Integrated Instrumentation Plan for 

Assessing the Seismic Response of StructuresA Review of the Current USGS Program 

Integrated Instrumentation Plan for 

Assessing the Seismic Response of StructuresA Review of the Current USGS Program 

By Mehmet Celebi, Erdal Safak, A. Gerald Brady, 

Richard Maley, and Vahid Sotoudeh 

U.S. GEOLOGICAL SURVEY CIRCULAR 947 

7987 
Department of the Interior 

DONALD PAUL HODEL, Secretary 

U.S. Geological Survey 

Dallas L. Peck, Director 

Library of Congress Cataloging-in-Publication Data 

Main entry under title: 

Integrated instrumentation plan for assessing the seismic response of 

structures. 

(U.S. Geological Survey Circular 947) 

Bibliography 

Supt. of Docs. no.: I 19.4/2:947 

1. Structural dynamics. 2. Earthquake engineering Instruments. I. Celebi, 

Mehmet. II. Geological Survey (U.S.) III. Series. 

TA654.6.I56 1985 624.1'762 85-600306 

Free on application to the Books and Open-File Reports Section, 

U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225 
Page 

Introduction----------------------- 1 

Objectives-------------------------- 1 

Current programs for instrumentation of structures---------------- 2 

San Francisco Bay Area instrumentation program----------------- 2 

Steps in instrumenting a structure- 4 

Site visit---------------------- 5 

Ambient-vibration tests--------- 6 

Forced-vibration test----------- 6 

Dynamic analysis---------------- 6 

Installation of instruments----- 6 

Maintenance--------------------- 6 

Instrumentation for structural 

response studies-------------- 7 

Data processing----------------- 7 

Examples of structural instrumentation schemes----------------- 8 

Pacific Park Plaza, Emeryville- 8 

Transamerica Building, San 

Francisco--------------------- 8 

Great Western Savings Building, 

Berkeley--------------------- 10 

Computer analysis---------- 10 

Ambient-vibration test----- 10 

Instrumentation------------ 14 

Case studies of data from instrumented structures---------------- 14 

Imperial County Services 

Building--------------------- 14 

Meloland Road-Interstate 

Highway 8 overcrossing------- 15 

Future instrumentation needs------- 16 

Conclusions------------------------ 19 

References cited------------------- 19 

CONTENTS 

Page 

Appendix A--Measurement of 

structural motion---------------- 20 

Introduction------------------- 20 

Motion of a rigid body--------- 20 

Large rotations------------ 20 

Small rotations------------ 22 

Plane motion------------------- 23 

Large rotation------------- 23 

Small rotation------------- 24 

Flexible structures------------ 24 

Application to vibration of 

buildings-------------------- 25 

Rigid floors--------------- 25 

Flexible floors------------ 26 

Appendix B--Analysis of ambientvibration data------------------- 27 

Natural frequencies and 

damping---------------------- 27 

Modal amplitudes--------------- 27 

Phases------------------------- 27 

Example------------------------ 27 

Reference---------------------- 29 

Appendix C--Forced-vibration 

testing-------------------------- 29 

References cited--------------- 30 

Appendix D--Draft scheme for 

instrumenting the Bay Bridge----- 31 

Introduction------------------- 31 

Proposed scheme---------------- 31 

East suspension span------- 31 

Main truss span------------ 34 

Total instrumentation------ 34 

Appendix E--Instrumentation 

related to soil-structure 

interaction---------------------- 35 

References cited--------------- 36 

ILLUSTRATIONS 

Figure 1. 

2. 

3. 

4. 

5. 

Page 

Organizational flow chart of USGS instrumentation program-------------- 5 

Typical plan view and cross section used to determine instrumentation 

scheme, Pacific Park Plaza, Emeryville, California----------------- 8 

Mode shapes and frequencies from vibration testing, Pacific Park 

Plaza, Emeryville, California-------------------------------------- 9 

Instrumentation scheme, Pacific Park Plaza, Emeryville, California----- 10 

Typical elevation sections and floor plans showing locations and 

types of instrumentation, Transamerica Building, San Francisco, 

California--------------------------------------------------------- 11 

m 
Figure 6. 

7. 

a. 

9. 

10. 

11. 

12. 

13. 

Al-A6. 

A7-A8. 

Bl-B2. 

B3. 

Dl-02. 

El. 

E2. 

Page 

Mode shapes and frequencies from vibration, testing, Transamerica 

Building, San Francisco, California-------------------------------- 12 

Floor plan of Great Western Savings Building, Berkeley, California----- 12 

Transducer locations for ambient-vibration testing, Great Western 

Savings Building, Berkeley, California----------------------------- 13 

Photograph of Imperial County Services Building, El Centro, 

California, showing structural damage suffered during the 1979 

Imperial Valley earthquake----------------------------------------- 14 

Plan view showing deployment of FBA accelerometers and SMA-1 

accelerograph at Imperial County Services Building, El Centro, 

California, and adjacent free-field site--------------------------- 15 

Part of CRA-1 strong-motion accelerogram recorded on October 15, 1979, 

in Imperial County Services Building, El Centro, California-------- 16 

Plan view showing deployment of FBA accelerometers at Meloland 

Road-Interstate Highway 8 overcrossing----------------------------- 17 

Part of October 15, 1979, strong-motion accelerogram recorded at 

Meloland Road-Interstate Highway 8 overcrossing-------------------- 18 

Sketches showing: 

Al. Motion of a rigid body in space----------------------------------- 20 

A2. Rotational components of motion----------------------------------- 21 

A3. Measurements for motion in space---------------------------------- 22 

A4. Measurements for motion in plane---------------------------------- 23 

AS. Measurements for flexible structures------------------------------ 25 

A6. Measurements for structure flexible only in x-z and y-z planes---- 25 

Instrumentation plans for: 

A7. A rigid floor of a building--------------------------------------- 26 

AS. A floor rigid in y and ey directions, and flexible in 

remaining directions---------------------------------------------- 27 

Examples of: 

Bl. Acceleration versus time history of ambient-vibration data-------- 28 

B2. Autocovariance of function of unsealed acceleration time history-- 28 

Power spectral density plot of acceleration time history--------------- 29 

Diagrams showing: 

Dl. General layout of San Francisco-Oakland Bay Bridge---------------- 32 

D2. Elevations of San Francisco-Oakland Bay Bridge used to 

determine instrumentation scheme---------------------------------- 33 

Vertical cross section of an integrated instrumentation scheme and 

plan view of horizontal free-field array-------------------------- 37 

Qualitative description of instrumentation around boundary and 

below foundation of a building------------------------------------ 38 

TABLES 

Page 

Table 1. Preliminary list of structures for seismic instrumentation developed 

by the San Francisco Bay Region Instrumentation Advisory Committee--- 3 

2. Frequencies of vibrational modes, Great Western Savings Building, 

Berkeley, California-------------------------------------------------14 

Dl. Suggested instrumentation for suspension span between Yerba Buena 

Island and San Francisco---------------------------------------------34 

D2. Suggested instrumentation for cantilevered truss span between 

Yerba Buena Island and Oakland---------------------------------------35 

Any use of trade names and trademarks in this publication is for 

descriptive purposes only and does not constitute endorsement by the 

U.S. Geological Survey. 

