 

(1994) compared the design criteria, code requirement, 

and the elastic and nonlinear dynamic response due to the 

earthquake. They also compared current U.S. and Japanese design procedures and requirements for this type of 

building and analyzed probable performance under more 

severe base motions. In order to achieve these objectives, 

linear elastic and nonlinear dynamic response analyses 

were conducted using both simplified and detailed analytical models. The results have been compared with Japanese design procedures. Contrary to others, the authors 

conclude that soil-structure interaction was insignificant 

for Pacific Park Plaza during the earthquake. 

The response of the building was also found to be sensitive to the dominant orientation of the maximum energy 

of Lorna Prieta ground motions. For this building, the 

orientation was similar to the rupture direction of the earthquake. A significant effect of the orientation of the ground 

motion for an unsymmetrical three-winged building such 

as Pacific Park Plaza was that it exhibited a disproportionate (as much as three times) response in one wing of 

the building compared to another, as shown in figure 8 

(Celebi, 1992). Therefore, the propagation direction of 

different waves (in most cases, surface waves) arriving at 

a building can be significant. As a general conclusion, 

because the energy of the ground motions can be azimuthally variable, structures with wings or unsymmetrical structures can be significantly affected by it. 

SIX-STORY OFFICE BUILDING (SAN BRUNO) 

A general view and the instrumentation scheme a sixstory, reinforced concrete framed building in San Bruno 

(SBR) is shown in figure 9 (Gelebi, 1996). The building 

is rectangular in plan and has four moment-resistant frames 

in the exterior and one in the interior in the transverse 

direction (355'). Anderson and Bertero (this chapter) developed three-dimensional, linear elastic models of the 

building and studied its response under recorded base 

motions and code-prescribed lateral forces. They reported 

that under Loma Prieta motions the models confirmed 

limited inelastic behavior, as was observed in the building 

following the earthquake. 

Phan and others (1994) also studied the building by 

using a mathematical model with a fixed base and with 

springs simulating soil-structure interaction effects. Their 

analyses showed that the frequency of the building determined from Loma Prieta response data (0.98 Hz, northsouth, and 1.17, east-west) was approximately 69 percent 

of the frequency determined from ambient test data (1.41 

Hz, north-south, and 1.72 Hz, east-west) (Marshall and 

others, 1991,1992; Celebi and others, 1991, 1993, Celebi, 

1996). Furthermore, the frequency determined from ambient vibration data matches the analysis results of the model 

with a fixed base. On the other hand, the Loma Prieta 

frequency matched the results of the model with soil-structure interaction springs. They concluded that soil-structure interaction plays a significant role in the response of 

this building. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

?- 

I I V I I I 1 -40. 5 10 15 20 25 30 35 40 45 

TIME (SEC) 

PACIFIC PARK PLAZA --RELATIVE DISPLACEMENTS 

TIME (SEC) 

I ma=-T.8 cm, mih= 6.3 cm 

max=3.4 cm, min=-3.1 cm 

1 

-zoo L I I I 1 I I I 

I 1 

5 10 15 20 25 30 35 40 45 

TIME (SEC) 

Figure 8,-Relative displacements at the wings of 30th floor of Pacific Park Plaza. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

Figure 9.-A, Six-story commercial building in San Bruno. B, Three-dimensional schematic of San Bruno building. 
c20 PERFORMANCE OF THE BUILT ENVIRONMENT 

RESPONSE OF STEEL STRUCTURES 

TRANSAMERICA BUILDING (SAN FRANCISCO) 

The response of one of the landmarks of San Francisco, 

the pyramidal Transamerica Building, 97 km from the 

epicenter of the earthquake, was recorded through an array of strong-motion instruments deployed by the USGS 

in 1985. The building was designed according to code 

requirements of that time; however, design evaluation was 

made using a site-specific design- response spectrum with 

seismic forces that were higher than the code requirements (R. Clough, personal commun., 1990). The building is 60 stories, 257.3 m (844 ft) high, and square in 

plan. At ground level, the plan dimensions are 53x53 m 

(174x174 ft). This plan starts reducing at the second floor 

to 44x44 m (145x145 ft) at the fifth floor and then follows an exterior wall slope of 1 to 11 upward. A perimeter truss system decorates and supports the building 

between the second and fifth floors. In addition to the 

exterior frame system, interior frames extend to the top of 

the structure, with some of them ending at the 17th and 

45th floors. The exterior pre-cast concrete panels are attached structurally to the exterior frames. The basement 

(three levels below the ground level) consists of a very 

rigid shear wall box system. The foundation of the building consists of a 2.7-m (9 ft)-thick basemat without piles. 

The underlying soil media consists, in general, of clays 

and dense sands. Below the ground level to a depth of 8 

m (25 ft), there is weak and compressible sand and rubble 

fill and recent bay deposits of sand and clay. Below 20 m 

(60 ft), the sands are partially cemented. The bedrock is 

between 48 and 60 m (145- 185 ft) below the present street 

grade. 

A general view, a three-dimensional schematic, overall 

dimensions, the instrumentation scheme, and recorded accelerations and displacements at some locations of the 

building are shown in figure 10. The instrumentation 

scheme was designed and implemented to study the response and associated dynamic characteristics of the building, including its translational, rocking, and torsional 

motions. There are a total of 22 channels2 Three triaxial 

strong-motion accelerographs with a total of nine channels are deployed synchronously with 13 uniaxial forcebalance accelerometers, all connected to a central recorder 

with common-time recording capability. The three triaxial 

accelerographs are located on the 49th, 29th, and 

basement levels. At the 21st, 5th, and ground levels, 

three uniaxial accelerometers are deployed, two parallel 

2~t is noted herein that channels 11 and 12 of the central recording 

system did not function properly. However, the remaining records are 

sufficient to perform analyses of the response of this important building. 

to one another at the nominal west and east ends (nominal 

north-south orientation-actually 35 1 ' clockwise from 

true north) and the third with a nominal east-west 

orientation (081' clockwise from true north). These orientations are coincident with the orientations of the horizontal channels of the three triaxial accelerographs at the 

49th, 29th, and basement levels. The remaining four 

uniaxial accelerometers are deployed in the basement; one 

each is positioned vertically at three corners of the 

building, and one is positioned horizontally and parallel 

to the nominal north-south horizontal channel of the triaxial 

accelerograph in the basement. The senses of the orientations of the channels are also shown in figure 10. The 

perpendicular distance between the two parallel vertical 

sensors at the basement level is 58.98 m (193.5 ft). 

In summary, there are parallel pairs of horizontal accelerometers in each of the 21st, 5th, ground, and basement 

levels and another single accelerometer deployed orthogonally to the pair in the horizontal direction at the same 

levels. 

The response of the Transamerica Building has been 

studied in detail by Celebi and Safak (1991) and Safak 

and Celebi (1991). The peak accelerations and displacements derived from the processed data are summarized in 

table 3. The fundamental frequency (period) is 0.28 Hz 

(3.6 s) in both the north-south and east-west directions, as 

extracted from the spectral analyses and system-identification techniques. Other frequencies are 0.5, 1.2, 1.5, and 

1.8 Hz for the east-west direction and 1 .O, 1.35, 2.0, and 

2.6 Hz for the north-south direction. Figure 11 shows the 

results of the application of the system-identification technique for the Transamerica Building records at the 49th 

floor as output and at the basemat as input (Celebi, 1996). 

The match between the observed and calculated response 

is excellent, as evidenced by comparison of the calculated 

and observed responses at the 49th floor and by comparison of the amplitude spectra of these responses. The critical viscous damping ratios extracted from the 

system-identification analyses corresponding to the 0.28 

Hz first mode frequency are 4.9 percent (north-south) and 

2.2 percent (east-west) (Celebi, 1996). 

The analyses of the records showed that there is no 

significant torsional motion, as evidenced by the differences in the parallel accelerations and displacements on 

each floor. These relative displacements or the relative 

accelerations, as nominal torsional motions (and their corresponding Fourier spectra, not shown here), are negligible compared to those of the translational components. 

The possibility of rocking was investigated using both 

the vertical motions recorded at the basemat and the horizontal motions recorded at the ground level and the 

basemat. Shown in figure 12 are the coherency, phase 

angle, and cross-spectrum plots for both north-south (351') 

and east-west (081') directions of pairs of horizontal acceleration on the 21st floor and vertical acceleration of 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c2 1 

the basemat. It is observed from these that the rocking at the 21st floor and the vertical motion in the basement 

motion occurs at 2.0 Hz (or 0.5 s) in the north-south (351') are coherent and in phase. These frequencies are also obdirection and at 1.8 Hz (or 0.56 s) in the east-west (081') served in the Fourier amplitude spectra for the horizontal 

direction, since at these frequencies the horizontal motion acceleration components (both directions) at the roof level, 

Figure 10.-A, Transamerica Building. B, Three-dimensional schematic of Transamerica Building and recorded 

accelerations and displacements (Celebi, 1992, 1996). 
PERFORMANCE OF THE BUILT ENVIRONMENT 

as shown in figure 11. Figure 13 shows (A) the east-west 

component of acceleration at the 49th floor, (B) its amplitude spectrum, (C) the east-west displacement at the 49th 

floor, (D) the rocking contribution of acceleration at the 

49th floor, (Â£ its amplitude spectrum clearly displaying 

the 2 Hz (0.5 s) rocking frequency (period), and (F) the 

rocking contribution of displacement at the 49th floor. It 

is noted that amplitudes of the rocking contribution (calculated by multiplying the rotation by the total distance 

between the basemat and the 49th floor) to the 49th floor 

displacement are very small. 

The maximum vertical displacement due to rocking motion (the difference in the two vertical displacements at 

the two corners of the basemat) is 0.313 cm or 5.31xl0-~ 

radians (0.003') when divided by the distance between 

the two vertical sensors. The peak relative horizontal displacement between the ground level and the basement is 

0.59 cm in the 081' (east-west) direction and 0.77 cm in 

the 35 1 ' (north-south) direction, which translates into 

4.6xl0-~ radians (or 0.026' of rotation around the 351' 

axis and 6.0xl0-~ radians (or 0.034') around the 081' 

axis. All of these peaks occur at approximately 11 s into 

the record. These rotations are shown in figure 14. The 

rotation of the wall around the north-south axis is approximately tenfold that of the rotation of the basemat 

around the same axis. 

The comparison of the rotations around the north-south 

(351') axis shows that there is a very significant difference between the peak rotations calculated from the difference of the vertical displacements at the basemat and 

those calculated from the difference of east-west direction 

displacements at the ground floor and the basemat. This 

disparity could be due to (1) the bending of the basemat, 

(2) the shear deformation and bending of the shear walls 

TRANSAMERICA BUILDING 

2.0E3 49TH FLOOR 

I^- GROUND FLOOR 

0 l- BASEMENT 

DISPLACEMENTS (CM) 

Figure 10.-Continued. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY C23 

Table 3.-Peak accelerations and displacements for Transamerica and columns in the three levels below the ground level, 

Building and (3) perhaps, the effect of the smaller stiffness of the 

--, only one channel in this direction; +++, not placed in this direction; embedment in the horizontal direction as compared to 

SMA, triaxial strong-motion accelerograph; FBA, force-balance acceler- the vertical direction. Another possibility is the presence ometer; CH, channel] of integration errors introduced during processing of the 

Floor Acceleration Displacement digitized data. The large ratio of the wall-to-basemat rota- O8l035l035l0U~ O8l035l035l0U~ tion may be important in assessing forces used in the 

(?I (?I (.e) (8) (cm) (cm) (cm) (cm) 

49 (SMA) 0.28 0.29 - - - 0.13 18.6 11.3 --- 1.22 

29 CSMA) 0.14 0.16 --- 0.11 12.9 7.7 --- 0.92 

design of basements. A detailed study of this issue was 

carried out by Soydemir and Celebi (1992). In usual Base (sMA) 0.10 0.10 - - - 0.05 5.2 1.9 - -- 1.10 

21 (FBA) 0.19 0.13 0.14 +++ 8.5 4.4 4.8 +++ practice, the basements are designed for seismic forces 5 (WA) 0.19 0.26 0.26 +++ 3.5 2.0 2.3 +++ 

Ground (FBA1 0.17 0.14 0.16 + + + 3.3 2.0 2.0 + + + similar to that of the ground floor. No difference in the 

Base(~~13)(kB~) +++ +++ +++ 0.04 +++ +++ +++ 1.0 

Base (CH7)(FBA) +++ +++ +++ 0.07 +++ +++ +++ 0.9 

SYSTEM IDENnFICATION--MSAMERICA BUILDING--LPE , /-"s 

. FL.--NS(351) Calculated "- 

BASEMAT--NS(351) Recorded 

0 10 20 30 40 50 60 

TIME (SEC) 

OUTPUT-49TH FL-NS(35 1) 

Recorded - 

3 4000 w .* - Calculated -. - 8 2000 

O012345 

FREQ (HZ) FREQ (Hz) 

SYSTEM IDENTIFICATION--TRANSAMERICA BUILDING-LPE 

W(26l)rded 

s 

u 49TH. FL.--EW(261) Calculated 

d 0 

U BASEMAT--EW(261) Recorded ^ -500 

0 10 20 30 40 50 60 

TIME (SEC) 

FREQ (Hz) FREQ (Hz) 

Figure 11 .-System identification applied with accelerations recorded at 21st floor and basement 

of Transamerica Building. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

TRANSAMERICA BLDG. CH 1 (21 ST FL) NS & CH 13 (BASEMAT (UP) 

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 

FREQ (HZ) 

- ^ ......................... 5000 - 2 

.- 5000 . ......................... ... 

w 

2 4000 - ..... :. ..... :. .... .: ..... . ....................... . -4000 

3 

u :. .... ..... ..... :. ... .:. :. ... .: .. - 3000 ... .... ..... ..... .... 

...:..... : .... - 2000 

1 I 1 

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 

FREQ (HZ) 

TRANSAMERICA BLDG. CH 3 (21 ST FL) EW & CH 13 (BASEMAT (UP) 

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 

FREQ (HZ) 

FREQ (HZ) 

Figure 12.-Rocking investigated with cross-spectrum, coherency (solid line), and phase angle (dashed line) 

plots of horizontal motions at 21st floor and vertical motion at basemat of Transamerica Building. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY C25 

seismic forces andlor accelerations are considered between 

the ground level and the embedded basement levels. 

However, the data set from the Transamerica Building 

shows that significant differences may occur between the 

motions at the ground floor and those at the basemat level 

of embedded basements and that the deformations of 

basemat and basement walls can also be significantly different. To compensate for this in design, a simplified 

approximate procedure has been developed by Soydemir 

and Celebi (1992). A similar disparity in the deformation 

of the basemat and walls of the basements has been observed from the data of Embarcadero Building in San 

Francisco, discussed later in this paper. 

Although very small in amplitude, the rocking motions 

significantly influenced the motions at the basement and 

the ground level. This is evidenced by the normalized 

response spectra (fig. 15) for the records from the three 

components of the triaxial accelerograph at the basement. 

Thus, this type of response may be pertinent to the incorporation of response spectra used in the design process of 

! buildings. The design response spectra represents, in general, free-field motions assumed to be applicable at the 

foundation level of a structure; while it may include site 

effects, it should not include the effect of the vibration of 

the structure or soil-structure interaction. In this case, it 

was shown that the motions at the foundation (basemat) 

level are influenced by the soil-structure interaction effects. 

TRANSAMERIW49FL (EW) 

f RECORDED. TO 

J 0 

Forced vibration tests and dynamic analyses were performed on the Transamerica Building in 1972-73 by 

Stephen and others (1974). Ambient vibration tests were 

performed by Kinemetrics (1979). The data from these 

investigations permit (1) the comparison of the actual earthquake response characteristics with those from small amplitude tests and (2) the assessment of the validity of 

various assumptions made in the dynamic analyses. The 

results of these small-amplitude tests and related analyses 

are summarized later in this paper. It is important to note 

that both the forced-vibration and ambient-vibration tests 

were performed when the construction of the building was 

just completed and the building was not yet occupied. 

Therefore, it did not contain nonstructural partitions and 

live load. 

The dynamic analysis was performed with a mathematical model that considered only one-quarter of the building 

above the plaza (ground) level with appropriate boundary 

conditions. Translational and rotational responses were assumed to be uncoupled (Stephen and others, 1974). This 

assumption is valid for a symmetrical structure such as 

the Transamerica Building. Although the attachments of 

exterior panels were detailed with the intention to minimize their effect on the lateral stiffness of the building 

during small amplitudes of vibration, Stephen and others 

(1974) concluded that the panels contributed significantly 

to the lateral stiffness. 

TRANSAMERIC2k49FL. (EW) 

TIME (SEC) TIME (SEC) 

Figure 13.-A, East-west component of acceleration at 49th floor. B, Amplitude spectrum of east-west component at 49th floor. C, East-west displacement 

at 49th floor. D, Rocking contribution of acceleration at 49th floor. E, Amplitude spectrum of rocking contribution of acceleration at the 49th floor clearly 

displaying the 2 Hz (0.5 s) rocking frequency (period). F, Rocking contribution of displacement at 49th floor. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

TRANSAMERICA BUILDING 

i A An GROUND FL (081) - CH 10 

6oth FLOOR 844'- 8" 

57th FLOOR 795'-2" 

54th FLOOR 743'-8" 

5 lst FLOOR 693'- 2" 

5oth FLOOR 676'-4" 

4Bth FLOOR 640'- 6" 

- 

BEAMS VARY 

FROM W 14 

TO W 30 

- TIME (SEC) dn BEAMS 

GROUND FL (171) - CH 9 

S 55- 

g BASEMAT (171) 

J 00- 

83 GROUND FL. (171) - BASEMAT (171) 

-5 -^vu.-- -

0 10 20 30 40 50 60 

TIME (SEC) 

42 th FLOOR 559'-6" 

GIRDER 

36"' FLOOR 478'-6" 

30"' FLOOR 397'-6" 

27th FLOOR 357'-0" 

24'" FLOOR 316'-6" 

TRANSAMERICA BUILDING 

BASEMAT (VERTICAL) - CH 7 

BASEMAT (VERTICAL) - CH 13 1 

d 00 BASEMAT (VERTICAL) - (CH 7 - CH 13) I FLOOR 235'-6" 

TIME (SEC) 12"' FLOOR 154'-6" 

5 FLOOR 60' 

LOBBY OR 0' 

PLAZA LEVEL <c 1 ROTATION= BASEMAT VERTICAL(CH7-CH 13115898 

TIME fSEC1 

EAST ELEVATION 

Figure 14.-Rotation of basemat and basement walls at Transamerica Building. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

- TRANSAMERICA : NORMALIZED RESP. SPECTRA [LOMA PRIETA] 

PERIOD (S) 

Figure 15.-Normalized response spectra for records from three components of the triaxial accelerograph at basement of Transamerica 

Building. 
C28 PERFORMANCE OF THE BUILT ENVIRONMENT 

EMBARCADERO BUILDING 

(SAN FRANCISCO) 

The 47-story Embarcadero Building (No. 4) is the first 

building in the United States constructed with eccentric 

bracing. The building, approximately 97 km from the epicenter of the earthquake, was constructed in 1979 in 

accordance with the UBC 1976 design provisions but using design response spectra relating to much higher seismic performance requirements. 

The Embarcadero Building is 172 m (564 ft) high. The 

building actually consists of two structures; an abovegrade 47-story building and an above-grade 3-story building. The below-grade 2-story shear walls and diaphragms 

are common to the two structures. The 47-story tower, 

referred to as the Embarcadero Building herein, is a momentresisting steel-framed structure. Four center north-south 

frames are eccentrically braced (two up to the 41st floor 

and two up to the 29th floor). Plan dimensions decrease 

above the 39th and again above the 41st floors. Eccentric 

diagonal braces consist of double angles which are offset 

by approximately 1.5 m (4.5 ft) from the column-beam 

intersections. Recent codes (Uniform Building Code, 199 1) 

term this type of structural framing as special momentresisting frames. A general view of this building, the vertical sections showing the eccentric bracing and a typical 

plan view with significant dimensions, and a three-dimensional view of the building and instrumentation scheme 

are provided in figure 16 (Celebi, 1993). 

The Embarcadero Building is located in the Lower Market area of San Francisco, which is reclaimed fill area 

well known for its soft-soil characteristics that amplify 

ground motions originating at long distances. Due to these 

amplified motions, the Embarcadero Freeway, located 

within 100 m of the Embarcadero Building suffered extensive damage during the earthquake and was razed in 

1991 (fig. 16A). The building was not damaged during the 

earthquake. 

The building base is a 1.67-m (5 ft)-thick, reinforced 

concrete mat supported by approximately 50-67-m (150- 

200 ft)-long composite concrete and steel bearing piles. 

The underlying soil media consists of approximately 8.5 

m of a top layer of silty fine sand fill with rubble (estimated shear wave velocity, V , 200 m/s, followed by 25 

m of soft very dark greenish-gray Holocene silty clay 

(Bay mud, V 150 mls), 9 m of sand (V , 250 mls) and 

21.5 m of very stiff to hard silty clay (old Bay mud, Vs 

230 m/s ). The rock at 64 m depth is sandstone with V of 

approximately 1,000 m/s (T. Fumal, personal commun., 

1992). The transfer function, from rock to the surface, 

based on estimated shear-wave velocities of the layered 

media (provided by Fumal) and calculated by Haskell's 

formulations (Haskell, 1953, 1960; Silva, 1976) using software by C.S. Mueller (personal commun., 1992), is shown 

in figure 17. From this, a site frequency (period) of 0.8 

Hz (1.25 s) is inferred. Alternatively, using the wellknown formula Ts = 4HIV , a depth (H) of 64 m, and an 

average shear-wave velocity (Vs) of 200 m/s, an approximate site frequency (period) of 0.77 Hz (1.3 s) is obtained 

(Celebi, 1993). The match between the calculated site periods is very good. The significance of the site period is 

discussed later in this section. 

The site characteristics of any area are deemed very 

important. To further assess the impact of soft-soil characteristics on a densely built-up urban environment, such 

as the Lower Market area of San Francisco, the USGS has 

installed downhole accelerograph arrays within 40 m of 

the Embarcadero Building (between the building and the 

razed Embarcadero Freeway). 

For the design of the building, site-specific design 

response spectra based on two levels of performance of 

the building were used. The first level of performance 

requires elastic response without structural or nonstructural 

damage under a moderate earthquake (M=7) that is likely 

to occur during the economic life of the building. The 

second level of performance demands that the structure 

will not collapse under the most severe (major) earthquake (M=8.3) that could occur during the economic life 

of the building. Substantial structural and nonstructural 

damage (without collapse) is considered acceptable 

under such an event. Design response spectra based on 

United States Nuclear Commission Regulatory Guide 1.60 

(United States Nuclear Regulatory Commission, 1973) of 

earthquake level I (anchored at zero period acceleration, 

0.3 g and 3 percent damping) and earthquake level I1 

(zero period acceleration, 0.5 g and 7 percent damping) 

are provided in figure 18, which also shows the 1976 

Uniform Building Code spectrum for comparison (Celebi, 

1993). 

The strong-motion instrumentation scheme implemented 

in 1985 by the California Strong Motion Instrumentation 

Program of the California Division of Mines and Geology 

at six different levels of the building is shown in figure 

16C (Shakal and others, 1989). There are 6 digital seismic accelerographs (DSA-1) with a total of 18 channels 

of syncronous, uniaxial (FBA-11) and biaxial (FBA-21) 

force-balance accelerometers within the structure. One of 

the unidirectional accelerometers is in the adjacent building basement. The reference north-south orientation is 345' 

clockwise from true north (fig. 16C). 

The processed 120 seconds of the recorded acceleration 

and the calculated displacements at different levels are 

shown in figures 19 and 20. This data has been band-pass 

filtered with ramps at 0.07-0.14 Hz and 23-25 Hz (Shakal 

and others, 1989). Peak accelerations and displacements 

are summarized in table 4. In figures 19 and 20 and in 

table 4, it is noted that the north-south peak responses 

(accelerations and displacements) at the 39th floor are 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

Figure 16.-A, Embarcadero Building. Also shown is one of the damaged columns of Embarcadero 

Freeway, razed in 1991. B, Typical plan view and vertical sections of Embarcadero Building. C, 

Three-dimensional view and instrumentation scheme of Embarcadero Building. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

LONGITUDINAL SECTION TRANSVERSE SECTION 

(FRAMES 5 & 6 - 

TO 41ST FLOOR) 

TRANSVERSE SECTION 

(FRAMES 4 & 7 - 

TO 29TH FLOOR) 

ALL DIMENSIONS 

IN METERS 

Figure 16.Ã‘Continued 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

Figure 16.Ã‘Continued 
PERFORMANCE OF THE BUILT ENVIRONMENT 

SITE TRANSFER FUNCTION OF 47-STORY OFFICE BLDG. -- SF 

2 2.5 3 

FREQ (HZ) 

Figure 17.-Site transfer function of Embarcadero Building site. 

DESIGN RESPONSE SPECTRA -- 47-STORY OFFICE BLDG. (SF) 

11- [M = 8.3, ZPA = 0.5 g, DAMP = 7 %] 

UBC 76 - 

PERIOD (SEC) 

Figure 18.-Design response spectra of Embarcadero Building. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

1 47-STORY OFFICE BLDG.(SF)Ã‘LPEÃ‘ (345) 

39TH. FL:m14 (WEST END) - NS (345) 

47-STORY OFFICE BLDG.(SF)Ã‘LPE--N (345) 

1500 

39TH. FL:CH15 (EAST END) - NS (345) 

TIME (SEC) 

47-STORY OFFICE BLDG.(SF)Ã‘LPEÃ‘ (075) & VERTICAL 

-200 

:CHI (NORTH WALL) - UP 

0 20 40 60 80 100 

TIME (SEC) 

Figure 19.-Recorded accelerations. This data has 

! been band-pass filtered with ramps at 0.07-0.14 

Hz and 23-25 Hz (Shakal and others, 1989). 
c34 PERFORMANCE OF THE BUILT ENVIRONMENT 

47.STORY OFFICE BLDG.(SF)--LPEÃ‘NS(345 

47.STORY OFFICE BLDG.(SF jLPEÃ‘NS(345 

44TH.FL(EAST END) 

CH8 GROUND LEVEL (WEST END-NS345) 44 

CH6 BASEMENT (EAST END-NS345) -50 0 

TIME (SEC) 

47-STORY OFFICE BLDG.(SF)-LPE-EW(075) & VERTICAL 

I BASEMENTtCH2 (SOUTH WALL) - UP 

0 20 40 60 80 100 1 

TIME (SEC) 

Figure 20.-Recorded displacements. This data 

has been band-pass filtered with ramps at 0.07- 

0.14 Hz and 23-25 Hz (Shakal and others, 1989). 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c35 

Table 4.-Peak accelerations and displacements for Embarcadero Building 

Level Sensor Direction Acceleration Displacement 

44 16 EW 

44 17 NS 

44 18 NS 

39 14 NS 

39 15 NS 

16 11 EW 

16 12 NS 

16 13 NS Podium 9 NS 

Podium 10 NS 

GR 7 EW 

GR 8 NS 

BasementB 4 NS 

Basement B 5 EW 

BasementB 6 NS 

BasementB 1 UP Basement B 2 UP Basement C(*) 3 EW 

* At C level below adjacent building. 

less than the north-south peak responses at the 16th floor. 

This may be attributed to several reasons, including the 

discontinuity of stiffness at the 39th and 41st floors, as 

discussed later. 

Figure 21 shows results of system-identification analysis, using 80 seconds of the recorded basement accelerations as input and the recorded 44th floor accelerations as 

output. Figure 22 shows the same, using displacements to 

obtain better identification at the lower frequencies. Both 

approaches exhibit excellent match between the calculated 

and observed outputs (Celebi, 1993). 

The modal acceleration contributions are extracted from 

the system identification analyses and compared to the 

total response at the 44th floor for the most significant 

four frequencies that contribute to the overall motions of 

the building (fig. 23, 0.19, 0.57, 0.98, and 1.33 Hz for the 

north-south direction; fig. 24, 0.16, 0.46, 0.77, and 1.06 

Hz for the east-west direction). The figures also show the 

Fourier amplitude spectrum of each mode superimposed 

on that of the total response. The damping values, also 

determined from system identification procedures, are provided in these figures and summarized in table 5. It is 

noted that the four significant modal periods in each of 

the principal axes of the building follow the general rule 

of thumb approximation of T, Tl3, Tl5, and Tl7. For all 

four modes in each of the two principal axes, modal damping percentages determined by system identification vary 

between 1.4 and 3.7 percent (table 5). As expected, the 

damping percentages of the north-south (braced) direction 

are lower than the east-west (unbraced) direction. The 

east-west modal damping percentages vary between 2.5 

percent and 3.7 percent. The damping percentage of the 

east-west fundamental mode, when determined alternatively by the logarithmic decrement approach using the 

44th floor displacements, yields 3.5 percent. The fundamental mode damping percentages are 2.5 percent and 3.7 

percent for north-south and east-west, respectively (table 

5). In summary, such low modal damping percentages 

explain why the response records are longer than the processed 120 seconds (figs. 19 and 20) (Celebi, 1993). 

The extracted modal displacements at the top three 

north-south instrumented floors are grouped and plotted 

in figure 25. A separate system identification analysis is 

performed for each of the additional input-output pairs 

(39th and ground floor, and 16th and ground floor). This 

is done primarily to investigate why the 39th level northsouth motions are smaller than those of the 16th floor. It 

is noted in figure 25 that the third-mode contribution to 

the 39th floor motions is much smaller than at the 44th or 

the 16th floor. On the other hand, while the second-mode 

contribution is comparable in amplitude for the 44th, 39th, 

and 16th floors, it is in phase between the 44th and the 

39th floor, 180' out of phase between the 39th and 16th 

floors. This anomaly of reduced motions at the 39th floor 

may be directly attributable to the discontinuity of the 

stiffness at the 40th floor, causing (1) the 39th floor to 

behave as the upper nodal point of the third mode, (2) a 

whipping effect of the top floors, and/or (3) a resonating 

appendage effect of the floors above the 39th floor, where 

the bracing is discontinued and the plan of the floors gets 

smaller (in stiffness and mass). Whatever combination 

occurs, the main reason is the change in stiffness (Celebi, 

1993; Astaneh and others, 1991; Chen and others, 1992). 

The explanations provided above are also related to 

discussions on drift. For the north-south direction, figure 

26 shows superimposed drift ratio time-history between 

the 44th and 39th floors and between the 44th floor and 

street level. Figure 26 shows superimposed drift ratios 

between the 16th floor and street level, between the 39th 

and 16th floors, and between the 44th floor and street 

level. When average drift ratios between the 44th floor 

and street level are considered, the peak is approximately 

0.0014 or less (28 percent of 1976 Uniform Building Code 

allowable). Average drift ratio exceeds the 0.005 allowable only between the 44th and 39th floors (with a peak 

of 0.006). This may be attributed to either the discontinuity of the eccentric bracing above the 40th floor or the 

whipping effect at the top floor, which cannot be confirmed since there are no sensors at the two consecutive 

top floors. Similarly, for the east-west direction, the superimposed drift ratio time-histories between the 16th floor 

and street level, between the 44th floor and the 16th floor, 

and between the 44th floor and street level are provided 

in figure 26. Since, in the east-west direction, there are no 

sensors on the 39th floor or the two top consecutive floors, 

it is not possible to determine whether the code-allowable 

drift ratio of 0.005 was exceeded. However, there appears 

to be a mostly consistent average drift ratio with a peak of 

less than 0.002 in this direction. Maximum drift ratio is 

0.0023 between the 44th and 16th floors. Therefore, drift 

ratios on the average were about 40 percent (or less) of 
PERFORMANCE OF THE BUILT ENVIRONMENT 

47-STORY OFFICE BLDG. (SF) -- LOMA PRIETA EQ. 

TIME (SEC) 

44TH.FL :CHI 7 (NS) GR0UND:CHS (NS) I I I I 

FREQ (HZ) FREQ (HZ) 

47-STORY OFFICE BLDG. (SF) -- LOMA PRIETA EQ. 

1000 I I I I 1 I I 

44TH.FL.--CHl6--EW--RECORDED 

44TH.FL.--CHl6--EW--CLCUMTED 

- 

GROUND FL.--CH7--EW--RECORDED 

I I I I I I I 

0 10 20 30 40 50 60 70 80 

TIME (SEC) 

- 44TH.FL.tCH16 (EW) GROUND:CH7 (EW) I I I 

FREQ (HZ) FREQ (HZ) 

Figure 21 .-System identification using ground-floor accelerations as input and 44th-floor accelerations as output for Embarcadero Building. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY C37 

47-STORY OFFICE BLDG. (SF) -- LOMA PRIETA EQ. 1 I I I I I 

40 60 

TIME (SEC 

44TH.FLCH17 (NS) I I I 1 1 

1 0.19 Hz 

5' s s 

% 4000 recorded 

I4 

n calculated 

2 2000 0.98 

GROUND:CH8 (NS) 

I I 1 I 1 

recorded . 

"0 0.5 1 1.5 2 2.5 "0 0.5 1 1.5 2 2.5 

FREQ (HZ) FREQ (HZ) 

47-STORY OFFICE BLDG. (SF) -- LOMA PRIETA EQ. 

TIME (SEC) 

44TH.FLCH16 (EW) GROUND:CH7 (EW) 1 I 1 1 I I 

calculated 

recorded 

U 300v 

recorded - 

- 

L - 

2 2.5 

FREQ (HZ) FREQ (HZ) 

Figure 22.-System identification using ground-floor displacements as input and 44th-floor displacements as output for Embarcadero Building. 
C38 PERFORMANCE OF THE BUILT ENVIRONMENT 

NS - MODE 1 CONTRIBUTION 

500 7 I I 1 

TIME (SEC) 

500 1 NS - MODE 2 C0N'I'R.IBUTION I I I 

. . 

TIME (SEC) 

500, NS - MODE 3 CONTRIBUTION n I I I . . I 

TIJME (SEC) 

500, NS - MODE 4 CONTRIBUTION I 1 I I 

,. 

- -500; I 1 I I 20 40 60 SO 

TIME (SEC) 

XI@ NS - SPECTRA ? {I 1 MODE 1 

FREQ (HZ) 

XI@ NS - SPECTRA I! I 

5; MODE 2 

FREQ (HZ) 

4 11 I c MODE 3 A 

"0 1 2 3 

FREQ (HZ) 

XI@ NS - SPECTRA 1 1 

MODE 4 

FREQ (HZ) 

Figure 23.-Significant north-south modal accelerations (and their amplitude spectra) of the 44th-floor for Embarcadero Building extracted from 

system identification analysis and comparison with the total 44th-floor acceleration. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c39 

MODE 1 

0.16 

CONTRIBWrIOh. 

-; 4 It- - :*: 

1 

0 20 40 60 "0 1 2 3 

TIME (SEC) FREQ (HZ) 

EW - MODE 2 CONTRIBUTION I I 1 ; : . . 

TIME (SEC) FREQ (HZ) 

400 1 I 1 1 I EW - MODE 3 CONTRIB~ON EW - SPECTRA 

.mol n I1 I 1 

TIME (SEC) FREQ (Hz) 

400 I 

EW - MODE 4 CONTRIBUTION . I 

. I I 1 

TIME (SEC) FREQ (HZ) 

Figure 24.-Significant east-west modal accelerations (and their amplitude spectra) of the 44th-floor for Embarcadero Building extracted from system 

identification analysis and comparison with the total 44th-floor acceleration. 
C40 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 5.-Identified dynamic characteristics for 

Embarcadero Building 

NS EW MODE f T C f T C 

code-allowable ratios during the earthquake. It is safe to 

predict that the allowable drift ratios could be exceeded 

for the design level I1 earthquake. This conclusion is from 

studies of Astaneh and others (1991), Chen and others(l992), and celebi (1993). 

Torsional response and rocking of the building is insignificant (Astaneh and others, 1991; Chen and others, 1992; 

celebi, 1993). 

The amplitude at approximately 0.75-0.8 Hz (1.25- 1.33 

s), noted in the Fourier amplitude spectra (figs. 21, 22), is 

attributed to belonging to the site because the amplitude 

at the roof does not increase significantly when compared 

to that at the ground floor. Furthermore, the site period, 

estimated to be around 1.25-1 -40 s, falls within the range 

of modal periods of the building, and therefore the building may have been subjected to a double-resonance effect. It is noted that the site-specific response spectra (fig. 

18) do not adequately reflect the resonating site period 

(fig. 17) (celebi, 1993). 

MODAL amWONS47mRY OFF'ICE BIDG.(SF)-NS 

1m.m 

-15 

Figure 25.-Comparison of modal dis- 

-mo I I 

20 40 60 80 100 placement contributions at the top three 120 north-south instrumented floors for ErnTIME (SEC) barcadero Building. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c4 1 

x~o-~ 47-STORY OFFICE BUILDING (SF) - NS -- LPE 

........................................................ .................................................. ..*..... .......................................... ............................. 

I I 

.............- 

DASHED: BETWEEN 44TH & 39TH FLOORS 

0 

SOLID: BETWEEN 44TH & STREET LEVEL ....................................... ............................ ...!.............. ................................... ................................................ .......................................! .................... 

TIME (SEC) 

xi o-~ 47-STORY OFFICE BUILDING (SF) -- NS -- LPE 

4 DASHED: BETWEEN 39TH & 16TH FLOORS 

DASHDOT: BETWEEN 16TH & STREET LEVEL - 

SOLID: BETWEEN 44TH & STREET LEVEL 

TIME (SEC) 

x~o-~ 47-STORY OFFICE BUILDING (SF) - EW -- LPE 

0 20 40 60 80 100 120 

TIME (SEC) 

Figure 26.-Drift ratios for Embarcadero Building. 
C42 PERFORMANCE OF THE BUILT ENVIRONMENT 

In figure 27, rotation time-histories (calculated from 

displacement time-histories) of the mat as well as the basement walls are shown. The significance of this plot 

is that (1) the mat rotation has a peak value of 0.000057 

radians (0.0033' ) while the peaks of the wall rotations 

are (north-south) 0.0004 radians (0,023') and (east-west) 

0.00048 radians (0.028'), and (2) the frequencies of these 

rotations are the same as those of significant translational 

frequencies in respective directions, thus confirming the 

presence of structural frequencies in the basement motions. The anomaly here is that the rotations of the walls 

are 7-8 times larger than the rotations of the mat. Although there may be some numerical errors related to data 

processing, these ratios are too high to ignore. Similar 

magnitude peak rotations were observed from the response 

data of the nearby Transamerica Building (Celebi and 

Vafak, 1991; Soydemir and Celebi, 1992). 