IV 
Integrated Instrumentation Plan for Assessing 

the Seismic Response of StructuresA Review of the Current USGS Program 

By Mehmet Celebi, Erdal Safak, A. Gerald Brady, 

Richard Maley, and Vahid Sotoudeh 

INTRODUCTION 

There are two main approaches to evaluate 

seismic behavior and performance of structural 

systems. One requires a laboratory in which 

subsystems, components, or (if the facility is large 

enough) prototypes or large, scaled models of 

complete systems are tested under static, quasistatic, or dynamic loading. This approach does not 

necessarily demand a time-dependent testing 

scheme, such as a shaking table or hydraulically 

powered and electronically controlled loading 

systems; however, testing of structural systems 

under controlled simulated dynamic environments is 

desirable. Since the early 1950's such laboratory 

research has increased both in quantity and quality, 

with engineering colleges in the United States 

playing a key role. Laboratory testing has also 

contributed substantially to our understanding of 

dynamic soil properties and the interaction 

phenomenon between the soil and structure. 

The second approach to evaluate behavior and 

performance of structural systems is to use the 

natural laboratory of the Earth, by observing and 

studying damage to structures from earthquakes. 

By determining why specific designs lack 

earthquake resistance and then by using extensive 

laboratory testing of modified designs, significant 

progress in improved designs can be achieved. 

For such design studies a natural laboratory 

would be a seismically prone area that offers a 

variety of structural systems; in optimum test 

areas, strong ground motions as well as moderatelevel motions would be experienced frequently. 

Integral to the "natural labor a tory" approach is the 

advance instrumentation of selected structures so 

that their responses can be recorded during future 

earthquakes. Thus it is essential that integrated 

arrays of instrumentation be planned and installed 

to assess thoroughly the relation of ground motion 

that starts at a source and is transmitted through 

various soils to a substructure and finally to a 

1 

superstructure. The direction for seismologists and 

engineers working together is clear: to develop 

integrated networks which measure the seismic 

source, the transmittal of ground motion, and the 

structural response processes. 

This report is intended to consider issues that 

are related to instrumentation-strong-motion 

arrays, free-field instrumentation near structures, 

and structural instrumentation schemes. Ultimately 

the data obtained should reveal the complete 

phenomena of motion originating at a source and 

response of a structure at a particular site. 

OBJECTIVES 

The main objective of seismic instrumentation 

program for structural systems is to improve our 

understanding of the behavior and potential for 

damage of structures under the dynamic loads of 

earthquakes. As a result of this understanding, 

design and construction practices can be modified 

so that future earthquake damage is minimized. 

An instrumentation program should provide 

enough information to reconstruct the response of 

the structure in enough detail to compare with the 

response predicted by mathematical models, the 

goal being to improve the models. In addition, the 

data should make it possible to explain the reasons 

for any damage to the structure. The nearby freefield and ground-level time history should be known 

in order to quantify the interaction of soil and 

structure. More specifically, a well-instrumented 

structure for which a complete set of recordings has 

been obtained should provide useful information to 

(1) check the appropriateness of the dynamic model 

(both lumped-mass and finite-element) in the elastic 

range, (2) determine the importance of nonlinear 

behavior on the overall and local response of the 

structure, (3) follow the spreading nonlinear 

behavior throughout the structure as the response 

increases and determine the effect of this nonlinear 
behavior on frequency and damping, (4) correlate 

the damage with inelastic behavior, (5) determine 

ground-motion parameters th~t correlate well with 

building response damage, and (6) make 

recommendations eventually to improve seismic 

codes. 

Various codes in effect in the United States 

recommend different types and schemes of 

instrumentation depending upon its purpose. For 

example, the Uniform Building Code (UBC) of 1976 

recommended that, for seismic zones 3 and 4, a 

minimum of three accelerographs be placed in every 

building over six stories with an aggregate floor 

area of 60,000 square feet or more, and in every 

building over ten stories regardless of the floor 

area. The purpose of this requirement by the UBC 

was to monitor rather than to analyze. In 1976 the 

City of Los Angeles adopted the UBC's 

recommendation but in 1983 revised this 

requirement to only one accelerograph. 

The UBC-type instrumentation, because it is 

designed for monitoring, is not necessarily a useful 

first stage for the instrumentation being discussed. 

Experiences from past earthquakes show that the 

UBC minimum guidelines do not ensure sufficient 

data to perform meaningful model verifications. As 

an example, three horizontal accelerometers are 

required to define the horizontal motion of a floor 

(two translations and torsion). Rojahn and 

Matthiesen (1977) concluded that the predominant 

response of a high-rise building can be described by 

the participation of the first four modes of each of 

the three sets of modes (two translations and 

torsion); therefore, a minimum of 12 accelerometers would be necessary to record these modes. 

If vertical motion and rocking are expected to be 

significant and need to be recorded, at least three 

vertical accelerometers are also required at the 

basement level. Furthermore, high-precision record 

synchronization must be available within a structure 

if the response time histories are to be used 

together to reconstruct the overall behavior of the 

structure. Rojahn and Raggett (1981) provided 

some additional guidelines for the instrumentation 

of bridges, and instrumentation for earth dams has 

been addressed by Fedock (1982). 

Like the superstructure, the foundation 

system needs to be instrumented to study its 

response. This is easily accommodated along with 

the instrumentation scheme of the superstructure. 

Placing sensors at critical locations of the 

foundation to capture all its relevant motions will 

at a minimum facilitate study of its behavior. 

However, more information is required to 

interpret the motion of the foundation substructure 

relative to the ground on which it rests. Engineers 

use free-field motions as input motion at foundation 

level, or they obtain the motion at foundation level 

by convoluting the motion through assumed or 

determined layers of strata to base rock and 

deconvoluting the motion back to foundation level. 

To confirm these processes requires downhole 

2 

instrumentation near or directly beneath a 

structure. Downhole data are especially scarce, 

although a few such arrays have been developed 

outside of the United States. These downhole 

arrays will serve to yield data on (1) the 

characteristics of ground motion at bedrock at a 

defined distance from a source and (2) the 

amplification of seismic waves in layered strata. 

Selection of particular sites for downhole studies 

requires more extensive consideration; therefore, it 

will be dealt with in the section "Future 

Instrumentation Needs" and appendix E. 

CURRENT PROGRAMS FOR INSfRUMENTATION 

OF SfRUCTURF.S 

Programs for instrumentation of structures 

can be classified into three categories: 

1. Federal Programs. The U.S. Geological 

Survey (USGS) has its own nationwide 

instrumentation program. In addition, if 

requested, USGS will coordinate, install, 

maintain, and process the data acquired 

from strong-motion arrays and structures 

instrumented by various Federal agencies. 

2. State Programs. In California, the State 

Division of Mines and Geology (CDMG) has 

the responsibility to develop strong-motion 

arrays and to instrument typical structures 

within the state (Shakal, 1984). Other 

states (for example, Alaska) have similar 

programs. 

3. Private Institutions. Some private institutions 

such as International Business Machines 

(IBM) and University of Southern California 

(USC) have developed their own 

instrumentation programs. 

Through these programs, approximately 400 

structures are known to be instrumented (National 

Research Council, 1982). Although these networks, 

particularly the USGS, CDMG, and USC networks, 

were designed with full cooperation, maintenance of 

these instruments and data processing are done by 

each program separately; therefore, at present 

there is no national coordination of efforts 

(National Research Council, 1982). Similar 

concerns were aired during a national strong-motion 

instrumentation workshop (Iwan, 1981). 

SAN FRANCISCO BAY AREA 

INSfRUMENTATION PROGRAM 

The USGS, in addition to strong-motion 

arrays, began a pilot program in 1983 in the San 

Francisco Bay area through the Instrumentation 

Advisory Committee, composed of USGS engineers 
from the strong-motion program and practicing 

engineers in other public or private sectors. The 

objective of this pilot program was to develop a list 

of structures for seismic instrumentation and to 

rank them according to a rational set of parameters 

and criteria (Celebi and others, 1984). For the 

preliminary list of selected structures in table 1, 

two terms, which were added subsequently, were 

assigned values according to the following 

parameters: 

1. Structural parameters: Considering the 

2. 

construction material, structural system, 

geometry, discontinuity, and age, a 

weighting term of 3 or less was assigned to 

each structure, whether building, bridge, or 

dam. 

Site-related parameters: 

a. Severity-of-shaking factor was assigned 

to each structure on the basis of its 

closeness to one or more of the main 

faults within the boundaries of the San 

Francisco Bay area (San Andreas, 

TABLE 1.--Preliminary list of structures for seismic instrumentation developed by the 

San Francisco Bay Region Instrumentation Advisory Committee 

Buildings 

Berkeley 

*1. Great Western 

2. Wurster Hall (Univ. Calif.) 

3. Underfield Parking (Univ. Calif. 

Campbell 

4. Pruneyard Towers 

Emeryville 

5. Pacific Park Plaza 

Hayward 

*6. City Hall 

7. City Hall Parking 

Millbrae 

8. SFO (airport) Parking 

Mountain View 

9. Moffett Field Hangar 

Oakland 

1 o. 

11. 

Arena 

City Hall 

Palo Alto 

12. Hewlett Packard 

Richmond 

13. Bulk Mail Facility 

San Jose 

*14. Santa Clara County Office Bldg. 

*15. IBM Facility 

16. Water Control Plant 

San Francisco 

17. Bank of America Building 

18. 45 Fremont (Bechtel Bldg.) 

19. 1 Metro Plaza (Bechtel Bldg.) 

20. City Hall 

21. Embarcadero Center (No. 4) 

San Francisco (con.) 

22. Fairmont Hotel 

23. Hartford 

24. Levi Plaza 

25. Moscone Center 

26. St. Francis Hotel 

27. Shaklee 

*28. Standard Oil (575 Market St.) 

29. Sutter Street Garage 

*30. Transamerica 

*31. 101 California St. 

32. Cow Palace (Daly City) 

33. Student Union SF State Univ. 

Santa Clara 

34. Leavey Center (Univ. Santa 

Clara) 

35. 

36. 

*37. 

*38. 

39. 

40. 

41. 

42. 

*43. 

44. 

45. 

46. 

47. 

*48. 

49. 

50. 

51. 

52. 

53. 

54. 

Bridges 

Bay Bridge 

Carquinez 

Dum barton 

Golden Gate 

Hegenberger Overhead 

Hayward-San Mateo 

Richmond-San Rafael 

San Joaquin River 

Sierra Point Viaduct 

Junction Interstates 101 and 92 

Junction Interstates 280 and 92 

Crystal Springs Creek 

Tunnels 

BART (Trans bay Tube) 

Caldecott 

Dams 

San Pablo 

Upper San Leandro 

Calaveras 

Leroy Anderson 

Chabot 

Briones 

*Instrumented or in the process of being instrumented as of September 1985. 

3 
Hayward, and Calaveras faults). A map 

prepared for this purpose was used to 

obtain these factors (Borcherdt and 

others, 1975). 

b. Probability of a large earthquake (M = 6.5 

or 7) occurring on the fault(s) within the 

next 30 years was obtained. A study 

made for this purpose (Lindh, 1983) was 

used to obtain the probabilities. 

c. Expected value of strong shaking at the 

site was then determined as the product 

of a and b. This product was then 

multiplied by a factor to upgrade the 

maximum expected value of strongshaking factor to 3, which was the 

maximum value used for the structural 

parameter. Thus the structural 

parameter and site-related parameters 

are given equal weights. 

The final product of the developed 

methodology is provided in table 1, which presents 

different structures in different regions of the San 

Francisco Bay area according to their priority for 

instrumentation. Some of the structures have 

already been or are in the process of being instrumented. 

The activities associated with the USGS 

strong-motion instrumentation program are 

supervised by the Chief of the Branch of 

Engineering Seismology and Geology (ESG), a unit 

within the Office of Earthquakes, Volcanoes, and 

Engineering. Associated with the branch is the 

Instrumentation Advisory Committee (lAC). After 

the lAC formulates its recommendations, they are 

transmitted to the Branch Chief and to the 

engineering staff of ESG through technical and 

planning reports for implementation (fig. 1). The 

engineering staff in turn obtains instrumentation 

permits for selected structures, gathers information 

relative to the project including structural plans and 

design and model information, and directs structural 

evaluation and if necessary performs ambient 

response studies. This integrated set of data is then 

used as a basis for determining transducer locations 

that will adequately define the response of the 

structure during a strong earthquake. The actual 

selection of measurement points in the structure is 

carried out with the members of the strong-motion 

section installation team. Mter the sensor locations have been agreed upon by the engineering 

staff, the installation team, a representative of the 

owner of the structure, and an electrical contractor 

are called in to plan placement of the data cable. 

The installation team works with the contractor 

during this phase and subsequently calibrates and 

installs sensors and recording systems. A final step 

is a complete documentation of each transducer 

location and orientation, characteristics of total 

system response, and any peculiarities of the 

instrumentation or access to required sites. A 

4 

summary description of the completed system is 

then available to members of ESG and to other 

interested engineers and scientists. These steps are 

described in more detail in the following section. 

STEPS IN INSTRUMEN'ftNG A STRUCTURE 

Once it is decided to instrument a structure, a 

series of studies, deductions, and decisions follow. 

It is important to optimize the instrumentation 

schemes from the points of view of both cost and 

required data. Too few instruments do not yield 

sufficient data to record the performance of a 

structure during an earthquake. This has been the 

case for many buildings in the past. As mentioned 

above, even now the Uniform Building Code recommends only three accelerographs for buildings 

meeting certain qualifying characteristics, although 

the UBC does not intend that a detailed structural 

analysis be possible from its recommended 

instrumentation. Rojahn and Matthiesen (1977) and 

Hart and Rojahn (1979) have made studies related to 

instrumentation needs of a structure. A detailed 

study of how to determine the quantity and 

locations of sensors to obtain necessary data of 

structural performance during earthquakes is 

pre sen ted in appendix A, which is directly from 

Erdal Safak (written commun., 1985). 

In order to optimize the instrumentation it is 

important to study the expected dynamic behavior 

of the structure. The preliminary studies include 

the following steps: 

(1) study of available design and analysis 

information after permission for instrumenting is 

granted by the owner, 

(2) site visit, and 

(3) required analytical studies and tests. 

It is not automatic for an owner of a structure 

to grant permission for an outside agency to install 

instruments. Some owners willingly cooperate; 

others fear that there may be some hidden 

objectives that will cause them financial losses. In 

general, however, per mission is granted once the 

objectives of the program are explained fully. This 

in turn makes it possible to obtain structural 

information. Ideally, the following information, if 

available, will be required: relevant blueprints and 

design calculations, dynamic analysis (mode shapes 

and frequencies), forced-vibration test results, and 

ambient-vibration test results. 

Seldom is all this information available for 

any structure. For the structure to be 

instrumented, a minimum requirement is that 

ambient-vibration tests be performed to obtain 

mode shapes and frequencies. If forced-vibration 

tests have been performed to obtain the mode 

shapes and frequencies, ambient vibration tests are 

not normally needed. 
Site Visit 

A general scheme can be prepared after a 

study of the blueprints and other available 

information related to dynamic characteristics. 

However, the general scheme for locating 

instruments needs to be confirmed by a site visit. 