In addition to detailed analyses presented above, Chajes 

and others (1992) used an approximate technique to conduct dynamic analysis of the Embarcadero Building. Their 

analysis utilizes a continuum methodology to create a reduced-order representation of the building. The accuracy 

of the approximate dynamic analysis is established by comparing the computed results to the actual response recorded 

during the earthquake. 

Astaneh and others (1991) and Chen and others (1992) 

performed detailed dynamic elastic time-history and response spectrum analyses of the Embarcadero Building 

and compared the calculated responses with actual 

recorded responses. They varied several parameters in the 

development of their mathematical models. The model 

buildings were then subjected to ground accelerations 

recorded at the basement level of the building during the 

earthquake. The set of analyses using the mathematical 

models produced floor displacement time-histories 

which were then compared with the recorded values. The 

results show that at Loma Prieta level ground motions, 

the design seems sufficient. They also performed nonlinear dynamic analyses using two-dimensional models 

of a typical frame, whereby the mathematical models were 

subjected to base excitations of (1) actual recorded motions during the earthquake, (2) motions represented by 

scaling-up of the peak accelerations of the earthquake 

record, and (3 ) motions of a hypothetical San Andreas 

M=8+ plus event to simulate a major earthquake. The 

trends in development of plastic hinges throughout the 

structures for these cases indicate that significant inelastic 

behavior occurs at the floors between the 36th and 42d 

levels and is attributed to the change in stiffness at those 

levels. The development of plastic hinges for cases 2 and 

3 described above are exhibited in figures 28 and 29 

(Astaneh and others, 1991; Chen and others, 1992; 

Bonowitz and Astaneh, 1994). These figures are important in that they indicate the behavior that may be expected from the Embarcadero Building during future 

earthquakes with large input motions (either due to larger 

magnitude earthquakes or earthquakes at closer distances 

to the building). 

TIME (SEC) 

Figure 27.-Rotation of basemat compared to rotation of basement walls for Embarcadero Building. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

45 

40 

35 

30 

25 

20 

15 

10 

5 

2 

Mezz 

Pod 

Lob 

st. 

A 

ENVELOPE OF PLASTIC HINGES - E-W FRAME 

(2.75~ LOMA PRIETA RECORD) 

Note: Not to Scale Accumulated P.H. Rotation: 

Â Plastic Hinge < .005 Radians 

,005 - ,010 Radians 

,010 - .020 Radians 

> .020 Radians 

Figure 28.-Development of plastic hinges formed in the east-west frame modeled by Astaneh and others 

(1991) for 2.75 times the Loma Prieta earthquake motions (with permission of A. Astaneh). 
45 

40 

35 

30 

25 

20 

15 

10 

5 

2 

Mezz 

Pod 

Lob 

Str 

A 

PERFORMANCE OF THE BUILT ENVIRONMENT 

ENVELOPE OF PLASTIC HINGES - E-W FRAME 

(SAN ANDREAS EVENT) 

Note: Not to Scale Accumulated P.H. Rotation: 

Â Plastic Hinge < .005 Radians 

Â .005 - .010 Radians 

.0 10 - .020 Radians 

> .020 Radians 

Figure 29.-Development of plastic hinges formed in the east-west frame modeled by Astaneh and others 

(1991) for a hypothetical M=8+ San Andreas event (with permission of A. Astaneh). 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c45 

SANTA CLARA COUNTY OFFICE BUILDING 

(SAN JOSE) 

One of the buildings that benefited from studies of its 

recorded responses during strong-motion events is the 

Santa Clara County Office Building in San Jose (fig. 30). 

A decision to retrofit was based on response data of the 

building recorded during the Loma Prieta earthquake and 

two previous but smaller earthquakes, the 24 April 1984 

Morgan Hill (Ms=6. 1) and the 3 1 March 1986 Mt. Lewis 

(Ms=5.5) (Huang and others, 1985; R. Darragh, personal 

commun., 1991). The set of data from these three earthquakes has been studied by Lin and Papageorgiou (1989), 

Boroschek and others (1 WO), Boroschek and Mahin 

(1991), and Celebi (1994a). Although this building did 

not suffer structural damage during these earthquakes, it 

suffered extensive nonstructural and contents damage. 

Rihal (1994) studied the performance of nonstructural 

members of the building during these three earthquakes, 

when the occupants were uncomfortably and severely 

shaken during the prolonged and resonating vibration of 

the building. Consequently, in 1994, the building was retrofitted with viscous elastic dampers (Crosby and others, 

1994). 

A general view of the Santa Clara County Office Building is seen in figure 30A. The location of the building, the 

epicentral locations of the three earthquakes referred to 

above, and the instrumentation scheme are shown in figure 305 (Celebi, 1994a). This 13-story, 56-m-tall, moment-resisting steel-framed building was built in 1975 

according to the Uniform Building Code (1970). There 

are six column lines in each direction of the approximately 51x51m plan of the building. 

Figure 31 shows acceleration responses at the roof recorded during the three earthquakes. The figure clearly 

indicates the long-duration and resonating responses of 

the structure and the beating effect observed in the responses. Figure 32 shows typical system identification 

analysis results (Celebi, 1994a) for only Loma Prieta data. 

Peak accelerations at the roof and basement levels, the 

fundamental translational and torsional frequencies (and 

periods), and damping percentages extracted by system 

identification techniques (for all three earthquakes) are 

presented in table 6. The second and the third modes (at 

approximately 1.45 and 2.50 Hz) contribute very little to 

the overall response; therefore, the remainder of the discussion will be devoted to the fundamental modes only. 

The small differences between the periods determined from 

the three earthquake response data are noted. Since the 

structural characteristics of the building during the three 

earthquakes are very similar, only Loma Prieta is chosen 

to further characterize the structural response. 

Figure 33A shows acceleration responses at the roof 

level, as well as the difference of parallel accelerations at 

the roof level (nominal torsional accelerations-actual torsional accelerations can be calculated by dividing the nominal torsional accelerations by the distance between the 

two parallel sensors). Figure 335 shows the amplitude 

spectra of the unidirectional responses clearly peaking at 

the translational frequency (period) 0.45 Hz (2.22 s). Figure 33C shows amplitude spectra of the nominal torsional 

accelerations (calculated from parallel records in the northsouth and east-west directions respectively) that peak at 

0.57 Hz (1.69 s) and 0.45 Hz (2.22 s). Figures 33D and E 

show the coherence, phase angle, and normalized crossspectra of the unidirectional response and confirm the 

translational frequency at 0.45 Hz. At this frequency, the 

motions are coherent and are in phase, clearly indicating 

that they are related. Figure 33F, on the other hand, confirms the torsional frequency at 0.57 Hz that were identified from the nominal torsional accelerations represented 

by the differential accelerations of two parallel sensors at 

a floor. There is unity coherence and the phase angle is 

zero at 0.57 Hz (Celebi, 1994a). The proximity of the 

torsional frequency (period) at 0.57 Hz (1.69 s) to the 

translational frequency (period) at 0.45 Hz (2.2 s) causes 

the observed coupling and beating effect. 

Lin and Papageorgiou (1989) studied the Santa Clara 

County Office Building response data from the Morgan 

Hill earthquake and concluded that strong beating-type 

phenomena occur in buildings with identifiable closecoupled torsional and translational modal characteristics, 

as in the case of the Santa Clara County Office Building. 

Boroschek and Mahin (1991) further elaborated on this 

issue. In their investigation of the behavior of this lightly 

damped, torsionally coupled structure, they developed 

three-dimensional linear and nonlinear numerical models 

and performed several elastic and inelastic computer analyses. As in the recorded data, Boroschek and Mahin found 

that the calculated responses of the building are characterized by (1) long duration, narrow-banded periodic motions with strong amplitude modulation, (2) large 

displacements and torsional motions, (3 ) large amplification of the input ground motions, and (4) slow decay of 

the building's dynamic responses. They concluded that 

the unusual response characteristics of the building are 

due to design parameters that produced a structural system with low equivalent viscous damping, resonance, and 

beating. 

The close-coupling of the torsional and translational 

frequencies at low damping percentages (lightly damped 

system) clearly explains that the translational and torsional 

modes reinforce one another during vibration, with only 

small dissipation, and that beating occurs with a period of 

Tb=2TlT2/(Ti-T2)=2(2.22)(1 .69)/(2.22-1.69)~ 14.2 s 

(Borosheck and Mahin, 199 1; Celebi, 1994a). 

The mat foundation of the building rests on alluvial site 

conditions. The depth to bedrock at the site is estimated 

to be between 270 m and 500 m. Figure 34 shows the site 

transfer function plots for two estimated depths to 
PERFORMANCE OF THE BUILT ENVIRONMENT 

Figure 30.-A, Santa Clara County Office Building (SCCOB). B, Location of Santa Clara County Office Building, its overall dimensions and 

instrumentation, and epicenters of three nearby earthquakes. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c47 

bedrock and shear-wave velocities (Vs) assigned to each 

layer based on available geotechnical reports (Earth Sciences Associates, 1971). The figure indicates that the site 

is capable of generating resonating surface waves at low 

frequencies that are, as will be shown, close to the frequencies of the building. In a study of the Santa Clara 

Valley following Loma Prieta, Frankel and Vidale (1992) 

concluded that 2-5 second long-period motions in the basin can be generated during earthquakes. 

Figure 35 shows the coherence, phase angle, and normalized cross-spectra for the roof and basement motions 

calculated only for the east-west direction, since at the 

basement the two parallel sensors are in this direction. 

The proximity of the site frequency at 0.33-0.38 Hz and 

the fundamental frequency at 0.45 Hz is the cause of the 

resonating (more or less steady-state surface wave) motions of the building. This is simply explained using the 

relationship for the amplification (A) of a damped system 

(in percent), with a frequency ratio (r ) of ground to structure): 

For a lightly damped system with r=0.33/0.45=0.73 or 

0.38/0.45=0.84, significant amplification in the response 

can be expected (for example, ~=l/(l-r2)=2.14-3.40) 

(Celebi, 1994a). 

The average drift ratios calculated between the roof 

and the basement and between the 12th floor and 2nd 

floor reach 0.8 percent and exceed allowable code drift 

ratios (0.5 percent). The drift ratio between the roof and 

12th floor is smaller than the average drift ratio or the 

code allowable. 

In a recent study, Porter (1996) showed that the observed structural response of the building can be explained 

by the geometrical configuration between earthquake epicentral location and the orientation of the structure. 

Rihal (1994) studied nonstructural damage in the building following the earthquake to correlate the recorded California Strong Motion Instrumentation Program response 

data with observed nonstructural component damage. A 

methodology is presented to assess the performance and 

behavior of nonstructural building components during 

earthquakes. One main objective of this case study was to 

investigate the relationship between seismic response parameters (for example, peak response acceleration levels, 

frequency content, and inter-story drift levels) and corresponding nonstructural component damage observed during the earthquake. Significant nonstructural component 

damage was observed to have occurred, particularly at the 

7th and 11th floor levels. Comparison of the observed 

nonstructural damage and peak recorded accelerations at 

the 7th floor and at the 12th floor show the thresholds of 

response accelerations that produce nonstructural component damage. Rihal proposed a nonstructural component 

damage index expressed as a percentage of components 

damaged to characterize observed nonstructural component damage data. 

In retrofitting the building, Crosby and others (1994) 

installed viscous elastic dampers in selective bays of the 

building. Figure 36A and B show the plan view and vertical sections. The bays of the framed structure where the 

dampers were installed are indicated in the figures. Figure 

36C shows a typical viscous elastic damper (Crosby and 

others, 1994). Celebi and Liu (1997) performed ambient 

tests of the building following the retrofit; preliminary 

results show that for low-amplitude excitation, the dampers produce small changes in the dynamic characteristics 

but are expected to alter the dynamic behavior significantly during strong shaking. Before the retrofit, Marshall 

and others (1991) and Celebi (1996) observed significant 

differences in the dynamic characteristics of the building 

for strong- and low-amplitude shaking. This is discussed 

later in this paper. 

RESPONSE AR ROOF OF SCCOB: CH6 (NS) - BEFORE RETROFIT 

I I I I I I I 

............................... 

RGAN HILL EQ.(I 984) 

LOMA PRIETA EQ. (1989) 1 

0 20 40 60 80 100 120 

TIME (S) 

Figure 31.-Acceleration responses at the roof of Santa Clara County Office Building recorded during the 

Lorna Prieta, Morgan Hill, and Mt. Lewis earthquakes. 
C48 PERFORMANCE OF THE BUILT ENVIRONMENT 

In summary, there are three causes for the long-dura- standing the cumulative structural and site characteristics 

tion response of the Santa Clara County Office Building: that affect the response of the building is important in 

(1) basin effect and site characteristics that contribute to assessing earthquake hazards to other similar buildings 

resonating excitation, (2) the close-coupled translational- ((y'elebi, 1994a). The results emphasize the need to better 

torsional mode that causes beating phenomena to occur, evaluate structural and site characteristics in developing 

and (3) the inherent low-damping of the building. Under- earthquake resisting designs that avoid resonating effects. 

SAN JOSE-13-STORY GOVERNMENT OFFICE BLDG.-LPE 

~105 ROOF-OUTPUT-NS ~104 LOWER LV.-INPUT-NS 

Recorded 

solid: Recorded 

dashed: Calculated 

1 2 3 4 5 

1000 SAN JOSE-13-STORY GOVERNMENT OFFICE BLDG.-LPE 

6s 

500 

u 0 

d 

r> -500,l - - -- A 

TIME (SEC) 

ROOF-OUTPUT-EW 

0,45 $Iz I , 7 

CH4 

solid: Recorded - 

dashed: Calculated 

I I - 

xl04 LOWER LV.-INPUT-:EW 

"[ WI Recorded 

Figure 32.-Typical system identification analysis results of Santa Clara County Office Building for Lorna Prieta data 

(celebi, 1994). 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

Table 6.-Peak accelerations and dynamic characteristics for Santa 

Clara County Office Building 

Earthquake 

Loma Prieta Morgan Hill Mt. Lewis 

Peak Acceleration (g) Roof (NS) 0.34 0.17 0.32 

Roof (EW) 0.34 0.17 0.37 

Base (NS) 0.10 0.04 0.04 

Base (EW) 0.09 0.04 0.04 

Translational Period [T (s)] 2.22 2.17 2.08 

Frequency [f (Hz)] 0.45 0.46 0.40 

Damping [c (pet.)] 2.70 1.95 2.12 

Torsional Frequency [f (Hz)] 0.57 0.59 0.58 

Damping [c (pet.)] 1.69 1.70 1.72 

SJ: ROOF: LPE 

D TIME (SEC) 

CH6 & CH7 (NS) 

FREQ (HZ) 

xl05 SPECTRA: ROOF: LPE - - xW SPECTRA: ROOF: WE 

0.45' I I 

3 - 1'10.57 : 

FREQ (HZ) 

FREQ (HZ) FREQ (HZ) 

Figure 33.-A, Acceleration time histories of roof of Santa Clara County Office Building. B, C, Amplitude spectra. D, E, F, Coherence (solid line), 

phase angle (dashed line) plots with cross-spectra, Sxy, (dashed-dot line) superimposed to distinguish translational and torsional frequencies. 

SITE TRANSFER FUNCTION: SCCOB: ESTIMATED PARAMETERS 

7 1 1 

DEPTH (M) Vs 

FREQ. (HZ) 

Figure 34.-Site transfer function for Santa Clara County Office Building. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

LPErROOF: CH4 & CH5 (EW) LPE:BASE: CH20 & CH21 (EW) 

Figure 35.-Cross-spectra (dotted line), coherence (solid line), and phase angle (dashed line) plots for east-west motions at the roof and basement of 

Santa Clara County Office Building to distinguish structural and site frequencies. 

&?*?-w BCDEFGHJKL -^-TO? 

SE15MIG DAMPER BAY LOCATION DIAGRAM (Plan) @ NOT TO SCALE A 

Figure 36.-A, Plan view of Santa Clara County Office Building. B, Vertical sections of building 

show bays where viscous elastic dampers have been installed for retrofitting the structural system to 

alter its dynamic behavior. C, Typical damper installed. (All figures courtesy of P. Crosby, The 

Crosby Group, Redwood City, Calif.) 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c5 1 

MECHANICAL 

ELEVENTH FL. 

. TENTH FL. 

NINTH FL. 

,EIGHTH FL 

SEVENTH FL. 

SIXTH FL. 

FIFTH FL. 

. FOURTH FL. 

THIRD FL. 

SECOND FL. 

GROUND FL. 

.LOWER LEVEL 

NOT TO SCALE 

SEISMIC DAMPER BAY LOC.AT10N 6RID LINE 2 5EISMIC DAMPER BAY =ATION GRID LINE M 

B 

SECTION O SQR. BARS I SECTION Â VISC^-ELASTIC MATERIAL Â¥CJUbÂ¥Â¥>I' c Â¥CMJkrir-o 

Figure 36.Ã‘Continued 
C52 PERFORMANCE OF THE BUILT ENVIRONMENT 

CHEVRON BUILDING (SAN FRANCISCO) 

The 42-story Chevron Building at 575 Market St. is 

a steel, moment-resisting framed, slender, rectangular 

in-plan building. It has two levels of basements and is 

built on 10-m-long precast pile clusters. A three-dimensional schematic of the building and its instrumentation 

scheme is shown in figure 37 (Celebi, 1992). Recorded 

accelerations at different levels of the building and corresponding displacements are shown in figure 38. System 

identification analyses results are shown in figure 39. 

Identified first-mode frequencies (periods) are 0.16 Hz 

(6.25 s) in the 225' direction and 0.21 Hz (4.76 s) in the 

135' direction. Figure 40 shows coherency and phaseangle plots for (1) two parallel 225'-oriented motions (horizontal plane) at the roof, (2) two parallel 225' motions 

(in the vertical plane), and (3) two parallel 135' motions 

(also in the vertical plane) of the building. From these 

plots, we conclude that (1) torsion is insignificant since 

the motions in the horizontal plane at the roof are in 

42nd Floor 

40th Floor 

34th Floor 

25th Floor 

3rd Floor 

1st Floor 

(Ground) 

2nd Basement 

33'2' 1 

A 

. 

CO 'all 

phase and coherent, (2) 0.55 Hz and 1.0 Hz in the 225' 

direction are the second and third modal frequencies since 

the phase angles are 180' and -180' out of phase, and 

(3) similarly for the 135' direction, 0.61 Hz and 1 Hz are 

the second and third modal frequencies. The recorded peak 

accelerations and extracted frequencies (periods) and 

damping percentages are summarized in table 7 (Celebi, 

1992). 

Safak (1993) studied the building in detail using a system identification method based on the discrete-time 

linear-filtering and the least-squares estimation techniques. 

He concluded that higher modes contribute significantly 

to the overall response and that soil-structure interaction 

occurs at 1.0 Hz. Anderson and Bertero (this chapter) 

studied the building also and concluded that the 

building remained elastic during the earthquake. 

They attributed this to the fact that the designers of the 

building opted to use site-specific design response spectra 

that was more conservative than the minimum code 

requirements. 

ma 

(46 Jm) Figure 37.-General three-dimensional view and 

instrumentation scheme of Chevron Building, San 

Francisco. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

575 MARKET ST SAN FRANCISCO - LOMA PRIETA EQ. 575 MARKET ST.. SAN FRANCISCO - LOMA PRIETA EO 

i 

! 

40- i BASE (135) Basement 'AWnfvs 

I 

.1oooO 5 10 15 24 25 30 35 40 45 40 45 j( TIME (SECT TIME (SEC) 

-Im0 5 I0 l3 24 s 30 35 40 45 Â¥so 5 I0 I5 10 2.5 30 3'5 40 45 i0 

TIME (SEC) TIME (SEC) 

BASEMENT (VERTICAL) 

~ -w&++-+-- 1 

Figure 38.-Acceleration and displacement time-histories of Chevron Building. 

Table 7.-Peak responses and dynamic 

characteristics of chevron Building 

A. Peak responses 

Accel. (g) Displ. (cm) 

Roof 0.3 1 18.6 

Basement 0.12 3.3 

B. Dynamic characteristics 

Mode ~irection f (Hz) T (s) 6 ( pct.) 

1 225 0.16 6.25 4.1 
PERFORMANCE OF THE BUILT ENVIRONMENT 

575 MARKET ST., SF -- LPE -- SYSTEM IDENTIFICATION 

200 

!^ 

6G 

S 0 

u .1 Recorded [SMA-CHI (135)l 

U -200 

u 

TIME (SEC) 

575 MARKET ST., SF -- LPE --SYSTEM IDENTIFICATION 

200r , T 

- I 

@ OUTPUT 

up------ 

I 

f -8 -400.--- 5 10 15 20 25 30 35 40 45 50 

TIME (SEC) 

Recorded [CHI01 

FREQ (Hz) FREQ (Hz) 

Figure 39.-System identification for Chevron Building. 

A xlv CROSS-SPECTRUM 

a 5 ^ 1Â° Oo 1 2 3 

xl@ CROSS-SPECTRUM l:EII 

Oo 1 2 3 

' xlO- CROSS-SPECTRUM 

1 2 3 

FREQ (HZ) FREQ (HZ) FREQ (HZ) 

COHERENCE & PHASE 180 COHERENCE & PHASE 180 COHERENCE & PHASE 

8 $5 

'"'> '' \, ; \f 

1 2 3 

FREQ (HZ) FREQ (HZ) 

Figure 40.Ã‘Cross-spectrum coherence, and-phase angle plots for (A) parallel (225') motions at 42d floor, (B) 

parallel (225') motions at 42d and 34th floors, and (C) parallel (135') motions at 42d and 34th floors. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c55 

MIXED CONSTRUCTION 

CSUH ADMINISTRATION BUILDING 

(HAYWARD) 

The 13-story California State University at Hayward 

(CSUH) Administration Building was constructed in 197 1 

in accordance with the Uniform Building Code (1967) 

design provisions. It is approximately 70 km from the 

Loma Prieta epicenter and is located within less than 5 

km of the Hayward fault, also capable of generating large 

earthquakes. The general location of the building relative 

to the epicenter is shown in figure 1 (marked as HAYW). 

A general view of the building is shown in figure 41A. A 

three-dimensional view of the building and instrumentation scheme are shown in figure 41 5. The availability of a 

free-field station on the university stadium grounds provides an opportunity to assess site and soil-structure interaction effects, if any. The building is 61.29 m high, with a 

structural system consisting of interior core moment steel 

frame and exterior perimeter concrete moment frame. The 

plan dimensions are 34.29~34.29 m. Up to the second 

floor, there are concrete shear walls around the elevator 

shafts. The building has a two-story extension bridge struc- 

(recorded input) and roof accelerations (recorded output) 

of the building are shown. System identification calculations were also performed using the 5th floor accelerations as recorded output. The three significant frequencies 

(periods) and damping values also determined from system identification procedures are summarized in table 9. 

Figure 45 shows the structural frequencies in the spectral 

ratios calculated from amplitude spectra of roof and basement motions. It is noted that the three significant modal 

periods in each of the principal axes of the building follow the approximation of T, Tl3, and Tl5. 

For all three modes in each of the two principal axes, 

modal damping percentages determined by system identification vary between 1.3 and 6.4 percent (table 9). 

ture (enclosed on the second story) at its east side connecting it to an adjacent building (fig. 415). The bridge 

structure is free to move on friction bearings at its juncture with the adjacent structure. The building sits on bearing piles with a 45-cm-thick reinforced concrete mat on 

grade (celebi, 1994b). 

Channels 2, 7, and 10 (parallel sensors in the northsouth direction at the roof, 2d, and 1st floors, respectively) (fig. 4 15) malfunctioned during the earthquake 

(Shakal and others, 1989); therefore, identification of 

torsional motions of the structure cannot readily be made. 

The processed 40 seconds of the recorded acceleration 

and the displacements at different levels are shown 

in figure 42. The data from the building have been 

band-pass filtered with ramps at 0.2-0.4 Hz and 23-25 

Hz (Shakal and others, 1989). The band-pass filter of the 

free-field data has a low-frequency ramp at 0.08-0.16 Hz. 

It is noted that the peak accelerations and displacements 

summarized in table 8 show significant differences, 

particularly in the displacement of the free-field versus 

the basement, due to the 0.08-0.16 Hz band-pass ramped 

filter of the free-field records. Also, longer periods 

are distinct in displacement plots of the free-field in figure 43. Such filtering errors and inconsistencies can 

provide misleading interpretations and can lead to wrong 

results in calculations using displacement (for example, 

drift). 

In figure 44, calculated output of system identification 

analysis using 40 seconds of the basement accelerations 

Figure 41.-A, California State University (Hayward) Administration 

Building. B, Three-dimensional schematic of building 
PERFORMANCE OF THE BUILT ENVIRONMENT 

The possibility of torsion was investigated by coherence function and phase angle plots as shown in figure 

46. At 0.76 Hz, the two horizontal orthogonal motions at 

the roof are not coherent and are not in phase, which may 

imply that torsion is insignificant. However, this may be 

misleading if the two horizontal sensors are located at or 

close to the center of rigidity of the floor, in which case 

the torsional contributions would not show in the unidirectional motions. Thus, the importance of acquiring two 

parallel motions at a floor is emphasized. 

Figures 465 and C show the peaks of the first three 

modes in the cross-spectrum of north-south and east-west 

motions at the roof and 5th floor, respectively, indicating 

that the coherence is unity for all modes and the phase 

angles are 0' at the fundamental frequency and 180Â°a the 

second and third modal frequencies. 

Figures 47A and B show 5 percent damped response 

spectra of the north-south and east-west free-field and 

basement motions of the building. Figure 47C shows the 

basement north-south and east-west response spectra superimposed. It is clear from these spectra that both freefield and basement have peaks in the range 0.25-0.3 s 

(3.3-4.0 Hz). Figure 47C shows roof and basement response spectra. Clearly, the first three modal frequencies 

(periods) can be identified directly from these spectra. 

In the absence of a geotechnical report, available 

geotechnical and geological data of the strong-motion 

station site at the university stadium are used. It is assumed that the site conditions at the Administration Building are similar to those at the stadium. The top 3-4 m 

consists of sandy clay and clay (estimated V of 175 m/s) 

followed by 9-10 m of deeply weathered rhyolite (V of 

315 m/s), followed by moderately weathered rhyolite (Vs 

of 825 m/s) (Fumal, 1991). The recommended average (to 

30 meter depth) V is 525 m/s (Fumal, personal commun., 

1991). 

Due to absence of field measurements of site period, 

and considering only the top layer, a site period of approximately 0.1 seconds is estimated using T = 4HIV. 

This estimated period implies that this particular building 

would not be significantly influenced by the site effects. 

Furthermore, no site amplification is expected. 

Figure 41 .-Continued. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c57 

HAYWARD--13 STORY SCHOOL OFFICE BLDG.--LPE HAYWARD--13 STORY SCHOOL OFFICE BLDG.--LPE 15 r 7 

1 ROOF: CH3 fNS1 . . I 

0 

1ST. FL: CHI1 (NS) 

5 

BASEMENT: CHI6 (NS) 

TIME (SEC) TIME (SEC) 

HAYWARD--13 STORY SCHOOL OFFICE BLDG.--WE HAYWARD--13 STORY SCHOOL OFFICE BLDG.--WE 15 800 

600 10 

s 

u 

d 0 J 

&! 0 

-200 ' 1ST. FL: CHI2 (EW) 

-400 -5 - 

-600 -10 

0 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40 

TIME (SEC) TIME (SEC) 

Figure 42.-Acceleration and displacement time-histories of California State University (Hayward) Administration Building. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 8.-Accelerations and displacements 

at California State University (Hayward) 

Administration Building 

Location Sensor Direction 

Roof 3 NS(3207 

4 

5th. floor. 5 

6 

2d floor. 8 

9 

1st floor. 11 

12 

Basement 1 

13 

16 

14 

15 

Free Field FF1 

FF2 

- FF3 

Accel. (g) Dipl. (cm) 

0.142 3.50 

CSUH: LPE: COMPARISON OF BASEMENT (SOLID) & FREE-FIELD (DOTTED) 

- a 

I I I I I I I 

0 5 10 15 20 25 30 35 40 

TIME (SEC) 

I I 1 , I 

5 10 15 20 25 30 35 40 

TIME (SEC) 

Figure 43.-Comparison of basement (solid line) and free-field (dash-dot lines) motions for California 

State University (Hayward) Administration Building. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

HAYWARD--13 STORY SCHOOL OFFICE BLDG.--LPE 

TIME (SEC) 

@ 

le 

s 

8 

4 -500 

x104 OUTPUT--ROOF--CH3 

dashed: Calculated 

246810 

FREQ (Hz) FREQ (Hz) 

I I r I I I I 

OUTPUT ROOF--NS-CH3-Recorded 

500Ã‘^^-^'-w't'^AA/Vl^iV^A/VS/^ 

INPUT BASEMENT--NS--Wl--Recorded 

HAYWARD--13 STORY SCHOOL OFFICE BLDG.--LPE I I I I I 

500 

I I 

0 5 10 15 20 25 30 35 40 

FREQ (Hz) FREQ (Hz) 

u 

Figure 44.-System identification for California State University (Hayward) Administration Building. 

U 

4 -500 INPUT BASEMENT--EW--CHI 4.-Recorded 

0 5 10 15 20 25 30 35 40 

TIME (SEC) 
PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 9.-Identified dynamic characteristics for California State University 

(Hayward) Administration Building 

NS EW 

MODE f T 6 f T 6 

(Hz) (s) (pet.) (Hz) (s) (pet.) 

Figure 45.-Spectral ratios for California State University (Hayward) Administration Building. 

LPE: CSUH: ROOF TO BASEMENT: SOLID (NS), DASHED (Ev 

20 - I 1 I I I I I 1 I 

A 

x104 CROSS-SPECTRUM XIO* CROSS-SPECTRUM CROSS-SPECTRUM 

ft0 

O012345 O012345 O012345 

15 

FREQ (HZ) FREQ (HZ) FREQ (HZ) 

FREQ (HZ) FREQ (HZ) FREQ (HZ) 

/,,.76 (Ns&EW); 

Figure 46.-Cross-spectrum, coherence, and phase-angle plots for California State University (Hayward) Administration 

Building. 

I 

-,,, ....,,,,.,.... I 

1 1 T I 

II 

11 I1 

!,,, ,,. .,,.,. . ,..,........,....,. ;..,,.....,,..... .. ..,,...,,....,.~ ................................... i ................................... i ...,,,.,,.,..... .................. 4 ............................ ...... ; .................................... i. .......................... . .... ; ..... . . . . .......... .. .- "' '' 

: 

: 

: 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

A 

CSW FF & BASE: NS (5 %I CSUH: FF & BASE: EW (5 %) 

c PERIOD (SEC) 

PERIOD (SEC) 

D PERIOD (SEC) 

CSUH: ROOF & BASEMENT (5 %) 

10-1 1 o1 

PERIOD (SEC) 

Figure 47.-Comparison of response spectra for California State University (Hayward) Administration Building. A, B, 

Free-field vs, basement. C, Orthogonal motions of basement, D, Roof and basement. 
C62 PERFORMANCE OF THE BUILT ENVIRONMENT 

TWO-STORY OFFICE BUILDING (OAKLAND) 

McClure (1991) analyzed the data from the two-story 

Oakland office building designed by him in 1964. The 

building was designed according to the 1961 Oakland 

City Building Code (same as 1961 Uniform Building 

Code). The building is essentially structural steel framed 

with reinforced concrete block masonry infill walls. A 

three-dimensional schematic of the building, its dimensions, and the instrumentation scheme is shown in figure 

48 (celebi, unpub. data, 1997). As showm in figure 48, 

the building had a severe plan torsional irregularity. The 

building was subjected to large peak accelerations (ground 

0.26 g, second-floor 0.54 g, and roof 0.69 g). Recorded 

accelerations and their peaks are shown in figure 49. 

The building suffered no damage. Its inherent stiffness 

provided by infill walls reduced the drift ratio. The 

three-dimensional computer model analyses by McClure 

(1991) showed that the building behaved elastically when 

subjected to Lorna Prieta motions, The time-histories 

of the translational and torsional roof accelerations 

and corresponding amplitude spectra are shown in figure 

50 (celebi, unpub. data, 1997). The amplitude spectra by 

themselves reveal frequencies at approximately 0.8, 1.7, 

and 1.95 Hz, To distinguish these frequencies, spectral 

ratios are shown in figures 51A and B. It is noted that 

the spectral ratio in the north-south direction is approximately unity up to 2 Hz because the east wall of the 

building does not amplify the structural response (figure 

51B). The building was subjected to severe torsional 

behavior that is close-coupled with the translational 

mode at approximately 1.67 and 2 Hz (0.67 and 0.5 s). 

These frequencies are identified in the spectral ratios of 

torsional accelerations of the roof (figs. 51C and D). The 

weak peak at approximately 0.7 Hz observed in the 

amplitude spectra is attributed to the site frequency (celebi, 

unpub. data, 1997). McClure (1991) stated that the ambient vibration tests of the building performed in 1965 by 

the USGS (then U.S. Coast and Geodetic Survey) 

indicated a period of 0.47 s for the building, Later, in 

1966, forced vibration tests of the building by Bouwkamp 

and Blohm (1966) yielded a first-mode period of 0.416 s. 

The mathematical model McClure used to perform dynamic analyses of the building was based on matching 

the period of the building to one of those from the lowamplitude tests. However, McClure noted that the building periods were 0.5-0.6 s during Loma Prieta and 

attributed this to possible disengagement of the 

nonstructural elements that were not well connected to 

the structural frame (199 1). 

ncrete 

Wa I I 

Figure 48.-Three-dimensional schematic of two-story building in Oakland. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY C63 

TILT-UP BUILDINGS AND BUILDINGS 

WITH FLEXIBLE DIAPHRAGMS 

Particularly used as industrial and storage facilities, tiltup buildings commonly are designed with large aspect 

ratios and flexible long span plywood roof diaphragms. 

Bouwkamp and others (1991) studied some of the buildings with flexible diaphragms, including a warehouse in 

Hollister (which also has records from the 1984 Morgan 

Hill earthquake and the 1986 Hollister earthquake), West 

Valley College gymnasium in Saratoga (which has records 

also from the 1984 Morgan Hill earthquake), and a twostory building in Milpitas (which also has records from 

the 1988 Alum Rock earthquake). The availability of data 

from several earthquakes for these tilt-up buildings allowed comparison of the frequencies and the loss of stiffness and the amplification of motions at the center of the 

roof diaphragm compared to its edges and the base. 

Bouwkamp and others (1991) reported that the stiffness 

of the Hollister building during Loma Prieta was approximately 50 percent of that calculated from the records of 

the 1984 Morgan Hill earthquake. The amplifications of 

motions at the walls were negligible, as they should be 

PEAK ACCEL. 0.16 - g 

- 

- 2 ROOF: NE - 

- 

I, 2ND FLOOR: NE CORNER (W) - 

- 5 2ND FLOOR 

- 

- 6 GROUND FLOOR: NE CORNER (W) 

2ND FLOOR: NE CORNER (S) -%- 0.22 g 

2 

to GROUND FLOOR: NE CORNER (S) - - 

Figure 49.-Recorded responses of two-story building in Oakland. 

OAKLAND: 2-STORY OFFICE BUILDING: ROOF 

TIME (SEC) 

Figure 50.-Roof responses and amplitude spectra of two-story building in Oakland, 

2xlV Om ROOF 

NS: CH8 

1.5 - 

1.66 ,, 1.95 

FREQ (HZ) 
C64 PERFORMANCE OF THE BUILT ENVIRONMENT 

since the walls are extremely rigid in their planes. Furthermore, the code equations for estimating the diaphragm 

displacement did not match well with the recorded responses. 

The West Valley College gymnasium was studied in 

detail by celebi and others (1989) using the 1984 Morgan 

Hill earthquake records, A general view and schematic of 

the gymnasium and its instrumentation scheme is shown 

in figure 52 (celebi and others, 1989). The Morgan Hill 

earthquake records are compared to the Loma Prieta 

records in figure 53. In short, the records of the building 

are greatly influenced by the diaphragm frequency at approximately 4 Hz. Significant peak accelerations of the 

gymnasium are summarized in table 10 for the two earthquakes (celebi, 1990). As a result of these studies, the 

design requirements for the restraints at the edges of the 

roof diaphragm were increased in the 1991 Uniform Building Code by 50 percent. 

A 

OAKLAND: 2-STORY BLDG: NS 5, I t I I I I 1 1 I 

B FREQ (HZ) 

15 OAKLAND: 2-STORY BLDG: EW 

a: ROOF: (CH2)/GR.FL(CH6) ;-; 

/ 'I b: ROOF:(&I~)/GR.FL(CH~) ,/' 1.; \ b ;: 

1, 

FREQ (HZ) 

c 

1.5 XI@ OAKLAND: 2-STORY BLDG: DIFFERENIIAL NS MOTION I I , I , I I 

1 - a: ROOF: CH3-CH2 , 

b: GRFL: CH7-CH6 

0.5 - 

D FREQ (HZ) 

, , 2-7YB, LDG., , , 

ROOF(CH3-CH2)/GR.=(CH7-C56) 

FREQ (HZ) 

Figure 51.-Spectral ratios of amplitude spectra of two-story building in Oakland. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY C65 

Figure 52.-A, West Valley College Gymnasium. B. Three-dimensional 

Valley College Gymnasium, 

schematic of West 
C66 PERFORMANCE OF THE BUILT ENVIRONMENT 

WEST VALLEY COLLEGE (SARATOGA, CA.) GYMNASIUM 

CDMG LOMA PRIETA RECORD (27 km from the epicenter) 

1; GROUND FLOOR 

N. END UP 0.278 I 

1 

v-- NE CORNER (W> 0.33~ 1 

m44 I 

CDMG MORGAN HILL RECORD (30 km from the epicenter) 

- 1 around Door - Up 0.03 q 

- 2. UP 0.03 9 

Structure Rtf. Ortentatton: M460*. tf270Â 

Figure 53,-Recorded responses of West Valley 

TIME (SEC) College Gymnasium for two earthquakes. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY C67 

Table 10.-Peark acceleration responses for two earthquakes from West Valley College (Saratoga) Gymnasium 

Morgan Hill M, =6.1 Lorna Prieta M, =7.1 

April 24, 1984 October 17, 1989 

Location Direction Accel (g) Accel. (g) 

Ground NS 0.10 0.26 

EW 0.04 0.33 

Roof edge NS 0.14 0.36 

EW 0.065 0.43 

Roof center NS 0.42 0.77 

EW 0.20 0.87 

Wood and Hawkins (this chapter) also investigated the 

trends in seismic behavior of two tilt-up buildings (the 

same two-story building in Milpitas and the same onestory building in Hollister). Both buildings were constructed by different methods, and both were relatively 

new buildings (constructed according to codes between 

1979 and 1989). Both buildings are within 50 km of the 

Loma Prieta epicenter and both recorded similar responses. 