The structure may present various constraints that 

affect safe installation and reliable performance of 

the sensors. The site visit enables the technical 

personnel to make relevant changes in the prepared 

schemes. 

Branch Chief, 

Engineering Seismology 

and Geology 

.. 

Technical planning 

and reports 

Instrumentation 

Advisory Committee 

Chief, 

Strong Motion 

Instrumentation ---•1 Documentation 1•--- Structural 

engineering 

staff 

1•---------r---lnst-rume-nt ~•------- -1 

lnsta llation 

cabling, electrical work, 

and sensor selection 

1 

Maintenance 

installation 

permit 

l 

Instrumentation 

scheme, development, 

and 

sensor (type) 

selection 

Structural 

design study 

1 

Ambient -vibration 

studies 

FIGURE 1.-0rganizational flow chart of USGS instrumentation program. 

5 
Ambient-Vibration Tests 

Preliminary analytical studies usually give a 

fairly accurate view of the structure's expected 

dynamic behavior. Using this information, the next 

step is to per form an ambient-vibration test on the 

structure. The test is done fairly quickly using 

portable recorders at three to five locations that 

are expected (from analytical studies or other 

information) to have maximum amplitudes during 

lower vibrational modes. 

The ambient-vibration data are processed 

using the procedure given in appendix B. The 

structural parameters (natural frequencies, 

damping, and mode shapes) are calculated through a 

random-vibration approach. 

The importance of ambient-vibration analysis 

is that it gives the elastic properties of the 

structure. During a strong earthquake the structure 

may experience nonlinear behavior; thus, if the 

parameters of linear behavior are known 

beforehand, it is easier to analyze the nonlinear 

behavior. 

Forced-Vibration Test 

A forced-vibration test is more difficult than 

an ambient-vibration test. The required equipment 

(vibration generator with control consoles, weights, 

recorders, accelerometers, and cables) is heavier, 

and the test takes longer than the ambient-vibration 

test. Furthermore, state-of-the-art vibration 

generators do not necessarily have the capability to 

excite to resonance all significant modes of all 

structures. A general description and some of the 

many references on forced-vibration testing is 

provided in appendix C. 

Dynamic Analysis 

If a dynamic analysis was not prepared by the 

designers of a structure or the information is 

unavailable, then a simplified finite-element model 

is developed to obtain the elastic dynamic 

characteristics. This is performed with any one of 

the several tested computer programs available 

(such as SAP, STARDYNE, ANSYS, and STRUDL). 

Installation of Instruments 

After approximate sensor locations are 

chosen, a USGS technician and the owner's 

representative review each site to determine exact 

sensor locations satisfactory to both parties. In 

general, final criteria must consider long-term 

accessibility, potential interference with the 

occupant's space, placement of data cable runs, and 

aesthetic requirements of the owner. 

Next the USGS technician inspects the entire 

structural scheme with an electrical contractor who 

6 

will install the data cable and terminal boxes at 

each sensor site. Actual cabling by the contractor 

is monitored by the USGS and the owner's 

representative to be sure the cable is installed as 

desired and that all building code regulations are 

followed. 

The cable-termination box is prepared in the 

USGS shop and includes data and calibration 

circuits, batteries, battery chargers, radio circuit 

board or time-code generator, digital-event 

counter, and electronic test points. This box, 

normally mounted on the wall above the recorder, 

contains external connectors for all data channels 

so that signals may be amplified and used to record 

the ambient response of the structure at any time. 

The recorder location is selected on the basis of 

security, typically in a telephone or electrical 

switch room, and in some circumstances is enclosed 

with separate fencing in an open area. 

The instrumentation undergoes a preliminary 

calibration in the strong-motion laboratory and is 

then installed in the structure with appropriate test 

procedures including a static tilt sensitivity test for 

each component and determination of direction of 

motion for upward trace deflection on the record. 

These data are permanently stored on film as part 

of the system description. Other documentation 

includes precise sensor location, period and damping 

of each unit, location of cable runs, access 

information, and circuit diagrams. 

Maintenance 

It is essential to have periodic and consistent 

maintenance of instruments in order to have a 

successful program. Therefore, routine 

maintenance is conducted every 3 months initially 

and may be extended to every 12 months if 

circumstances and experience so allow. This 

maintenance includes the following: 

1. Remote calibration of period and damping. 

2. Inspection of battery terminals, load voltage, 

and charge rate (batteries are replaced every 

3 years). 

3. Check of lamp intensity and voltage and the 

trace positions on the recording film. 

4. Measurement of threshold of triggering system 

and length of recording cycle. 

5. Check of film take-up and supply (the film is 

replaced each year). 

6. Inspection for operation and synchronization of 

the WWVB radio receiver or time-code 

generator. 

As a final maintenance procedure, a 

calibration record is returned for developing and 

then examined for the desired characteristics. All 

inspection procedures are recorded in duplicate with 

one copy left at the recorder site and the second in 

the permanent station file at the laboratory. 
Instrumentation for Structural 

Response Studies 

Instrumentation intended for strong-motion 

structural-response studies can be broadly classified 

under two categories according to recording 

techniques: (1) analog recording on 7-in. and 70-mm 

film, recording 12 and 3 channels, respectively, of 

data, and (2) digital recording on magnetic tape 

cassette at 100 or 200 samples per second, 

recording data in multiples of three channels per 

unit. 

The use of digital recording systems in 

structures, begun by the USGS in 1975, has proved 

somewhat less than satisfactory, although new 

digital recorders are considerably improved. The 

development over many years of highly reliable, less 

costly analog recorders has led to their installation 

in most instrumented structures. Since this 

discussion refers only to the recording mode and not 

to the sensors or manner of data transmission, the 

collection of data in an analog mode is not 

necessarily a permanent feature in structural 

studies; if digital recorders achieve the same high 

reliability and competitive prices, they will be 

installed in new structures and could replace any 

outdated recorders presently in use. 

The analog recorder usually installed for 

structural-response studies is the Kinemetrics CRA1 capable of recording 12 or 13 channels of data on 

a single 7-in. film. In this system, incoming signals 

are directly transmitted to 150-Hz galvanometers, 

which in turn deflect a light beam across the 

moving film strip. The instrument, triggered by a 

vertical starter with a nominal threshold of 0.01 .[ 

between 1 and 10 Hz, has a total recording time of 

25 minutes. The recorder continues to operate for 

approximately 20 seconds (shop adjustable) after the 

last occurrence of vertical ground motion exceeding 

the triggering threshold, in order to record the 

earthquake fully. An internal clock impresses halfsecond time marks on the edge of the film. A 

second timing system, installed as an optional 

feature by the USGS, puts real time on the opposite 

edge of the film from an internal time-code 

generator or by using a WWVB radio code signal. In 

the latter case the instrument must operate for a 

minimum of 60 seconds in order to accommodate 

the entire WWVB code. Power is supplied by floatcharged batteries located in a nearby battery box. 

A rotary key switch provides for periodic testing of 

natural frequency and damping of the remotely 

located sensors. 

Sensors usually installed in the USGS program 

are 5Q-Hz, 0. 7 critically damped Kinemetrics forcebalance accelerometer models FBA-11 and FBA-13 

(respectively uniaxial and triaxial transducers). The 

accelerometers are bolted to the building frame or 

floor, and sensed data are transmitted to the 

central recording location by shielded cable. In 

some applications Terra Technology SA-102 servo 

7 

accelerometers with similar response characteristics have been used. 