The authors report that the transverse accelerations at the 

center of the roof of both buildings were approximately 

three times that at the base of the buildings. This indicated that such buildings designed according to codes prior 

to Loma Prieta resulted in very flexible diaphragms. Therefore, the 1991 changes made in the Uniform Building 

Code are warranted. 

The findings of Wood and Hawkins (this chapter) are 

similar to those of Tena-Colunga and Abrams (1992) and 

Abrams (1995), who studied the flexible diaphragm problem of a two-story, masonry office building located at 

Palo Alto, California. The latter investigations provide 

further insight into the role of floor or roof diaphragms in 

dynamic response of unreinforced masonry building systems and into deliberations on the impact of these responses to the design of masonry buildings. 

BUILDINGS WITH NEW 

TECHNOLOGIES 

At the time of the Loma Prieta earthquake, there were 

no buildings in the greater San Francisco Bay area that 

were constructed or retrofitted with base-isolation technology. Only an overpass bridge (Sierra Point in South 

San Francisco) had been retrofitted with base isolation 

(Kelly and others, 1991). However, since Loma Prieta, a 

number of base isolated buildings have been either recently constructed (for example, San Francisco Main Library Building) or retrofitted with this technology (for 

example, Court of Appeals Building in San Francisco, a 

four-story apartment building in the Marina District of 

San Francisco, and Oakland City Hall). Also, the Santa 

Clara County Office building in San Jose has been retrofitted with viscous elastic dampers (see section "Santa 

Clara County Office Building (San Jose)") (Crosby and 

others, 1994). 

LOW-AMPLITUDE TESTING 

The dynamic characteristics of five of the previously 

discussed buildings in the San Francisco Bay area that 

recorded the Loma Prieta earthquake were studied by recording their responses to low-amplitude ambient vibratory motions (Marshall and others, 1991, 1992; Celebi 

and others, 1993; Celebi, 1996). The five buildings studied are: (1) the Pacific Park Plaza Building , (2) the 

Transamerica Building, (3) the Santa Clara County Office 

Building , (4) an office building in San Bruno, and (5) the 

California State University (Hayward) Administration 

Building . The primary reason for selecting these five buildings 

was that they recorded the earthquake and therefore lowamplitude tests provide an opportunity to compare their 

dynamic characteristics under strong- and low-amplitude 

motions. The Loma Prieta response of all these buildings 

has been investigated by several investigators, as summarized earlier in this paper. 

The location of these five buildings relative to the Loma 

Prieta epicenter is shown in figure 1. The general dimensions and instrumentation schemes of each of the five 

buildings are provided previously in this paper. However, 

significant characteristics of these buildings (structural 

type, foundation type, and general dimensions), distance 

from the epicenter, number of channels of strong-motion 

sensors within the superstructure, and peak accelerations 

at ground level (or foundation level) and at roof level are 

summarized in table 11. The reference north building orientation in degrees clockwise from true north, provided in 

the table, is different than the adopted nominal northsouth and east-west directions shown in the table. Results 

from dynamic analyses and low-amplitude tests performed 

on the Transamerica Building (Stephen and others, 1974; 

Kinemetrics, 1979) and Pacific Park Plaza Building 

(Stephen and others, 1985) prior to the earthquake and 

dynamic characteristics for the Santa Clara County Office 

Building extracted from response data of two earthquakes 

that occurred prior to the earthquake (R. Darragh, personal commun., 1991) are included in comparative studies by Marshall and others (1991, 1992), Celebi and others 

(1993), and Celebi (1 994a). 

The testing of each building was conducted from a recording room that contained the junction box of the cables 

of the force-balance accelerometers permanently deployed 

throughout the superstructure. These cables were hooked 

up to a digital (PC-based) data acquisition system (Marshall 

and others, 1991, 1992; Celebi and others, 1993). This (1) 
C68 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 1 1 .-Building characteristics and Lorna Prieta peak accelerations 

[Data from Marshall and others (1992) and Celebi (1996). The reference north building orientation in degrees clockwise from true north is different from 

adopted nominal north-south and east-west directions. Number of floors (NA, above ground level; Ng, below ground level). Height of building=H. 

Distance to epicenter=D. Number of channels in instrumented building=n. Orientation of reference north (clockwise from true north)=N] 

Building Comments H NA/NB D n Loma Prieta Peak 

(instrumentation (m) (m) Accel. (g) 

administrator, in Nom. Gr. Roof 

Office Building 

(CDMG) 

N=337O 

San Bruno Office 

Building 

(CDMG) 

N=335O 

California State 

University 

(Hayward) 

Admistration 

Building 

(CDMG) 

N=320Â .. . . . . - . - - - -. - 

Pacific Park Plaza Reinforced concrete 

(USGS) moment resisting frame 

N=350Â (1.5-m-thick concrete 

mat on piles). 

Transamerica Bldg. Steel frame, 48th floor is 

(USGS) the top occupied floor 

N=351Â (2.75-m-thick concrete 

mat - no piles). 

Santa Clara County Moment-resisting steel 

frame (Concrete mat - 

no piles] 

Reinforced concrete 

moment resisting frame 

(individual spread 

footing) 

Steel moment-frame 

core; exterior reinforced 

concrete moment frame 

(0.45-m-thick slab on 

grade and bearing piles) 

facilitated easy access to various floors of the building 

without actually going to those floors or without having 

to provide temporary cables and sensors, (2) allowed ready 

access to record response data, (3) made it possible to 

compare directly the ambient and strong-motion response. 

The damping ratios are extracted by system identification analyses of Loma Prieta data in accordance with the 

procedures outlined by Ghanem and Shinozuka (1995) and 

Shinozuka and Ghanem (1995) (see "Methods of Analyses" section). The low-amplitude (ambient) vibration data 

was analyzed by conventional spectral analysis techniques. 

System identification techniques were not applied to the 

ambient vibration data because of the unknown system 

input characteristics. Furthermore, because of the lower 

signal-to-noise ratio, system identification techniques do 

not minimize the errors simply and solutions may be unreliable. The results of analyses of the strong-motion response data and the ambient vibration data are summarized 

in table 12. Also included for comparison are results of 

previous tests and analyses for the Pacific Park Plaza and 

Transamerica Buildings. 

In each of the five buildings tested, the first-mode periods associated with the strong-motion records are longer 

than those associated with the ambient vibration records. 

The highest first-mode period ratio (Loma Prieta earthquakelambient) is 1.47. Also, the percentages of critical 

damping for the first mode for the ambient data are significantly smaller than those from the strong-motion data. 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY 

Table 12.-Dynamic characteristics of the five buildings discussed in report 

[Data from Marshall and others (1992) and Celebi (1996). f, frequency (Hz); T, period (s); x, damping (percent)] 

Building Nominal Pre-Loma Prieta test Loma Prieta Post-Loma Prieta 

direction and analyses data Ambient -- 

f/(T) Â (pct.1 f/m ^(PC~.) f/(T) ^(PC~.) 

Pacific Park Plaza 

[PPPI 

Transamerica Building 

[TRAI 

Santa Clara County Office 

Building 

[SCCOB] 

San Bruno Office Building 

[SBRI 

California State University 

(Hayward) Administration 

Building [CSUH] 

These differences in periods and damping percentages 

may be caused by several factors, including (1) possible 

soil-structure and/or possible pile-foundation interaction, 

which is more pronounced during strong-motion events 

than during ambient excitations, (2) nonlinear behavior of 

the structure (such as microcracking of the concrete at the 

foundation or superstructure), (3) slip of steel connections, and (4) interaction of structural and nonstmctural 

elements. 

In the case of the Santa Clara County Office Building, 

the largest critical damping ratio assessed from the 

records is about 2.7 percent (tables 4, 12). Damping from 

the low-amplitude test data could not be determined due 

to its low signallnoise ratio. It is noted again that this 

building is now retrofitted with visco-elastic dampers to 

change its dynamic characteristics so that the global 

damping and the fundamental frequency both will increase. 

The resulting shift in the fundamental period and increase 

in damping is expected to eliminate excessive responses 

and the beating effect (Crosby and others, 1994). Lowamplitude tests on the now retrofitted building recently 

performed by Celebi and Liu (1997) preliminarily indicate that there is improvement in the dynamic characteristics. 

For the Pacific Park Plaza, the post-Loma Prieta frequency (0.48 Hz) is lower than the pre-Loma Prieta frequency (0.59 Hz), but both are larger than the Lorna Prieta 

frequency (0.38 Hz). For Transamerica Building, the frequencies from the pre- and post-LPE low-amplitude tests 

are in good agreement (0.34 vs. 0.34 and 0.32 Hz) and are 

larger than the Loma Prieta frequency (0.28 Hz) (Stephen 

and others, 1974; Kinemetrics, 1979; Stephen and others, 

1985). The immediate conclusion drawn from table 12 is 

that the damping ratios and periods are consistently and 

significantly larger for the strong-shaking than for the lowamplitude vibrations. 

In addition to these general conclusions, a specific characteristic for Pacific Park Plaza is noted: The damping 
C70 PERFORMANCE OF THE BUILT ENVIRONMENT 

ratios extracted from the system identification analyses 

corresponding to the 0.38-Hz first mode frequency are 

11.6 percent (north-south) and 15.5 percent (east-west). 

Such unusually high damping ratios for a conventionally 

designedlconstructed building require explanation. The 

building with its large mat foundation in a relatively soft 

geotechnical environment is capable of energy dissipation 

in the soil due to radiation (or foundation) or material 

damping. Therefore, the 0.38-Hz frequency should be considered as the fundamental frequency that incorporates 

soil-structure interaction at the level of amplified shaking 

summarized in table 1. Analyses showed that rocking was 

insignificant. 

Accepting 0.38 Hz (2.63 s) as the frequency (period) 

with soil-structure interaction and 0.48 Hz (2.08 s) or 

0.57 Hz (1.75 s) as that without soil-structure interaction, 

then it is possible to make some quantification of the 

amount of foundation damping using equations developed 

by Veletsos (1977) and included in the Applied Technology Council's publication ATC 3-06 (Applied Technology Council, 1978). The area (A ) of the foundation mat 

is approximately 1,600 m2. The equivalent circular radius 

of the foundation is calculated as r = (A/r)Oe5 = 22.6 m 

(this number is possibly too low considering that the Pacific Park Plaza is a three-winged building, and therefore 

the effective radius is possibly larger than the conversion 

provided above). The height of the building (h) is 89.2 m; 

the effective height (heff) as defined by the Applied Technology Council (1978) is hff = Olh = 62.4 m. For hf4r 

= 62.4122.6 = 2.8, and for TSJT = 2.6312.08 = 1.26 or 

2.6311.75 = 1 SO, the foundation (including radiation and 

material) damping can be approximated at 5-6 percent 

from figure 54 (adopted from Applied Technology Council, 1978). Inserting these into Veletsos' equation for effective damping (1977): 

The value of iS; here is the structural damping without 

soil-structure interaction, normally accepted from empirically prepared tables to be between 3 and 7 percent. Therefore with TssilT varying between 1.26 and 1.50, the 

structural damping is reduced to 29-50 percent of this 

accepted value. Thus, the estimated foundation damping 

of 5 to 6 percent is not unusual, given the fact that the eeff 

determined from observed response data is 1 1 - 15 percent. 

More detailed discussions on foundation damping can be 

found in Veletsos (1977), Luco (1980), Roesset (1980), 

Dobry and Gazetas (1985), Todorovska (1992), and very 

recently in Wolf and Song (1996). It can therefore be 

categorically stated that radiation damping is beneficial in 

reducing responses of structures. 

It should be stated that consideration of the effect of 

soil-structure interaction in estimation of damping for 

analysis and design purposes has not yet found its way 

into design offices, except for those involved with critical 

structures. In the past, during designlanalysis processes of 

engineered structures, it was assumed that a structure's 

foundation is fixed to the underlying media. State-of-theart knowledge and analytical approaches require, when 

warranted, the structure-foundation system to be represented by mathematical models that include the influence 

of the sub-foundation media. Identification of beneficial 

and adverse effects of soil-structure interaction is a necessity. Adverse effects of soil-structure interaction during 

Figure 54.-Foundation damping in terms of 

ratio of periods and ratio of building heightto-equivalent radius of foundation (from 

Veletsos (1977) and Applied Technology 

Council (1978)). 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c7 1 

the 1985 Michoacon (Mexico) earthquake was addressed 

by Tarquis and Roesset (1988), who showed that lengthened fundamental periods due to soil-structure interaction 

of mid-rise buildings (5-15 stories) in the Mexico City 

lakebed area placed them close to the resonating 2-s site 

period. This study points out that by neglecting the effect 

of soil-structure interaction, the dynamic characteristics, 

and specifically damping in structures can be underestimated if the values of damping assessed from low-amplitude testing are used in lieu of values supposedly for strong 

motions. 

To repeat, current analyses and design procedures using estimated damping values may not be realistic due to 

(1) change in construction and design practices and (2) 

the fact that soil-structure interaction effects in most cases 

are ignored for buildings that are not considered to be 

critical facilities. However, results presented herein suggest that the critical damping percentages used for analyses and design of some buildings may not have been 

properly estimated (specifically, overestimated for the 

Santa Clara County Office Building and underestimated 

for the Pacific Park Plaza) and possibly not even considered for some buildings. Furthermore, changes in damping values and fundamental values commensurate with 

inferred strong-motion values should be considered to improve design and analyses results. 

OTHER STUDIES 

POUNDING OF STRUCTURES 

Kasai and others (1991) surveyed the damage due to 

pounding in the San Francisco Bay area following the 

Loma Prieta earthquake. They determined that significant 

pounding of buildings occurred at sites ranging up to more 

than 90 km from the epicenter and discussed the implications of this finding in forecasting future possible catastrophic damage that may occur during earthquakes having 

epicenters closer to dense urban areas. They also present 

analytical research on pounding that includes development 

of dynamic analysis programs that incorporate pounding. 

Parametric studies were performed on building pounding 

response as well as appurtenance response. These analyses included a spectrum method to obtain peak pounding 

responses, actual case studies, and a spectrum method to 

determine required building separations to preclude pounding. Their analytical studies did not relate to any recorded 

pounding responses of buildings. This may be due to the 

fact that there is no specific instrumentation scheme implemented, by either California Strong Motion Instrumentation Program or U.S. Geological Survey, in adjacent 

buildings with possibility of pounding to record their responses during earthquakes. 

MASONRY BUILDINGS 

Hart and Jaw (1991) investigated the performance of a 

tall (approximately 87 ft) reinforced masonry building located in Santa Cruz, near the epicentral region. They observed that the building was not damaged, in spite of a 

probable maximum ground motion of 0.3-0.5 g. 

OVERTURNING FORCES 

Chen (1992) presented a newly developed time-delay 

method for earthquake analysis which incorporates the 

attribute of finite wave speed propagating upward into a 

structure. He showed verification of the method using data 

from the California Strong Motion Instrumentation 

Program applied to calculate the overturning moment 

of structures. He used one detailed three-dimensional 

model and nine simplified two-dimensional models for 

this purpose. 

Gates and others (1994) investigated three high-rise 

shear wall buildings to evaluate overturning forces in the 

shear walls under three recent northern California earthquakes: 1984 Morgan Hill, 1986 Mt. Lewis, and 1989 

Loma Prieta. The buildings are in the 9-10 story range, 

with three different shear wall configurations of (1) perimeter walls, (2) core walls, and (3) distributed walls. 

They employed two methods of data reduction and analysis to assess the significance of soil-structure interaction 

on building overturning forces: (1) simplified data analysis procedures using recorded motion, mode shapes, and 

building weights to assess dynamic performance and (2) 

three-dimensional linear elastic dynamic analyses using 

soil-structure models for the shear walls and foundation 

systems. They reported that the recorded responses show 

ample evidence of foundationlshear wall rocking under 

the moderate and strong shaking provided by the cited 

earthquakes. They also reported that the lengthening in 

the periods of the buildings take place due to both rocking 

and inelastic behavior. The analytical results are compared with code procedures for predicting the periods of 

the structures as well as the distribution of overturning 

forces. They showed that careful modeling of buildings 

refined by system identification techniques using actual 

recorded responses provides useful steps in the evaluation 

of buildings. 

CODE ASSESSMENT AND SERVICEABILITY 

REQUIREMENTS 

Fenves (1990) evaluated the lateral force procedures 

for buildings with irregular plans or vertical irregularities. 

He used recorded strong-motion response data from two 

instrumented buildings to determine vibration properties 
C72 PERFORMANCE OF THE BUILT ENVIRONMENT 

and distribution of lateral forces. One of the two buildings is a four-story hospital with records from both Morgan Hill earthquake (1984) and Loma Prieta and the other 

with only Loma Prieta records. Fenves reported that due 

to the irregular configuration of the buildings, the distribution of lateral forces changes substantially with changes 

in the amplitude of earthquake response. 

Uang and Maarouf (1991), Werner and others (1992), 

and Beck and others (1992) investigated the Uniform 

Building Code and serviceability requirements from building response data. They all investigated several buildings 

and compared their analyses with observed performance 

of each building during the Loma Prieta earthquake and 

other earthquakes for which data are available. 

3. The data base indicates that there is great variation of the 

damping ratios. The variation is larger for concrete buildings 

(2-14 percent) than for steel buildings (1-6 percent). 

4. For reinforced concrete and steel framed buildings, the 

maximum story drift occurs at a middle or lower story, but for 

shear wall buildings the maximum story drift occurs at a 

middle or higher story. 

5. Rocking occurs in some buildings as a result of translational vibration. For stiffer buildings, rocking can occur as a 

distinguishable mode of vibration. 

6. Large variations in building frequency can be detected 

within the time-history of a response record of a building. This 

may or may not be a symptom of damage but can be used as 

an indicator of possible damage. 

7. There is strong correlation between change of frequency 

and variation of drift. 

SOIL-PILE-STRUCTURE INTERACTION 

Gould and Ahn (1991) studied one of the six-story wings 

of the Clarion Hotel near San Francisco International Airport. The reinforced concrete shear-wall and framed building rests on pile foundation penetrating into stiff, clayey 

soils underlying the soft Bay mud. With the premise that, 

in general, lengthening of the building period results in 

lower inertial forces, they studied the building with mathematical models that includes soil-pile-structure interaction. They used ground motion recorded at the San 

Francisco International Airport as surrogate input base 

motion to the subject building model. To differentiate the 

effect of the soil-pile-structure interaction, they performed 

fixed-base analyses of the structure. Their analyses show 

that the base shear and the base overturning moment are 

significantly reduced with lengthening of the building period from 0.35 s (fixed-base) to as much as 1.33 s due to 

soil-pile-structure interaction. 

TRENDS IN BEHAVIOR OF REGULAR 

BUILDINGS 

Li and Mau (1997) recently completed a study of 21 

regular (typical, symmetric) buildings which have recorded 

response records. Of these, 11 buildings are from the San 

Francisco Bay area with records from the Loma Prieta 

earthquake and 9 are from the Los Angeles area with 

records from the 1987 Whittier earthquake. With this data 

base, they performed extensive analyses using system identification techniques as a tool. They arrived at the following conclusions: 

1. Estimates of building periods by code formulas is less 

reliable for shear wall buildings than for framed buildings. 

2. Estimates of building frequencies by code formulas for 

longitudinal and transverse direction is variable and is dependent on the structural framing and presence and distribution of 

shear walls. 

CONCLUSIONS 

This paper summarizes studies of performances of buildings during the Loma-Prieta earthquake. The studies referred to herein are mostly on the recorded responses of 

instrumented buildings. The conclusions derived from these 

studies are as follows: 

1. Instrumentation of structures as part of hazard reduction 

programs is very beneficial, as studies of this type will help to 

better predict the performance of structures in future earthquakes. 

a. Studies of recorded responses of buildings help 

researchers and practicing professionals to better understand the cumulative structural and site characteristics that 

affect the response of buildings and other structures. Such 

studies impact mitigation efforts. 

b. In turn, the behavior that may be expected from 

buildings during future earthquakes with large input motions (either due to larger magnitude earthquakes or earthquakes at closer distances to the building) can be forecast. 

This is particularly true for the San Francisco Bay area 

wherei. The probability of magnitude 7 or larger earthquakes occurring on major faults, including the San 

Andreas and Hayward faults, is considered to be approximately 67 percent or higher within a 30-year 

period (Working Group, 1990). 

ii. There is a large inventory of buildings within 0- 

10 krn of the two major faults capable of generating 

M>7 earthquakes. This is particularly important because, very recently, the Structural Engineers Association of California (SEAOC) issued the 1996 edition of 

the "Recommended Lateral Force Requirements and 

Commentary ," which has provisions for increasing the 

design base shear by 0- 100 percent depending on the 0- 

10-km distance of a building from a fault. This implies 
PERFORMANCE OF BUILDING STRUCTURES-A SUMMARY c73 

that the forecasting of performance of buildings within 

0-10 krn of major faults must be done more informatively. This requisite information can be achieved only 

through acquiring and studying response data from 

buildings during earthquakes. 

iii. Furthermore, some of the building response data 

are from tall buildings that are on soft soils. The motions 

at the soft-soil sites of some of the important tall 

buildings (for example, Pacific Park Plaza, Transamerica 

Building, Embarcadero Building, and Chevron Building) are amplified by 3-5 times within the periods of 

engineering interest when compared with the motions 

of Yerba Buena Island, a rock site approximately the 

same distance away from the epicenter of the earthquake. 

2. There is an acute need to better evaluate structural and 

site characteristics in developing earthquake resisting designs 

of building structures. Studies in this paper show, as in the case 

of Santa Clara County Office Building, that designs of buildings with low structural damping, resonation, and beating 

effects caused by closely coupled translational and torsional 

modes must be avoided. Also, as expected in most tall buildings, higher modes are excited. As in the case of Embarcadero 

Building, higher modes play an important role in the response 

of building structures and therefore must be carefully evaluated to assess their future performances. 

3. Drift ratios calculated from observed data in certain 

cases exceed code drift limitations for part or all of the 

structural systems. Assessing the drift exposure of structural 

systems are ever more important since the designlanalyses of 

buildings are recently being shifted toward a performancebased design procedure. 

4. Soil-structure interaction is one of the least understood 

actions that affects structural behavior. There may be beneficial or detrimental effects of this interaction to the overall 

behavior of structures. It stands to be prominent in the behavior of several buildings (for example, Pacific Park Plaza, 

Santa Clara County Office Building, Transamerica Building, 

and others) as assessed from studies of the recorded responses 

of buildings during the Loma Prieta earthquake. Therefore, 

two specific issues are that (1) identification of beneficial and 

adverse effects of soil-structure interaction is a necessity and 

(2) design offices must be informed and trained in consideration of the effect of soil-structure interaction in estimation of 

fundamental period and damping of a building, as this is not 

yet the case. Specifically, the damping percentages are overestimated for the Santa Clara County Office Building and 

underestimated for the Pacific Park Plaza, and possibly not 

even considered for some buildings. 

5. Development of design response spectra deserves more 

intensive consideration by geotechnical engineers since site 

effects play an important role in the response of building 

structures. There are significant discrepancies in the comparison of the response spectra derived from recorded motions 

with the actual design response spectra. Amplified motions due 

to soft soil conditions, site-specific resonating, and frequency 

content must be kept in mind in the development of design 

response spectra (for example, the Embarcadero Building). 

6. The propagation direction of surface waves arriving at 

buildings affect particularly unsymmetrical buildings or buildings with wings (for example, Pacific Park Plaza and Santa 

Clara County Office Building). 

7. The basemat rotation of tall buildings with basements 

calculated by the displacements in the comers of the basemat 

is considerably smaller than the rotation of the basement walls 

calculated by the displacements derived from the horizontal 

sensors at the street level and basemat. This is observed for 

both the Transamerica Building and Embarcadero Building. 

The implication is that the current practice which assumes that 

the inertial forces at ground level and basemat level to be the 

same is not correct. 

8. Low-amplitude tests have been conducted on five buildings that recorded the Loma Prieta earthquake. Results indicate, as expected, that the first-mode periods extracted from 

strong-motion response records are longer than those associated with the ambient vibration records. Similarly, the percentages of critical damping for the first mode for the ambient 

data are significantly smaller than those from the strong-motion 

data. These differences may be caused by several factors 

including (1) possible soil-structure interaction which is more 

pronounced during strong-motion events than during ambient 

excitations, (and similarly, in buildings with pile foundations, 

possible pile-foundation interaction which may not occur 

during ambient excitation), (2) nonlinear behavior of the 

structure (such as microcracking of the concrete at the foundation or superstructure), (3) slip of steel connections, and (4) 

interaction of structural and nonstructural elements. Changes 

in damping values and fundamental period values commensurate with inferred strong-motion values should be considered 

to improve design and analyses results. 

9. In processing of data of recorded responses for buildings, use of different filters for basement versus free-field 

causes gross differences in displacements (as in the case of 

California State University, Hayward). A common rational 

filtering for basement and associated free-field motions is 

recommended. 

10. Specific instrumentation schemes of some of the 

already instrumented buildings and of those buildings yet to 

be instrumented must be improved andlor implemented so 

that the response characteristics expected of that building can 

be captured (for example, soil-structure interaction, pounding, and variation of drift due to abrupt changes in stiffness). 

When applicable, specific buildings should be specially 

instrumented extensively to better capture their behavior 

in response to actions such as pounding and soil-structure 

interaction. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

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THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

PERFORMANCE OF THE BUILT ENVIRONMENT 

BUILDING STRUCTURES 

MEASURED RESPONSE OF TWO TILT-UP BUILDINGS 

By Sharon L. Wood, University of Texas; and 

Neil M. Hawkins, University of Illinois 

CONTENTS 

ABSTRACT 

Acceleration response histories were recorded in two 

tilt-up buildings during the earthquake. The acceleration 

and displacement data are used to identify trends in the 

seismic behavior of tilt-up construction. Both buildings 

were less than 10 years old at the time of the earthquake, 

yet they represent significantly different construction practices. The one-story warehouse in Hollister is representative of traditional construction with a plywood roof 

diaphragm and cast-in-place pilasters. The two-story industrial building in Milpitas has window openings in every panel, includes a metal-deck floor diaphragm and a 

wood roof, and represents modern trends in tilt-up construction. 

Structural damage was not observed in either building 

following the Loma Prieta earthquake. The general nature 

of the measured dynamic response is similar. Transverse 

accelerations measured at the center of the roof diaphragm 

were approximately three times larger than the corresponding ground accelerations. In-plane accelerations were transmitted up the wall panels with essentially no amplification. 

INTRODUCTION 

Tilt-up construction is used throughout the U.S. for lowrise commercial and industrial buildings. Construction 

costs are low due to the efficient system of casting wall 

Page 

C77 

77 

7 8 

78 

83 

86 

8 8 

89 

90 

panels in a horizontal position on site and then lifting the 

panels into their final vertical position. Although economical to build, tilt-up construction is vulnerable to seismic 

damage. Following the collapse of a number of tilt-up 

buildings during the 197 1 San Fernando earthquake, building code provisions were modified to strengthen critical 

connections between the wall panels and the roof diaphragm. 

Tilt-up buildings typically include a number of different construction materials. The reinforced concrete wall 

panels are often combined with wood or metal-deck roof 

diaphragms to form a boxlike system. Design procedures 

for tilt-up construction tend to focus on proportioning the 

individual components, such as the thickness of the wall 

panels or the nailing patterns in the plywood roof diaphragm. Response histories recorded in two tilt-up buildings during the Loma Prieta earthquake provide a rare 

opportunity to study the seismic response of complete tiltup systems (Shakal and others, 1989). 

Both buildings were located within 50 km of the epicenter. Peak horizontal ground accelerations were approximately 0.36 g at the base of a one-story warehouse in 

Hollister and 0.14 g at the base of a two-story industrial 

building in Milpitas. Ground motions recorded at the two 

sites are shown in figure 1. The duration of the strong 

ground motion was between 5 and 10 seconds at both 

locations. Linear response spectra indicate that spectral 

accelerations and displacements in Hollister were higher 

than those in Milpitas (fig. 2). 

The general nature of the measured dynamic response 

of the two tilt-up buildings is described in this paper. The 

two buildings represent extremes in styles of tilt-up construction. The building in Hollister represents a traditional 

tilt-up structural system. The one-story structure is a warehouse and little attention was paid to architectural features during design. In contrast, the building in Milpitas 

represents a recent trend of using tilt-up construction for 

multistory buildings. The tilt-up wall panels in the completed structure are more representative of frames than 

walls, due to frequent openings for windows. 

Structural damage was not reported in either structure 

following the Loma Prieta earthquake. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

0.3 Hollister Warehouse - NS A 

^ 0.3r Hollister Warehouse - EW 

<b 0.3,- 0 Milpitas Industrial Building - NS 

0.3 ,- Milpitas Industrial Building - EW 

Time, sec Figure 1 .-Horizontal ground accelera0.0 5.0 10.0 15.0 20.0 25.0 30.0 tions recorded in Hollister and Miloitas. 

Hollister - NS - - - - - - - Hollister - EW --- Milpitas - NS 

Milpitas - EW 

0.0 1.0 2.0 3.0 

Period, sec 

Figure 2.-.Linear response spectra recorded in Hollister and Milpitas. 

HOLLISTER WAREHOUSE 

The warehouse in Hollister was built in 1979; however, 

the structural details are more representative of construction in the early 1970's. The building is 300 by 100 ft in 

plan (fig. 3). Concrete wall panels 6 in. thick are connected by cast-in-place pilasters which form continuous 

connections between adjacent wall panels. Cambered 

glulam beams run in both directions of the building and 

are supported on a single line of pipe columns. Purlins 14 

in. deep are spaced at 8 ft on center in the roof and overlain with one-half in. structural plywood. The building 

was designed with nine openings: four overhead doors 

for fork-lift access and five personnel doors. 

Typical reinforcement details are shown in figure 4. 

The panel reinforcement comprised a single layer of #4 

bars spaced at 12 in. on center in each direction. Two #5 

bars were added around panel openings. Chord reinforcement for the diaphragm was located in the wall panels. 

The reinforcing bars extended beyond the panel boundaries and were welded to chord reinforcement from adjacent panels. The chord reinforcement and horizontal panel 

reinforcement were then encased in the pilasters. 

MILPITAS INDUSTRIAL BUILDING 

The two-story industrial building in Milpitas was constructed in 1984. The building is 168 by 120 ft in plan 

(fig. 5). with the first story used as a warehouse and the 

second for offices. Nearly every wall panel has window 
Ref N 

(N 25 W) 

Beams 

r 6" Concrete Wall Panels (typical) 

0. H. Door 

' 

8" 0 STD pipe ~olurnns 

0. H. Door 

O.H. Door I 

O.H. Door 

Floor Plan 

West Elevation 

Figure 3.Ã‘Structura configuration of Hollister warehouse. 
C80 PERFORMANCE OF THE BUILT ENVIRONMENT 

openings in both the first and second stories. Each wall Eighteen structural tube columns carry the vertical loads 

panel varies in thickness between 8 and 16 in. for archi- from the floor and roof into the foundation. Open-web 

tectural reasons. steel girders span in the longitudinal direction of the buildfi Bars at 12" OC 

Each Way (Typical) 

TYPICAL 

CHORD BAR SCHEDULE 

PANEL DETAILS 

- - 

PANEL LOCATION 1 BAR SIZE 

Trans verse Wail Panels 1 1 - d5 Bar 

End Lon itudinal Wall Panels 1 - fg ear (8 total) 

Middle Longitudinal Wall Panels _ ug Bars (24 total) 

Chord bars: 

Lapped and welded 

r Cast-in- Place Pilaster 

Bend in alternate panel 

horizon fa1 bars. 

TYPICAL PANEL-TO-PANEL 

CONNECTION 

wall panels 

Figure 4.-Typical structural details in Hollister warehouse. 
MEASURED RESPONSE OF TWO TILT-UP BUILDINGS C8 1 

ing at the second floor level. Open-web steel joists spaced 2 112-in.-thick concrete slab. The pitched roof is supported 

at 8 ft on center run in the transverse direction between by glulam beams running in the transverse direction. Puradjacent girders and support a composite metal deck and lins 16 in. deep are spaced at 8 ft on center in the longituJ- 8x8 Structural Tube Columns 

Second-Story Plan Ti1 t-Up Wall Panels 

L-dL--J 

O.H. Door O.H. Door 

Open Web Steel Girders 

................ 0 ............... 

First-Floor Plan 

Ref. N 

(N 25 W) 

North Elevation 

Figure 5,Ã‘Structura configuration of Milpitas industrial building. 
C82 PERFORMANCE OF THE BUILT ENVIRONMENT 

dinal direction and are overlain with one-half-in, structural plyWood. 

Typical reinforcement details are shown in figure 6. 

The panel reinforcement may be divided into six categories: distributed vertical and horizontal panel reinforcement, vertical jamb bars along the panel edges, vertical 

and horizontal trim bars around the openings, diagonal 

reinforcement around the openings, closed ties in the wall 

boundary elements near the openings, and chord reinforcement. A pocket was left around the chord reinforcement 

near the panel boundary at the second-story and roof 

levels to provide access to the chord reinforcement. Chord 

reinforcement from adjacent panels was welded to steel 

angles to create a lapped connection. Adjacent wall 

panels were connected only at the second-floor and 

roof levels. 

Panel 

T 

2nd Floor IFinished I Floor 

1 Detail A 

oc 

Face 

Face 

1'-6" 16-in. Thick Wall Panel w/ #4 Bars at 12'' OC Each Way, Each Face 

FI 8-in. Thick Wall Panel w/ #4 Bars at 12" OC Each Way 

TYPICAL PANEL DETAILS 

CHORD REINFORCEMENT SCHEDULE 

PANEL LOCA TION 1 SECOND FLOOR 1 ROOF 

Grout after 

Welding 

Connection 

Trans verse Wall Panels 

Longitudinal Wall Panels 

L 2 1/2 x 2 1/2 x 3/8 x 

Chord Rein f. 

Tilt-Up Wall Panels 

Figure 

(3) #9 

(2) #8 

6.-Typical structural details in Milpitas 

(2) #8 

(2) #6 

DETAIL A 

industrial building. 
MEASURED RESPONSE OF TWO TILT-UP BUILDINGS C83 

MEASURED ACCELERATION 

RESPONSE 

Records from 13 strong-motion instruments were obtained from each building during the earthquake (California Strong Motion Instrumentation Program, 1991). In the 

Hollister warehouse, five instruments recorded the ground 

motion, seven monitored motion at the roof, and one measured the out-of-plane response of a longitudinal wall panel 

at mid-height (fig. 7). In the Milpitas building, five instruments recorded the ground motion, four measured the 

response at the roof, and four measured the response of 

the second floor (fig. 8). Instrument locations are summarized in tables 1 and 2 for the Hollister and Milpitas buildings, respectively. 

Measured acceleration histories from the Hollister warehouse are shown in figure 9. Response in the transverse 

(east-west) direction is shown at the base of the structure 

(channel 7), at the top of the transverse walls (channel 3), 

at mid-height of the center longitudinal wall panel (channel 6), and at the center of the roof (channel 4). In the 

longitudinal (north-south) direction, response histories are 

shown at the base of the structure (channel 13), the top of 

the longitudinal walls (channel 12), and the center of the 

roof (channel 11). Acceleration maxima are presented in 

table 1. 

Predominant frequencies were identified for each acceleration history from Fourier amplitude spectra (Carter 

and others, 1993) and are also reported in table 1. 

The data indicate a significant amplification of the transverse accelerations at the center of the roof for the Hollister 

warehouse (fig. 10). The peak roof accelerations were approximately three times the ground peak accelerations at 

the center of the diaphragm. In contrast, the in-plane 

accelerations measured at the top of the transverse wall 

panels were essentially the same as the ground motions. 

Ref N 

(N 25 W) bÃ‘Ã‘Ã‘Ã‘Ã‘Ã‘Ã‘ 300'-0' Ã‘- 

Roof Plan 

Slab Plan 

T Elevation - West Wall 

Sensors 4 and 11 are mounted on glulam beams. 

Sensor 13 is mounted on the floor slab. 

All other sensors are mounted on the wall panels. 

Figure 7.-Location of strong motion instruments in Hollister warehouse. 

Roof Plan 

Second Floor Plan 

Slab Plan 

Figure 8.-Location of strong motion instruments in Milpitas industrial 

building. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 1.-Summary of strong motion data from Hollister warehouse 

[-, Relative displacement data were calculated by subtracting the appropriate ground displacement from the structural 

displacement. Therefore, relative displacement were not calculated for channels recording ground motion.] 

Channel Location Elevation Direction 

number 

1 Center North Wall Ground Vertical 

2 114 Point South Wall Roof EW 

3 Center North Wall Roof EW 

4 Center Roof EW 

5 Center West Wall Roof EW 

6 Center West Wall Midpanel EW 

7 Center West Wall Ground EW 

8 Center North Wall Ground EW 

9 114 Point South Wall Ground EW 

10 Center West Wall Roof NS 

11 Center Roof NS 

12 Center East Wall Roof NS 

13 Center North Wall Ground NS 

Maximum Predominant Maximum 

acceleration frequency relative Reference 

(g) (Hz) displacement channel (in.) 

Table 2.-Summary of strong motion data from Milpitas industrial building 

[-, Relative displacement data were calculated by subtracting the appropriate ground displacement from the structural 

displacement. Therefore, relative displacement were not calculated for channels recording ground motion.] 

Channel 

number 

Location Elevation 

North End of East Wall 

South End of East Wall 

Center of East Wall 

Center of North Wall 

Center of West Wall 

Center of East Wall 

Center of North Wall 

Center of West Wall 

Center of East Wall 

Center of West Wall 

Center of East Wall 

Center of East Wall 

Center of East Wall 

Ground 

Ground 

Roof 

Roof 

Roof 

2nd Floor 

2nd Floor 

2nd Floor 

Ground 

Ground 

Roof 

2nd Floor 

Ground 

Maximum Predominant Maximum 

Direction acceleration frequency relative Reference 

(g) (Hz) displacement channel (in.) 

Vertical 0.08 0.4 - - 

Vertical 0.08 0.4 - - 

Transverse accelerations measured at mid-height of the 

longitudinal wall were amplified with respect to the base, 

but did not exceed the amplitude of the roof accelerations. 

Significant differences in the frequency content of the 

transverse accelerations were also observed between the 

ends and the center of the Hollister roof diaphragm. The 

predominant frequency of the transverse accelerations 

measured at the top of the north and south walls was 0.6 

Hz, the same as that of the ground motion. However, the 

predominant frequency of the transverse accelerations 

measured at the center of the diaphragm and at the center 

of the west wall was 1.1 Hz. 

Similar amplification was not observed in the longitudinal direction of the Hollister warehouse. The peak longitudinal acceleration response at the center of the roof 

was approximately 1.25 times the peak ground accelera- 
MEASURED RESPONSE OF TWO TILT-UP BUILDINGS C85 

tion, while the longitudinal acceleration histories measured 

at the top of the longitudinal walls were essentially the 

same as the ground accelerations. The predominant frequency of the longitudinal acceleration response at all 

three locations on the roof was the same as that of the 

ground motion. 

Measured acceleration histories in the Milpitas industrial 

building are shown in figure 11. ~esponse in the transverse (north-south) direction is characterized by the ground 

motion (channel 9), the in-plane response of the transverse walls at the second-floor and roof levels (channels 6 

and 3), and the out-of-plane response of the longitudinal 

walls at the second-floor and roof levels (channels 7 and 

4). In the longitudinal (east-west) direction, measured response at the ground (channel 13) is shown along with the 

out-of-plane response at the second-floor and roof elevaTRANSVERSE ACCELERATION RESPONSE 

0.8 Channel 4 - Roof, Center 

0.8 ,- Channel 6 - Midpanel, West Wall 

$ 0.8- \ 

Channel 3 - Roof. North Wall 

0 

0 

0 0.0 A i 

-? 

-0.8L 

OS8 r Channel 7 - Ground, West Wall 

-0.8L Time, sec 

0.0 5.0 70.0 75.0 20.0 25.0 30.0 

LONGITUDINAL ACCELERATION RESPONSE 

0.8 - Channel 77 - Roof, Center 

0.0 -- 7 -I 

^-0.8 - 

0.8- Channel 72 - Roof. East Wall 

0 * 

p 0.0 & - -- 

<b 

\ 

8 -0.8 - 0.8 Channel 73 - Ground, North Wall 

0.0 

-0.8 Time, sec 

Figure 9.-Measured acceleration records 0.0 5.0 7 0.0 75.0 20.0 25.0 30.0 in Hollister wNehouse. 
C86 PERFORMANCE OF THE BUILT ENVIRONMENT 

tions (channels 12 and 13). Peak accelerations and predominant frequencies are presented in table 2 for all channels. 

In contrast to the Hollister warehouse, the measured 

data indicate significant amplification of both the longitudinal and transverse accelerations at the roof of the Milpitas 

industrial building (fig. 12). Amplification factors for peak 

roof response relative to the base exceeded four in the 

longitudinal direction and three in the transverse direction. Amplification factors for peak response at the second floor were approximately one-half those at the roof. 

Similar to the Hollister warehouse, the in-plane acceleration response of the wall panels was not amplified with 

height above the base. 

The predominant frequency of the ground motion was 

0.2 Hz in the transverse direction and 0.4 Hz in the longitudinal direction. Predominant frequencies identified from 

the in-plane wall response were the same as those identified from the ground motion. Predominant frequencies 

identified from the out-of-plane wall response were 3.7 

North West Center South 

Wall Wall Wall 

North West Center East 

Wall Wall Wall 

Ground Acceleration 

Ref. N @ Panel Acceleration 

O(Mid-height) 

'\ Roof Acceleration 

Figure 10.-Measured peak accelerations in Hollister warehouse. 

Hz in the transverse direction and 4.5 Hz in the longitudinal direction. 

The measured acceleration response of both the Hollister 

warehouse and the Milpitas industrial building highlight 

important differences between the in-plane and out-ofplane response of tilt-up wall panels. Little, if any, amplification of in-plane accelerations was observed with height 

above the base. The predominant frequency of the inplane acceleration response measured at the top of the 

panels was the same as the predominant frequency measured at the base. Therefore, tilt-up wall panels may be 

considered to behave as rigid bodies when excited in plane. 

In contrast, the out-of-plane accelerations measured at the 

top of the wall panels exhibited significant amplification 

relative to the base, and the predominant frequencies of 

the out-of-plane response differed from the predominant 

frequencies at the base. The out-of-plane response at the 

top of the wall panels and the acceleration response at 

the center of the roof indicate the inherent flexibility of 

floor and roof diaphragms commonly used in tilt-up 

construction. 

MEASURED DISPLACEMENT 

RESPONSE 

The digitized data provided by the California Department of Conservation included displacement histories that 

were obtained by integrating and filtering the corrected 

acceleration response (California Strong Motion Instrumentation Program, 1991). Recorded displacement data at 

the base of the two buildings and at the centers of the 

roofs are shown in figure 13. The peak ground displacement at both sites was approximately 10 in. With the exception of the transverse response of the Hollister 

warehouse, the absolute displacement response at the centers of the roofs of both buildings was approximately the 

same as the ground displacements. Amplification of the 

transverse displacements at the center of the roof in the 

Hollister warehouse may be observed. 

An estimate of the displacements sustained by the buildings was made by subtracting the ground displacement 

from the structural displacement. The resulting relative 

displacement histories were then filtered to remove noise 

attributable to the ground motion (Carter and others, 1993) 

Due to the nature of the integration process used to obtain 

displacements from accelerations and the subsequent data 

manipulation, the relative displacement response histories 

should be considered to be approximate. Peak relative 

displacements are also presented in tables 1 and 2 for the 

Hollister and Milpitas buildings, respectively. 

Relative displacement histories for the Hollister warehouse are shown in figure 14. Peak relative transverse 

displacements at the center of the roof exceeded 4 in., or 
MEASURED RESPONSE OF TWO TILT-UP BUILDINGS 

TRANSVERSE ACCELERATION RESPONSE 

0.3 1 Channel 3 - Roof, East Wall 

^-0.3'- 

C- Oe3r 

Channel 7 - Second Floor, North Wall 

0 

05 

u -0.31 

0.3, Channel 6 - Second Floor, East Wall 

-0.3L 

0.3 r Channel 9 - Ground, East Wall 

-0.3L Time, sec 

0.0 5.0 70.0 15.0 20.0 25.0 30.0 

LONGITUDINAL ACCELERATION RESPONSE I 1 I I 

Channel 77 1- Roof, East Wall 

0.3 

Q Second Floor, East Wall 

*- .<-, 

p 0.0 

(b 

\ 

(b 

u -0.3L 

u 

Â¥ 0.3, Channel 73 - Ground, East Wall 

-0.3L Time, sec 

0.0 5.0 70.0 I 75.0 20.0 25.0 30.0 

Figure 1 1 .-Measured acceleration records in Milpitas industrial building. 
C88 PERFORMANCE OF THE BUILT ENVIRONMENT 

East North West 

Wall Wall Wall 

Ground Acceleration 

2nd Level Acceleration 

Roof Acceleration 

East 

Wall 

Figure 12.-Measured peak accelerations in Milpitas industrial building. 

1 percent of the height of the building. The displacement 

response in the longitudinal direction and in the transverse direction at the top of the transverse walls was negligible. Elastic deformations within the plane of the 

diaphragm must occur to accommodate these displacements without structural damage. 

Relative displacement histories for the Milpitas building are shown in figure 15. The amplitude of the peak 

response did not exceed one-fourth in., which is the same 

order of magnitude as the expected error in converting 

from accelerations to displacements (Shakal and Ragsdale, 

1984). Therefore, the signal to noise ratio was considered 

to be too small to have confidence in these relative displacement data. 

SUMMARY 

The measured response of two tilt-up buildings located 

within 50 krn of the epicenter of the earthquake were 

presented. Although the structural systems used in these 

buildings differ significantly, their dynamic response was 

similar. 

Transverse accelerations were amplified by a factor of 

approximately three between the base and the center of 

the roof in both buildings. Peak transverse accelerations 

measured at mid-height of the longitudinal wall panels in 

the Hollister warehouse and at the second-floor level of 

the longitudinal wall panels in the Milpitas building were 

1O.Or Hollister Warehouse - NS 

Hollister Warehouse - EW Ground ----- \ Roof 

Milpitas Industrial Building - NS 

r 

'Oeo r Milpitas Industrial Building - EW 

Time, sec 

Figure 13.-Absolute displacement response of ground and roof. 
MEASURED RESPONSE OF TWO TILT-UP BUILDINGS C89 

greater than one-half the peak transverse accelerations 

measured at the center of the roof. 

In the longitudinal direction, the peak roof accelerations in the Hollister warehouse were approximately 1.3 

times the peak ground accelerations, while the peak roof 

accelerations in the Milpitas building exceeded the peak 

ground accelerations by a factor of 4. 

Essentially no amplification of in-plane accelerations 

over the height of the wall panels was observed. The magnitude and frequency content of the in-plane acceleration 

histories measured at the roof of the buildings were essentially the same as the ground motion. 

Displacement of the roof relative to the ground was 

more pronounced in the transverse, than the longitudinal 

direction. Maximum transverse displacements at the roof 

of the Hollister building exceeded 1 percent of the height 

of the building. 

ACKNOWLEDGMENT 

The work described in this paper was funded by the 

National Science Foundation under Grant No. BCS9120281 at the University of Illinois. The California Department of Conservation, Strong Motion Instrumentation 

TRANSVERSE DISPLACEMENT RESPONSE 

Channel 4 - Roof, Center 

A 

- 

4.0r 

Q, 

Channel 6 - Midpanel, West Wall 

u Channel 3 - Roof. North Wall 

3 0 0 --+=-A - - 

(b 

i 

(b 

-4.0- Time, see 

0.0 5.0 70.0 75.0 20.0 25.0 30.0 

.S LONGITUDINAL DISPLACEMENT RESPONSE 

^ 4.0- E Channel 77 - Roof, Center 

(b 

- 8 0.0. - -I 

Channel 72 - Roof. East Wall 

Time, see 

Figure 14.-Relative displacement response of Hollister warehouse. 
C90 PERFORMANCE OF THE BUILT ENVIRONMENT 

Program is thanked for providing acceleration records and 

structural drawings. 

REFERENCES 

California Strong Motion Instrumentation Program, 1991, Processed 

strong-motion data from the Loma Prieta earthquake of 17 October 

1989-Building records: California Department of conservation, 

Division of Mines and Geology, Office of Strong Motion Studies, 

Report No. OSMS 91-07, 196 p. 

Shakal, A., Huang, M., Reichle, M., Ventura, C., Cao, T., Sherburne, 

R., Savage, M., Darragh, R., and Peterson, C., 1989, CSMIP strongmotion records from the Santa Cruz Mountains (Loma Prieta), California earthquake of 17 October 1989: California Department of 

Conservation, Division of Mines and Geology, Office of Strong 

Motion Studies, Report No. OSMS 89-06. 

Shakal, A.F., and Ragsdale, J.T., 1984, Acceleration, velocity and displacement noise analysis for the CSMIP accelerograrn digitization 

system, in Proceedings, Eighth World Conference on Earthquake 

Engineering, V. 11, San Francisco: Earthquake Engineering Research 

Institute, p. 111-118. 

Carter, J.W., Hawkins, N.M., and Wood, S.L., 1993, Seismic response 

of tilt-up construction: University of Illinois, Urbana, Structural 

Research Series No. 581, 224 p. 

TRANSVERSE DISPLACEMENT RESPONSE 

0.25 ,- Channel 4 - Roof, North Wall 

0.25[ Channel 3 - Roof, East Wall 

0.25, .. Channel 7 - Second Floor, North Wall 

-0.25 1- Time, sec 

0.0 5.0 70.0 75.0 20.0 25.0 30.0 

-*Ãˆ- 

$ 0.ov 

$ 

.S LONGITUDINAL DISPLACEMENT RESPONSE I I I I I I I 

4 

P 

:-- 

- 

g 0.25, Channel 72 - Second Floor, East Wall 

a -0.25 - 

(b Channel 6 - Second Floor, East Wall -44 

5 

$ 

0.00 4 

-0 -0.25 - Time, sec 

(1) 

(b 

-*Ãˆ0.0 5.0 70.0 75.0 20.0 25.0 30.0 Figure 15.-Relative displacement reE sponse of Milpitas industrial building. 
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

BUILDING STRUCTURES 

PERFORMANCE OF THE BUILT ENVIRONMENT 

SEISMIC RESPONSE OF A SIX-STORY REINFORCED 

CONCRETE BUILDING 

CONTENTS 

By James C. Anderson, 

University of Southern California; and 

Vitelmo V. Bertero, 

University of California, Berkeley 

Page 

C9 1 

9 1 

92 

92 

92 

92 

92 

94 

96 

97 

97 

9 8 

98 

102 

103 

104 

far exceed code design values and indicate that a nonlinear response analysis will be required to accurately evaluate the building response. 

ABSTRACT 

A six-story reinforced concrete building located in San 

Bruno was instrumented with 13 strong motion accelerometers at the time of the earthquake. Lateral resistance 

is provided by moment-resisting perimeter frames and 

one interior moment frame. Spectral analysis procedures 

were used to evaluate the dynamic properties of the 

building based on the response recorded during the earthquake. 

Linear elastic models of the building were developed 

for use with commercially available computer programs. 

Building responses calculated using these models compared well with the recorded response, indicating that the 

response was predominately linear elastic for this earthquake. Base shears calculated using accelerations recorded 

at the base of the structure exceed code values by a factor 

of 3.6 in the longitudinal direction and 2.7 in the transverse direction, indicating that some cracking and limited 

yielding may have occurred. Base shears developed by 

other earthquake ground motions recorded in the free field 

INTRODUCTION 

It has been pointed out by Bertero and others (1991) 

that most human injury and economic loss due to moderate or severe earthquake motions are caused by failures of 

civil engineering facilities, particularly buildings, many 

of which were presumably designed and constructed to 

provide protection against such natural hazards. The two 

most effective ways to mitigate the destructive effects of 

earthquakes are the improvement of present methods for 

designing, constructing, and maintaining new structures 

and the development of effective methods of seismic upgrading of existing facilities which do not meet current 

code requirements. 

One of the main steps toward the improvement of the 

design of new facilities and the selection of technically 

and economically efficient strategies for the seismic upgrading of existing facilities is the ability to predict in a 

reliable manner the dynamic behavior of the structures 

when they are subjected to critical seismic excitations that 

can occur during their expected service life. In order to 

achieve such improvements, it is necessary to compare 

the results of analytical calculations with the results of 

large-scale experiments. The best large-scale experiment 

is when an earthquake occurs and properly placed instruments record the response of the building to ground motions recorded at the base. This report summarizes the 

studies conducted on one such structure, a six-story reinforced concrete building. 

The Loma Prieta earthquake occurred about 9 miles 

northeast of Santa Cruz and 60 miles south of San Francisco. Following the earthquake, the U.S. Geological Survey (USGS) (1989) obtained building-response records in 

approximately eight instrumented structures. At the same 
C92 PERFORMANCE OF THE BUILT ENVIRONMENT 

time, the California Strong Motion Instrumentation Program (CSMIP) of the California Division of Mines and 

Geology (CDMG) (1989) obtained recorded response 

records from some 30 buildings. 

Two reinforced concrete buildings from this group of 

instrumented structures were selected for detailed study 

by the authors. The first of these was a 30-story condominium tower which is described in detail by Anderson 

and others (1991). The second building is a six-story, reinforced concrete office building located in San Bruno, 

approximately 50 miles from the epicenter. 

OBJECTIVES 

The main objectives of these studies are the following: 

(1) to evaluate the reliability of the analytical models presently available for analyzing the dynamic response of 

buildings; (2) to estimate through static and dynamic analyses the damage that the reference building may have experienced under the recorded ground motion; (3) to 

estimate the response of the building under critical ground 

motions to which it may be exposed during its service 

life; and (4) to assess the implications of the obtained 

results regarding the reliability of present seismic code 

regulations for the design of such buildings. 

SEISMIC DESIGN CRITERIA 

The structure was built in 1978, and it is assumed that 

the governing building code was the 1976 Uniform Building Code (UBC76). Lateral seismic design forces specified in this code will be compared to those currently 

required by the 1991 edition (UBC91). 

UBC76 SEISMIC FORCES 

Lateral seismic loads specified in the 1976 UBC were 

expressed in terms of the base shear which was defined as 

where C is the design seismic-resistance coefficient and 

We is the effective seismic dead load. The design seismic 

resistance coefficient is in turn given by the formula 

where 

C= 1/(15dr)=0.086; 

T=O. 1 *N=0.6 s; 

K=0.67; 

Z=S =I =1 .o. 

Using these values, the design seismic resistance 

coefficient becomes 

BUILDING DETAILS 

The six-story building, which is 78 feet in height and 

has a rectangular plan of 90 by 200 feet, was built in 

1978. Lateral resistance is provided by ductile, momentresistant concrete frames, four which are located on the 

perimeter and one which is located on the interior in the 

transverse direction. Typical beam spans are 16 feet. The 

exterior concrete columns are cast into precast finish shells 

which served as forms. Gravity loads are carried to the 

exterior columns and a center column by prestressed beams 

having a 42-foot simple span. The foundation under the 

moment frames consists of heavy grade beams and spread 

footings. Concrete strength for the moment frames is 

5,000 psi with Grade 60 reinforcing steel. Inspection of 

the structure following the earthquake showed some hairline cracking of the concrete finish shells around the columns. There was no visible damage noted on the interior 

of the building. 

The weight (mass) at the roof level and typical floor 

level are estimated as follows: (1) roof level; 2,790 kilopounds (kips) (7.22 k-s21in), (2) typical floor; 3,032 kips 

(7.85 k-s2/in). Using these values, the total self weight of 

the structure becomes 17,950 kips. 

This states that the total lateral force requirement will 

be 5.8 percent of the effective seismic dead load, which 

for most types of building occupancy is equal to the total 

self weight of the structure. Using the estimated self weight 

of the building of 17,950 kips, the base shear requirement 

becomes 

V = O.O58* 17950 = 1,041 kips. 

UBC91 SEISMIC FORCES 

The 199 1 UBC defines the design seismic-resistance 

coefficient as 

where the period, T, can be estimated as either 
SEISMIC RESPONSE OF A SIX-STORY REINFORCED CONCRETE BUILDING c93 

T(computer)=l -032 s (E-W) 

and the site coefficient, C, is specified by the formula 

The site coefficient based on the empirical formula for 

the period is the same for both directions and has the 

value 

whereas the values for this parameter obtained using the 

computed fundamental periods are 

The code further requires that these values be at least 

80 percent of those calculated using the empirical formula. It can be seen that this requirement is satisfied. 

Using Z=0.4; ^,,,=I2 and S=I=l .O, the design seismic resistance coefficients become 

and the corresponding base shears 

It should be noted that these values are 22.4 and 29.3 

percent less respectively than the lateral force requirement used to design the building in 1976. The authors do 

not feel this reduction in lateral force requirement is justified. 

v , Penthouse 

S/N Elevation 

Ground Level Plan 

CÃ§ntÃ Moment FremÃ 

/ 

Roof Plan 

5th Floor Plan 

Structure ~eterence 2nd Floor Plan 

Orientation: N= 335' 

Figure 1 .-Location of strong motion instrumentation. 
c94 PERFORMANCE OF THE BUILT ENVIRONMENT 

INSTRUMENTATION AND RECORDED 

RESPONSE 

The building was instrumented by the California Strong 

Motion Instrumentation Program (CSMIP) with 13 strong 

motion accelerometers at the time of the earthquake. These 

instruments were distributed throughout the building as 

shown in figure 1 which is taken from the California Department Mines Geology (California Division of Mines 

and Geology, 1989). The time histories of the recorded 

accelerations are shown in figures 2 to 14. The vertical 

accelerations recorded at ground level near the center of 

the building are shown in figure 2. Here it can be seen 

that the vertical acceleration has a peak value of 0.12 g, 

which is almost equal to the peak horizontal acceleration 

of 0.14 g recorded in the north-south direction (fig. 14). 

The transverse acceleration recorded at the roof, near 

the center of the building, is shown in figure 3. Here it 

can be seen that the peak acceleration reaches 0.31 g and 

Figure 2.-Recorded acceleration, vertical, base level, center. 

that the duration of strong motion response is approximately 17 s. The transverse accelerations recorded at the 

roof level near the north end of the building are shown in 

figure 4. At this location the peak acceleration reaches 45 

percent of gravity. A comparison of this record with the 

one just discussed indicates that there is a torsional response of the structure at this level. Similar acceleration 

data for the fifth story level is shown in figure 5 for the 

center and in figure 6 for the north end. At the center, the 

peak acceleration is 0.20 g and at the north end it increases to 0.23 g, which also indicates a torsion response 

but not as much as the roof level. Transverse acceleration 

data at the south, center, and north locations at the second-floor level is shown in figures 7 to 9. At these locations the peak recorded accelerations are 0. l l, 0. l l, and 

0.14 g, respectively. The recorded base acceleration at the 

center of the building in the transverse (east-west) direction is shown in figure 10. Here it can be seen that the 

peak acceleration is 0.1 1 g and that the duration of strong 

ground motion is about 7 s. 

n 

G> 0-3. SAN BRUhO (LOMA ^RlElA) - 

w I I 0.3 1 

1 I I I I I I 

I I - 1 ROOF CENTER EW L -0.3 

Figure 4.-Recorded acceleration, east-west, roof level, north. 

Figure 3.-Recorded acceleration, east-west, roof level, center. 

I I I 11-0.4 

2 5 3 0 

Figure 5.-Recorded acceleration, east-west, fifth level, center. 
SEISMIC RESPONSE OF A SIX-STORY REINFORCED CONCRETE BUILDING c95 

In the longitudinal (north-south) direction, all record- the longitudinal (north-south) direction of 0.25 g which 

ings were from instruments located near the center of the compares with 0.32 g in the east-west direction. Recall 

building. The acceleration recorded at the roof level is that the direction of fault propagation was approximately 

shown in figure 11, which indicates a peak acceleration in north-south. Also, at this location, the duration of strong 

Figure 6.-Recorded acceleration, east-west, fifth level, north. Figure 7.-Recorded acceleration, east-west, second level, south. 

0 5 10 15 20 2 5 3 0 

Figure 8.-Recorded acceleration, east-west, second level, center. Figure 9.-Recorded acceleration, east-west, second level, north. 

0 5 10 15 2 0 25 3 0 o.4 IIXI 1i,iIiii111111 ill1 llil 0.4 

n SAN 6RUh0 (LOMA 'RICTA) - 

o.3 n 

0 0.3 0 w w 

o.4 n 

U 0.3 -./ - 

111, 

-0.3 

- ROOF CE-HER NS - -0.3 

-0.4 t , , , , ,I, , , , , 0 5 3 -0.4 

25 3 0 TILE (~%coN~s) 

1i.I 

L 

Figure 10.-Recorded acceleration, east-west, base level, center. Figure 11 .-Recorded acceleration, north-south, roof level, center. 

11.1 

SAN BRUh0 

.Ill 

a -o.2 

2 -0.3 - - 

0.4 

' o,3 

11l.i.1.l 

(LOMA 

GRD CEN'ER 

'RIETA) 

EW 

-0,2 

-0.3 

- 
C96 PERFORMANCE OF THE BUILT ENVIRONMENT 

motion response is approximately 10 s compared with 17 

s in the transverse direction. Acceleration data at the fifth 

and second floors is given in figures 12 and 13. The recorded base acceleration is shown in figure 14, where it 

can be seen that the peak acceleration is 0.14 g compared 

to 0.11 g transverse. 

Linear elastic response spectra (LERS) showing the 

spectral accelerations for the three components of motion 

recorded at the base of the building are shown in figure 

15 along with the acceleration spectrum of the 1976 UBC. 

Based on this review of the recorded acceleration data, 

the following observations are made: 

1. The vertical accelerations recorded at the base have a peak 

amplitude which is similar to that of the horizontal accelerations at the same location. 

2. Peak ground acceleration occurred in the longitudinal 

(north-south) direction, whereas peak building response 

occurred in the transverse (east-west) direction. 

3. Examination of the acceleration data recorded at the roof 

level indicates that the fundamental period of vibration in 

both directions is in the range of 0.9 to 1.0 s. 

Figure 12.-Recorded acceleration, north-south, fifth level, center. 

Figure 13.-Recorded acceleration, north-south, second level, center. 

4. Comparisons between accelerations recorded at the center 

of the building and those recorded at the ends in the 

transverse direction indicate that there may be a torsional 

component in the dynamic response of the upper levels. 

MATHEMATICAL MODELS FOR 

ELASTIC RESPONSE 

Three dimensional models were developed for the structure using the SAP90 computer program (Wilson and 

Habibullah, 1989), and the ETABS computer program 

(Habibullah, 1989), although it is recognized that several 

alternative programs could have been used for this phase 

of the response analysis. 

The version of the ETABS program available at the 

time this phase of the study was conducted did not have 

the capability to produce time-history response plots and 

therefore did not permit a comparison of recorded response 

with calculated response. The current version has this capability and could have been used for all response and 

design analyses for this structure. 

An isometric view of the analytical model used in the 

SAP90 program to represent the lateral force-resisting 

system of the building is shown in figure 16. Here it can 

be seen that there are two moment resistant frames in the 

longitudinal direction (north-south) and three moment 

resistant frames in the transverse direction (east-west). The 

floor is modeled as a rigid diaphragm. Views of the building model used in the ETABS program are shown in 

figure 17. An isometric view of the ETABS model, shown 

in figure 17A, includes the gravity framing in addition to 

the lateral force framing. A plan view indicating the two 

basic types of frames and the location of the column lines 

is shown in figure 175. 

0.1 

0.0 

-I I'I I I -0.1 

TIME (SECONDS) 

Figure 14.-Recorded acceleration, north-south, base level, center. 
SEISMIC RESPONSE OF A SIX-STORY REINFORCED CONCRETE BUILDING c97 

The static deflected shape of the building in the longitudinal (north-south) direction obtained using the SAP 

model with the lateral seismic loads specified in UBC76 

is shown in figure 18A. Here it can be seen that the maximum deflection at the roof level is almost 1 inch. A similar plot showing the deflected shape in the transverse 

(east-west) direction is presented in figure 185. In this 

direction the maximum deflection is seen to be 1.3 inches. 

The static deflected shapes obtained using the ETABS 

program and the code lateral loads are shown in figure 

19. The deflected shape in the north-south direction is 

shown in figure 19A and the static deflected shape in the 

east-west direction is shown in figure 195. Comparing 

the results shown in figure 19 with those shown in figure 

18 indicates the following: (1) The roof deflection in the 

longitudinal direction calculated using the SAP90 model 

is 0.996 inches and that using the ETABS model is 0.92 

inches and (2) the roof deflection in the transverse direction obtained from the SAP90 model is 1.30 inches and 

that obtained from the ETABS model is 1.23 inches. 

ELASTIC DYNAMIC RESPONSE 

ANALYSES 

MODAL PERIOD DETERMINATION 

In order to better evaluate the recorded response, spectral analyses were conducted in both the time domain (response spectra) and in the frequency domain (Fourier 

spectra) in an effort to identify the predominate periods of 

BASE N-S 

---- 

- - 

BASE E-W 

- UBC 1976 BASE UP 

PERIOD (sEc.) 

Figure 15.-Spectra of recorded motions vs. code requirement. 
C98 PERFORMANCE OF THE BUILT ENVIRONMENT 

vibration. The response spectra are generated by passing 

the recorded floor accelerations through a single degree 

of freedom oscillator having 5 percent of critical damping. The Fourier spectra are transfer functions which consider the base input and the floor level output. 

Using the three-dimensional analytical models developed for the SAP90 and ETABS programs, the mode 

shapes and frequencies for the first six modes of vibration 

were evaluated. Results indicated that these six modes 

accounted for 95.3 percent of the effective mass in the 

north-south direction and 94.6 percent in the east-west 

direction. These analyses indicate that the first mode is a 

translational mode in the transverse (east-west) direction, 

the second mode is a translational mode in the longitudinal (north-south) direction and the third mode is a torsional mode. The fourth mode is a second translational 

mode in the east-west direction, the fifth mode is a second translational mode in the north-south direction, and 

the sixth mode is the second torsional mode. 

The modal periods identified using linear elastic response spectra (LERS) and Fourier transfer functions (FTF) 

are compared with the modal periods obtained from the 

mathematical models in table 1. These results show a good 

correlation between those obtained from the recorded data 

and those obtained from the analytical models. 

MODAL VERIFICATION ANALYSES 

In these analyses, the three-dimensional SAP90 model 

was simultaneously subjected to accelerations recorded in 

Figure 16.-Lateral force framing system. 

the north-south and east-west directions at the base. Timehistory comparisons are presented and response spectra 

are used for a more critical comparison of the recorded 

and calculated motions. As mentioned previously, the spectra are generated by passing the recorded and calculated 

accelerations through a single-degree-of-freedom oscillator having 5 pecent of critical damping. 

The time-history of the accelerations recorded at the 

roof level in the north-south direction near the center of 

the building are compared with the calculated values in 

figure 20A. Here the match with the frequency is quite 

good; however, the amplitudes of the calculated values 

tend to exceed the recorded values for a couple of cycles 

in the 10- to 15-s region of the time-history. The corresponding response spectra are shown in figure 20B. Here 

it can be seen that the spectra obtained from the calculated accelerations tends to fall above that of the recorded 

accelerations over much of the period range but the difference is not large. 

Similar comparisons in the east-west direction are shown 

in figure 21. The time histories are shown in figure 21A, 

where it can be seen that in this direction, the recorded 

values tend to exceed the calculated values during a couple 

of cycles in the 10- to 15-s time period. The corresponding spectra are shown in figure 21B where the spectral 

values for the recorded response tend to fall above the 

calculated values. A similar pattern to that just described 

for the roof level is seen in the data for the fifth floor, 

which is presented in figure 22 (north-south) and figure 

23 (east-west), and for the second floor, which is presented in figure 24 (north-south) and figure 25 (east-west). 

In general the match between the recorded and calculated values is very good over the complete height of the 

building. The calculated acceleration amplitudes tend to 

overestimate the recorded response in the north-south direction and to underestimate the recorded response in the 

east-west direction. This may by due in part to the fact 

the exact location of the recording instrument is unknown, 

and therefore the calculated results may not be at the same 

location as the recorded results. As noted previously, there 

appears to be a torsional response in this building, and that 

may be influencing the comparisons due to the uncertainty 

of the exact location of the instrument "near the center." 

STRONG MOTION ANALYSES 

The elastic dynamic behavior of the structure is evaluated by comparing the response to the UBC91 static lateral forces to the time-history demands of the earthquake, 

which were recorded at the base of the structure, and to 

the time-history demands of three additional earthquake 

ground motions. The three ground motions are the following: (1) the north-south component of the ground motion 
SEISMIC RESPONSE OF A SIX-STORY REINFORCED CONCRETE BUILDING c99 

recorded in El Centre (1940), the north-south component histories for these three ground motions are shown in figof the ground motion recorded at Hollister during the Loma ure 26. Elastic response spectra for these three motions 

Prieta earthquake (1989) and the S. 45O W. component of for 5 percent of critical damping are given in figure 27. 

the ground motion recorded at James Road during the Envelopes of maximum response are shown for lateral 

Imperial Valley earthquake (1979). The acceleration time displacement, interstory drift index, inertia force, and story 

Figure 17.-Gravity and lateral force framing. A, Isometric view; B, plan view, 
0-- 

PERFORMANCE OF THE BUILT ENVIRONMENT 

sunbr'3 

DEFORMED 

SHAPE 

LOAD 1 

MINIMA 

X 0.0000Et00 

Y-B. P?)S7f..-fl I 

Z-0.4 158E-01 

MAX I MA 

X 0.9956Et00 

Y 0,3215E-01 

Z 0.4158E-01 

sunbr'3 

DEFORMED 

SHAPE 

LOAD 1 

MINI MA 

X-0.3294E-0 I 

Y 0.0000E+00 

Z-0.5805E-0 1 

MAX I MA 

X 0.3611E-El 

Y 0. 1295Et0 1 

7 0.5805E-0 1 

Figure 18.-Static lateral force deflections, SAP90. A, Longitudinal (north-south); B, transverse (east-west). 
SEISMIC RESPONSE OF A SIX-STORY REINFORCED CONCRETE BUILDING 

SANBRU. PST 

DEFORMED 

STAT I C 

LOAD A 

TOP 6 

BOT I 

MINIMA 

X 0. 1919E+00 

Y-0,816lF- l l 

Z-0.3933E-0 I 

MAX I MA 

X 0.9131E+00 

Y 0.9247E-11 

Z 0.3933E-0 1 

ETABS 

SANBRU, PST 

DEFORMED 

STAT I C 

LOAD 8 

TOP 6 

BOT I 

MINIMA 

X-0.328 1E-0 1 

Y 0, 1799Et00 

Z-0.492 1E-0 I 

MAX I MA 

X 0.328 1E-0 1 

Y 0. 1091E+01 

Z 0.4921E-01 

ETABS 

Figure 19.-Static lateral force deflections, ETABS. A, Longitudinal (north-south); B, transverse (east-west). 
c 102 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 1 .-Periods of vibrations 

Mode 1 2 3 4 5 6 

LERS 1.00 0.90 - 0.30 - - 

FTF 1.05 0.85 0.70 0.34 0.27 - 

SAP90 1.06 0.91 0.71 0.34 0.29 0.23 

ETABS 1.03 0.89 0.71 0.33 0.29 0.23 

shear. Comparisons are made between the static analysis 

for UBC 1991 lateral forces and the dynamic response 

analyses for the four earthquake ground motions. The response envelopes in the transverse (east-west) direction 

are shown in figure 28, and those for the longitudinal 

(north-south) direction are shown in figure 29. 

The envelopes of maximum lateral displacement, shown 

in figure 28A, indicate that the displacement due to the 

recorded base motions is 2.35 inches compared to the 

0.68 inches due to code loads, a factor of 3.4 larger. The 

largest displacement is due to the Hollister motion and 

has a value of 13 inches at the roof. The envelopes of 

interstory drift index are shown in figure 285. Under the 

recorded motion, the maximum drift occurs at the second 

story level and reaches a value of 0.4 percent; however, a 

drift of more than 2 percent occurs at the second story 

level under the Hollister ground motion. This amount of 

drift would certainly cause damage to both structural and 

nonstructural components. 

The lateral inertia forces specified by the code are compared with those developed during the four time-history 

analyses in figure 28C . Here it can be seen that the forces 

developed at the building, based on the recorded base 

motion, are more than three times larger than those used 

for the design. In the case of the Hollister ground motion, 

the forces at the top of the structure are almost 20 times 

larger than the code values. The envelopes of maximum 

story shear are shown in figure 28D. Here it can be seen 

that the story shears developed during the Loma Prieta 

earthquake are 2.7 times larger than the code values. As 

in the previous response parameters, the Hollister motion 

develops a significant increase in the base shear which is 

20 times the code values. 

The envelopes of maximum lateral displacement in the 

longitudinal direction are shown in figure 29A. This figure indicates that the displacement under the recorded base 

motion is approximately three times larger than that produced by the code lateral loads. The largest displacement 

is due to the Hollister ground motion and has a value of 9 

inches at the roof level, 13 times larger than the code 

values. Similar data for the interstory drift index is shown 

in figure 295. The maximum drift in this direction under 

the recorded base motion is 0.34 percent at the second 

story level. Under the Hollister ground motion, this value 

increases to 1.37 percent. 

The envelopes of maximum inertia force are shown in 

figure 29C. The recorded base motions produce an inertia 

force at the roof level that is approximately 3.6 times the 

code value. When the effect of the Hollister ground motion is considered, this value increases to 13 times the 

code value. The envelopes of maximum story shear, shown 

in figure 29D, appear very similar to those discussed previously for the transverse direction. However, in this direction, the base shear due to the recorded base motions is 

3.7 times the design value. 

SUMMARY AND- CONCLUSIONS 

This study has investigated the dynamic response of a 

six-story reinforced concrete building which was instrumented with 13 strong motion accelerometers at the time 

of the Loma Prieta earthquake. The building has a rectangular plan with lateral resistance provided by four moment-resistant frames on the perimeter and one momentresistant frame on the interior in the transverse direction. 

Recorded peak accelerations at the base were 0.1 1 g in 

the transverse (east-west) direction and 0.13 g in the longitudinal (north-south) direction. Only limited damage 

was reported as a result of this motion. 

Spectral analyses of the recorded acceleration data were 

used to identify the predominant periods of vibration. Three 

dimensional, linear elastic models of the building were 

developed to study the behavior under the recorded base 

motion and to estimate the effects of stronger ground motions recorded in the free field during previous earthquakes. 

Comparisons are made between the dynamic responses 

and the forces and deformations obtained from code prescribed seismic forces. 

On the basis of these studies, the following general 

conclusions are presented: 

1. The maximum story shears obtained from the elastic 

response under the recorded base motions, exceed the code 

values by 3.7 times in the north-south direction and by 2.7 in 

the east-west direction. This amount of story shear must have 

resulted in considerable cracking of the concrete members and 

may have produced some minor yielding of the main reinforcement. 

2. The design resistance coefficient specified by current 

building codes has been reduced substantially for a building of 

this type when compared with the value used in the original 

design in 1977. It is not clear to the investigators how this 

reduction can be justified. 

3. It is clear that the base motions recorded during the 

earthquake were not strong relative to other recent earth- 
SEISMIC RESPONSE OF A SIX-STORY REINFORCED CONCRETE BUILDING C103 

quakes. However, they were still sufficient to drive the struc- rotation demands of the stronger ground motions. These 

ture to the yield level and perhaps a little beyond. Under considerations are discussed in a recent report by the authors 

stronger ground motions, such as those recorded closer to the (1997). 

epicenter at Hollister, the response is much more severe and 

would have undoubtedly led to severe damage. ACKNOWLEDGMENTS 

4. A static nonlinear analysis needs to be performed on this 

structure to estimate its lateral force capacity, and nonlinear This report is based on the results of a series of 

dynamic analyses need to be done to estimate the ductility and studies that have been conducted by the authors at the 

1 SAN BRUNO 6 N-S 0.3 

Figure 20.-Roof response, north-south. A, Calculated vs. recorded accelerations; B, calculated vs. recorded 

LERS. 
C104 PERFORMANCE OF THE BUILT ENVIRONMENT 

University of California, Berkeley, and the University REFERENCES 

of Southern California. This study is part of a KSearch Anderson, J.C., Miranda, E., and Bertero, V.V., 1991, Evaluation of 

project entitled "Implications of Response of Structures seismic performance of a thirty-story RC building: Report No. UCB/ 

to Ground Motions Recorded During the Loma Prieta EERC-91/16, Earthquake Engineering Research Center, University - 

Earthquake: Evaluation of Structural Response Factors," of California, Berkeley. 

Anderson, J.C., and Bertero, V.V., 1997, Seismic performance of a six which was supported by a research grant provided by the story reinforced concrete building: Report No. UCBIEERC-93/01, National Science Foundation. This support is greatly ap- Earthquake Engineering Research Center, University of California, 

predated. Berkeley. 

SRN BRUNO SIX 

n 

0 0.3 w 

Figure 21.-Roof response, east-west. A, Calculated vs. recorded accelerations; B, Calculated vs. recorded 

LERS. 