Triaxial self -contained accelerographs are 

often installed in conjunction with the remotesensor system either to record free-field ground 

motions or to supplement the structural 

instrumentation when more than 12 chtJ.nnels are 

required. This instrument, generally a Kinemetrics 

SMA-1, records data optically onto 70-mm film 

from 25-Hz flexure-type accelerometers. The SMA1 has the same trigger system, recording capability, 

and real time options as described for the CRA-1. 

Some Teledyne RFT-250 accelerographs, with 

characteristics similar to the SMA-1, are used at 

free-field sites after extensive modification by the 

USGS. 

The approximate cost of instrumenting a 

typical building (defined as a 25-story structure) 

with a 12 channel CRA-1, a free field SMA-1, and 

all previously described options is approximately 

$40,000 excluding USGS salary costs. The use of 

digital recording would increase this figure by 

$14,000. 

Data Processing 

Records are processed according to the 

descriptions in the computer program AG RAM 

developed by Converse (1984). Briefly, the steps 

are as follows: 

1. The film records are digitized at a commercial 

firm on a trace-following, computer-controlled 

laser scanner. Unequal time spacing, at an 

average of 600 samples per second, is used. 

2. The separately digitized, 10-second frames are 

reassembled using specially inserted vertical 

lines; each vertical line is digitized twice, once 

in each adjacent frame. 

3. The uncorrected data are prepared by 

subtracting out the reference traces, using the 

time marks for the x-coordinates, and 

subtracting the average value. Plots of the 

uncorrected data are prepared. 

4. The data from both film and digital recordings 

are then passed through a correction algorithm 

that applies a high-frequency filter (50 Hz), 

instrument corrections, and decimation to 200 

samples per second. A low-frequency high-pass 

Butterworth filter removes all periods longer 

than a predetermined period from the data. 

This period is chosen after consideration of the 

strong-motion duration of the records, any 

distortion during pre-event memory on the 

digitals, displacements calculated at specific 

sites, and displacements of adjacent film and 

digital recordings at specific sites. Plots of the 

corrected acceleration, velocity, and 

displacements for the three components of 

each recording are prepared. 

5. Response spectra are calculated for periods up 

to the long-period limit. Linear plots of 

relative-velocity response spectra and the log-
log tripartite plots of pseudo-velocity response 

are prepared. 

6. Fourier amplitude spectra, calculated by fast 

Fourier transform, are presented on linear axes 

and log-log axes. 

EXAMPLES OF STRUCTURAL 

INSTRUMENTA'llON SCHEMES 

Pacific Park Plaza, Emeryville 

This 30-story reinforced concrete building 

(fig. 2) with a 5-ft mat foundation resting on 

friction piles was subjected to dynamic analysis and 

forced-vibration testing by means of a vibration 

generator (Stephen, written commun., 1984). The 

important vibrational characteristics (mode shapes 

and frequencies) are provided in figure 3 (Stephen, 

1984). 

From the study of the mode shapes, 

vibrational data from ground level, the thirteenth 

and twenty-first floors, and the roof were 

determined to be most useful. Therefore 

instruments placed on each of these levels (fig. 4) 

include two orthogonal sensors at the core as well 

as an additional sensor at the end of each wing at 

levels other than the ground level. By obtaining 

translational accelerations at the core and the 

wings, it will be possible to determine the 

translational modes of the core and differential 

translation or torsional vibrations of the wings. 

Although analytical models of this building indicate 

that the assumption of flexible foundations does not 

alter the vibrational characteristics of the building 

(Stephen, 1984), it is essential to confirm it by 

means of four vertical sensors (one at the core and 

one at each wing) at ground level. In addition, three 

orthogonal sensors are placed in the (nominal) free 

field at an adjacent ground-level site. Thus, a total 

of 24 sensors (FBA-11) linked to two central 

recorders (CR-1) provide a practical 

instrumentation scheme for this building. 

An ambient-vibration testing of this building 

will be performed by using the sensors permanently 

in place. 

Transamerica Building, San Francisco 

The pyramid-shaped Transamerica Building 

(fig. 5) has 48 floors with an additional 204 ft of 

tower (49th-60th floors) and is of steel-frame 

construction erected on a 9-ft-thick basemat 174ft 

square. 

Dynamic analysis and forced- and ambientvibration tests of the Transamerica Building have 

been performed by Stephen and others (1973); the 

results are summarized in figure 6. 

On the basemat of the building, an SMA-1 

(two horizontal and one vertical channel) is installed 

near the center core. Three vertical sensors are 

placed at three of the four corners of the basemat; 

an additional horizontal sensor is placed in one of 

these corners. 

8 

At the plaza level, fifth floor, and twentyfirst floor, three FBA-11's are installed horizontally 

(two orthogonally on each floor near the core and 

another horizontal sensor near the exterior on the 

same floor but parallel to one of the sensors near 

the core). Thus it will be possible to detect all 

significant translational and torsional motions. 

SOUTH WING 

r--

r< 

-

1- -

,- - - ,- - -

- -

K 

-=L r-.., I 

r - -<- - -Ll L_-· 

I fr-

-

-

- --

- -

- ---

I 

J.--.....__ 

B 

ELEV MA 

I -

--

--

PENTHOUSE 

CHINE ROOM 

~ 

30 

--~ 

___ 1.§._ 

__ _!Q_ 

FLOOR ___ 5 __ 

GA 

HEALTH CLUB 

MEZZANINE 

OUNO FLOOR 

FIGURE 2.-Typical plan view (A) and cross section 

(B) used to determine instrumentation scheme, 

Pacific Park Plaza, Emeryville, Calif. 
In addition, one SMA-1 is placed on the 

twenty-ninth floor and one on the forty-ninth floor 

(fig. 5). These sensors, along with others, will 

enable the detection of the effects of the changes 

in the building's cross section at the fifth, twentyfirst, and twenty-ninth floors. 

0.595 

30 

27 

24 

21 

18 

15 

11 

7 

4 

_J 

w 

> w 

_J 

>-

a: 

0 

ten 

(!) 

z 

i5 

_J 

5 0.565 CD 

30 

27 

24 

21 

18 

15 

11 

7 

4 

TRANSLATIONAL (EAST-WEST) MODE SHAPES 

2 3 4 

1.675 3.12 4.39 

TORSIONAL MODE SHAPES 

2 3 

1.690 3.16 

EXPLANATION 

Calculated-Rigid foundation 

Calculated-Flexible foundation 

• Measured 

4 

4.53 

5 MODE 

6.19 FREQUENCY (Hz) 

MODE 

FREQUENCY (HZ) 

FIGURE 3.-Mode shapes and frequencies from vibration testing, Pacific Park Plaza (from R.M. 

Stephen, written commun., 1984). 

9 
Great Westem Savings Building, 

Berkeley 

This is a 13-story office building located at 

2150 Shattuck Avenue, in downtown Berkeley. The 

primary load-supporting structural members, not 

apparent from the exterior of the building, are twin 

reinforced concrete cores, each 20 by 36 ft in plan 

and 88 ft apart (fig. 7). At the top of these cores, a 

steel-framed grid supports a total of 16 vertical 

hanging straps from which all floors above the third 

are suspended. Each floor is a horizontal structural 

steel frame with a steel-decked concrete slab. 

Floors 1 through 3 are supported on columns from 

ground level. A description of the vibrational 

characteristics and instrumentation for this building 

follows (from V. Sotoudeh and A. G. Brady, written 

commun., 1985). 