, 

; : . II I 

SAN BRUNO 6 E-W 

0.3 
SEISMIC RESPONSE OF A SIX-STORY REINFORCED CONCRETE BUILDING C105 

Bertero, V.V., Anderson, J.C., Krawinkler, H., and Miranda, E., 1991, 

Design guidelines for ductility and drift limits: review of the stateof-the-practice and state-of-the-art on ductility and drift-based earthquake resistant design of buildings: Report No. UCBIEERC 91/15, 

Earthquake Engineering Research Center, University of California, 

Berkeley. 

California Division of Mines and Geology, CSMIP strong-motion records 

from the Santa Cruz Mountains (Loma Prieta) California earthquake of October 17, 1989: Report OSMS 89-06. 

Habibullah, A., 1989, ETABS three dimensional analysis of building 

systems: User's manual: Computers and Structures, Inc., Berkeley, 

Calif. 

International Conference of Building Officials, 1976, Uniform Building 

Code [I976 Edition]: Whittier, Calif. 

International Conference of Building Officials, 199 1, Uniform Building 

Code [I991 Edition]: Whittier, Calif. 

U.S. Geological Survey, 1989, Strong motion records from northern 

California (Loma Prieta) earthquake of October 17, 1989: U.S. Geological Survey Open-File Report 89-568. 

Wilson, E.L., and Habibullah, A., 1989, SAP90, A series of computer 

programs for the static and dynamic finite element analysis of structures: User's manual: Computers and Structures, Inc., Berkeley, Calif. 

SRN BRUNO SIX 

n 

0 0.3 

10-2 10-1 1 00 1 02 

PERIOD B 

Figure 22.-Fifth floor response, north-south. A, Calculated vs. recorded accelerations; B, calculated vs. 

recorded LERS. 

SAN BRUNO 6 N-S 

0.3 
PERFORMANCE OF THE BUILT ENVIRONMENT 

PER I OD B 

Figure 23.-Fifth floor response, east-west. A, Calculated vs. recorded accelerations; B, calculated vs. recorded LERS. 
SEISMIC RESPONSE OF A SIX-STORY REINFORCED CONCRETE BUILDING 

SRN BRUNO SIX 

10-2 

CRLCULRTED 2ND NS 

n ----- RECORDED 2ND NS 

n 

0 0.3 

w 

2 

0.2 

PER I OD B 

Figure 24.-Second floor response, north-south. A, Calculated vs. recorded accelerations; B, calculated vs. 

recorded LERS . 

SAN BRUNO 6 N-S 

0.3 

0.2 
PERFORMANCE OF THE BUILT ENVIRONMENT 

SRN BRUNG SIX 

10-1 1 00 101 1 02 lllllll I ll l11lll I 1111"4-"2 

CALCULATED 2ND EN ----- RECORDED 2NO EM 

n 

C") 0.3 w 1 

Figure 25.-Second floor response, east-west. A, Calculated vs. recorded accelerations; B, calculated vs. 

recorded LERS. 

SAN BRUNO 6 E-W 0.3 
SEISMIC RESPONSE OF A SIX-STORY REINFORCED CONCRETE BUILDING 

-~.~O:IIII~IIII~IIII~IIII~IIII~II~I~ 

0.0 5.0 10.0 15.0 20.0 25.0 30.0 

Time (sec) 

- I I I I I I I 

I I I I I I I 

-0.40 IIII~IIII~IIII~IIII~IIII~IIII~IIII~IIII~ 

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 

Time (sec) 

Figure 26.-Recorded free field ground motions. A, El Centro, 1940; B, Hollister, 1989; C, James 

Road, 1979. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

- JAMES 

PERIOD 

Figure 27.-Spectra of free field motions. A, El Centro, 1940; B, Hollister, 1989; C, James Road, 1979. 
SEISMIC RESPONSE OF A SIX-STORY REINFORCED CONCRETE BUILDING Clll 

DISPLACEMENT (IN.) A 

%........ .. vv...m. 1...1..m. I I '0 

1 0.6 1 od2 0.b 

INTERSTORY DRIFT RATIO B 

MAX. INERTIA FORCE r< 

I I I 

I I I I I I 

0 ,.,.',,..'..,.',... 0 sob0 lo600 15600 20000 

STORY SHEAR (KIPS) D 

Figure 28.-Maximum response envelopes, transverse. A, Lateral displacement; B, interstory drift index; C, inertia force; D, story shear. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

0 

6 

6 

4 

3 

2 

1 

0 

0 

MAX. INERTIA FORCE /^ 

I I I 

I I I 

I I I I I I . . 0 

5ob0 i 0600 I&O 2doo 

STORY SHEAR (KIPS) D 

Figure 29.-Maximum response envelopes, longitudinal. A, Lateral displacement; B, interstory drift index; C, inertia force; D, story shear. 
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

PERFORMANCE OF THE BUILT ENVIRONMENT 

BUILDING STRUCTURES 

SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING 

By James C. Anderson, 

University of Southern California; and 

Vitelmo V. Bertero, 

University of California at Berkeley 

CONTENTS 

ABSTRACT 

An instrumented 42-story steel moment resisting space 

frame located in San Francisco is analyzed in detail. The 

building was instrumented with 14 strong-motion accelerometers deployed throughout the structure at the time of 

the earthquake. Inspection of the building following the 

earthquake did not reveal any damage. Critical comparisons are made between recorded response, calculated response, seismic design criteria, and code lateral force 

requirements. The response of the building to other more 

critical ground motions is also investigated. 

Results indicate that the response of this building during the earthquake was entirely linear elastic. The building designers opted to use a site-specific design spectrum 

that was more conservative than the code minimum requirements. However, the study indicated that there may 

be considerable uncertainty in developing site-specific 

design spectra for soil sites. It also indicated that for this 

tall building, the higher vibration modes have a significant effect on the building response. Use of the code 

design procedure, which requires scaling of the design 

Page 

Cll3 

113 

114 

114 

115 

116 

118 

118 

124 

125 

126 

127 

134 

136 

138 

139 

139 

spectrum based on the calculated base shear, may lead to 

unconservative design forces in the upper stories of tall 

buildings due to the effects of the higher modes on the 

shear distribution over the height. Use of the minimum 

code design requirements for the design of this building 

would have resulted in increased damage potential under 

service-level earthquakes such as the one experienced by 

this structure. 

INTRODUCTION 

It has been noted that most human injury and economic 

loss due to moderate or severe earthquake ground motions are caused by failures of civil engineering facilities, 

particularly buildings, many of which were presumably 

designed and constructed to provide protection against such 

events. Building codes specify minimum lateral-force requirements, which represent the inertia forces that develop 

in the structure during ground shaking. Economic considerations usually dictate that these minimum lateral forces, 

which are predicated on the concept of life safety, become the structural design forces. The relatively high economic loss incurred from the earthquake has created a 

renewed interest in a two-level design criteria in which 

damage control under moderate ground shaking is considered along with life safety under strong ground motion. 

In order to improve the design criteria used for the 

design of new construction and the seismic upgrading of 

existing structures, it is necessary to accurately predict 

the dynamic behavior of such facilities when they are 

subjected to seismic excitations that can be expected to 

occur during their service life. Once this is done, the lateral forces determined from current building codes can be 

compared with those which develop in the structure during actual earthquakes. 

In order to evaluate the accuracy of the seismic provisions of current building codes, it is necessary to compare 

the results obtained using current analytical methods with 

the results of large-scale experiments. The best large-scale 

experiment occurs when instruments located in a structure 
C114 PERFORMANCE OF THE BUILT ENVIRONMENT 

reliably measure the building response to the ground motions recorded at the base during an actual earthquake. 

The Loma Prieta earthquake produced records of strong 

ground motion in more than 46 well-instrumented buildings. This collection of data included the strong motion 

recorded in the 42-story steel building which is the subject of this study. 

OBJECTIVES 

The main objectives of this study are the following: 

(1) to evaluate the accuracy of current analytical modeling techniques for determining the dynamic response of a 

tall building; (2) to estimate the damage that the reference 

building may have experienced under recorded ground 

motions; (3) to estimate the response of the building under critical ground motions to which it may be exposed 

during its service life; (4) to evaluate the reliability of the 

seismic design criteria with respect to recorded response 

and code requirements; (5) to evaluate the influence of 

modeling discretizations on the computed response; and 

(6) to assess the implications of the obtained results regarding the reliability of present seismic code requirements for the design of such buildings. 

In order to achieve these objectives, three-dimensional 

analytical models were developed and linear elastic dynamic analyses were conducted using the recorded base 

motions for the Loma Prieta earthquake and recorded freefield motions for other strong motion earthquakes. Results are presented in terms of response spectra, 

time-history-response comparisons and maximum response 

envelopes, which include lateral displacement, interstory 

drift index, inertia forces, and story shears. 

BUILDING SYSTEM AND 

INSTRUMENTATION 

The 42-story building is a 550-foot-tall office building 

located in the financial district of San Francisco. The structure has .a rectangular plan (79 by 15 1 ft) with a steel 

superstructure which can be classified as a special moment-resistant space frame. The building, which was designed in 1972, uses rolled W sections and built-up 

members of A36 steel for the beams and girders which 

have typical spans of 27 feet. Welded box sections fabricated from A572 Grade 42 steel in the bottom 33 floors 

and A36 steel in the upper floors are used for the columns. The box sections vary from 20 by 20 inches to 26 

by 26 inches with wall thickness that varies from 112 to 3 

318 inches. Internal stiffners, equal in thickness to the thickness of the girder flange are used where girders deeper 

than 30 inches frame into the column. The beams and 

girders are connected to the column by full-penetration 

welds at the top and bottom beam flanges, and the beam 

web is bolted to the face to the column with a shear plate. 

The building is supported on 491 precast concrete piles 

which are 16 inches square. Pile groups consisting of 13 

to 25 piles are located under the 29 column lines. Pile 

groups are capped by concrete pile caps which are 78 to 

84 inches thick. The 29 pile groups are interconnected by 

concrete tie beams which are typically 24 by 27 inches. 

An isometric view of the three- dimensional computer 

model of the building used for the elastic analyses is shown 

in figure 1. 

At the time of the earthquake, the building was instrumented with 14 strong-motion accelerometers located at 

the base, ground, 25th, 34th, and 42d levels. Each of these 

levels included a longitudinal and transverse component 

Figure 1 .-Schematic plan of 42 story 

building. 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING C115 

located at the geometric center of the tower. In addition 

the base included a vertical component, and the 25th, 34th, 

and 42d floors included a transverse component at the end 

of the tower. Unfortunately, no vertical-motion accelerometers are located in the building, which would have 

permitted the effect of vertical ground motion to be evaluated. The location of the instrumentation is summarized 

in figure 2. 

SEISMIC DESIGN CRITERIA 

The building was designed in 1972 for the requirements 

of the building code for the City and County of San Francisco (CCSF). At that time, the code lateral-force requirement for this building was an equivalent static loading 

which was defined in terms of the base shear. Lateral 

forces defined in terms of the base shear have the general 

form 

where C is the design seismic resistance coefficient and 

We is the total seismic dead load, which for an office 

occupancy is equal to the total dead load, W. At the time 

the building was designed, the CCSF code required a design base shear given by the expression 

V = (KQW 

where K = 0.67, C = 0.05/(7)'~ and T = O.1N where N is 

the number of stories. Using N equal to 42 gives an estimate of the period (7) of 4.2 s. Using this estimate, C 

EAST SIDE 

- GROUND 

42ND.34TH. & 2STH FLOORS 

GROUND FLOOR 

NORTH SIDE 

SENSORS 

VERTICAL 

HORIZONTAL *Â¥.am 

BASEMENT 

Figure 2.-Instrument locations in building. 
C116 PERFORMANCE OF THE BUILT ENVIRONMENT 

becomes 0.031 and results in a design seismic resistance 

coefficient (C) of 0.021, or 2.1 percent of the total dead 

load. With the total dead load estimated to be 62,473 

kilopounds (kips), the base shear required by the 1972 

CCSF code would have been 1,312 kips. 

In addition, the owner and the structural engineer decided to use a site-specific design spectrum and a dynamic analysis to determine the lateral design forces. The 

design criteria required that the structural frame should 

withstand the lateral forces due to the base design spectrum with stresses less than the yield stress. The design 

earthquake was specified as having an effective peak acceleration on rock of 0.2 g, which was considered representative of an M=7 earthquake on either the San Andreas 

or Hayward fault systems. This design earthquake led to 

the development of a site-specific spectrum representative 

of rock motions. The rock spectrum was further modified 

to reflect the effect of local soil conditions using the methods suggested by Ruiz and Penzien (1969). This led to 

the development of the base design spectrum (fig. 3) which 

was used for the design of the structure. The use of this 

spectrum resulted in design base shears of 3,469 kips longitudinal (north-south) and 3,996 kips transverse (eastwest). It must be recognized that the CCSF code 

requirements are based on allowable stresses, whereas the 

design spectrum is based on yield stresses. 

MATHEMATICAL MODELS FOR 

ELASTIC RESPONSE 

Detailed three-dimensional finite-element models of the 

building were developed in order to evaluate the response 

to the two horizontal components of ground motion recorded at the base. Several computer programs are currently available on a commercial basis for analyzing the 

elastic dynamic response of three-dimensional structures 

using a personal computer. The SAP90 program developed by Wilson and Habibullah (1989) was selected for 

use in the initial phase of this study for the following 

reasons: (1) An extended memory version, SAP90 Plus, 

was available which permitted modeling the structure in 

detail for computation on a personal computer and (2) the 

program allows the user to plot the time-history response 

at any node point. This feature was crucial for comparison of the calculated results with the recorded response. 

The completed SAP90 model of the structure consisting of 1,260 nodes, 3,116 elements, and 3,690 degrees of 

Figure 3.-Site specific design spectra for building. 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING C117 

freedom is shown in figure 4. An overall isometric view 

is shown in figure 4A and a plan view is shown in figure 

45. This view shows that the structure is symmetrical 

about the transverse axis but has some nonsymmetry in 

the stiffness about the longitudinal axis. An elevation view 

in the transverse (east-west) direction is shown in figure 

4C and an elevation view in the longitudinal (north-south) 

direction is shown in figure 4D 

A second three-dimensional model of the building was 

developed for use with the ETABS-Plus program, which 

is also discussed by Habibullah (1989). This model was 

used for making comparisons between the earthquake response and the building code requirements in terms of 

lateral displacement, interstory drift, lateral force, and story 

shear. It was also used for checking the deflection of the 

building for wind loading and for performing a stress check 

of all the main steel members in the building. It should be 

noted that since this study was completed, a time-historyresponse capability has been added to this program so 

that if the study were to be done today, there would be no 

Figure 4.-Building configuration: isometric 

view (A); plan view (B); transverse elevation 

(C); longitudinal elevation (D). 
c118 PERFORMANCE OF THE BUILT ENVIRONMENT 

need to use the SAP90 program for the analysis of this 

structure. 

An evaluation of the dynamic properties of the building 

requires that the self weight (mass) of the structure be 

estimated as accurately as possible. The self weight was 

estimated as follows: 

42d level.-The building has a two story, mechanical 

penthouse having a 53 by 125 foot plan located above the 

42d level. The weight of this structure was added to the 

weight at the 42d level, resulting is a total weight (mass) 

of 2,593 kips (6.71 kip-s2/in.). 

41st level.-This floor is also a mechanical equipment 

level, and the general notes on the structural plans indicated an equipment weight of 125 psf. Using this value, 

the total weight (mass) at this level was estimated to be 

2,633 kips (6.8 1 kip-s2/in.). 

25th-40th level.-General notes on the structural plans 

for these typical floors indicated a partition load of 20 

psf. Using this value and estimating the flooring and ceiling weights, the total weight (mass) is estimated to be 

1,368 kips (3.54 kip-s2/in.). 

3rd-24th level.-Weights of these typical floors are 

slightly higher than those above due to an increase in the 

steel weight. The total weight (mass) is estimated to be 

1,495 kips (3.87 kip-s2/in.). 

2nd level.-The weight of this mezzanine floor was 

estimated to be higher due to increased loading indicated 

in the general notes. The total weight (mass) is estimated 

to be 2,464 kips (6.38 kip-s2/in.). 

Summing the weights of all the individual floors results 

in an estimated building weight (mass) of 62,473 kips 

(161.7 kip-s2/in). 

DYNAMIC CHARACTERISTICS 

In order to determine the periods of vibration from the 

recorded data, analyses are conducted in both the time 

and frequency domains. The recorded responses are evaluated in the time domain using a linear elastic-response 

spectra with 5 percent of critical damping. Analyses in 

the frequency domain are done using nonparametric time 

invarient and nonparametric time varient system-identification techniques. The modal periods identified from the 

recorded data are compared with the calculated values in 

figure 5. Periods for the transverse direction are compared in figure 5A and periods for the longitudinal direction in figure 5B. 

From these analyses of the recorded response of the 42- 

story building, obtained during the earthquake, the following observations can be made regarding the dynamic 

response: 

1. Translational modes in the two principal directions have 

the following periods of vibration in seconds forthe first 

four modes: (a) transverse (east-west)-5.4, 1.8,l. 1, and 0.7; 

(b) longitudinal (north-south-5.1, 1.7, 1.0 and 0.7. 

2. The second and third modes of vibration of this building 

make a significant contribution to the response. 

3. At the ground level, the peak recorded acceleration was 

approximately 12 percent of gravity in each direction. 

4. Peak accelerations recorded at the 25th level were equal 

to or larger than those recorded in the upper stories, including 

the roof. 

5. The 34th level has little response from the second mode 

in either direction, indicating it may be near a modal node. 

6. The torsional response of the building was very small and 

did not make a significant contribution to the total response. 

ANALYTICAL MODEL VERIFICATION 

Significant advances have been made during the past 

20 years in the development of computers and programs 

which can be used for the seismic response analysis of 

structures. Along with these developments, it is necessary 

to evaluate the accuracy of these programs and mathematical modeling techniques in predicting the seismic response 

of actual buildings. In order to accomplish this, it is necessary to compare the calculated response with that obtained from experiments. The best experiment is obtained 

when instruments reliably record building response due to 

motions recorded at the base. The time-history accelerations recorded at the ground-floor level (fig. 6) indicate 

that the peak values were approximately 0.12 g in both 

horizontal directions. This recorded acceleration data will 

serve as input to the dynamic models when calculating 

the building response to this earthquake. 

Using the accelerations recorded on the ground floor as 

input, the time-history response of the model was evaluated, and the calculated accelerations and displacements 

were compared to those recorded in the building. In these 

analyses, accelerations recorded in the east-west (X) and 

north-south (Y) directions were applied simultaneously to 

the model, and the dynamic response was calculated using the modal superposition, time-history approach. A total of 21 modes of vibration (seven translational modes in 

each principal direction and seven torsional modes) were 

used in the dynamic analyses. These modes represented 

97 percent of the reactive mass. It should be noted that 

only nine modes of vibration would have been required to 

meet the code requirement that 90 percent of the reactive 

mass participate in the response. 

In order to better evaluate the comparison between the 

frequency content of the recorded and the calculated floor 

accelerations, floor response spectra were generated for 

both motions. This was done by passing the recorded floor 

accelerations through a single-degree-of-freedom (SDOF) 

oscillator and generating a response spectrum with 5 percent of critical damping. In a similar manner, the floor 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING C119 

accelerations which were calculated using the 3-D finiteelement model were passed through the SDOF oscillator 

generating another floor response spectrum. These spectra 

were then plotted on the same tripartite graph for comparison. Initially, the damping in the structure was taken 

as 6 percent of critical in all modes. Review of the initial 

comparisons between recorded and calculated response 

indicated that a lower damping was appropriate, particularly in the higher modes. Therefore, the final damping 

values were taken as 5 percent of critical for the first 

three modes and 3 percent of critical in the higher modes. 

The data recorded at the 42d level in the east-west (transverse) direction are compared with the calculated results 

in figure 7. It can be seen in figure 7A that the maximum 

calculated acceleration (0.21 g) is slightly larger than the 

recorded value (0.19 g); however, the frequency match is 

very good. The comparison of displacements (fig. 75) 

shows that the maximum calculated displacement is 5.5 

inches versus 5.8 inches recorded. Here the frequency 

match is good for the first 20 seconds, and beyond that 

point the period of the recorded values appears to lengthen. 

Considering the floor spectra (fig. 7C) the comparison is 

good over the entire period range. 

Similar data at the 42d level in the north-south (longitudinal) direction are compared with the calculated results 

in figure 8. The time-history plots of the acceleration (fig. 

8A) indicate that in this direction, the calculated response 

tends to overestimate the recorded response, although the 

frequency comparison is reasonably good. A similar result is shown in figure 85 for the displacement response, 

where the maximum calculated displacement is 5.3 inches 

compared with 2.8 inches recorded. The floor spectra (fig. 

8C) indicate that the calculated response exceeds the recorded response over the entire period range. It should be 

Period (Sec.) 

Mode 1 Mode 2 Mode 3 Mode 4 

Response Spectrum Fourier Spectrum 0 SAP90 3-D 

Period (Sec.) ft. I 

5 

4 

3 

2 

1 

0 

Figure 5.-Identified and calculated periods of vibration: 
PERFORMANCE OF THE BUILT ENVIRONMENT 

noted that this calculated response could be reduced by 

increasing the damping for the modes in this direction. 

However, this caused an excessive reduction in the response in this direction in the floors below this level. 

The recorded response data at the 34th level in the eastwest direction are compared with the calculated results in 

figure 9. The time-history results are compared in figure 

9A Here it can be seen that the comparison with both 

amplitude and frequency is very good, with a maximum 

calculated value of 0.16 g compared to recorded value of 

0.148 g. The displacement time history is shown in figure 

9B. The maximum calculated displacement is 3.9 inches 

compared to a maximum recorded value of 3.1 inches. As 

discussed previously, the period match is good for the 

first 20 seconds, but beyond this point the period of the 

recorded data tends to lengthen. The floor spectra (fig. 

9C) indicate that the comparison is very good over the 

complete period range. 

Similar comparisons at the 34th level for motions in the 

north-south direction are shown in figure 10. Considering 

0.4 

n 

0 0.3 w 

2 

0.2 

0 0.1 

2 0.0 

cr 1 

-0.1 

_J - w " -0.2 

-0.3 

-0.4 

0 5 10 30 35 

TI$ ({Fco~Ds) 40 

Figure 6.-Recorded base motion (ground level). 

Ill1 Ill1 li[l ll[[ 1111 

RECORDED 

11,1 

GROUND NS 

- 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING 

the time-history response (fig, 10A) the maximum calcu- The recorded response data at the 25th level in the eastlated acceleration is 0.168 g compared with a recorded west direction is compared with the calculated values 

value of 0.183 g, The displacement response (fig, 10B) in figure 11. The acceleration time-history (fig. 11A) 

indicates a maximum calculated displacement of 3.0 inches indicates a peak calculated acceleration of 0.18 g comcompared with a maximum recorded value of 2.5 inches. pared to a recorded value of 0.22 g. Frequency compariThe floor spectra (fig. 10C) compare favorably over the 

entire period range. 

Figure 7.-Recorded response vs. calculated, 42d level, east-west. 

PERI OD 

- - I I I I I I I 

Figure 8,-Recorded response vs. calculated, 42d level, north-south. 

E 

0 

-10.0 - 10.0 

0 4 lb 1'5. 2'0 2'5 3'0 3'5 4'0 

TIME (SECONDS) 

B 

I I - CALCULATED 42 NS I I I--.--- I I I ~RECOR~ED 4q NS I I I I I I I 

- 

- 
PERFORMANCE OF THE BUILT ENVIRONMENT 

sons are very good, particularly for the first 20 seconds. in the floor spectra (fig. 11C). Here it can be seen that the 

The time-history of the displacement (fig, 11B) indicates calculated spectrum is below the recorded spectrum over 

a peak calculated displacement of 3.2 inches compared much of the period range. 

with a recorded value of 4.6 inches, This underestimation Similar data at the 25th level in the north-south 

of the response by the calculated results is also reflected direction are presented in figure 12. The time-history 

TIME (SECONDS) A 

PER I OD c 

Figure 9.-Recorded response vs. calculated, 34th level, east-west. 

PERIOD c 

Figure 10.-Recorded response vs. calculated, 34th level, north-south. 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING 

data (fig. 12A) indicate a peak calculated acceleration of of 2.0 inches. The overestimation of the response by the 

0.188 g compared with a recorded value of 0.163 g. The calculated results is also indicated in the floor spectra 

time-history of the displacements is shown in figure (fig. 12C). Here it can be seen that the calculated 

12B. Here it can be seen that the maximum calculated spectrum falls above the recorded spectrum in most pedisplacement is 2.9 inches compared with a recorded value riod ranges. 

TIME (SECONDS) A 

TIME (SECONDS) B 

PER I OD 

Figure 11.-Recorded response vs, calculated, 25th level, east-west. 

PER 100 c 

Figure 12.-Recorded response vs. calculated, 25th level, north-south. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

INPUT ENERGY 

In order to evaluate the relative importance of earthquake motions in the north-south and east-west directions, 

the seismic energy input to the structure in these two directions was evaluated. This was done by considering the 

product of the recorded base displacement, v and the 

calculated base shear, V. In incremental form txis can be 

expressed as 

Input energy = ZV x 6vg. 

The time-history of the base shear in the east-west direction is shown in figure 13A, and the corresponding 

values for the north-south direction are shown in figure 

13B. The figures indicate that the maximum base shears 

in both directions are almost equal, being 2,000 kips in 

the east-west direction and 2,200 kips in the north-south 

direction. A comparison of the elastic input energies (fig. 

13C) indicates that there is about 50 percent more input 

energy in the north-south 

east-west direction. 

direction as compared to the 

IHE XI0 

I 

l,ll~,,~l1l~~~~,ll,,,l,, 

I 

20.00 40.00 60.00 

TIME (SECONDS) f' 

Figure 13 Elastic energy input: base shear time history, transverse (A); base shear time history, longitudinal (B); elastic input energy time 

history (C). 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING C125 

MODAL RESPONSE CONTRIBUTION 

The relative contributions of the first four modes of 

vibration in the east-west (transverse) direction to the absolute acceleration at level 42 are shown in figure 14. The 

absolute acceleration due to the first mode response (fig. 

14a) indicates that the peak acceleration due to this mode 

is only 0.072 g. Inclusion of the second mode response 

increases the peak acceleration to 0.17 g (fig. 14b), indicating an increase of more than 100 percent from the 

second mode. Inclusion of the third mode response (fig. 

14c) indicates a peak acceleration of 0.255 g which represents another increase of 50 percent. With the inclusion of 

the first four modes of vibration (fig. 144 the peak acceleration increases another 49 percent to a value of 0.38 g. 

These results indicate that for this tall building, the acceleration response at the top of the building is primarily due 

to the 2d, 3d, and 4th modes with a minimal contribution 

from the 1st mode. Note that had the code guideline of 

considering participation of 90 percent of the reactive mass 

been followed, the fourth mode would have been neglected, 

with a significant truncation of the acceleration at the 42d 

level. 

Displacement data for this level is presented in figure 

15. The relative displacement response due to the first 

mode (fig, 15A has a maximum value of 4.75 inches. 

Inclusion of the second mode response (fig. 15B) increases 

the maximum displacement to 5.5 inches. With three 

modes, the displacement increases to 5.7 inches (fig. 15C). 

Considering four modes of vibration, the maximum displacement becomes 6.2 inches (fig. 15D). These results 

indicate that the displacement response is primarily due to 

the first mode; however, inclusion of the fourth mode 

increased the displacement by 9 percent. 

The absolute acceleration response at the 25th level is 

shown in figure 16. The absolute acceleration due to the 

first mode (fig. 16A) is very small (0.026 g), Inclusion of 

the second mode response increases the maximum acceleration to 0.10 g (fig. 16B). With the inclusion of the 

third mode (fig. 160 the maximum acceleration reaches 

0.23 g. Consideration of the first four modes increases the 

peak acceleration to 0.35 g (fig. 16D). These results indi0 5 10 15 20 25 30 35 40 

0.4 

0.2 

0.0 

-0.2 

-0.4 

TIME (SECONDS) 

Figure 14.-Modal accelerations, roof level, transverse: first mode (A); first and second modes (B); first, second, and third modes (C) first, second, 

third, and fourth modes (D). 
PERFORMANCE OF THE BUILT ENVIRONMENT 

cate the absolute acceleration at this level is primarily due 

to the second, third, and fourth mode response with almost no contribution from the first mode. 

The relative displacement response at this level is shown 

in figure 17. The displacement response due to the first 

mode has a maximum value of 2.5 inches (fig. 17A). Inclusion of the second mode response increases the maximum displacement to 4.2 inches (fig. 175). Adding the 

displacement response of the third mode, (fig. 17C) results in no change in the maximum displacement, and 

inclusion of the fourth mode actually decreases the maximum displacement at this level to 4.05 inches (fig. 17D). 

These results indicate that the displacement-response at 

this level is primarily due to the first and second modes of 

vibration in this direction. 

These results (figs. 13-17) indicate the importance of 

the higher modes of vibration in the response of tall buildings. They also indicate that the modal cutoff specified in 

the current code at participation of 90 percent of the reactive mass may truncate a significant part of the dynamic 

response for tall buildings such as the one considered in 

this study. The authors feel that this requirement should 

be increased to at least 95 percent. 

STRESS CHECK 

In order to verify the design condition which required 

that stresses in the members remain below yield under 

the lateral forces developed by the design spectrum, a 

stress check was performed using the postprocessing program STEELER, developed by Habibullah (1989). This 

required that the gravity loading be represented as accurately as possible. Based on the geometry of the framing 

system, the gravity load was distributed to the members 

of each floor level as shown in figure 18. Since the design 

spectra is representative of yield load and not allowable 

load, use of the stress check program, which is based on 

allowable stress, requires that the applied load be reduced 

by a factor of 0.66. 

Results of the stress check are summarized in figures 

19 and 20. The stress ratios in a typical transverse frame 

are shown in figure 19, where a ratio of less than unity 

represents elastic behavior. Stress ratios for levels 41 

through 27 (fig. 19A) represent the upper third of the 

structure. In this region, the maximum value is 1.05 in an 

exterior column. Values for levels 27 through 13 are shown 

in figure 195. In this region, the stress ratios are slightly 

Figure 15.-Modal displacements, roof level, transverse: first mode (A); first and second modes (B); first, second, and third modes (C); first, 

second, third, and fourth modes (D). 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING 

lower, with a maximum value of 0.99 near the 27th level. 

Stresses in the bottom third of the structure, levels 1 

through 13, are shown in figure 19C, where it can be seen 

that the maximum value of 0.93 occurs in an exterior 

beam at the third-story level. 

The stress ratios in a typical longitudinal frame are 

shown in figure 20. Stress ratios for levels 41 through 27 

are shown in figure 20A. The maximum value in the columns is 0.84, and the maximum value for the girders is 

0.87. Values for the middle third of the structure are shown 

in figure 205 for levels 13 through 27. In this region the 

stress ratios in both the beams and the columns are lower 

than those in the upper third of the structure. In the bottom third of the structure, levels 1 through 13, the stresses 

in the girders increase with the maximum value reaching 

0.92 in the exterior girder at the fourth story level (fig. 

20Q. 

The stress check indicates that, with the exception of a 

few members which are stressed just above nominal yield, 

the design criteria that required stresses in the members to 

remain below yield under the site-design spectrum is generally satisfied for these two frames, which are representative of the remaining frames. 

ELASTIC RESPONSE ANALYSES 

The elastic response of the reference structure is evaluated considering the static lateral force requirements specified by the CCSF code, the dynamic lateral-force 

requirements of the site-design spectrum, the time-history 

demands of the Loma Prieta accelerations recorded at 

ground level, and the time-history demands of the Loma 

Prieta accelerations recorded at Hollister. The performance 

is evaluated in terms of the following response envelopes: 

lateral displacement, interstory drift index (IDI), inertia 

force, and story shear. 

The two horizontal components of the ground accelerations recorded at Hollister are shown in figure 21. Here it 

can be seen that the north-south component (fig. 21A) of 

this ground motion is considerably stronger than the eastwest component (fig. 215). These two components are 

applied simultaneously to the base of the structureand the 

resulting response is calculated for comparison with the 

other responses. 

The response envelopes for the east-west (transverse) 

direction are given in figure 22. The envelopes of lateral 

displacement (fig. 22A) indicate that the displacement due 

Figure 16.-Modal accelerations, 25th level, transverse: first mode (A); first and second modes (5); first, second, and third modes (C); first, second, 

third, and fourth modes (D). 
C128 PERFORMANCE OF THE BUILT ENVIRONMENT 

Figure 17.-Modal displacements, 25th level, transverse: first mode (A); first and second modes (B); first, second, and third modes (C); first, 

second, third, and fourth modes (D). 

Figure 18.-Gravity load distribution. 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING 

Figure 19.-Stress ratios, typical transverse frame: levels 41 through 27 (A); 

levels 27 through 13 (B); levels 13 

c through 1 (Q. 
c 130 PERFORMANCE OF THE BUILT ENVIRONMENT 

Figure 20.-Stress ratios, typical longitudinal frame levels 41 through 27 (A); levels 27 through 13 (B); levels 13 through 1 (C). 

to the recorded base motion is less than that due to the 

lateral design forces. It is of interest to note that the displacements due to the CCSF code loads, when factored to 

represent yield, exceed the lateral displacements due to 

the design spectrum in the upper half of the structure. The 

roof displacements due to the Hollister motion exceed 

those due to the design spectrum by a factor of 2.4 and 

recall that the east-west component is the smaller of the 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING C131 

two. Envelopes for the maximum interstory drift index 

are shown in figure 225. As before, the ID1 due to the 

recorded base motion is less than that due to the design 

spectrum. For this parameter, the indices due to the design spectrum exceed those due to the factored CCSF 

lateral forces over most of the building height with the 

exception of floor levels 15 to 22. The envelope due to 

the Hollister ground motion is considerably larger than 

the others. 

The envelopes of maximum inertia force over the building height are shown in figure 22C. It can be seen that the 

inertia forces due to the design spectrum exceed those 

due to the recorded ground motion with the exception of 

three floors near the top of the structure (floors 32-34). It 

can also be seen that the inertia forces due to the Hollister 

ground motion exceed those due to the design spectrum at 

all floor levels except the 39th. The inertia forces specified in the CCSF code represent the minimum values, 

which are exceeded by all others. 

The envelopes of maximum story shear are shown in 

figure 22D. This figure indicates that the story shears due 

to the design spectrum exceed those developed by the 

recorded base motions at all story levels. This tends to 

indicate that the behavior of the structure in this direction 

is linear elastic. However, it is of interest to note that had 

the minimum lateral-force requirements of the CCSF code 

been used for design, the story shears due to the recorded 

base motions would have exceeded the design values in 

the upper 15 floor levels. The story shears due to the 

Hollister motions exceed the design values over most of 

the building and indicate yielding of the structural frames, 

which will have to be investigated using a nonlinear analysis. It is also of interest to note that the decrease in story 

shear, indicated by the design spectrum at the 20th story 

0.4 

0.2 

0.0 

-0.2 

O4 Figure 2 1 .-Ground acceleration recorded at 

Hollister, Loma Prieta earthquake: north-south 

component (A); east-west component (B). 
C132 PERFORMANCE OF THE BUILT ENVIRONMENT 

level, does not occur in the response envelope of the 

Hollister motion. Such a decrease, if used to reduce member sizes, could result in a weak section of the structure, 

although this was not the case for this building. 

The response envelopes for the north-south (longitudinal) direction are shown in figure 23. The envelopes of 

maximum lateral displacement (fig. 23A) are similar to 

those just discussed for the transverse direction. In this 

case, the displacements due to the design spectrum exceed those due to the CCSF code over the height of the 

structure, and both of these exceed the displacements due 

to the recorded base motions. The displacement at the 

roof level due to the Hollister ground motion exceeds that 

due to the design spectrum by a factor of 1.9, which is 

slightly less than in the transverse direction. The envelopes of ID1 are shown in figure 235. The values due to 

the recorded base motions are less than those due to the 

design spectrum over the height of the structure; how- 

) .,.;...* l~~~l~~~r,~llllll4r~~~ll~~~l~~

0 

0.000 0.005 0.010 0.0 15 0.020 

INTERSTORY DRIFT INDEX 

B 

o~ll#,lllll~ll~lllll~~llllll~lll~l,lll,ll+ll#~~~~ 0 

0 1000 2000 3000 4000 5000 

STORY SHEAR 

D 

Figure 22.-Transverse response envelopes: lateral displacement (A); interstory drift index (B); inertia force (C); story shear (D). 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING C133 

ever, they do exceed those due to the CCSF code in the 

upper 10 floors. The ID1 due to the Hollister motions is 

considerably larger than those due to either the design 

spectrum or the recorded base motions. 

The envelopes of maximum inertia force are shown in 

figure 23C. As before, the forces specified in the CCSF 

code represent minimum values. It is Interesting to note 

that the inertia forces due to the recorded base motion 

exceed those due to the design spectrum between floors 

levels 26-33. The forces due to the Hollister motions exceed all others by a substantial margin. The envelopes of 

story shear (fig. 23D) indicate that the shears due to the 

recorded motions are less than those due to the design 

spectrum over the total building height. This indicates 

-------* -;lli,ll,l~lili#iI,ll~l~i~ll,lll,~lllI~o 0 500 1000 1500 2000 

INERTIA FORCE 

c 

0.01 5 0.020 

~llifilt~lltlslltL - 40 

- 35 

- 30 

- 25 

- 20 

1.4 x CCSF -15 

- DESIGN SPECTRUM RECORDED BASE - 

-** HOLLISTER -10 

- 5 

0 ,lll,l,ll,,,llll,,i,~lll~l~ll,l~~~~~~l~ 0 

0.000 0.005 0.01 0 0.015 0.020 

INTERSTORY DRIFT INDEX 

Figure 23.-Longitudinal response envelopes: lateral displacement (A); interstory drift index (B); inertia force (C); story shear (D). 
PERFORMANCE OF THE BUILT ENVIRONMENT 

that the behavior of the building in this direction was 

most likely linear elastic. As in the transverse direction, 

the shears due to the Hollister motions exceed those due 

to the design spectrum over the height of the building, 

indicating inelastic behavior. 

CURRENT CODE DESIGN 

REQUIREMENTS 

In the current edition of the Uniform 

(1994), the design base shear is given as 

Building Code 

where C = 1.25~/7*'~, I= 1.0, Rw = 12, and Z = 0.4. 

The fundamental period estimated by code formula is 

whereas the value calculated using SAP90 is 5.75 s. The 

code requires that the value of the period not be more 

than 30 percent greater than the value given by the above 

formula. Therefore, using 5.16 s for the period and a site 

factor of 1.5 (soil type 3), the value of C is calculated as 

0.628. Using this value for C, the design seismic resistance seismic coefficient is calculated as 

which represents 2.1 percent of the dead load and is the 

same as the CCSF lateral force requirement. However, an 

additional requirement in the 1994 UBC code requires 

that 

C > 0.075Rw = 0.9, 

which has the effect of placing a lower bound on the base 

shear for tall buildings. Using this value, the design seismic resistance coefficient becomes 

and results in a base shear of 1,874 kips. 

The lateral force requirements in the UBC require a 

dynamic analysis for a building of this height (550 ft, 

>240 ft). If a site-specific design spectrum is not provided, the design spectrum provided in the code can be 

used with the appropriate soil type. 

4.0 

PERIOD 

Figure 24.-Design spectra vs. recorded motion. 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING C135 

The code design spectrum and the site-specific design 

spectrum are compared with the spectra of the recorded 

ground motions in figure 24. The code spectra with a 

structural system coefficient (Rw=l) results in the highest 

spectral values, whereas using a system coefficient of 

R-12, which is typical for this type of structure, results 

in the lowest spectral values. Hence the code design spectra envelope the other spectra depending on the value of 

the structural system coefficient that is used. Based on the 

results presented, it would appear that the use of a value 

of 12 is excessive. 

The site spectrum has a peak spectral acceleration of 

0.5 g in the period range of 0.2 to 0.4 s, which is representative of rock motion at the site. The base design spectrum reflects the influence of local soil conditions on the 

response. It has a peak spectral acceleration of 0.87 g at a 

period of 0.4 s and a second peak of 0.48 g at 1.25 s. The 

spectrum for the recorded ground motion in the east-west 

direction has a peak spectral acceleration of 0.57 g at a 

period of 0.9 s. The spectral values of the two components of recorded base motion in the east-west and northsouth directions are very similar, with the exception of 

the period range around 0.9 sec where the spectral acceleration in the north-south direction is truncated and only 

reaches 0.36 g. 

These results emphasize the uncertainty that surrounds 

the selection of an appropriate structural system factor 

and the determination of the soil period for site-specific 

spectra. It is noteworthy that the recorded base accelerations were no greater than 0.12 g, which would certainly 

be considered a service loading condition. 

If the base shear obtained using a dynamic, responsespectrum analysis differs from that obtained using the static 

lateral force procedure, it may be scaled to the following 

percentage of the static value: (1) One hundred percent 

for irregular buildings or (2) ninety percent for regular 

buildings, except that the base shear shall not be less than 

80 percent of that determined using the calculated period. 

The code further states that "All corresponding response 

parameters, including deflections, member forces and moments shall be increased proportionately." In the analyses 

that follow, the base shear has been scaled to 100 percent 

of the static value. This result is identified as "Scaled 

UBC" in the figures that follow. 

The lateral displacements and forces obtained using the 

code spectrum for a type 3 soil are compared with the 

static lateral force requirements and the recorded values 

in figure 25 for the east-west (transverse) direction. The 

envelopes of maximum lateral displacement (fig. 25A) indicate that the displacements obtained from the static lateral forces far exceed those obtained from the scaled 

spectrum. This reflects the influence of the code requirement that places up to 25 percent of the base shear at the 

top of the structure. The displacement response due to 

Loma Prieta is less than that obtained using the scaled 

spectrum. The envelopes of maximum interstory drift index (IDI) are shown in figure 255. As before, the ID1 for 

the static lateral loads exceeds the spectral values, with 

the exception of the bottom four floor levels. The response due to the scaled spectrum is equal to or greater 

than the recorded response, with the exception of the upper six floors. 

The envelopes of the maximum lateral inertia forces 

are shown in figure 25C, and the corresponding envelopes 

of maximum story shear are shown in figure 25D. It can 

be seen in figure 25D that the base shears due to the static 

forces and the scaled spectrum are equal as a result of the 

scaling procedure. By coincidence, the base shear due to 

the recorded ground motion is also approximately equal 

to the code value. However, it is of particular interest to 

note that story shears due to the recorded ground motion 

exceed the scaled code spectrum in the upper half of the 

structure, with peak differences at the 38th level and the 

28th level. This clearly indicates the importance of the 

response of the higher modes of vibration and their 

influence on the scaling procedure, which is based on 

base shear. The higher modes tend to increase the response at the base and in the upper portions of the structure. Scaling upward, based on the base shear, can result 

in reduced design shear forces over much of the building 

height. 

The lateral displacements and forces obtained using 

the code spectrum are compared with the static lateral 

force requirements and the recorded Loma Prieta values 

in figure 26 for the north-south (longitudinal) direction. 

The lateral-displacement response shown in (fig. 26A) is 

very similar to that of the previous case, wherein the displacements of both the static and scaled spectra exceeded 

the displacements due to the recorded motions over the 

height of the structure. The envelopes of maximum ID1 

are shown in figure 265. It can be seen that the ID1 due to 

the recorded base motion exceeds that of the scaled spectrum for all stories above the 32d floor. It even exceeds 

that of the equivalent static loading for floor levels 33 

to 40. 

The envelopes of maximum lateral inertia forces are 

shown in figure 26c where it can be seen that the forces 

due to the recorded base motion exceed those of the scaled 

spectrum over the entire building height. The envelopes 

of maximum story shear are given in figure 26d. As required by the scaling procedure, the shear due to the static 

load and that due to the scaled spectrum are equal at the 

base. This figure indicates that in the upper nine floors 

the story shears due to the recorded base motion exceed 

those due to both the static forces and the scaled spectrum. The shears of the scaled spectrum are also exceeded 

at mid-height, between stories 17 and 25. Toward the base 

of the structure, both the static and scaled spectrum values are exceeded in the bottom three stories. This indicates that had this structure been designed according to 
C136 PERFORMANCE OF THE BUILT ENVIRONMENT 

the current code requirements, the damage due to the rela- EARTHQUAKE DAMAGE POTENTIAL 

tively low base motions recorded at the site would have 

been much more substantial. As mentioned previously, In general, the base shear is not a very accurate paramthe building was designed for a site-specific design eter for estimating the severity and damage potential of 

spectrum which was substantially higher than current code earthquake ground motions. A more sensitive parameter 

requirements. is the elastic input energy, which can be calculated as the 

0 10 20 30 11111111~11~~-~~~~~1~~~~1~~1~1 

-10 

- 

------ CODE STATIC 

- CODE SPECTRUM ~5 SCALED SPECTRUM - *-**-** LOMA PRIETA EW 1 

------ CODE SPECTRUM - r5 

SCALED SPECTRUM - *-**--* LOMA PRIETA EW 1 

- SCALED SPECTRUM 

*-**-** LOMA PRIETA EW 

0.000 0.002 0.004 0.006 0.008 

INTERSTORY DRIFT INDEX 

STORY SHEAR 

D 

Figure 25.-Code design vs. recorded response, transverse: lateral displacement (A); interstory drift index (B); inertia force (C); story 

shear (D). 
I 

SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING C137 

product of the base shear and the base displacement as 1989). The input energy in the east-west direction is shown 

mentioned previously, The elastic input energy for this in figure 27A, which clearly indicates that the motion from 

structure due to the Loma Prieta base motion is compared the Loma Prieta earthquake which was recorded at the 

with that of three other strong motion earthquakes in fig- base of this structure did not input a significant amount of 

ure 27, These include James Road (Imperial Valley, 1979), energy to the structural system. It does indicate that the 

SCT (Mexico City, 1985) and Hollister (Loma Prieta, James Road ground motion, discussed previously by 

0 10 20 30 llt.l~~ll~~~.~~'~~~~~lllllt~ll 

- 40 

- 35 

- 30 

- 25 

- 20 

-15 

-10 

- 

------ CODE STATIC 

- CODE SPECTRUM ;5 SCALED SPECTRUM - LOMA PRIUA NS 

l~,zl~~~~~ll 0 

20 30 

DISPLACEMENT 

INERTIA FORCES c 

- 

*----- 

- SCALED SPECTRUM 

*-**-** LOMA PRIUA NS 

0 ~1,,,,,1,1,111,,,,~ b,lrtlll ~~l11t11911 ti 0 

0.000 0.002 0.004 0.006 0.008 

INTERSTORY DRIFT INDEX 

B 

STORY SHEAR 

D 

Figure 26.-Code design vs. recorded response, longitudinal: lateral displacement (A); interstory drift index (B); inertia force (C); story 

shear (D). 
c 138 PERFORMANCE OF THE BUILT ENVIRONMENT 

Anderson and Bertero (1994), and the Hollister ground 

motion discussed in this paper are much more severe. It 

can be seen that the energy input due to James Road 

occurs with a sudden jump, whereas that due to the SCT 

ground motion builds up over a long period of time. Recall that the James Road record contains large acceleraLorna ~lieta L 

8~111~111-1~~118111111 

0.00 20.00 40.00 60.00 

TIME (SECONDS) 

TIME (SECONDS) 

Figure 27.-Elastic input energies, strong earthquakes: transverse (eastwest) (A); longitudinal (north-south) (B). 

tion pulses and is representative of near-fault motion, 

whereas the SCT record is almost sinusoidal in character, 

starts very slowly, and is representative of a large, distant 

earthquake. Elastic input energy in the north-south 

direction has similar characteristics and is shown in 

figure 27B. 

CONCLUSIONS 

This paper has discussed some of the response studies 

conducted on an instrumented 42-story steel building. Attention has been focused on the elastic response analysis. 

More complete details and the results of nonlinear analyses are to appear in a future University of California, 

BerkeleyKarthquake Engineering Research Center report. 

The results presented in this paper suggest the following 

general conclusions: 

1. Estimation of the soil periods at a given site is very 

difficult. This uncertainty should be recognized in the development of a design spectra by using a smoothed spectra with 

a wide-period band. 

2. Few recorded ground motions have a high spectral 

acceleration at the fundamental period of a tall building such 

as the one considered in this study (5 s). Therefore the 

influence of the second and higher lateral modes becomes of 

increasing importance and must be considered in the design 

process. 

3. This building was designed for a base spectrum which 

resulted in lateral design forces which were substantially 

above the minimum values required in the building code. For 

this reason the response due to the recorded Loma Prieta base 

motions was linear elastic with no damage. Had the minimum 

code requirements been used, some inelastic behavior may 

have occurred in the upper half of the building. 

4. The code guideline for considering enough modes of 

vibration to represent 90 percent of the reactive mass is not 

adequate for tall buildings. Such a requirement for this building would eliminate the need to consider the fourth translational mode in each direction, and it has been shown that these 

modes make a significant contribution to the response. Inclusion of the fourth mode can increase the acceleration by as 

much as 50 percent and the displacement by 10 percent. It is 

recommended that this requirement be increased to 95 percent 

for tall buildings, 

5. Scaling the dynamic response based on the shear at 

the bottom story is an oversimplification which can result 

in reduced design forces over much of the height of a tall 

building. It would be preferable to use a "correct" design 

spectrum directly. Currently, a reasonable spectrum is scaled 

down by the structural system coefficient, Rw, and then 

scaled up by the ratio of the base shears. In the process, the 

design forces over much of the building height are reduced 

to values that are substantially lower than the static design 
SEISMIC RESPONSE OF A 42-STORY STEEL BUILDING C139 

procedure. This reduction does not appear to be reasonable or 

justifiable and therefore should be eliminated. 

ACKNOWLEDGMENTS 

This report is based on the results of a series of studies 

that have been conducted by the authors at the University 

of California at Berkeley and the University of Southern 

California. This study is part of a research project entitled 

"Implications of Response of Structures to Ground Motions Recorded During the Loma Prieta Earthquake: Evaluation of Structural Response Factors," which was supported 

by a research grant provided by the National Science Foundation. 

The authors are grateful to the consulting firm of H.J. 

Brunnier Associates in San Francisco and particularly to 

Mr. Edwin Zacher of this firm for providing the drawings 

and needed information regarding the design of the building. Records of the recorded motion were provided by the 

U.S. Geological Survey in Men10 Park. This support is 

also greatly appreciated. 

REFERENCES 

Anderson, J.C., and Bertero, V.V., 1994, Lessons learned from an instrumented high rise building, in Proceedings, Fifth National Conference on Earthquake Engineering, 2: Chicago, Ill., Earthquake 

Engineering Research Institute, 

Habibullah, A., 1989, ETABS, three dimensional analysis of building 

systems: Berkeley, Calif., Computers & Structures, Inc., 

Habibullah, A., 1989, STEELER, stress check of steel frames:" Berkeley, Calif., Computers & Structures, Inc. 

Ruiz, R., and Penzien, J., 1969, Probabilistic study of the behavior of 

structures during earthquakes: Earthquake Engineering Research 

Center, University of California, Berkeley, Calif., UCBEERC-69/03 

Wilson, E.L., and Habibullah, A., 1989, SAP90, a series of computer 

programs for the static and dynamic finite element analysi5 of structures: Berkeley, Calif., Computers and Structures, Inc. 
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

PERFORMANCE OF THE BUILT ENVIRONMENT 

BUILDING STRUCTURES 

A SUMMARY OF UNREINFORCED MASONRY BUILDING DAMAGE 

PATTERNS-IMPLICATIONS FOR IMPROVEMENTS IN 

LOSS ESTIMATION METHODOLOGIES 

By Bret Lizundia, Rutherford & Chekene Consulting Engineers, San Francisco, Calif.; 

Weimin Dong, Risk Management Solutions, Men10 Park, Calif.; 

William T. Holmes, Rutherford & Chekene Consulting Engineers, San Francisco, Calif.; and 

Robert Reitherman, The Reitherman Company, Half Moon Bay, Calif 

CONTENTS 

ABSTRACT 

This paper summarizes the results of a study of the 

performance of unstrengthened, unreinforced masonry 

bearing wall buildings damaged in the earthquake. The 

collected data, for over 4,800 buildings, are compared 

with similar damage data obtained from past earthquakes 

elsewhere in the United States. Observed damage patterns 

are correlated with modified Mercalli intensity in these 

past events. In the Loma Prieta event, a number of other, 

more quantitative measures of ground motion have been 

developed from strong motion instrument recordings and 

are correlated with observed damage patterns. The results 

are used to help develop a new methodology for loss estimation. 

INTRODUCTION 

In an earthquake, unreinforced masonry (URM) bearing wall buildings are generally considered to be one of 

the most dangerous building types. They have been responsible for significant property damage and loss of life 

in previous earthquakes, both in the United States and 

around the world. In the Loma Prieta earthquake, many 

URM bearing wall buildings were seriously damaged and 

were the cause of nine deaths. 

Due to the interest and foresight of the City of San 

Francisco, a large amount of damage data was collected 

on URM bearing wall buildings in San Francisco in conjunction with postearthquake emergency building evaluations, using a form created by Rutherford & Chekene, a 

San Francisco engineering firm. The earthquake affected 

not only San Francisco but many other cities and communities as well, Funding was obtained from the National 

Science Foundation and the California Seismic Safety 

Commission after the earthquake to further investigate 

URM damage. Substantial data were collected from numerous communities. The results of that work were summarized in Lizundia and others (1991). Lizundia and others 

(1993) involves more detailed analysis of the damage data 

and correlation to various ground motion parameters as 

well as application of the data to and discussion of improvements in loss estimation methodologies. Collected 

data were organized into computer files that are available 

to interested researchers. 

In this paper, we have organized a presentation of the 

results of that study into four sections: (1) a discussion of 

the data collected, (2) comparison of the data with that 

from previous U.S. earthquakes, (3) correlation of the damage data with various quantitative measures of ground 

motion, and (4) development of a new proposed methodology for property loss estimation. 

DATA COLLECTED 

Lizundia and others (1991, 1993) discussed in detail 

the methodology and sources used to collect the data. 

Telephone calls, letters, building department records, field 

surveys by other engineers, and our own field surveys 
c 142 PERFORMANCE OF THE BUILT ENVIRONMENT 

were used to collect data. In general, the more damage 

that the buildings in a community suffered, the more time 

we devoted to obtaining damage data. 

The collected data have been categorized into three 

levels of quality. The lowest level, Level 1, has data from 

113 communities in the nine-county Bay Area-a region 

which easily encompasses the modified Mercalli 

intensity (MMI)=VI-VII contour line on the USGS intensity map (U.S. Geological Survey, 1989). These communities are shown in figure 1. Level 1 data have the greatest 

number of building records, but the information is restricted to "general damage statusu-the number of damaged, vacated, and demolished buildings in each 

community. In these nine counties, there were at least 

4,824 unstrengthened URM bearing wall buildings, of 

which at least 758 were damaged. Of those damaged, at 

least 384 were vacated, and of those vacated, at least 54 

were demolished. Using data from the 78 communities 

with four or more URM buildings, we have generated 

geographic contour maps showing the percentage of damaged and vacated buildings (figs. 2, 3). 

Level 2 data cover a subset of nine cities and 2,356 

buildings, but have ATC- 13 (Applied Technology Council, 1985) "damage states" as well as other information on 

building characteristics and damage patterns. The cities 

included are Campbell, Gilroy, Hollister, Los Gatos, Oakland, Salinas, San Francisco, Santa Cruz, and Watsonville. 

Table 1 organizes the Level 2 data in the ATC- 13 format; 

table 2 organizes the data by general damage status versus MMI. Table 3 compares damage states with general 

damage status. 

Table 1 also includes the sample mean, standard deviation (biased), and coefficient of variation (COV) for the 

damage ratio at each MMI. As expected, the mean damage ratio increases as the MMI increases. Another important finding is that the COV is quite large, indicating that 

there is significant scatter in the collected data. The COV 

is, in fact, larger than unity for each MMI. As the table 

shows, the data are not symmetric about the mean; particularly for MMI's of VI, VII, and VIII, there are a large 

number of very small damage ratios (those equal to zero 

or 0.005) and a smaller number of higher damage ratios 

(those equal to 0.055, 0.2, 0.45, 0.8, and 1.0). The result 

of this distribution is that the mean is close to the low 

values, but the scatter or variance caused by the higher 

values being farther from the mean leads to a relatively 

large standard deviation. Although probabilistic models 

have not been developed in this paper to represent the 

data, it is clear that a normal distribution would be an 

inappropriate representation of the damage ratio. A beta 

distribution would appear to be a likely candidate; the 

beta distribution was used to develop the damage probability matrices in ATC-13. 

The best set of data, Level 3, covers only the 896 buildings in San Francisco; it includes cracking patterns, other 

damage information, ATC- 13 and ATC-20 (Applied Technology Council, 1989) ratings and building characteristics. Note that over 1,000 buildings which have been 

addressed by the city's parapet strengthening program have 

not been included in this study because they are not truly 

"unstrengthened." Statistical t-testing has been done in an 

attempt to correlate ATC-13 levels of damage in these 

buildings with site soils and building attributes. We found 

that soil conditions and mean story height had a statistically significant influence on the average damage ratio 

(defined as the ATC-13 assumed repair cost divided by 

replacement value). Table 4 provides a comparative summary of results. For additional details, see Lizundia and 

others (1993). 

COMPARISON WITH PREVIOUS U.S. 

EARTHQUAKES 

To place the Loma Prieta event in a larger context, we 

compare our collected damage data with that obtained 

from past U.S. events. Foreign damage data was deliberately excluded because of concerns about comparability 

of building construction details. The resulting compilation of earthquake data is believed to be one of the most 

comprehensive ever. A rigorous attempt has been made to 

use a consistent format for data presentation so that damage at different MMI levels can be compared with current 

prediction tools such as the ATC-13 damage probability 

matrices (DPM's). Table 5 summarizes the results. We 

are aware that table 5 contains significant simplifications 

of the concepts in ATC-13. The information in the literature is not specific enough to allow separating ATC-13 

damage states 1 and 2, or 3 and 4, or MMI=VIII and IX. 

Table 5 includes data sets from the following earthquakes: 

1886 Charleston (South Carolina), 1906 San Francisco, 

1933 Long Beach, 1952 Bakersfield, 1971 San Fernando, 

1983 Coalinga, 1983 Mackay (Idaho), 1984 Morgan Hill, 

and 1989 Loma Prieta (Level 2 data only). Data from the 

1987 Whittier and 1994 Northridge earthquakes are not 

included. 

CORRELATION OF DAMAGE WITH 

GROUND MOTION AND SITE SOILS 

Because of the absence of strong motion instruments in 

most past earthquakes, correlation of damage with ground 

motion has typically been done using intensity. For 

the Loma Prieta earthquake, more than 20 quantitative 

measures of ground motion developed from 90 California 

Strong Motion Instrumentation Program and U.S. 
A SUMMARY OF UNREINFORCED MASONRY BUILDING DAMAGE PATTERNS C143 

x I 

RIO VISTA 121.5O 

x PLEASAN TON 

APPROXIMATE CAMPBE1 

SURFACE 

PROJECTION LOS GATOS 

OF RUPTURE 

PLANE 

\ 

MORGAN HILL 

x GILROY 

8 0 8 16 24 Kilometers i 

MONTEREY 

8 0 8 16 24 Miles 

km-w-' 1 

PACIFIC 

MONTEREY (CITY) 

GONZALES 

x CARMEL 

SOLEOAO 

GREENFIELD 

121.5O 

Figure 1.- Cities with URM buildings and with known numbers of damaged and vacated buildings 
C144 PERFORMANCE OF THE BUILT ENVIRONMENT 

8 0 8 16 24 Kilometers 
A SUMMARY OF UNREINFORCED MASONRY BUILDING DAMAGE PATTERNS C145 

8 0 8 16 24 Kilometers 

Figure 3.- Contour percentages of total URM buildings which were vacated (cities with four or more total buildings) 
C146 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 1 .-Level 2 damage state data 

Damage Damage Assumed MMI 

state state central 

description damage ratio VI VII VIII IX 

# % # % # % # % 

1 None 0 

2 Slight 0.005 

3 Light 0.055 

4 Moderate 0.20 

5 Heavy 0.45 

6 Major 0.80 

7 Destroyed 1 .OO 

Total with known damage state 

Total with unknown damage state 

Mean damage ratio 

Standard deviation 

Coefficient of variation 

Ill 7.24 

Table 2.-General damage status for Level 2 data vs. MMI 

MMI 

General damage status 

VI VII VIII IX 

- -- 

Damaged (but not vacated or demolished) 115 68.86 128 41.56 5 5.38 1 3.85 

Vacated (but not demolished) 49 29.34 165 53.57 56 60.22 25 96.15 

Demolished 3 1.80 15 4.87 32 34.41 0 0 

Total with known damage status 167 100 308 100 93 100 26 100 

Unknown damage status 58 74 10 1 
A SUMMARY OF UNREINFORCED MASONRY BUILDING DAMAGE PATTERNS 

Table 3.-Comparison of damage states and general damage status in Level 2 data 

Damage states General damage status 

Damaged (but 

not vacated or Vacated (but 

Damage state demolished) not 

Damage state description demolished) Demolished Unknown 

1 None 

2 Slight 

3 Light 

4 Moderate 

5 Heavy 

6 Major 

7 Destroyed 

Total 

Table 4.-Influence of site soil type and mean story height on damage 

Variable Mean damage Number of Number of 

ratio (%) buildings buildings 

with known with 

damage unknown 

ratios damage 

ratios 

1 0.49 45 0 

1991 UBC site soil classification 2 4.78 660 12 

3 or 4 6.68 171 8 

5 4.9 m (16 ft) 3.95 637 16 

Mean story height > 4.9 m (16 ft) 7.90 , 239 4 
C148 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 5.-Comparison of historical data with ATC-13 

[Source: The Reitherman Company, Half Moon Bay, Calif.] 

MMI 

Assigned central VII VIII-IX 

Damage state damage ratio Historical ATC-13' Historical ATC- 13 

- - 

# % % # % 

1 and 2 0.005 1,352 8 7 1 712 16 

3 0.05 116 7 56 1,047 24 

4 0.20 43 3 43 1,608 37 

5 and 6 0.65 48 3 1 73 8 17 

7 1 .OO 0 0 0 267 6 

Totals 1,559 100 100 4,372 100 

Mean damage factor3 --- 3 12 --- 25 

'From facility class = 75, low rise (1-3 stories). 

~ean of the MMI=VIII-IX values. 

wean damage factor here is the sum of the assigned central damage ratio times the percentage in the damage state. 

Geological Survey free-field strong motion recordings have 

been correlated with observed damage patterns. These measures include the maxima and mean of horizontal and 

vertical components of acceleration, velocity, 

pseudospectral acceleration and pseudospectral velocity, 

"frequency" (defined as the number of zero crossings in 

the acceleration record), bracketed duration, Arias Intensity, the "destructiveness potential factor" of Araya and 

Saragoni (1985), and the number of horizontal acceleration peaks over 0.10 g and the sum of those excursions. 

Selected strong motion stations are shown in figure 4. 

Several different correlation techniques were used. The 

first technique was a visual comparison of the building 

damage contour maps (figs. 2, 3) with similar maps of the 

various ground motion parameters. Pseudospectral acceleration (SPA) and pseudospectral velocity (SPV) maps at 

a 0.5s period were found to provide potentially the best 

match with the damage maps. They are shown in figures 

5 and 6. Figure 7 shows the maxima of the two horizontal 

acceleration records (AHMAX) plot when all soil types 

are included. Strong motion stations were, in fact, assigned to three different site soil types. Soil Types 1 and 

2 correspond to 1991 UBC categories. Soil Type 3 is a 

combination of UBC categories 3 and 4 since there are so 

few records for these soft soil sites. Comparison of plots 

of ground motion instruments from one soil type with 

another clearly shows the impact of surficial soils in influencing the recorded ground motion. See figures 8, 9, 

and 10 for the influence on AHMAX. 

Other correlation techniques were used to quantitatively 

compare damage and ground motion. Before this could 

occur, however, it was necessary to interpolate the ground 

motion at each building site since the strong motion locations do not correspond to the building locations. A rigorous process was used to take into account the proximity 

of the instrument to a building, the attenuation of the 

ground motion, and the site soil characteristics. Details 

are contained in Lizundia and others (1993). Following 

the interpolation process, one alternative correlation 

technique involved sorting the Level 2 damage data by 

groups within the ground motion parameters. The characterization of damage states uses the ATC-13 classifications and provides similar DPM's. Table 6 provides an 

example for horizontal acceleration. Note that the mean 

damage ratio does not consistently increase as the acceleration increases, and, like table 1, the standard deviations are quite large. 

The last and the most rigorous correlation procedure 

involved the use of multivariate regression analysis and 

discriminant analysis discussed in Miller (1962). Using 

this procedure, we found pseudospectral velocity and, to a 

lesser extent, frequency to be statistically significant predictors of the average damage ratio. For details on this 

process, see Lizundia and others (1993). 

PROPOSED METHODOLOGY FOR 

PROPERTY LOSS ESTIMATION 

Popular loss estimation methodologies currently in use, 

such as ATC-13, directly equate a damage state with a 
A SUMMARY OF UNREINFORCED MASONRY BUILDING DAMAGE PATTERNS C149 

APPROXIMATE 

SURFACE 

PROJECTION 

OF RUPTURE 

PLANE 

Figure 4.- Location of all stations selected 
Figure 5.-Pseudospectral acceleration 

I 

(T=0.5 s)-all soil types 

I I 
A SUMMARY OF UNREINFORCED MASONRY BUILDING DAMAGE PATTERNS c151 

Figure 6.-Pseudospectral velocity (T=0.5 s)-all soil types 
C152 PERFORMANCE OF THE BUILT ENVIRONMENT 

Figure 7.-Maximum horizontal acceleration-all soil types 
A SUMMARY OF UNREINFORCED MASONRY BUILDING DAMAGE PATTERNS - --- 

121.5* 

80816 

-a 

38- 

8 0 8 16 24 I Mile 

Figure 8.-Maximum horizontal acceleration-Soil Type 1 
C154 PERFORMANCE OF THE BUILT ENVIRONMENT 

Figure 9.-Maximum horizontal acceleration-Soil Type 2 
A SUMMARY OF UNREINFORCED MASONRY BUILDING DAMAGE PATTERNS C155 

Figure 10.-Maximum horizontal acceleration-Soil Type 3 
C156 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 6.-Correlation of Level 2 damage data with AHMAX 

Assumed AHMAX 

Damage central 

state damage ratio <O.lg 0.1 -0.2g 0.2-0.3g 0.3-0.4g >0.4g 

1 0 

2 0.5 

3 5 

4 20 

5 45 

6 80 

7 100 

Totals 

Mean damage ratio 

Standard deviation 

damage ratio. This is reasonable if dollar losses in regional loss estimates are intended to measure out-of-pocket 

expenditures directly resulting from only repair of damage. Based on our observations of activity following the 

Loma Prieta earthquake, however, we believe that attempting to translate the state of observed damage to a dollarloss value presents the most significant problem in 

establishing dollar losses, primarily because of the ambiguity and variability of acceptable criteria to define what 

work is actually required as a result of the earthquakecaused damage. This work may not merely be associated 

with "repair." To address this issue, we propose a new 

model, as shown in figure 1 1. 

Step 1 involves the traditional approach of correlating 

ground motion with a damage state. Many methods can 

be used: fragility curves, Markov models, or DPM's. An 

example of a Markov model approach is Thiel and Zsutty 

(1987). We use DPM's here. 

Once a baseline DPM is established in Step 1, if information is available on building characteristics such as story 

heights, height- to-thickness wall ratios, adjacency conditions, mortar quality, etc., then Step 2 is where the baseline 

DPM is separated into final DPM's associated with subpopulations of the total group of buildings under study. 

Rutherford & Chekene (1990) provides an example of 

this process for San Francisco. 

We believe the damage ratio depends significantly on 

the course of action taken by an owner following an earthquake. There are many ways that linkage between damage 

state and course of action can occur. In Step 3 we use a 

"Course-of-action probability matrix," which covers common postearthquake actions the owner could take (table 7). 

In using a matrix like table 7, it is expected that the 

loss estimator will make appropriate modifications using 

local standards of practice or existing local regulations. 

Significantly different results would be anticipated, for 

example, in a community like Oakland which has enacted 

an ordinance requiring strengthening of damaged URM 

bearing wall buildings when the lateral capacity has been 

reduced by more than 10 percent. 

There is, of course, no direct correlation between the 

percentage loss of lateral capacity and the seven damage 

states. Determination of these percentage-loss-of-capacity 

figures in Oakland has been quite controversial. Nonetheless, we can make an attempt to qualitatively show how 

such an ordinance would alter table 7. Table 8 is the 

result. 

In table 8, we assume that the 10 percent loss-of-strength 

trigger lies in the vicinity of damage states 3 and 4. Any 

buildings in these damage states with more than 10 percent damage will require repair as well as strengthening. 

It is expected that all of the damage state 5 and 6 buildings would be over the 10 percent trigger. As table 8 

shows, the result of an ordinance like Oakland's is to 

increase the damage ratio and thus the losses in an earthquake. Of course, in the next earthquake, losses would 

presumably be much lower because of the presence of 

strengthened buildings. 
A SUMMARY OF UNREINFORCED MASONRY BUILDING DAMAGE PATTERNS C157 

Once the course of action is chosen, then the damage 

ratio can be established. Traditional methodologies, such 

as ATC-13, generally focus only on the repair costs of the 

"evaluation and repair" course of action. Table 9, by contrast, shows the great variety of property loss costs which 

can actually be triggered by postearthquake rehabilitation 

activity. Table 9 includes only structural and nonstructural 

property losses; not included are contents losses; the many 

kinds of economic losses; and tax, housing, and social 

losses. 

Step 1 

Step 2 

Step 3 

Ground Motion 

At the Site 

Fragility Curves, 

Damage Probabilit Matrices or Markov dodels 

Baseline Damage 

State Model I - Building Vulnerability 

u Characteristics 

Final Damage 

Course of Action 

and Damage Ratio 

Figure 1 1 .-Proposed loss estimation model 

One of the key concepts shown in tables 7 and 8 and 

observed following the Loma Prieta earthquake is that 

when the cost of strengthening a damaged building rises 

to a certain level-what we call here the economic critical loss ratio (ECLR)-then an owner will choose not to 

repair and strengthen the building. The building may lie 

vacant for a long period of time, or it may be demolished 

and replaced. Little effort has been applied to determining 

the ECLR, but Rutherford & Chekene (1990) used 40 

percent. Regardless of the exact value, it will certainly be 

much less than 100 percent. Using the ECLR concept, we 

can represent the resulting probability mass and density 

functions (PMF and PDF) for a selected damage state. 

Figure 12 is a PMF representation where each course of 

action in tables 7 and 8 is given a discrete damage ratio. 

Figure 13 smooths out the nonzero portion of the damage 

ratios into a PDF to provide a range of damage ratios for 

each course of action. Note the gap between the ECLR 

and 100 percent. A similar gap between 0 percent and the 

"Evaluation Only" line occurs because even the most minimal evaluation by a consulting engineer has a nonzero 

cost. Figures 12 and 13 apply only to a single building 

and are expected in an actual loss estimate to be aggregated across the total population of buildings, resulting in 

a smoothed, nonzero PDF measuring "average" damage 

for the population. 

REFERENCES 

Applied Technology Council, 1985, Earthquake damage evaluation data 

for California: Redwood City, Calif., Applied Technology Council 

Report ATC-13,492 p. 

Applied Technology Council, 1989, Procedures for postearthquake safety 

evaluation of buildings: Redwood City, Calif., Applied Technology 

Council Report ATC-20, 152 p. 

Araya, R., and Saragoni, G.R., 1985, Earthquake accelerogram destruc- ' tiveness potential factor, in Proceedings of the Eighth World Conference on Earthquake Engineering: San Francisco, Earthquake Engineering Research Institute, p. 835-842. 

Lizundia, Bret, and others, 1991, Damage to unreinforced masonry buildings in the October 17, 1989 Loma Prieta earthquake, Technical 

report to the National Science Foundation and the California Seismic Safety Commission, 65 p. 

Lizundia, Bret, Dong, W., Holmes, W.T., and Reitherman, R.K., 1993, 

Analysis of unreinforced masonry building damage patterns in the 

Loma Prieta earthquake and improvement of loss estimation methodologies: Technical report to the U.S. Geological Survey, 239 p. 

Miller, R.G., 1962, Statistical prediction by discriminant analysis: Meteorological Monograph 4, no. 25. 

Rutherford & Chekene, 1990, Seismic retrofitting alternatives for San 

Francisco's unreinforced masonry buildings: Estimates of construction cost and seismic damage, San Francisco Department of City 

Planning, 35 1 p. 

Thiel, Charles, and Zsutty, T., 1987, Earthquake parameters and damage statistics: Technical report, San Francisco, Calif., Forell/Elsesser 

Engineers, 264 p. 

U.S. Geological Survey, 1989, Lessons learned from the Loma Prieta, 

California earthquake of October 17, 1989: U.S. Geological Survey 

Circular 1045. 
c 158 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 7.-Sample course-of-action probability matrix 

Damage state Damage state No action Evaluation Evaluation Evaluation Demolition 

description taken only and repair repair and 

strengthening 

1 None 8 0 20 0 

2 Slight 20 60 20 

3 Light 10 20 60 

4 Moderate 3 5 4 5 

5 Heavy 0 0 10 

6 Major 0 0 10 

7 Destroyed 5 0 0 

Table 8.-Course-of-action probability matrix for a community with a loss of strength trigger of 10 percent 

Damage state Damage state No action Evaluation Evaluation Evaluation Demolition 

description taken only and repair repair and 

strengthening 

1 None 8 0 20 0 0 0 

2 Slight 20 60 20 0 0 

3 Light 10 20 5 0 20 0 

4 Moderate 3 5 10 8 0 2 

5 Heavy 0 0 0 70 30 

6 Major 0 

7 Destroyed 5 
A SUMMARY OF UNREINFORCED MASONRY BUILDING DAMAGE PATTERNS 

Table 9.-Costs of addressing earthquake-caused damage organized by the course of action taken 

Type of property loss costs Course of action - -- 

Evaluation ~valuation and ~valuation, Evaluation Evaluation 

only repair repair and and and 

strengthening demolition demolition 

for for 

economic imminent 

reasons life-safety 

reasons 

Engineering assessment 

Engineering study 

Architectural study 

Construction documents 

Structural costs of repair work 

Architectural refinishing 

costs of repair work 

Structural costs of strengthening work 

Architectural refinishing costs of 

strengthening work 

Building permits and testing costs 

Owner project management costs 

Asbestos and Soil contamination costs 

Accessibility requirements 

Fire upgrade requirements 

Additional costs for working 

in occupied building 

Additional costs for working 

in historical building 

Legal fees 

Financing costs (interest and fees) 

Repair of existing conditions 

(e.g., Dry rot and termites) 

Concurrent renovation work 

(can include mechanical and electrical) 

Demolition permit 

Demolition costs 

Building replacement 

Very likely 

Less likely 

Less likely 

No 

No 

No 

No 

No 

No 

Small 

No 

No 

No 

Minimal 

Minimal 

Unlikely 

No 

No 

No 

No 

No 

No 

Very likely 

Likely 

Possible 

Fairly simple 

Yes 

Yes 

No 

No 

Low 

Moderate 

Very unlikely 

Unlikely to be 

triggered 

Unlikely to be 

triggered 

Can add a 

premium 

Can add a 

premium 

Unlikely 

Less likely 

Less likely 

Unlikely 

No 

No 

No 

Very likely Likely 

Very likely Likely 

Likely Possible 

Extensive Possible 

Yes No 

Yes No. 

Varies widely No 

Varies widely No 

Higher No 

Higher Moderate 

Possible No 

Possibly triggered No 

Possibly triggered No 

Adds a bigger No 

premium 

Adds a bigger Can add 

premium premium 

Possible Possible 

More likely Less likely 

More likely No 

Possible No 

No Yes 

No Yes 

No Probable 

Very likely 

Unlikely 

Very unlikely 

No 

No 

No 

No 

No 

No 

Small 

Possible 

No 

No 

No 

No 

Unlikely 

Unlikely 

No 

No 

Yes 

Yes 

Probable 
PERFORMANCE OF THE BUILT ENVIRONMENT 

No Action Evaluation 

Taken and Repair 

Demolition 

v Gap v 

0 ECLR 100 

Note: ECLR = Economic Critical LOM Ratio 

DR = Damage Ratio 

Figure 12.-Probability mass function for a given damage state 

Demolition 

ECLR 

Note: ECLR = Economic Critical Lo*Ã Ratio *Ale + A2+ A3 + A4 + A5 = 100% 

DR = Dimtge Ratio 

Figure 13.-Probability density function (with discrete component at DR=O%) for a given damage state 
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

BUILDING STRUCTURES 

PERFORMANCE OF THE BUILT ENVIRONMENT 

HOUSING REPAIR AND RECONSTRUCTION AFTER THE EARTHQUAKE 

By Mary C. Comerio, University of California, Berkeley 

CONTENTS 

Page 

C161 

161 

162 

163 

164 

164 

165 

166 

166 

ABSTRACT 

This paper describes the housing losses that occurred 

and the attempt to provide emergency, temporary, and 

housing recovery services. The first section briefly describes the losses in four key areas, San Francisco, Oakland, Santa Cruz, and Watsonville. The second section 

describes the differences between single-family and multifamily housing needs as well as the services provided by 

relief agencies and the patchwork of one-time extra-agency 

housing-relief solutions. The final section reviews the cost 

and effectiveness of the overall assistance provided in the 

aftermath of the earthquake and discusses the lessons which 

can be learned to improve housing recovery in future disasters. 