Computer Analysis 

A finite-element analytical model of the 

Great Western Savings Building was developed from 

the building plans and design calculations using the 

structural analysis program SAP IV. The model 

provided shapes and frequencies for many of the 

vibrational modes; four modes were chosen for 

further study and instrumental verification. These 

included the first two bending modes of the cores in 

their longitudinal or stiffer direction, the first 

bending mode of the cores in their transverse or 

softer direction, and the first torsional mode of the 

structure, which combines bending and torsion of 

the cores coupled with twisting (in plan) of the floor 

grids (table 2). 

Ambient-Vibration Test 

Wind-excited ambient-vibration testing was 

carried out to confirm the capabilities of strongmotion transducers in low-amplitude tests and to 

verify the results of the analytical study. 

Transducers were temporarily installed in the 

western core at the basement, fifth floor, and roof 

levels and at the southwest corner of the building at 

the fourth and thirteenth levels. These locations 

were chosen as a result of the analytical results, 

permitting a direct comparison with calculated 

mode shapes and frequencies. 

Five portable digital recorders, seven singlechannel force-balance accelerometers, and two 

triaxial accelerometers were used to record the 

building vibrations (fig. 8). After the internal 

clocks of all the recorders were synchronized, eight 

sets of 30-second recordings were obtained over a 

2-hour period. 

The four modes selected from computer 

analyses as warranting further study could be 

identified from this ambient data (table 2) by the 

10 

procedure described in appendix B. These modes do 

not necessarily have the four lowest frequencies of 

all possible vibration modes, but they are the most 

identifiable modes, given the wind environment, the 

recording locations, and the data-analysis method. 

In the following mode descriptions, note that the 

shear cores are considered the primary structural 

feature and the directions indicated by 

"longitudinal" and "transverse" refer to the core 

cross section, or plan. These descriptions are not 

the same as would be used in describing the total 

building plan. 

The mode with a frequency of 1.15 Hz 

corresponds to the bending of the shear core in its 

transverse direction. In this mode, the motion of 

the core is all in phase, and there is no twisting or 

in-plane deformation of the floors. The motions of 

the two shear cores are anticipated to be identical, 

that is, asymmetric with respect to the midspan 

plane of symmetry. 

SOUTH WING 

EXPLANATION 

Direction of acceleratiOn recorded 

1... Free-f1eld site 

SOUTH WING 

FIGURE 4.-Instrumentation scheme, Pacific Park 

Plaza. 
49th floor II t

11 FLOOR •t3' • 2" 

!101h FLOOR .-T&'- 4" 

- - - - --..-'" 'Loci! .-.o-:a··- - - - - - - - - - - -1 SMA -I 

29th floor 

2 I st floor --------~~------------ 8§~------ 3 FBA-11 

J:i::: 11 ~ FLOOII 25!>'-15" a= 

LOCATION OF 

INSTRUMENTATION 

NORTH ELEVATION 

2 4 6 8 10 

EAST ELEVATION 

ELEVATIONS OF MAIN EXTERIOR FRAMES 

12 14 

TYPE OF 

INSTRUMENTATION 

I T I I I I I i 1

1

1

1 

1

1

1

3

1 

-s 

--c 

• • • • -D 

• • • • -E • • • • --E 

• -F 

• • • • • • • --G • -G 

• -H 

• • • • -J • • • • --J 

• -K 

• • • • -L • • • • --L 

• • • • -M -----.---------. --N 

I I I I I I 

---- • • -P 3 5 7 9 I I 13 

BELOW 2ND FLOOR ABOVE 5TH FLOOR 

BUILDING FLOOR PLANS 

FIGURE 5.-Typical elevation sections and floor plans (from Stephen and others, 1973) 

showing locations and types of instrumentation, Transamerica Building, San Francisco. 

Lettered and numbered grid system identifies columns (squares). 

11 
FLOOR 

60 

57 

54 

42 

36 

30 

24 

18 

12 

NORTH-SOUTH EAST-WEST 

First mode: f = 0.345 Hz 

FLOOR FLOOR 

• 

NORTH-SOUTH EAST-WEST 

Second mode: f = 0.635 Hz 

EXPLANATION 

Calculated 

Forced-vibration test 

• Ambient-vibration test 

NORTH-SOUTH EAST-WEST 

Third mode: f = 1.17 Hz (N-S) 

f = 1.13 Hz (E-W) 

FIGURE 6.-Mode shapes and frequencies from vibration testing, Transamerica Building (from Stephen and 

others, 1973). Horizontal lines correspond to floor levels in figure 5. 

~B .. u1ldmg North north 

I 

~ _L __ j_l_-L ___ J ............ ~~ 

f I I I I [ I I I lb"o"o 1 i ,--r-----r- ---------r-------r--:--1------l- "='" oore l------1-~ ~ -h J 

~ ----+- l Core +----+-l-+--- I ,,_ __ 3 __ ~~~~ ~ Hanging straps ....£-___ ....,. 

longitudinal I 

direction I 

FIGURE 7.-Floor plan of Great Western Savings Building, Berkeley, Calif. Grid system identifies vertical hanging 

straps from which all floors above the third are suspended. 

12 
A 

/1 X ~ 4 

us re -

~ 

9 .-~r 

... :ryiEw~ ::::}~h· ., .•.. 

;--~\~~ ~ 12--------

:-. . :: <:_{ o;;:c·;·<·~;:~:::}.::::'':·,· .\-;. > 

FLOOR FEET B 

- 188 

R- 170 

13- 158.5 

l5 1---- 3_j 

12- 147 

11 - 135.5 

10- 124 

9- 112.5 

8 - 101 

7- 89.5 

6- 78 

5 - 66.5 

4- 55 

~ 10 

a_j 

3- 42 

2- 28 

1 - 10 

~ :::.' B-0 =:;/ 11 ------ 1 

EXPLANATION 

~;~::;_{\ .:·~· ~ ;::;:F:~~::\_:·.~· ,. :::.::::;•;:;.;:._,.,:· .. ·.:-; .. , .. _,.,, ..•.. 

...-- Direction of acceleration 

recorded 

e Acceleration in direction 

perpendicular to 

plane of figure 

~ r.f:_;( 

n ,::).'~{:•. 

FIGURE B.-Transducer locations for ambient-vibration test, Great Western Savings Building. Western core (A) and 

southwest corner (B) of building were selected. Channels 1, 6, and 11 are shear-core longitudinal transducers; 

channels 2, 7, and 12, shear-core transverse; 3, 4, 5, triaxial package at thirteenth level; and 8, 9, 10, triaxial 

package at fourth level. R, roof; B, basement; t, centerline. 

13 
TABLE 2.--Frequencies of vibrational modes, Great Western Savings 

Building, Berkeley, California 

Mode description Frequency (Hz) 

Analytical Ambient 

Bending of core in longitudinal direction 1. 57 

5.31 

1.04 

3.04 

1. 44 

5.60 

1. 15 

1.84 

Second mode bending of core, longitudinal direction 

Bending of core in transverse direction 

Twisting of structure 

The mode at 1.44 Hz corresponds to the shear 

core bending in its longitudinal direction with a 

rigid in-plane translation of the floors. Again the 

motion of the core is all in phase. 

The mode at a frequency of 1.84 Hz has been 

predominant at all locations. It consists of 

asymmetric ail-in-phase longitudinal bending and 

twisting of the shear cores accompanied by an inplane rigid twisting of the floors. In this mode, 

modal amplitudes of the core increase 

monotonically from the bottom to the top of the 

building. 

The mode with a frequency of 5.60 Hz 

corresponds to the second bending mode of the 

shear core in its longitudinal direction together with 

a rigid in-plane translation of the floors. The 

motion of the shear core changes phase between the 

fourth floor and the roof. There is no twisting of 

the floors in this mode. 