HOUSING LOSSES 

Data from a variety of sources (De Monte, 1990; U.S. 

Government, 1991; U.S. Department of Housing and Urban Development, 1991) contribute to an estimate of approximately 12,000 housing units lost or severely damaged 

and another 30-35,000 moderately damaged in the ninecounty San Francisco Bay area. Of the 12,000 lost or 

severely damaged, approximately 7,000 (60 percent) were 

rental units and 4,500 (40 percent) were low-cost units 

(tables 1, 2). 

Media coverage of the earthquake focused on the dramatic collapse of apartments in San Francisco's Marina 

district, but this was not the whole story. There was extensive damage to older residential buildings throughout 

San Francisco. A total of 25,000 housing units incurred 

some damage; of these, 1,450 were officially declared 

uninhabitable (red-tagged), and almost 5,000 units were 

in need of substantial repair. Only 25 percent of those 

units lost or substantially damaged were located in the 

Marina district. Of the remaining 75 percent, half were in 

downtown neighborhoods-South of Market, Tenderloin, 

Bush Street corridor, and Chinatown-and half were distributed around the city, with concentrations in the Richmond and Sunset districts (City of San Francisco, 1989). 

The damaged buildings typically were three- and fourstory wood-frame structures with soft first stories, except 

in the downtown area. There the damaged buildings were 

older, four- to six-story unreinforced-masonry (URM) construction. 

The 1,300 units destroyed or significantly damaged in 

Oakland were in 10 unreinforced-masonry downtown 

single-room-occupancy (SRO) hotels. Here again, these 

hotels served as the last affordable housing resource for 

many minority and elderly residents in an urban community high property values, high rents, and few options for 

those at the bottom of the income ladder. The city estimates that the earthquake added 2,500 people to the roster 

of homeless in the city. By comparison, the 2,000 units 

which sustained moderate damage were primarily woodframe single-family houses with cracked chimneys, foundations, porches, and stairs. By and large, the inhabitants 

of the latter group did not need temporary or alternative 

housing. They were able to live with the damages and 

repair them over time (City of Oakland, 1993). 

The greatest concentrated loss of commercial and residential buildings was in the Pacific Garden Mall area of 

the City of Santa Cruz, a seaside community and university town of 48,000. In addition to the loss of approximately 25 commercial structures, four residential hotels 

with approximately 400 units were lost. Three of these 

housed low-income elderly tenants, and only one of the 

smaller hotels had a more transient population. Numerous 

single-family residences and small apartments were rendered unoccupiable, and throughout the county, something 
PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 1 .-Damages to housing units 

County Destroyed or significantly Estimate of in need of some Total 

damaged repair 

San Francisco 6,300 18,500 24,800 

Alameda 1,300 2,000 3,300 

Santa Cmz 3,000 10,000 13,000 

Santa Clara 400 600 1,000 

Others 500 500 1,000 

Total 1 1,500 3 1,600 43,100 

Table 2.-Estimated rental and low income units destroyed or significantly damaged 

Destroyed or significantly Rental Units Affordable Units 

damaged (PC~) (pet) 

San Francisco 6,300 75% 66% 

Alameda 1 ,OO 100% 100% 

Santa Cruz 3,000 33% 10% 

Santa Clara 400 0% 10% 

Others 500 0% 0% 

Total 1 1.500 60% 40% 

on the order of 10,000 housing units incurred moderate 

damage (Santa Cruz County, 1989). 

Watsonville sustained the greatest single-family housing loss. Some 850 housing units (10 percent of the city's 

housing stock) was severely damaged or destroyed. These 

were small wood-frame houses and apartment buildings 

literally knocked off their foundations: 40 percent were 

owner occupied; more than 75 percent were low-cost housing units. Largely occupied by farm worker and cannery 

worker extended families, these houses typically had one 

permanent ownerltenant as well as informal subtenants 

who were relatives or friends, some legal, some illegal, 

some permanent, and some migrant workers. There were 

reports of 30-40 people living in one house, and individuals and families living in converted garages and chicken 

coops. In inspecting the damage, city officials found as 

many as 300 "illegal" dwellings (Phillips, 199 1; Comerio, 

1992). 

One year after the earthquake, the damaged wood-frame 

single-family stock had largely been repaired or replaced, 

except in cases near the epicenter where geotechnical studies and land-use issues forced delays. The picture for masonry and wood-frame multifamily housing is quite 

different. Less than half of the units severely damaged or 

lost have been repaired or replaced. The reason is twofold. First, the existing programs designed to assist housing recovery are geared toward the single-family 

homeowner, and as such there is very little funding available that can be used for multifamily buildings. Second, 

the assumption built in to the existing programs is that the 

private market will adjust and provide alternative housing 

resources in the postdisaster period. The concentrated 

losses of inner-city affordable housing resulting from the 

earthquake proved this assumption false and forced a reevaluation of existing housing recovery and reconstruction programs. 

EMERGENCY SHELTERING 

Although the State of California and the local chapters 

of the American Red Cross (ARC) consistently receive 

high praise for their planning and execution of emergency 

services in the first 24 to 48 hours after a disaster, the 

emergency response to the earthquake left many victims 

with the perception that services were financially and racially biased. According to plan the ARC set up masscare shelters in schools and public buildings. Tents were 

also used in Watsonville and Santa Cruz. Although the 

procedures were routine, the levels of service and the attitudes of volunteers varied considerably. It was widely 

believed that volunteers gave "better" service to white 

middle-class owners and renters and lacked sensitivity to 

low-income, ethnic, minority, and transient disaster victims. In part, the problem is institutional in that the rental 

assistance for temporary housing provided by both the 

ARC and FEMA requires "permanent" resident status; that 

is, documentation of ownership or lease agreements for a 

period of longer than 30 days. As such, mass-care shelter 

operators sometimes applied the criteria to those seeking 
HOUSING REPAIR AND RECONSTRUCTION AFTER THE EARTHQUAKE C163 

shelter instead of following the "humanitarian care first, 

questions later" motto of senior ARC staff. 

The insensitivity problem is also a function of the pool 

of volunteers (locally and nationally) who are predominantly white and middle class and who have no training 

in working with people from culturally diverse backgrounds. In Watsonville, the ARC was unwilling to recognize the informal tent camps in town parks as official 

shelters for several days. There were few Spanish speaking volunteers and little tolerance for the victims' fear of 

returning to any structure, despite the documented problems in the milder Whittier earthquake of 1985, when 

many Hispanic victims camped in their cars and on street 

curbs for almost a month before returning to their homes 

and apartments. 

ARC and FEMA provide "temporary" shelter by providing up to 18 months of rental assistance for homeowners 

until they could package their loans, and/or insurance 

settlement, and rebuild their homes, and providing two 

months' rental assistance for renters. Unfortunately, a disproportionate number of the victims were living in lowincome rental accommodations without any security of 

tenure. For these victims, access to FEMA or Red Cross 

temporary rental assistance was blocked by their inability 

to produce a lease or other documentation of residency at 

the same time that the number of affordable units in their 

community had been seriously reduced. Although in some 

areas and in some cases, FEMA and the Red Cross accepted vouchers from clergy and other nontraditional proof 

of residency, the majority of people living in downtown 

San Francisco and Oakland hotels with 28-day rental limits, the elderly living in Santa Cruz hotels, the roommates, 

extended family members, and informal subtenants in 

Watsonville and Santa Cruz county houses did not qualify 

for any housing service other than emergency shelter. Most 

agency personnel now believe that the 30-day rule needs 

to be changed so that all victims can receive some temporary rental assistance, for the obvious reason that it is 

better, safer, and healthier to accommodate victims in traditional housing units rather than to keep large numbers 

of unrelated individuals and families in mass-care shelters 

in public buildings. 

REPAIR OF SINGLE-FAMILY HOUSES 

Individual homeowners with repairable damage found 

a variety of resources in Federal and state housing-recovery programs. The majority of the single-family housing 

reconstruction funding came through the Small Business 

Administration (SBA) loan program. Other programs included: a $5,000 minimum home repair (MHR) grant from 

FEMA for limited repairs to primary dwellings, and Individual Family Grants (IFG) combining FEMA and state 

funds (maximum $21,500) for real and personal property 

replacement. Mortgage assistance and Additional Living 

Expenses (ALE) were also available if needed through a 

FEMA program. Finally, if homeowners' needs were not 

met through these programs, they could apply for a loan 

from the California Disaster Assistance Program 

(CALDAP), administered by the state office of Housing 

and Community Development (HCD). The CALDAP program was initially set up was in two tracks: CALDAP-0 

for owner occupiers and CALDAP-R for rental housing 

owners; it was initially funded with $23 million in each 

track for loans and grants. Four years after the event, 

CALDAP-0 has provided $43 million in loans to 

homeowners, and there are still some loans applications 

pending (California Natural Disasters Assistance Rental 

Program, 1992). 

All in all, the resources available for homeowners to 

repair or rebuild single-family residences seem entirely 

adequate for a moderate disaster the scale of Loma Prieta. 

The major criticism of this aspect of the housing recovery 

program is that it is a bureaucratic nightmare. Each loan 

program has its own application forms and approval procedures, and each sends its own inspector to review and 

assess the damage. Further, an individual must be rejected 

from one program before they can apply for the next, 

saddling disaster victims with lengthy time delays and 

mountains of paperwork. 

Ironically, in the town of Watsonville, where more than 

800 single-family homes were destroyed or significantly 

damaged, very few victims applied for or received Federal and state assistance. Yet, one year after the earthquake, 75 percent of the houses were repaired or replaced. 

How? The city received over $1 million in donations 

from individuals, corporations, foundations, and their sister city in Japan. The Red Cross supplemented this fund 

with $2.5 million for affordable housing assistance. 

Watsonville officials recognized that the majority of their 

population would not qualify for SBA loans and probably 

would not attempt the complex process of applying for 

FEMA and state assistance. The city quickly decided to 

use the donations for small grants of $20,000 to $40,000 

to any residents in need of construction funds. 

Further, numerous volunteer organizations and religious 

groups (for example, the Mennonites, the Christian World 

Relief Committee, Habitat for Humanity, and others) donated time and funds in the rebuilding effort. Many weekends were marked by volunteer construction workers 

involved in "barn raising" new residences. The close-knit 

Latino community also contributed to the self-help atmosphere. In addition, the wood-frame small-scale buildings 

made it easy for unskilled labor to assist in the construction efforts, and the city decided to be "easy on permits 

and tough on inspections," allowing people to get on with 

the work without delays. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

REPAIR OF MULTIFAMILY 

STRUCTURES 

A simplistic view of any postearthquake rebuilding 

phase, as suggested by the name itself, assumes rebuilding the destroyed housing units as they were before, or 

replacing them with similar structures that will provide 

the same features, service, and value that the destroyed 

buildings provided. That is, however, not what happens, 

nor is it, in fact, what is needed. Virtually all the FEMA 

and SBA housing-assistance programs are designed for a 

prototypical, or ideal, victim, who owns or rents a housing unit which is adequate, conventional, and replaceable. 

Unfortunately, a significant portion of the individuals and 

families who lost housing in the Loma Prieta earthquake 

lived in old, poorly maintained, overcrowded, structurally 

unsound buildings because they could not afford better 

quality accommodations. 

The earthquake exposed the lack of specific programs 

targeted for the repair and reconstruction of multifamily 

and low-cost housing. The SBA is the only Federal agency 

making loans to apartment owners, and these are managed through the business (not the housing) loan program; 

moreover, loans will not be made if rents cannot cover 

the additional debt. The state created CALDAP to fill the 

gap of unmet housing needs. The program was based in 

the Department of Housing and Community Development 

(HCD) and staffed by individuals from other low-income 

housing programs. 

Unfortunately, the CALDAP rental program could not 

meet the needs of most private or nonprofit entities attempting the reconstruction of low-income housing. Several factors contribute to the problems CALDAP 

encountered. The program was new, with no set procedures or guidelines, and the rules changed each time the 

agency had to return to the legislature for additional funds. 

As a relief program, it was modeled after other relief 

programs which provide funds to assist in restoring residential units to preearthquake conditions. For run-down, 

old buildings, the requirement to return buildings to 

preearthquake conditions was unreasonable and illogical. 

For example, a 160-unit residential hotel in San Francisco was demolished. After a long process involving foreclosure and purchase by the Redevelopment Agency, the 

site was made available to a nonprofit housing agency. 

Zoning regulations limited the height to eight stories to 

protect a nearby park from shadow. Given the site size, 

the nonprofit could only build 140 of the smallest possible units within the zoning envelope. This discrepancy, 

along with the limitation on quality upgrades and the limited support for development costs, rent requirements, and 

tenant-relocation requirements imposed by CALDAP, led 

to innumerable delays and an ultimate resolution that 

CALDAP funding would only be used for 40 percent of 

the total project. 

Despite the fact that the CALDAP program became a 

source of frustration to all people trying to utilize it, it 

was the only source of financingfor the replacement of 

low-income housing in all areas affected by the earthquake. Overall, the CALDAP rental program loaned 

$44 million and provided 142 rental-property owners, a 

total of about 2,800 units, with loans averaging $15,000 

per unit. 

ONE-TIME SOLUTIONS 

The frustration experienced by local governments and 

affordable-housing advocates over the lack of resources 

for low-income victims took form in a frontal attack on 

FEMA by San Francisco Mayor Art Agnos and enormous 

pressure on the Red Cross to redirect all contributions 

directly to the Bay Area instead of reserving some portion 

of the funds raised for future disasters. The critical media 

blitz captured national attention and led to a number of 

special assistance programs that were unique to Loma 

Prieta. These special sources of funding for low-income 

housing renovation and reconstruction have become the 

primary mechanisms for aiding low-income victims, but 

unfortunately, each is perceived as a one-time solution by 

relief agencies. 

The Legal Aid Society of Alameda County sued FEMA 

to contest FEMA's interpretation of the relief regulations 

requiring 30-day residency for financial assistance, which 

disqualified many SRO dwellers, the lack of due process 

in the appeal procedure, and the incomplete information 

provided by FEMA regarding the assistance to which the 

disaster victims were entitled. While the strictly legal recourse to the grievance would have been to ask FEMA to 

provide rental assistance to the people concerned, Legal 

Aid recognized that the temporary rental assistance would 

not have solved the problem when no comparable units 

were available on the market. Therefore, Legal Aid decided to ask for money to replace the units lost in the 

earthquake. After agreeing to that solution, FEMA appeared to renege on the settlement, and it took another 

year of legal and media battles to create a second settlement, which provided $23 million to fund 2,200 SRO 

units. The settlement sum was arrived at by multiplying 

the number of SRO units lost by a unit cost of about 

$10,500. The money was divided among the various counties based on their proportional number of SRO units lost. 

The flexibility of the final settlement was ideal from the 

point of view of the local agencies, because the funds 

were provided in a block-grant fashion to the counties 

and as such allowed them the freedom to decide how to 

put the money to best use, with very few restrictions. 

Overall, 1,200 to 1,500 units were actually rebuilt using 

lawsuit funds. 
HOUSING REPAIR AND RECONSTRUCTION AFTER THE EARTHQUAKE C165 

Alameda County received the largest share, $1 1.8 million, of the settlement funds. Although $2 million was 

used for emergency sheltering costs, the remainder went 

to Oakland, where 1,300 of the lost SRO housing units 

were located. The city pledged $1 million to a multiservice 

homeless shelter and used the remainder to rehabilitate 

damaged buildings. Due to FEMA restrictions, the funds 

could not be used to assist private owners. Thus, the city 

assisted in arranging the sale of these buildings to 

nonprofits and committed city and community development block-grant funds to complete the projects. The transactions slowed the recovery process so that one year later, 

only one hotel with 84 units was completed. Four years 

after the event, two additional hotels and the multiservice 

center were completed for a total of 300 units restored to 

service. Six other hotels with approximately 600 units are 

in varying stages of construction, still leaving a deficit of 

400 units. 

The American Red Cross responded to political pressure to use some of the $52 million generated in the highly 

successful fund-raising drive after the earthquake for housing-recovery purposes. ARC set aside a special fund for 

various human service projects to earthquake victims, and 

about $13.15 million for housing-recovery projects. Of 

that, $900,000 was given as grants to low-income 

homeowners, $10 million was set aside for various multifamily projects, as part of the financing needed to rebuild, 

and $2.25 million was allocated as predevelopment loans 

to nonprofits to enable them to start the development process (American Red Cross, 1992). 

Many of the nonprofit agencies who purchased damaged buildings with the intent of renovating them with 

funds from CALDAP and/or the FEMA lawsuit settlement were quite skilled in financing low-income housing 

rehabilitation. These organizations leveraged ARC, 

CALDAP, or FEMA settlement funds to obtain additional 

financing from Redevelopment Agencies, tax credits, other 

state (HCD) housing funds, and private lenders. Because 

the disaster-recovery funding covered only 20 to 50 percent of the rebuilding costs, the funds provided by traditional low-income-housing lenders were not available for 

their original function, to increase the affordable housing 

stock. 

Further, the Federal programs at HUD were not made 

available for disaster recovery. Despite a memorandum 

from the Region IX office to Secretary Kemp in Washington, dated February 1990, describing the crisis and recommending additional allocations of section 8 vouchers 

and rental rehabilitation program funds to the affected 

cities, HUD was unwilling to assume the responsibility of 

assisting in the rebuilding of housing for low-income populations. Its only action was the allocation of 500 rental 

assistance vouchers and 664 moderate-rehabilitation vouchers to the area. This did not represent any extra allocation, 

only an attempt to speed up existing allocations, which 

because of bureaucratic delays, did not materialize until 

the summer of 1990. 

ECONOMICS OF HOUSING RECOVERY 

The reason housing is not rebuilt after natural disasters 

is not solely a function of recovery program design or 

implementation, it is tied with the economics of the marketplace. Someone, usually the building owner or potential lender, compares the cost of rebuilding with the 

potential for recovering those costs in the marketplace 

and decides the two just do not add up. 

Different types of housing are valued according to different attributes. Single-family homes are bought, sold, 

built-and following a disaster, rebuilt-on the basis of 

three attributes: the shelter provided (the space, the lot, 

the design features), the location (neighborhood, schools, 

proximity to work, etc.), and the wealth potential (appreciation, tax shelter, cash flow). Because the long-term value 

of an owner-occupied house is linked to its condition, 

homeowners have a powerful incentive to maintain their 

properties and to rebuild them after a disaster. Even at 

moderate appreciation rates of 2-4 percent, homeowners 

can absorb significant postdisaster rebuilding costs up to 

20 percent of the initial home value and still make a reasonable profit (Comerio and others, 1994). 

By contrast, renters rent housing for two reasons, 

shelter and location, but not for reasons of profit enhancement. Investors in rental housing (including developers, 

landlords, and partners) have a different set of concerns. 

Investors care about shelter and location only as much 

as they impact a particular property's profit potential, 

Unlike homeowners, investors have an incentive to maintain their properties only to the extent that the maintenance directly enhances cash flows. Following this 

logic, postdisaster rebuilding expenses are unlikely to be 

undertaken if they cannot be quickly recovered out of 

rents. 

Put simply, the likelihood of a single-family homeowner 

rebuilding a damaged or destroyed unit depends jointly 

on personal and financial concerns. The likelihood of a 

multifamily investor rebuilding a damaged building depends primarily on their ability to raise rents without losing tenants. Except in very tight housing markets, landlords 

have little ability to unilaterally raise rents, because if 

they do, the tenants will move elsewhere. Thus, when a 

building owner is faced with additional debt to cover damages for 20 percent of the initial building value, the owner 

would have to raise rents 8-10 percent in order to maintain predisaster levels of operating capital. The implication is clear: There is little if any economic incentive to 

rebuild multifamily housing, even with relatively favorable financing terms. The greater the repair cost, the more 

highly the owner is leveraged, the higher the market va- 
C166 PERFORMANCE OF THE BUILT ENVIRONMENT 

cancy rate, or the poorer the tenant base, the greater the 

incentives for property abandonment. This basic economic 

reality was clearly demonstrated after Loma Prieta. Of the 

12,000 units lost, 9,000 were in multifamily buildings. Of 

these, nearly 90 percent were still out of service one year 

after the earthquake, and 4 years later, about 50 percent 

of these remain unrepaired or unreplaced (Comerio and 

others, 1994). 

LESSONS 

Major disasters change the physical and social fabric of 

cities in unalterable ways. Agencies as well as individuals 

use the disaster as an opportunity to change the status 

quo, to their advantage. An individual rebuilding his house 

may want to make some changes that he was contemplating even before the disaster struck. A city may use funds 

available from earthquake reconstruction to finance homeless shelters and multiservice centers (as was the case in 

Oakland and San Francisco). A region may seize the opportunity of major destruction in the roads and infrastructure to transform and renew the infrastructure of the area. 

It is within this context that the design of assistance programs has to take place. The stress in the rebuilding phase 

is the rebuilding of the individual and community lives, 

not on the rebuilding of the particular artifacts that sustain it. 

The Loma Prieta earthquake deepened an already-existing housing crisis in the Bay Area. Many homeowners 

and multifamily building owners were probably carrying 

as much debt as the property could carry, and could not 

afford to take further loans to rebuild or rehabilitate the 

property. It was apparent that the normal single-family 

housing-oriented recovery programs would not be sufficient to enable real housing recovery. Furthermore, because the earthquake hit hardest in areas of concentrated 

multifamily low-income housing, the market was not able 

to provide alternative or replacement housing at affordable rents, without some assistance from the public. Overall, only 40 percent of the housing losses were served 

through the normal disaster-assistance process, and 60 percent can be described as a residue of unmet needs. Of 

these, half found some assistance through the one-time 

solutions, but these are not models for future disaster relief and recovery programs. 

Table 3 summarizes the expenditures made by various 

public agencies toward housing assistance and recovery 

in the Bay Area after the earthquake. The expenditures by 

government agencies and private insurance show that out 

of an estimated $1,130,000 spent on earthquake recovery, 

66 percent ($750 million) was spent on assistance for 

owner occupiers, although only 40 percent of the stock of 

damaged units was in single-family dwellings. Eliminating funds paid by insurance, and looking only at the public assistance money, homeowners still received proportionally more funding than renters, 62 percent of the funds 

compared to 40 percent of the damaged units. Furthermore, the aid that is given to rental housing is not direct 

aid that goes to the renter, but aid given to owners of 

rental housing. Some of it goes into predevelopment expenses and organizational overhead, not only into construction and repair. Therefore, the net sums used to restore 

the rental units is probably at least another 20 percent 

less. 

In summary, it seems that state and Federal governments should step in to aid local and county governments 

and individuals in the housing recovery process. Existing 

programs are biased toward homeowners. A more equitable approach would be to insure that the funds are distributed evenly across housing types. A proactive approach 

would insist that the earthquake presents an opportunity 

to redress some of the housing inequities and direct more 

funds on the basis of need, not on the basis of the losses 

incurred. The housing-recovery experience after Loma 

Prieta leads to three lessons to improve postdisaster housing recovery: 

1. Postearthquake housing recovery requires 

preearthquake planning.-Although California is rightfully 

proud of its planning and readiness for emergency response in 

the event of a disaster, Loma Prieta taught a valuable lesson in 

exposing the gaps in housing recovery and provides a roadmap 

for thinking about recovery planning. There is a need to 

promote hazards mitigation in all structures but particularly in 

vulnerable multifamily structures. Mitigation is the single 

most cost-effective method of reducing damage and limiting 

the human and financial costs after disasters. There is a need 

to create recovery funding programs for multifamily lowincome housing. This requires rethinking the current Federal 

and state programs and their relation to ongoing housing 

strategies, and it requires standardized reporting of damage 

and realistic budgeting. 

The entire structure of postdisaster housing-recovery needs 

to be rethought in terms of the method for estimating housing 

losses and recovery needs and in terms of the delivery system 

for assistance. A careful damage assessment based on common methods of data collection for each locale could be used 

to prepare a Federal housing recovery budget for the state, 

which could in turn develop its own assistance budget. These 

funds could be distributed through an agreed-upon delivery 

system which is already providing affordable housing assistance. The concept is one of state and Federal block grants 

administered through existing agencies in which state and 

local governments, which are familiar with the rules and 

regulations of the administering agencies, are notified as to 

how much they have to work with. 

2. Existing recovery programs should be streamlined to 

expedite services.-The single most important thing govern- 
HOUSING REPAIR AND RECONSTRUCTION AFTER THE EARTHQUAKE 

Table 3.-Summary of expenditures on building repair by client group 

Agency Owner/ Mean per Rental Mean per Total 

Occupier unit ($) ($ millions) unit ($) ($ millions) 

($ millions) 

Insurance 373.98 20,215 -152.00 N/A -525.98 

ARC 0.90 13,000 12.25 1 1,400 13.15 

FEMA 23.00 N/A 23.00 

IFG -23 .03 N/A -23 .03 N/ A 46.06 

SB A 309.14 26,900 -126.50 N/A 435.64 

CALDAP 43.31 13,000 43.63 15,865 86.94 

Total public 376.38 15,055 228.41 10,380 604.79 

assistance 

Total 750.36 380.41 1,130.77 

ment and relief agencies can do for disaster victims is to 

expedite the process of getting back to normal. The Disaster 

Application Centers (DAC s) are established to provide information and assistance in the recovery process to disaster 

victims, and each individual or family is assigned a case 

number when they come in. FEMA has created a single intake 

form which allows them the ability to computerize information on the victims which is shared through an electronic 

network with all the recovery funding agencies. There could 

be one similar application form for housing-recovery assistance which could be devised to cover all the information 

needed by each of the possible funding agencies. One agency 

could take the lead in assessing the actual damage through a 

single inspection by a qualified knowledgable contractor and 

then have a loan officer analyze which combination of loans, 

grants, and temporary assistance should be provided. 

3. Housing recovery programs will be most effective if 

they are administered at the local level.-Housing recovery 

involves the development and redevelopment of land and 

buildings, a process which requires localized construction 

expertise, the involvement of local private lenders, and local 

government approvals (as well as interactions among owners 

and tenants). This is not a process which can be managed by 

state and Federal agencies alone. 

A well-planned process for distribution of housing-recovery funds could avoid the mistakes and delays that occurred 

after Loma Prieta. The excursions taken by FEMA, and 

particularly ARC, into housing finance because of the political pressure to provide affordable housing assistance were 

taxing both for the organizations and for the clients. The 

agencies were afraid to make mistakes because of their lack of 

expertise in the field of housing, and as a result, officials 

tended to make stricter interpretations of regulations than 

what might have been necessary. Further, their tendency to 

distrust local housing officials created an atmosphere in which 

unusual andlor creative local solutions were perceived as 

fraudulent. The piecemeal housing aid and the process through 

which is was achieved is not a model for future disasters. The 

best and most expedient method for low-income housing 

recovery is to allow local governments the ability to make 

decisions on how best to serve the affected populations in 

their community and to provide funding through existing 

channels. 

ACKNOWLEDGMENTS 

The data for this paper were developed in a research 

project for the California Governor's Office of Emergency 

Services, conducted by the author with John Landis and 

Yodan Rofe. The final report, "Post-Disaster Residential 

Rebuilding," is published as Working Paper #608 by the 

Institute of Urban and Regional Development (IURD), 

University of California at Berkeley, February 1993. 

REFERENCES 

American Red Cross, 1992, Special disaster relief fund: Quarterly Report on Project Status (December 31), 21 p. 

California Natural Disasters Assistance Rental Program (CALDAP-R), 

1992, Activity summary and production report: Department of Housing and Community Development (June 30), 7 p. 

City of Oakland, 1993, Loma Prieta damaged properties list: Office of 

Planning and Building (March 18), 52 p. 

City of San Francisco, 1989, Earthquake impact on low and moderate 

income housing Rept. 4: Mayor's Office of Housing (November 

201, 4 P. 

Comerio, Mary, 1992, Hazards mitigation and housing recovery, in 

Disasters and the Small Dwelling: London: James and James, p. 

127-135. 

Comerio, Mary, John Landis, and Yodan Rofe, 1994, Post-disaster residential rebuilding: University of California, Berkeley, Institute of 

Urban and Regional Development, 85p. 

De Monte, Robert, 1990, Memorandum on proposed earthquake response 

by region IX: U.S. Department of Housing and Urban Development 

(Feb.), in Proceedings of the symposium on policy issues in the 

provision of postearthquake shelter and housing, January 26-28, 

1992: National Center for Earthquake Engineering and Bay Area 

Regional Earthquake Preparedness Project, 15 p. 

Housing and Urban Development, 1991, Status of recovery from the 

earthquake in San Francisco, Oakland, Santa Cruz and surrounding 

THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

PERFORMANCE OF THE BUILT ENVIRONMENT 

BUILDING STRUCTURES 

IMPACT OF THE EARTHQUAKE ON HABITABILITY OF 

HOUSING UNITS 

By Jeanne B. Perkins and Ben Chuaqui, 

Association of Bay Area Governments, Oakland, Calif. 

CONTENTS 

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ABSTRACT 

The Loma Prieta earthquake produced over 16,000 uninhabitable housing units throughout the Monterey and 

San Francisco Bay areas, including almost 13,000 in the 

Bay Area. Although this number is only about a third as 

high as the more than 48,000 units made uninhabitable in 

the 1994 Northridge earthquake, the impact on housing 

stock was significant. 

Data on habitability of housing structures were collected 

through a combination of telephone and in-person interviews with city and county building departments following the earthquake. Subsequent efforts were made to 

improve the quality of data regarding age, type of construction, and number of units associated with those structures. Similar data were collected following the 1994 

Northridge earthquake. The data collection effort focused 

on obtaining data for several categories of wood-frame 

construction, while minimizing the number of nonwood 

categories. 

These data were collected as part of a larger project 

and tied to ground-shaking modeling in order to predict 

the impacts of 11 future likely earthquakes on the housing 

stock of the San Francisco Bay region. 

These two damaging earthquakes emphasize the importance of creating housing-loss estimates for emergencyresponse planning in metropolitan areas subject to strong 

ground shaking. They also point out the impact of nonlife-threatening damage on people's lives. Design standards need further evaluation to better ensure functionality 

to these structures, most of which are nonengineered. 

BACKGROUND 

This data-collection effort on the impact of the Loma 

Prieta earthquake on habitability of housing units is part 

of a much larger program at the Association of Bay Area 

Governments (ABAG). The goal of the larger program is 

to develop estimates of the number of uninhabitable dwellings from future earthquakes affecting the San Francisco 

Bay area. This program began in 1991 with funding from 

the American Red Cross, the California Office of Emergency Services, and ABAG itself (Perkins, 1992). George 

Washington University (GWU) used these estimates to 

develop projections of shelter populations in future earthquakes in the San Francisco Bay area. More recently, funding from the National Science Foundation has supported 

our research using the data on housing impacts collected 

following the Loma Prieta and Northridge earthquakes, 

together with more sophisticated ground-shaking intensity 

models, to greatly improve the estimates of housing impacts following future earthquakes. These estimates also 

incorporate better data on all the existing residential housing stock, including location (see Perkins and others, 

1996). 

Data were collected on housing units designated as nonfunctional, or uninhabitable. "Uninhabitable" is defined 

as unable to be occupied due to structural problems. It is 

equivalent of the Applied Technology Council (ATC) redtagging definition for unsafe buildings where entry is prohibited for single-family homes. For multifamily units, 

the structure can be either red tagged (unsafe) or yellow 
C170 

tagged (entry restrict 

PERFORMANCE OF THE BUILT ENVIRONMENT 

,ed) (see Applied Technology Council, 1991). Building departments in California uniformly 

use the ATC definitions in their postearthquake tagging. 

Uninhabitable dwellings are not necessarily destroyed; 

most are repairable. 

The Loma Prieta earthquake provided extremely valuable information on the performance of several categories 

of housing when exposed to various levels of shaking. 

The residential structures tagged by local government 

building officials following the earthquake have been carefully examined and analyzed. 

Data on housing performance from a single earthquake 

are not nearly as useful as data from multiple events. 

Thus, ABAG staff collected similar information on habitability following the Northridge earthquake in the Los 

Angeles area. The ratios between green-, yellow-, and 

red-tagged housing units for different types of housing 

construction in the Los Angeles area obtained following 

the Northridge earthquake were combined with the Loma 

Prieta data and additional data from earlier earthquakes 

(from Dunne and Sonnenfeld, 1991) before being incorporated into the AB AG model which predicts housing 

impacts following future Bay Area earthquakes. 

Our goal has been to provide information that can improve mitigation, disaster response, and residential rebuilding efforts in future earthquakes. The overall program 

should serve to motivate retrofitting of homes and apartments, planning for responding to future earthquakes, and 

rebuilding faster after those earthquakes. 

TYPE OF STRUCTURES 

For the purposes of collecting data on the extensive 

Bay Area housing stock both habitable and uninhabitable 

following this earthquake, residential buildings have been 

broadly classified in table 1 according to three basic characteristics: (1) the building's structure, (2) the configuration or shape of the building, and (3) the age of the 

building. 

STRUCTURE 

Because of the large variation in housing stock in the 

nine Bay Area counties, implementing a detailed classification scheme that would account for both horizontal and 

vertical elements of the structure, as well as the methods 

by which they are connected, was not practical. Rather, 

this paper categorizes residential structures according to 

the material of their principal vertical elements. This "materiallstructure" classification follows established groupings from the building industry: 

1. Wood frame.-Includes buildings which consist of a 

concrete podium with a wood frame of several stories 

above; 

2. Masonry.-Includes brick and concrete block (reinforced 

or unreinforced); 

3. Steel frame.-Includes braced frames and moment-resisting frames, with and without concrete or masonry infill 

walls; and 

4. Concrete.-Includes cast-in-place and precast or a combination of the two. 

Since wood-frame buildings are by far the largest component of the housing stock in the Bay Area (up to 93.4 

percent), the structural types have been further regrouped 

into wood and nonwood classifications. Because the combined structural types of concrete, masonry, and steel frame 

represent only 1.5 percent of the Bay Area's total housing 

stock, they have been consolidated into a single structural 

type-nonwood. At the same time, since unreinforced 

masonry buildings have been identified by local building 

officials and they present specific seismic hazards, they 

have been isolated as a category in and of themselves. 

Two classifications used in the paper which do not follow the above rules are the categories of "mobile homes" 

and "other." Since mobile homes are accounted for in the 

U.S. Census and are a clearly distinguishable building 

type with particular seismic concerns, they have been classified into a category of their own. The category defined 

as "other" is used in accord with the U.S. Census Bureau 

Definitions (U.S. Bureau of the Census, 1991) and includes 

shelters such as boats, tents, caves, and railcars. 

The five categories used in this paper to classify the 

type of structure of the Bay Area's housing stock are 

therefore : 

1. Mobile homes.-Represent 2.8 percent of the total; 

2. Unreinforced masonry.-Represents 1.0 percent of the 

total; 

3. Wood.-Represents 93.4 percent of the total; 

4. Nonwood.-Includes concrete, reinforced masonry, and 

steel frame, and represents 1.5 percent; and 

5. Other.-Represents 1.3 percent of the total. 

CONFIGURATION 

While the structure of a building plays an important 

role in the mitigation of seismic forces, its configuration 

will likewise influence its earthquake resistance. Configuration deals with issues such as building height, building 

shape, and general width-to-height proportions. It might 

also include the location and arrangement of the major 

structural elements within the building. Due to the size of 

the Bay Area's housing stock, and due to the fact that 

height is perhaps the most important element in a building's 
IMPACT OF THE EARTHQUAKE ON HABITABILITY OF HOUSING UNITS C171 

configuration, this paper categorizes a building's configuration only by its height. Accordingly, for the purposes of 

this paper, the height of a building determines whether it 

is classified as: 

1. Low.-Three stories and under; 

2. Medium..-Between four and seven stories; and 

3. High..-Eight stories and over. 

When classifying a building according to its height, 

parking levels have consistently been included. Whether 

the parking level is below grade, half a level below grade, 

or above grade, it has been included in the story count of 

a building. 

Note that eight-story buildings are often considered highrise structures by the Uniform Fire Code for they exceed 

75 feet. In addition, the U.S. Bureau of the Census used 

these height classifications for data through 1980. (The 

question on number of stories was dropped from the 1990 

census.) 

AGE OF CONSTRUCTION 

The age of construction plays an important role in the 

seismic performance of buildings. Over time, building technologies have improved, and code requirements have increased. The age of a building can give clues as to how 

buildings were detailed and put together. The age of a 

building can also suggest information about the quality of 

the materials used and the quality of craftsmanship during 

construction. Since significant changes in construction 

practices over time typically have been triggered by earthquakes, the age of a building determines whether construction occurred before or after a code change or before 

or after the use of a particular building technology. Age, 

therefore, can serve as an indicator of how well a building 

will resist earthquakes in the future. 

The Uniform Building Code was first published and 

organized in 1927. The extensive use of plywood and 

other modern building materials, together with the wider 

existence and implementation of the building code, occurred with the large-scale developments following World 

War I1 (WW 11). It was not until suburbanization and the 

increase in demand for housing occurred that there were 

significant changes in construction practices (Gideon, 

1948; Kostof, 1985; Hayden, 1986). Note that the 1940 

cut-off date is extremely important. For example, foundation bolting of single-family wood-frame dwellings was 

more common after 1940. In the San Francisco Bay area 

almost 80 percent of the region's housing stock was built 

after 1940; this preponderance is slightly greater than the 

almost three-quarters value nationwide. 

One of the most noticeable differences between preand post-1940 buildings is the degree of parking both 

within multifamily and single-family buildings. It is common for most multifamily buildings built after WW I1 to 

have all or most of the ground floor dedicated to parking. 

(Residential buildings with tucked-under parking or firstfloor parking are more vulnerable to damage than those 

buildings without such parking.) 

Although 1940 is identified as the key cut-off point in 

this study, the actual decade of construction has been catalogued in our database. In addition, when the information 

was available and a precise record of construction existed, it was catalogued. Typically, the decade of construction was identified through field visits or from data 

obtained from building departments. Hence, a building 

built in 1936 has been identified as a 1930's building; a 

building built in 1967 as a 60's building, a building built 

in 1940 as a 40's building, and so on. 

The significance of WW I1 as the cut-off point in the 

classification of building age is corroborated by preliminary statistics from the Northridge earthquake. Insurance 

company statistics on percentage losses to single-family 

homes by decade of construction show that there was no 

significant difference in performance by year of construction other than pre- and post-WW I1 buildings. These companies speculate that, to some degree, improvements in 

the codes and construction practices have been offset by 

the extensive use of complex configurations, large amounts 

of glass, and cathedral ceilings in newer construction (Robert Dowen, oral commun., 1994). 