The frequencies listed in table 2 show good 

agreement between analytical and ambient figures 

for the translational modes; the comparison for the 

FIGURE 9.-lmperial County Services Building, El 

Centro, Calif., which suffered significant 

structural damage during the 1979 Imperial Valley 

earthquake. View north. 

14 

torsional mode is not good. The computer analysis 

reflects the difficulty of modeling the stiffness of 

this mode, in particular, the contributions to the 

stiffness at those points lying in the planes of 

symmetry and asymmetry. The discrepancy is 

attributed to approximations made in the analytical 

mode. 

Instrumentation 

As a result of ambient instrumentation tests, 

and the consequent understanding of at least the 

basic vibrational character of the structure, the 

permanent instrument locations were selected to be 

the same as the temporary ambient test locations. 

The floor instruments will be installed on the 

ceilings of the floors immediately below. The 

shear-core transducers will be installed in the 

stairwell of the core that does not contain the 

elevators. The installation is in progress at the date 

of this writing. 

CASE STUDIES OF DATA FROM 

INSTRUMENTED STRUCTURES 

Imperial County Services Building 

Of all the data recovered from buildings 

during those earthquakes providing damaging. 

structural motions, the most significant ones are 

those of the Imperial County Services Building (fig. 

9), which suffered significant structural damage 

during the October 15, 1979, Imperial Valley, Calif., 

earthquake (M = 6. 7) (Rojahn and Mork, 1981). This 

structure was instrumented by the California 

Division of Mines and Geology (CDMG) under their 

Strong Motion Instrumentation Program (CSMIP). 

Extensive studies (figs. 10 and 11) performed on the 

data from this reinforced concrete frame and shearwall building pointed out the following: 

1. Data obtained correlate with mathematical 

models that incorporate soil-structure 

interaction, not fixed-base assumptions. 

The maximum horizontal acceleration 

recorded at the nominal ground floor was 
approximately 60 percent higher than at 

the free-field site. 

2. The building's fundamental period of 

vibration increased significantly (of the 

order of 150 percent) during the 

earthquake, compared with the period 

determined from ambient-vibration tests. 

This in itself proved inelastic behavior. 

3. The timing of the initiation of the inelastic 

behavior and structural damage was 

estimated from the records. The damage 

pattern showed that soft first stories, as in 

this building, are vulnerable during earthquakes unless specific design 

considerations are made. 

The above conclusions show the benefits 

derived from data recorded from one wellinstrumented building during a strong-motion 

event. They also clearly show the need to obtain 

data related to soil-structure interaction for better 

confirmation of the effect of the foundation 

environment on the behavior of building systems. 

Plan views 

136'10" ·I 

lb T 

~ 1 C2J ~ ll'l 

I co 

l Roof iD 

a; 

8---

I 

Fourth floor 

\.Shear wall 

I 

I 

' f0 0L 

I 0 

N 

1 Second floor 

Melo1and Road-Interstate llighway 8 

Overcrossing 

A second set of structural response records 

during the 1979 Imperial Valley earthquake was 

obtained at the Meloland Road-Interstate Highway 8 

overcrossing (fig. 12), which was 0.8 km (0.5 mi) 

from the fault and 29 km (18 mi) from the epicenter 

(Rojahn and others, 1981). This structure is also a 

CDMG/CSMIP instrumented station. Acceleration 

records (fig. 13) were obtained from the sensors 

placed at the deck, base of midspan columns, 

embankment, and free-field sensors. Although the 

bridge did not sustain any significant structural 

damage, the following conclusions were derived 

from these records: 

1. Peak accelerations recorded on embankment 

sites adjacent to each abutment were as 

much as 30 percent higher than those 

recorded at the central support column. 

This showed that the motion of the 

embankment fill material influenced the 

East-west cross section Roof 

Sixth floor 

Fifth floor 

--- Fourth floor 

Third floor 

SMA-1 accelerograph 

Ground floor 

EXPLANATION FBA accelerometer-Arrow shows direction of acceleration 

recorded; dot, acceleration in direction perpendicular 

to plane of figure. Corresponding record-trace number 

shown in adjacent circle (see fig. 11) 

FIGURE 10.-Deployment of FBA accelerometers and SMA-1 accelerograph at Imperial County Services Building 

and adjacent free-field site (from Rojahn and Mork, 1981). 

15 
motion of the abutment. Also there was 

evidence of relative motion between the 

abutments and the surrounding fill 

material. 

2. The deck motions recorded show rotational 

as well as translational components. 

FUTURE INSTRUMENTATION NEEDS 

The data base of records from properly 

instrumented structures needs to be expanded in 

WWVB radio time code 

light of the benefits to the engineering 

community. Along with instrumentation of the 

superstructure, instrumentation for soil-structure 

interaction is necessary. 

The following steps are appropriate: 

1. Select other pilot areas within the United 

States from which to develop a list of 

structures to be instrumented. 

2. From these pilot areas select structures that 

are either instrumented to some degree or 

not yet instrumented, and that will be 

suitable for soil-structure interaction 

_ __. --- - - - ---- __ _. ____ - - --· . - - ~ - - - -. -- - - - - - - -- - Trace 

1 

2 

3 

4 

5 

12 

------.....·----------· . - . - - - - - --- . - - - - - - - - - - - - - - - - - - - - , 2 3 4--s--6--7--s--9-1a111213141s161718192o 

TIME, IN SECONDS 

FIGURE 11.-Part of CRA-1 strong-motion accelerogram recorded on October 15, 1979, in Imperial County 

Services Building (from Rojahn and Mork, 1981). Trace number at start of record (left) corresponds to 

accelerometer numbers in figure 10. Circles indicate maximum acceleration for a trace. 

16 
II 

II 

104'-0" 

208'-0" 

----Abu;-;e~,-·~-----.,===~-:::::-1~~--t....----===,; 

Embankment toe 

SECTION A-A' 

(I) 

c 

~ 

"0 

c 

::I 

0 

-e 

<ll 

(I) 

w 

EAST ELEVATION 

® 

20'-8" L 

GG 

PLAN 

II 

II 

--!~---

Abutment 3 

EXPLANATION 

FBA accelerometer-Arrow shows direction of acceleration recorded; dot, acceleration in direction 

perpendicular to plane of figure. Corresponding 

record-trace number shown in adjacent circle 

(see fig. 13) 

Note. Accelerometers 1, 2, and 4 mounted on footmg m 

ground vault; 10-12, 14-15, and 23-26 located m 

ground vault beneath ground surface; and 3, 5-9, 13, 

and 16-22 mounted on s1de of box g1rder supportmg 

roadway deck 

FIGURE 12.-Deployment of FBA accelerometers at Meloland Road-Interstate Highway 8 overcossing, which is 

29 km (18 mi) from epicenter of 1979 Imperial Valley earthquake (from Rojahn and others, 1981). 

17 
WWVB radio time code 

---. _ ....... ___ ~-.._.. ._, - .. __ --- ----- ~ - TlBasel Up_ _ • -~7 ~g • • .••. 

~-~·~~~~-------------------------------------

2 IBaseiW 

3 IDeckl W 

4 (Base) N 

11 ISEmbiW 

12 IS Embl N 

• "'~"'~ 1'\.../\r~ 

13(0eckiW 

028 9 

---<JV'\} 

141FrFidl U_:> • .J J.f~ 23 g O 

48 

g 

~·yu.,""..t~~ ... ~----,.......--~---------------

. . . . . . . . . . . . . - -- - --. ----- -- --- - ----- -- -. 0 

-------~-----~-----~---~--------------- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 

TIME. IN SECONDS 

FIGURE 13.-Part of October 15, 1979, strong-motion accelerogram 

recorded at Meloland Road-Interstate Highway 8 overcrossing. N 

(north), W (west), or Up (vertical) in trace identification denotes 

direction of positive acceleration. Circles indicate maximum 

acceleration for each trace. Accelerometer locations (1-26) are 

shown in figure 12 (from Rojahn and others, 1981). 