While the age of buildings has been identified by decade or by year in the database, the statistics in this study 

classify building age on the basis of pre- or post-WW I1 

(1940's) vintage. Thus, this study classifies the age of 

buildings according to two periods: 

1. Pre-1940; and 

2. 1940 to present. 

OVERALL CLASSIFICATION 

For the purposes of this study, the housing stock has 

broadly been classified according to the three sets of criteria: the material, the height or number of stories of the 

building, and the age of the building. These three categories enable classification of the Bay Area's housing stock 

into the 13 building types listed in table 1. Note that six 

of the 13 types are subdivisions of wood-frame construction. This classification system is extremely valuable for 

residential buildings in the Bay Area, where 93.4 percent 

of the housing stock is wood-frame construction, but 

unusual for earthquake studies. In the more typical Applied Technology Council study of earthquake damage 

(Applied Technology Council, 1985), only one of the 40 

categories was wood-frame construction. 
PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 1 .-Summary of characteristics of building types used in this paper 

Building Structure Configuration Age Single vs. 

type multifamily 

Type 1 Mobile Homes 1-3 stories Post 1940 Single Family 

TY pe 2 

Type 3 

TY pe 4 

Type 5 

TY pe 6 

TY pe 7 

TY pe 8 

Type 9 

Type 10 

Type 11 

Type 12 

Type 13 

Unreinforced Masonry 

Non-Wood 

Non-Wood 

Non-Wood 

Non- Wood 

Wood 

Wood 

Wood 

Wood 

Wood 

Wood 

Other 

Varies 

4-7 stories 

4-7 stories 

8 and up 

8 and up 

4-7 stories 

4-7 stories 

1-3 stories 

1-3 stories 

1-3 stories 

1-3 stories 

Varies 

Pre 1940 

Pre 1940 

Post 1940 

Pre 1940 

Post 1940 

Pre 1940 

Post 1940 

Pre 1940 

Post 1940 

Pre 1940 

Post 1940 

Varies 

Multi-family 

Multi-family 

Multi-family 

Multi-family 

Multi-family 

Multi-family 

Multi-family 

Multi-family 

Multi-family 

Single Family 

Single Family 

Varies 

POSTEARTHQUAKE HABITABILITY 

DATA COLLECTION 

LOMA PRIETA DATA 

A telephone survey of all jurisdictions affected by the 

Loma Prieta earthquake was conducted in 1991 in order 

to determine the extent of residential damage (other than 

mobile homes) and the displacement of residents. Unclear 

or incomplete data was rechecked with follow-up surveys 

in 1993 and 1994. Information was obtained from city 

and county building departments. These building departments were the agencies that immediately inspected and 

assessed the structural damage within their jurisdictions. 

Buildings were tagged with red (unsafe for occupancy), 

yellow (limited access), and green (safe). Survey respondents were asked to identify the residential red- and yellow-tagged buildings within their jurisdictions, as well as 

structure type and street address. 

One problem with the information was that some cities 

kept incomplete or no records of their tagging. Not all 

departments with red-tagged buildings could provide addresses. Some cities had extensive records, but these were 

disorganized to the point of being unusable. Other jurisdictions summarized their inspections very early on; therefore their summary of total red-tagged units may be low 

relative to the actual number (such as may have been the 

case in Gilroy and San Jose). Many building departments 

discovered new quake damage long after the earthquake 

as homeowners applied for repair permits. 

Local city and county building departments have no 

authority over mobile homes because they are not classified as buildings. However, data on uninhabitable mobile 

homes were centrally compiled by the State of California 

Department of Housing and Community Development 

(H&CD) (1991). 

Table 2 contains the results of the survey of cities, 

counties, and H&CD. It summarizes the total number of 

red-tagged housing units, as well as the yellow-tagged 

multifamily units, by jurisdiction for the nine-county San 

Francisco Bay area, as well as the three-county Monterey 

Bay areae1 Tables 3 and 4 contain the results of the building type classification used in table 1 by tagging type and 

by intensity level. The distribution of the red-tagged residential buildings is shown in figure 1. 

NORTHRIDGE DATA 

ABAG duplicated the process of collecting data on redand yellow-tagged residential buildings following the Loma 

Prieta earthquake in the Bay Area for the Northridge earthquake in the Los Angeles and Ventura County areas. As 

in the Loma Prieta survey, ABAG project staff conducted 

a telephone survey of the affected city and county building departments. This interview effort was supplemented 

by approximately 20 person-days in the impacted area to 

check data supplied by the building departments to ensure 

l~or those interested in the actual street address information, a PCformatted diskette of all red-tagged units in the nine-county Bay Area, 

as well as all multifamily yellow-tagged units, is available from ABAG 

as Publication P960001EQK. 
IMPACT OF THE EARTHQUAKE ON HABITABILITY OF HOUSING UNITS C173 

Table 2.-Results of ABAG survey conducted in 1994 of cities and counties related to unreinforced masonry buildings and housing damage from 

the Lorna Prieta earthquake 

County City No. of No. of No. of No. of units No. of No. of No. of red- No. of redURMs historic residential within yellow- yellow- tagged tagged 

URMs' URMs residential tagged tagged residential residential 

URMs residential residential units buildings 

units buildings 

Alameda County Alameda 82 7 9 64 0 0 0 0 

Alameda County Albany 58 0 1 46 0 0 0 0 

Alameda County Berkeley 444 0 41 897 150 150 45 3 

Alameda County Dublin 0 0 0 0 0 0 0 0 

Alameda County Emeryville 101 1 41 100 35 35 2 1 

Alameda County Fremont 32 5 5 14 50 50 0 0 

Alameda County Hayward 49 49 1 40 10 10 0 0 

Alameda County Livermore 48 3 1 1 0 0 0 0 

Alameda County Newark 0 0 0 0 0 0 0 0 

Alameda County Oakland 1,666 120 153 3,264 2,008 199 1,349 87 

Alameda County Piedmont 0 0 0 0 0 0 0 0 

Alameda County Pleasanton 36 0 0 0 0 0 0 0 

Alameda County San Leandro 59 0 1 24 1 1 0 0 

Alameda County Union City 7 0 0 0 0 0 0 0 

Alameda County Unincorporated 18 0 0 0 0 0 0 0 

Contra Costa Co Antioch 64 0 8 2 1 0 0 0 0 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Contra Costa Co 

Brentwood 

Clayton 

Concord 

Danville 

El Cerrito 

Hercules 

Lafayette 

Martinez 

Moraga 

Orinda 

Pinole 

Pittsburg 

Pleasant Hill 

Richmond 

San Pablo 

San Ramon 

Walnut Creek 

Unincorporated 

Marin County Belvedere 0 0 0 0 0 0 0 0 

Marin County Corte Madera 4 0 0 0 0 0 0 0 

Marin County Fairfax 4 0 3 15 0 0 0 0 

Marin County Larkspur 8 0 1 1 1 1 0 0 

Marin County Mill Valley 18 0 0 0 0 0 0 0 

Marin County Novato 2 0 0 0 0 0 0 0 

Marin County Ross 1 0 0 0 0 0 0 0 

Marin County San Anselmo 22 1 4 6 0 0 0 0 

Marin County San Rafael 50 0 17 109 1 1 0 0 

Marin County Sausalito 10 7 5 16 0 0 0 0 

Marin County Tiburon 1 0 0 0 0 0 0 0 

Marin County Unincorporated 1 0 0 0 0 0 2 2 
c 174 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 2.-Continued. 

County City No. of No. of No. of No. of units No. of No. of No. of red- No. of redURMs historic residential within yellow- yellow- tagged tagged 

URMs' URMs residential tagged tagged residential residential 

URMs residential residential units buildings 

units buildings 

Napa County American Canyon 0 0 0 0 0 0 0 0 

Napa County Calistoga 30 0 0 0 0 0 0 0 

Napa County Napa 45 12 0 0 0 0 0 0 

pa County Saint Helena 2 7 2 7 1 1 0 0 0 0 

Napa County Yountville 9 3 0 0 0 0 0 0 

Napa County Unincorporated 7 5 0 0 0 0 0 0 

San Francisco San Francisco 2,080 1,400 628 18,921 7,318 759 2,078 21 5 

San Mateo Co Atherton 1 1 0 0 0 0 0 0 

San Mateo Co Belmont 4 0 1 4 0 0 0 0 

San Mateo Co Brisbane 4 0 0 0 0 0 0 0 

San Mateo Co Burlingame 63 0 4 13 0 0 0 0 

San Mateo Co Colma 4 0 0 0 0 0 0 0 

San Mateo Co Daly City 1 0 1 3 20 20 0 0 

San Mateo Co East Palo Alto 0 0 0 0 0 0 0 0 

San Mateo Co Foster City 0 0 0 0 0 0 0 0 

San Mateo Co Half Moon Bay 2 1 0 0 26 26 0 0 

San Mateo Co Hillsborough 0 0 0 0 0 0 0 0 

San Mateo Co Men10 Park 2 0 0 0 0 0 0 0 

San Mateo Co Millbrae 2 0 0 0 150 150 0 0 

San Mateo Co Pacifica 0 0 0 0 3 3 0 0 

San Mateo Co Portola Valley 0 0 0 0 0 0 0 0 

San Mateo Co Redwood City 33 4 1 53 0 0 53 1 

San Mateo Co San Bruno 5 0 0 0 0 0 1 1 

San Mateo Co San Carlos 10 2 3 4 2 5 2 5 1 1 

San Mateo Co San Mateo 28 0 3 71 300 300 1 1 

San Mateo Co So. San Francisco 23 3 4 4 42 42 12 2 

San Mateo Co Woodside 0 0 0 0 0 0 0 0 

San Mateo Co Unincorporated 7 4 0 0 32 32 8 8 

Santa Clara Co Campbell 11 4 0 0 36 36 1 1 

Santa Clara Co Cupertino 1 0 0 0 50 50 0 0 

Santa Clara Co Gilroy 35 0 2 2 350 350 52 52 

Santa Clara Co Los Altos 1 0 0 0 1 1 1 1 

Santa Clara Co Los Altos Hills 0 0 0 0 65 65 10 10 

Santa Clara Co Los Gatos 2 1 6 2 2 1 500 500 2 5 24 

Santa Clara Co Milpitas 1 0 1 1 5 5 0 0 

Santa Clara Co Monte Sereno 0 0 0 0 0 0 1 1 

Santa Clara Co Morgan Hill 7 2 0 0 1 1 43 43 

Santa Clara Co Mountain View 25 7 2 24 0 0 46 11 

Santa Clara Co Palo Alto 49 0 1 26 0 0 2 1 

Santa Clara Co San Jose 196 0 0 0 18 18 8 8 

Santa Clara Co Santa Clara 24 0 0 0 0 0 0 0 

Santa Clara Co Saratoga 0 0 0 0 0 0 6 6 

Santa Clara Co Sunnyvale 86 30 0 0 10 10 3 3 

Santa Clara Co Unincorporated 14 0 0 0 303 303 210 208 
IMPACT OF THE EARTHQUAKE ON HABITABILITY OF HOUSING UNITS C175 

Table 2.-Continued. 

County City No. of No. of No. of No. of units No. of No. of No. of red- No. of redURMs historic residential within yellow- yellow- tagged tagged 

URMsl URMs residential tagged tagged residential residential 

URMs residential residential units buildings 

units buildings 

Solano County Benicia 39 18 0 0 0 0 0 0 

Solano County Dixon 12 0 0 0 0 0 0 0 

Solano County Fairfield 4 0 0 0 0 0 0 0 

Solano County Rio Vista 10 0 0 0 0 0 0 0 

Solano County Suisun City 24 0 0 0 0 0 0 0 

Solano County Vacaville 20 7 0 0 0 0 0 0 

Solano County Vallejo 64 8 6 56 2 2 0 0 

Solano County Unincorporated 2 0 0 0 0 0 0 0 

Sonoma County Cloverdale 18 2 0 0 0 0 0 0 

Sonoma County Cotati 0 0 0 0 0 0 0 0 

Sonoma County Healdsburg 18 0 2 5 0 0 0 0 

Sonoma County Petaluma 90 32 1 1 0 0 0 0 

Sonoma County Rohnert Park 0 0 0 0 0 0 0 0 

Sonoma County Santa Rosa 68 0 2 4 5 5 0 0 

Sonoma County Sebastopol 30 0 2 6 0 0 0 0 

Sonoma County Sonoma 51 29 3 19 0 0 0 0 

Sonoma County Windsor 0 0 0 0 0 0 0 0 

Sonoma County Unincorporated 174 16 7 2 8 0 0 0 0 

Monterey Carmel 22 0 0 0 0 0 0 0 

Monterey Del Rey Oaks 0 0 0 0 0 0 0 0 

Monterey Gonzales 3 1 0 0 1 1 0 0 

Monterey Greenfield 7 0 1 4 0 0 0 0 

Monterey King City 7 0 0 0 0 0 0 0 

Monterey Marina 0 0 0 0 0 0 0 0 

Monterey Monterey 60 0 6 30 0 0 0 0 

Monterey Pacific Grove 9 6 0 0 0 0 0 0 

Monterey Salinas 70 0 0 0 200 200 12 12 

Monterey Sand City 0 0 0 0 0 0 0 0 

Monterey Seaside 13 1 0 0 0 0 2 2 

Monterey Soledad 4 0 0 0 0 0 1 1 

Monterey Unincorporated 0 0 0 0 20 20 10 10 

San Benito Hollister 3 0 0 0 48 48 63 63 

San Benito San Juan Bautista 28 0 20 0 0 0 0 0 

San Benito Unincorporated 0 0 0 0 80 80 12 12 

Santa Cruz Santa Cruz 46 24 1 1 372 372 72 72 

Santa Cruz Capitola 1 0 0 0 0 0 6 6 

Santa Cruz Scotts Valley 0 0 0 0 160 160 11 11 

Santa Cruz Watsonville 72 1 0 0 200 200 275 275 

Santa Cruz Unincorporated 11 0 0 0 3,717 3,717 -2,619 2,619 

TOTALS 6,919 1,880 1,030 24,070 16,362 7,994 7,043 3,774 

I 

As defined by each individual city and county. At a minimum, these numbers include structures with state and 

national designations. 
C176 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 3.-Dwelling units red tagged due to the Lorna Prieta earthquake in the Sun Francisco Bay area 

TYPE MODIFIED MERCALLI INTENSITY 

v VI VII VIII IX x+ 

Mobile homes 0 0 94 7 0 0 

Unreinforced masonry 0 2 553 32 0 0 

Nonwood , 4-7 stories, <I 940 0 0 720 0 0 0 

Nonwood, 4-7 stories, > 1939 0 0 0 0 0 0 

Nonwood, 7+ stories, < 1940 0 0 403 0 0 0 

Nonwood, 74- stories, > 1939 0 0 0 0 0 0 

Wood frame, 4-7 stories, <1940, 0 68 456 29 1 0 0 

multifamily 

Wood frame, 4-7 Stories, > 1939, 0 0 22 0 0 0- 

multifamily 

Wood frame, 1-3 Stories, < 1940, 0 30 420 424 3 0 

multifamily 

Wood frame, 1-3 Stories, >1939, 0 18 27 14 0 0 

multifamily 

Wood frame, 1-3 Stories, < 1940, 3 33 74 38 76 77 

single family 

Wood frame, 1-3 Stories, >1939, 0 8 47 13 2 5 

single family 

"Other" (tents, caves, boats, etc.) 0 0 0 0 0 0 

consistency, add missing data (particularly number of units 

and age for multifamily housing), and improve accuracy. 

Minimal effort was made to field check and supplement data supplied by the City of Los Angeles because 

most of these data seemed to be remarkably accurate. The 

one problem with Los Angeles's data was the inconsistency in the labeling of number of stories. Based on a 

two-day field survey of residential multifamily buildings 

in the San Fernando Valley, ABAG staff estimated that 

approximately half of the units described as being in threestory structures were actually in three-story structures over 

a one-story parking garage (which, for purposes of this 

effort, should have been labeled as four-story structures). 

Rather than arbitrarily re-categorizing half of these structures as four-story buildings, we used available data on 

year built, number of units, construction type and square 

footage to assign the specific structures most likely to be 

four-story buildings to the four-story category. Approximately half of the total units in three-story buildings were 

reassigned to four-story buildings. Because we carefully 

documented the specific buildings involved, we will be 

able to field check these buildings as part of future datacollection efforts. In addition, a few obvious inconsistencies in the data were corrected (such as a 26-story 

wood-frame building with less than six units) that were 

probably data-entry errors by the city. Again, these 

changes were documented. 

This time-consuming data collection and field-checking process was not part of the original plan for the research. It had been anticipated that data on tagging would 

be readily available from the California Governor's Office of Emergency Services (OES) in Pasadena. However, 

in spite of the extensive data-collection effort conducted 

by the staff of this group, they were unable to release the 

data in a form that we could use due to restrictions placed 

on data release by some cities and counties, as well as by 

the management of OES. The data OES staff were able to 

release were aggregated by census tract and stripped of 
IMPACT OF THE EARTHQUAKE ON HABITABILITY OF HOUSING UNITS C177 

Table 4.-Dwelling units yellow tagged due to the Lorna Prieta earthquake in the Sun Francisco Bay area by construction type and intensity 

[Single-family units and mobile homes not rendered uninhabitable] 

TYPE MODIFIED MERCALLI INTENSITY - 

v VI VII VIII IX x+ 

Mobile homes 0 0 0 0 0 0 

Unreinforced masonry 0 127 1,180 42 0 0 

Nonwood , 4-7 stories, 4940 

Nonwood, 4-7 stories, >I939 

Nonwood, 7+ stories, 4940 

Nonwood, 7+ stories, >I939 

Wood frame, 4-7 stories, <l94O, 

multifamily 

Wood frame, 4-7 Stories, >1939, 

multifamily 

Wood frame, 1-3 Stories, <l94O, 

multifamily 

Wood frame, 1-3 Stories, >1939, 

multifamily 

Wood frame, 1-3 Stories, < 1940, 

single family 

Wood frame, 1-3 Stories, >1939, 

single family 

"Other" (tents, caves, boats, etc.) 

' Single family homes remain habitable if yellow tagged. Formal tagging does not occur with mobile homes, so a 

distinction between "red" and "yellow" has not been applied. 

the actual dwelling-unit count information essential to estimating displaced persons and peak shelter populations. 

OES staff was also unable to provide us with addresses so 

that we could collect the dwelling-unit data needed and 

add it to their databases. This extensive data-collection 

duplication effort could have been avoided if release of 

data to Federally-funded researchers was required as a 

condition of funding city and county, as well as state, 

emergency-response efforts. This data "security" issue 

was not present during our data collection efforts following the Loma Prieta earthquake. 

Table 5 shows ABAG data collected for housing impact analysis. It compares data collected by ABAG to two 

sources of data supplied by the OES Pasadena groupthe first was a FAX dated 2-7-95; the second was a 

report (EQE International and Office of Emergency Services, 1995) dated in April. Although the total numbers 

are within 15 percent of each other among the three 

sources, the data discrepancies for some individual jurisdictions are huge. We believe that the ABAG data are 

more accurate than the OES and EQEIOES data for several reasons. First, we collected the data later than OES, 

and therefore the cities and counties had a better understanding of the impact on their housing stock. In addition, 

the people collecting the data at ABAG had experience in 

collecting similar data after Loma Prieta, and therefore 

they knew the questions to ask and the types of people to 

talk to. Because ABAG knew the importance of collecting accurate data, a great deal of effort was made to ensure that all affected jurisdictions were contacted. Finally, 

to improve quality control and to ensure that data on housing units and construction type were as accurate as pos- 
C178 PERFORMANCE OF THE BUILT ENVIRONMENT 

sible given the time and money constraints, ABAG staff 

visited the impacted areas and checked samples of impacted buildings within the City of Los Angeles and 

checked all buildings with incomplete records in the remaining jurisdictions. If cities, counties, and the state cooperate in sharing these types of data following future 

earthquakes, more effort could be directed toward dataquality assurance. 

OVERALL RESIDENTIAL BUILDING 

STOCK DATA 

During 1994, ABAG staff developed an extensive inventory of existing residential buildings in the San Francisco Bay area. The work was conducted using the base 

date of April 1990 to make it consistent with the 1990 

census. This inventory groups buildings by numbers of 

dwelling units (not numbers of buildings) according to 

four sets of categories: material, number of stories (1-3, 

4-7, and 7+), age (by decade), and multifamily vs. single 

family. Material categories include: Wd (wood frame), 

MH (mobile home), LM (light metal), RM (reinforced 

masonry), UM (unreinforced masonry), SF (steel frame), 

RC (reinforced concrete, including ductile and nonductile 

frame), and CS (concrete or steel frame, unknown). These 

inventory data were created for each 1990 census tractcity map unit. 

The data on total number of dwelling units in 1990 

were obtained directly from the 1990 census. That census 

data were also used to obtain a count of mobile homes, as 

well as a breakdown of housing units by decade of construction and by single-family versus multifamily types. 

(See U.S. Bureau of the Census, 1991 and 1992.) 

The data on unreinforced-masonry residential buildings 

are based on actual inventory data collected by local governments and provided to ABAG during a telephone survey. Follow-up building surveys cleaned up the local data. 

For example, several jurisdictions did not collect data on 

type of use (i.e. residential vs. commercial) and even more 

did not obtain data on the number of residential dwelling 

units. 

Figure 1.-The Loma Prieta earthquake resulted in 13,000 uninhabitable Bay Area housing units, 

including 3,960 units in the 691 red-tagged residential buildings shown here. (From Perkins and 

others, 1996.) 
IMPACT OF THE EARTHQUAKE ON HABITABILITY OF HOUSING UNITS 

Table 5.-Northridge earthquake residential tagging data 

[Data from ABAG survey compared with OES FAX (2-7-95) and OES report (EQE and OES, April 1995)l 

Red-Tagged Residential 

ABAG OES EQEIOES 

(FAX) (Report) 

City Residential Single Multi-Family Total Units Mobile Total Residential Residential 

Buildings Family in Buildings Homes Residential Buildings Buildings 

Units 

Buildings Units 

Beverly Hills 18 12 6 130 142 0 142 22 2 1 

Burbank 12 8 4 47 55 0 55 30 3 

Calabasas 10 5 5 44 49 0 49 0 0 

Culver City 20 19 I 2 2 1 0 2 1 14 15 

Glendale 6 4 2 64 68 0 68 10 17 

Los Angeles 1,300 781 519 11,358 12,139 2,083 14,222 1,519 1,604 

Manhattan Beach 2 2 0 0 2 0 2 3 0 

Pasadena 1 0 1 12 12 0 12 4 5 

San Fernando 99 52 47 170 222 105 327 82 27 

Santa Clarita 80 64 16 184 248 1606 1854 60 83 

Santa Monica 27 12 15 30 1 31 3 0 31 3 60 67 

West Hollywood 3 0 3 13 13 0 13 3 4 

Unincorporated 46 44 2 7 5 1 193 244 0 28 

Fillmore 120 117 3 12 129 86 21 5 0 144 

Moorpark 1 1 0 0 1 3 4 0 2 

Simi Valley 13 9 4 22 3 1 51 9 550 0 25 

Thousand Oaks 53 47 6 18 65 0 65 0 55 

Unincorporated 14 14 0 0 14 0 14 0 14 

TOTALS 1,825 1,191 634 12,384 13,575 4,595 18,170 1,807 2.1 14 

Yellow-Tagged Residential 

ABAG OES EQEIOES 

(FAX) (Report) 

City Residential Single Multi-Family Total Units Mobile Total Residential Residential 

Buildings Family in Buildings Homes Residential Buildings Buildings 

Units 

Buildings Units 

Beverly Hills 84 68 16 95 163 0 163 80 53 

Burbank 31 24 7 80 1 04 0 1 04 7 1 7 

Calabasas 233 222 11 59 28 1 0 28 1 220 0 

Culver City 46 43 3 6 49 0 49 10 12 

Glendale 2 1 1 167 168 0 168 11 9 

Los Angeles 7,214 5,359 1,855 27,856 33,215 1,033 34,248 7,340 7,715 

Manhattan Beach 272 268 4 8 276 0 276 242 0 

Pasadena 15 2 13 179 181 0 181 13 13 

San Fernando 137 93 44 195 288 64 352 113 117 

Santa Clarita 188 163 25 226 389 0 389 150 184 

Santa Monica 91 28 63 890 91 8 0 91 8 21 2 239 

West Hollywood 1 1 0 0 1 0 1 5 8 

Unincorporated 201 159 42 639 798 15 81 3 0 85 

Fillmore 263 254 9 27 281 0 28 1 0 228 

Moorpark 59 59 0 0 59 0 59 0 22 

Simi Valley 203 171 32 96 267 50 31 7 0 299 

Thousand Oaks 213 205 8 29 234 0 226 0 121 

Unincorporated 37 36 1 3 39 0 39 0 47 

TOTALS 9,290 7,156 2,134 30,555 37,711 1162 38,873 8,467 9,159 
c 180 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 6.-Existing (1990) dwelling units by construction type in the Sun Francisco Bay area 

[Data for buildings over three stories that are not unreinforced masonry are estimated for San Francisco and Oakland] 

San San Santa Construction Type Alarneda Contra Marin Napa vÂ Mateo Solano Sonoma TOTAL 

Costa 

Mobile homes 

Unreinforced 

masonry 

Nonwood , 4-7 

stories, 4940 

Nonwood, 4-7 

stories, >I 939 

Nonwood, 7+ 

stories, < 1940 

Nonwood, 7+ 

stories, >I939 

Wood frame, 

4-7 stories, 

< 1940, 

multifamily 

Wood frame, 

4-7 Stories, 

> 1939, 

multifamily 

Wood frame, 

1-3 Stories, 

< 1940, 

multifamily 

Wood frame, 

1-3 Stories, 

> 1939, 

multifamily 

Wood frame, 1- 

3 Stories, 

~1940, single 

family 

Wood frame, 

1-3 Stories, 

>1939, single 

family 

"Other" 

(tents, caves, 

boats, etc.) 

TOTAL 5O5,3 16 3 16,145 191,635 44,195 328,848 25 1,782 54O,23 1 1 19,533 16 1,057 

20.6 % 12.8 % 7.8 % 1.8 % 13.4 % 10.2 % 22.0 % 4.9 % 6.5 % 2'458'742 

The location of tall (over three story) residential build- ing units. The most incomplete areas are the City of Oakings was collected by ABAG staff to the extent possible land and the City of San Francisco, where only some 

given time and budget constraints. The inventory includes buildings are identified by street address. 

information on location by street address, building con- The results of this process, aggregated to the county 

struction type and approximate age. and number of dwell- level, are provided in table 6. 
IMPACT OF THE EARTHQUAKE ON HABITABILITY OF HOUSING UNITS 

Table 7.-Composite percent of dwelling units red tagged by construction type and intensity 

TYPE MODIFIED MERCALLI INTENSITY 

v VI VII VIII IX x+ 

Mobile homes 0 0 0.87 40 90 100 

Unreinforced masonry 0 0.05 2.9 45 70 80 

Nonwood , 4-7 stories, <I940 0 0.30 8.0 45 70 80 

Nonwood, 4-7 stories, > 1 939 0 0 0 16 54 70 

Nonwood, 7+ stories, < 1940 0 0.30 8.0 45 70 80 

Nonwood, 7+ stories, > 1939 0 0 0 16 54 70 

Wood frame, 4-7 stories, 4940, 0 1.4 2.5 45 70 80 

multifamily 

Wood frame, 4-7 Stories, >1939, 0 0 0.09 10 15 25 

multifamily 

Wood frame, 1-3 Stories, <1940, 0 0.05 0.53 11 44 64 

multifamily 

Wood frame, 1-3 Stories, > 1939, 0 0.0 1 0.04 6.5 15 25 

multifamily 

Wood frame, 1-3 Stories, 4940, 0.01 0.04 0.12 1.8 8.4 12 

single family 

Wood frame, 1-3 Stories, >1939, 0 0 0.02 0.18 0.69 1.8 

single family 

"Other" (tents, caves, boats, etc.) 0 0 0 0 0 0 

Similar data were compiled for the area impacted by 

the Northridge earthquake in 1995 and early 1996. The 

data on tall (over three story) buildings are very incomplete. The information on these tall buildings is much 

more complete in the heavily impacted areas; we assumed 

that any residential building over three stories would have 

been inspected (and at least green tagged) in this area. 

Thus, we did not rely on the data for these less-common 

tall structures in the outlying less-impacted areas. 

CALCULATING VULNERABILITY BY 

CONSTRUCTION TYPE AND INTENSITY 

The purpose of collecting the data on uninhabitable units 

and total exposed housing stock for these two earthquakes 

was as data input for the analysis which ultimately produced the composite tables relating modified Mercalli intensity and building construction to percent of dwelling 

units made uninhabitable, as shown in tables 7 and 8 for 

red-tagged units and yellow-tagged multifamily units 

(Perkins and others, 1996). 

Data on modified Mercalli intensity for the Loma Prieta 

earthquake are from the intensity models developed, in 

part, using these same data (Perkins and Boatwright, 1995). 

Data on intensity exposures for the core impacted areas of 

the Northridge earthquake are based on tagging of post1940 single-family homes and extrapolating the data to 

other building types. (See J. Boatwright's research summarized in U.S. Geological Survey, 1996, p. 54-55, as 

well as Perkins and others, 1996.) 

Prior to ABAG's work on housing habitability, the only 

published matrix for relating these variables was developed by Dunne and Sonnenfeld (1991). The matrices developed for red-tagged and yellow-tagged units by Perkins 

and others (1996) have been modified from this earlier 

Dunne and Sonnenfeld (1991) matrix based on actual data 

from the Loma Prieta and Northridge earthquakes. Additional information on the analysis process is provided in 

Perkins and others (1996). The percentages are provided 

in tables 7 and 8 using two significant digits, with the 
C 182 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 8.-Percent of dwelling units yellow tagged by construction type and intensity 

[Single-family units are not rendered uninhabitable '1 

TYPE MODIFIED MERCALLI INTENSITY 

Mobile homes 

Unreinforced masonry 

Nonwood , 4-7 stories, -4940 

Nonwood, 4-7 stories, > 1939 

Nonwood, 7+ stories, <I940 

Nonwood, 7+ stories, > 1939 

Wood frame, 4-7 stories, <l94O, 

multifamily 

Wood frame, 4-7 Stories, > 1939, 

multifamily 

Wood frame, 1-3 Stories, <l94O, 

multifamily 

Wood frame, 1-3 Stories, > 1939, 

multifamily 

Wood frame, 1-3 Stories, <1940, 

single family 

Wood frame, 1-3 Stories, >1939, 

single family 

"Other" (tents, caves, boats, etc.) 

v VI VII VIII IX x+ 

0 0 0 0 0 0 

' Single-family homes remain habitable if yellow tagged. Formal tagging does not occur with mobile homes, so a 

distinction between "red" and "yellow" has not been applied. 

smallest value equal to 0.01 percent. These percentages 

are depicted graphically in figure 2. 

Note that the "0" values in these tables are actually 

greater than zero, but quite small. Although occasional 

dwellings are "tagged" in these categories, there are also 

large numbers of dwellings exposed to these relatively 

low levels of shaking. 

SAMPLE APPLICATION-ASSESSING 

IMPACTS AND POSSIBLE DAMAGE 

PATTERNS IN FUTURE EARTHQUAKES 

Given the percentages in tables 7 and 8, and estimates 

of modified Mercalli intensity levels in future earthquakes 

from Perkins and Boatwright (1995), ABAG produced estimates of uninhabitable housing units in future Bay Area 

earthquakes (Perkins and others, 1996). Small changes to 

these percentages have a significant impact on estimates 

of uninhabitable units. In particular, damage to wood-frame 

dwellings is an extremely significant component of the 

dwelling losses. The intensity maps used were based on 

the most recent version of ABAG's ground-shaking models (Perkins and Boatwright, 1995). These data were combined with the location of existing residential building 

stock (Perkins, 1994). A more complete description of 

this modeling process is contained in the full report for 

the research project (Perkins and others, 1996). The results of this modeling effort, by county, are shown in 

table 9. This table shows the models for the Loma Prieta 
IMPACT OF THE EARTHQUAKE ON HABITABILITY OF HOUSING UNITS 

earthquake, as well as the actual data from that earthquake, for comparison, along with the results for models 

of future expected Bay Area earthquakes. 

Another way of expressing the model results is in terms 

of construction type, rather than by county area. These 

numbers emphasize ways in which building retrofits can 

be targeted to reduce the number of uninhabitable dwelling units predicted. The results of the models for the Loma 

Prieta earthquake, as well as actual data from that earthquake, again along with model results of future expected 

Bay Area earthquakes, are provided in table 10. 

CONCLUSIONS 

This data-collection and analysis effort emphasizes the 

impact of both the Loma Prieta and Northridge earthquakes on nonengineered housing stock. Although singlefamily wood-frame homes are less susceptible to becoming 

uninhabitable than multifamily units, their predominance 

(over 60 percent of the existing building stock in the Bay 

Area) makes the impact significant (almost 1,200 in 

Northridge and 376 in Loma Prieta). For high shaking 

intensities, pre-1940 single-family homes are roughly 10 

onstruc n Type 

Figure 2.-Result of analysis and comparison of percentages of units rendered uninhabitable, by construction type, obtained by summing the percents in 

tables 7 and 8. 
C184 PERFORMANCE OF THE BUILT ENVIRONMENT 

Table 9.-Uninhabitable units in Bay Area counties for selected earthquake scenarios 

County data 

for Alameda Contra Marin Napa San San Santa Solano Sonoma TOTAL 

earthquake Costa Francisco Mateo Clara 

scenarios' 

Lorna Prieta, 

actual 

M = 6.9 

Lorna Prieta, 

modeled 

M = 6.9 

Hayward 

combined 

M = 7.3 

Southern 

Hayward 

M = 7.0 

Northern 

Hayward 

M=7.1 

Healdsburg 

Rodgers 

Creek 

M=7.1 

Maacarna 

M = 6.8 

Peninsula 

segment of 

San Andreas 

M=7.1 

San Gregorio 

M=7.1 

Northern 

Calaveras 

M = 6.9 

ConcordGreen 

Valley 

M=7.1 

Greenville 

M=7.1 

West Napa 

M = 6.7 

Expected moment magnitude provided based on length of fault segment. 

times more likely to become uninhabitable than post-1940 

homes. 

The results of this effort also point out the problems 

with mobile homes, which were responsible for almost 

4,600 of the 48,000 uninhabitable units from the Northridge 

earthquake. Because local city and county building departments have no jurisdiction over these units, 

preearthquake planning and retrofitting for this housing 

type are more difficult. 

Multifamily soft-story wood-frame housing units (that 

is, with tucked under or first-floor parking) were surprisingly vulnerable to becoming uninhabitable. They were 

responsible for 7,000 of the 16,000 housing units made 

uninhabitable by the Loma Prieta earthquake, and over 

11,000 of the 48,000 housing units made uninhabitable by 

the Northridge earthquake. 

Our efforts in using these data to predict the impact of 

future Bay Area earthquakes emphasize the need to collect data on the number of units in multifamily structures. 

Red Cross and other shelter providers require information 

on people displaced, not buildings impacted. The most 

practical means of collecting this information is to collect 

data on the number of housing units within the impacted 

buildings. 

Because of the relative scarcity of nonwood housing 

structures, it is extremely difficult to gather enough data 
IMPACT OF THE EARTHQUAKE ON HABITABILITY OF HOUSING UNITS c 185 

Table 10.-Uninhabitable units by residential construction type for selected earthquake scenarios 

Construction Mobile Unrein- Non- Non- Non- Non- Wood, Wood, Wood, Wood, Wood, Wood, 

tY Pe homes forced wood, wood, wood, wood, 4-7 4-7 1-3 1-3 1-3 1-3 

data for masonry 4-7 4-7 7+ 7+ stories, stories, stories, stories, stories, stories, 

earthquake stories, stories, stories, stories, < 1940 >I939 < 1940, > 1939, < 1940, >1939, 

scenarios' <I940 >I939 <I940 >I939 multi- multi- single single 

family family family family 

Loma Prieta, 

actual 

M = 6.9 

Loma Prieta, 

modeled 

M = 6.9 

Hayward 

combined 

M = 7.3 

Southern 

Hayward 

M = 7.0 

Northern 

Hayward 

M = 7.1 

Healdsburg 

Rodgers 

Creek 

M = 7.1 

Maacarna 

M = 6.8 

Peninsula 

segment of 

San Andreas 

M=7.1 

San Gregorio 

M=7.1 

Northern 

Calaveras 

M = 6.9 

ConcordGreen 

Valley 

M = 7.1 

Greenville 

M=7.1 

West Napa 

M = 6.7 

Expected moment magnitude provided based on length of fault segment. 

on these units for a pure statistical approach to estimate 

habitability. The matrices (tables 7 and 8) are least reliable for these types of structures. (See Perkins and others, 

1996, for a more complete description of this analysis 

effort.) 

Rigorous data-collection efforts in future earthquakes 

should continue to improve our knowledge of the functionality of our housing stock. These efforts should minimize duplication of data-collection efforts and maximize 

data sharing and quality control. 

ACKNOWLEDGMENTS 

We would like to thank the following individuals 

for their thoughtful comments on this paper: Linda Seekins 

and Paul Thenhaus, U.S. Geological Survey, 

and Edward Wyatt, ABAG. Additional comments 

were received on research which forms the basis for this 

work: Thelma Rubin, Councilmember, City of Albany 

(committee chair); Chris Arnold, Building Systems Development (BSD), Inc. ; Roger Borcherdt, U.S. Geological 
C186 PERFORMANCE OF THE BUILT ENVIRONMENT 

Survey; Laurence Kornfield, City and County of San Francisco; Frank McClure, Consulting Structural Engineer; 

Patrick McClellan, formerly with City of San Leandro; 

Gregg O'Ryon, American Red Cross, Bay Area; Paula 

Schulz, California Office of Emergency Services; Roy 

Schweyer, City of Oakland Office of Community Development; Martha Blair Tyler, Spangle and Associates; and 

Frannie Winslow, City of San Jose Office of Emergency 

Services. 

In addition, the analysis described in this paper would 

not have been possible without the support in data collection provided by Dan Friess and Edward Wyatt, ABAG, 

as well as the programming assistance of Paul Wilson of 

Mapframe Corp., and Fred Parkinson. 

The research which forms the basis of this paper was 

financed in part with National Science Foundation (NSF) 

Grants No. BCS-9302612 and CMS-941618 1. Any opinions, findings, conclusions, or recommendations expressed 

in this material are those of the authors and do not necessarily reflect the views of any Federal agency, including 

NSF. 

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