18 
studies. These ·studies would use 

horizontal and vertical spatially varying 

arrays that may include additional specific 

instruments for special purposes. 

3. Plan in advance for instrumentation of a 

future building to be erected at a suitable 

site. 

These sites should be near existing or planned 

ground-level arrays in seismically active regions so 

that data from an integrated scheme that includes 

the source, site soil-structure interaction, and 

superstructure will enable complete studies. 

At present, in addition to the San Francisco 

Bay area, other regions with strong shaking 

potential, such as San Bernardino County 

(California), Anchorage (Alaska), and Charleston 

(South Carolina) are being surveyed as separate 

pilot areas. Still other regions (New Madrid area, 

Missouri) are to be surveyed in the near future. A 

detailed description of instrumentation needs 

related to soil-structure interaction is provided in 

appendix E. 

CONCLUSIONS 

This report presents the current methods and 

status of instrumenting structures and. discusses the 

benefits derived from instrumenting a structure as 

well as the extent to which a structure should be 

instrumented. It also reviews some lessons derived 

from well-instrumented structures in earthquakes. 

Our main conclusions are two: 

1. It is better to instrument fewer structures 

with extensive instruments so that their 

performance can be fully recorded than to 

instrument many structures with minimum 

instruments. 

2. Soil-structure interaction has to be 

accounted for in the mathematical models of 

the structures. 

In line with these conclusions, a needed 

improvement is to provide borehole instrumentation 

near well-instrumented structures to obtain data on 

the vertical and horizontal spatial variation of 

earthquake motion along the immediate periphery 

of the buildings. The information derived certainly 

should answer questions related to soil-structure 

interaction, horizontal and spatial variation of 

earthquake motions, and building-to-building 

interaction where applicable. 

REFERENCES CITED 

Borcherdt, R.D., Gibbs, J.F., and Lajoie, K.R., 1975, 

Maximum earthquake intensity predicted for 

large earthquakes: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-709, scale 

1:125,000, 3 sheets. 

19 

Celebi, M., and others, 1984, Report on 

recommended list of structures for seismic 

instrumentation in the San Francisco Bay 

Region: U.S. Geological Survey Open-File 

Report 84-488. 

Converse, A.M., 1984, AGRAM: A series of 

computer programs for processing digitized 

strong-motion accelerograms: U.S. Geological 

Survey Open-File Report 84-525. 

Fedock, J.J., 1982, Strong-motion instrumentation 

of earth-dams: U.S. Geological Survey OpenFile Report 82-469. 

Hart, G., and Rojahn, C., 1979, A decision-theory 

methodology for the selection of buildings for 

strong-motion instrumentation: Earthquake 

Engineering and Structural Dynamics, v. 7, p. 

579-586. 

Iwan, W.D., ed., 1981, U.S. strong-motion 

earthquake instrumentation: U.S. National 

Workshop on Strong-Motion Earthquake 

Instrumentation: Santa Barbara, Calif., 

Proceedings, California Institute of 

Technology, 69 p. 

Lindh, A.G., 1983, Preliminary assessment of longtrem probabilities for large earthquakes along 

selected fault segments of the San Andreas 

fault system in California: U.S. Geological 

Survey Open-File Report 83-63. 

National Research Council, 1982, Earthquake 

engineering research-1982: Overview and 

recommendations: Washington, D.C., National 

Research Council. 

Rojahn, C., and Matthiesen, R.B., 1977, Earthquake 

response and instrumentation of buildings: 

Journal of the Technical Councils, American 

Society of Civil Engineers, v. 103, no. TCI, 

Proceedings Paper 13393, p. 1-12. 

Rojahn, C., and Mork, P.N., 1981, An analysis of 

strong-motion data from a severely damaged 

structure, the Imperial County Services 

Building, El Centro, California: U.S. 

Geological Survey Open-File Report 81-194. 

Rojahn, C., and Ragget, R.D., 1981, Guidelines for 

strong-motion instrumentation of highway 

bridges: Federal Highway Administration 

Report FHWA/RD-82/016. 

Rojahn, C., Ragsdale, S.T., Raggett, J.D., and 

Gates, J.H., 1981, Main-shock strong-motion 

records from the Meloland Road-Interstate 

Highway 8 overcrossing: U.S. Geological 

Survey Open-File Report 81-194. 

Shakal, A.F ., 1984, The California strong-motion 

instrumentation program; Status and goals: 

Earthquake Engineering Research Institute 

Seminar Proceedings, Pasadena, Calif. 

Stephen, R.M., Hollings, J.P., and Bouwkamp, J.G., 

1973, Dynamic behavior of multistory pyramidshaped building: Berkeley, Earthquake 

Engineering Research Center Report EERC 73-

17, 97 p. 

Uniform Building Code, 1976: Whittier, Calif., 

International Conference of Building Officials. 
APPENDIX A--MEASUREMENT OF STRUCTURAL MOTION 

Introduction 

Instrumentation of structures to measure their motion under various loading conditions 

is a widely used practice in engineering. The uncertainties in excitations and in 

structural parameters make the results of analytical models somewhat doubtful; the surest 

way of determining the exact errors involved is through measurements. Rapid developments in 

technology and computer science have resulted in more precise and easy-to-use instruments, 

and much faster and cheaper data processing, thus making instrumentation even more 

attractive for engineers. 

This section gives guidelines for the numbers and the locations of the instruments. 

Analytical expressions to describe the motion of the structure from the measured values are 

derived, both for three- and two-dimensional motions. Next, the application of the results 

to flexible structures, and to building vibrations, is presented. 

Motion of a Rigid Body 

Large Rotations 

Consider a rigid body R in a fixed Cartesian coordinate system as shown in figure A1. 

Let P be a point on the body with coordinates x, y, and z. Assume the following notations: 

Cartesian coordinates of point i. 

Displacements of point i in x, y, z 

directions, respectively. 

Rigid-body rotations with respect to 

x, y, z, respectively. 

Rigid-body translations in x, y, z 

directions, respectively. 

Let R1 denote the new configuration of the body after the motion. The new coordinates of 

point P now are P (x + Ux, y + U , z + U2 ). The new configuration R1 can be separated into 

its translational (UxO' UyO' 20 ~ and rotational (8 , 8 , 8 ) components as shown in figure 

A.1. Thus, the displacements Ux, UY, U2 of point Pxcanybe ~ritten as the sum of the 

X 

z R 

0 ~-------- y 

z' 

I 

I 

I 

I 

ezq) I 

I 

I 

/ 

,, 

\.. 

\'-\ ....... 

\ 

' 

R' 

I 

I 

I 

I 

I 

I 

I 

I / )_ - - - - - - - -{7 - - y' 

/o·<u u u > e Ox 0/ xo' yo' zo y 

x'/ 

FIGURE A1.--Motion of a rigid body in space. 

20 
contributions due to the rigid-body translations and the rigid-body rotations. These 

contributions will be shown by uxt' uyt' uzt due to translations and uxri' uyri' uzri due to 

rotation with respect to i (i = x,y,z). 

The contributions due to Uxo• UyO' Uzo• since they reflect a rigid-body translation, 

have the same effect at point P. Thus 

Uxt Uxo 

uyt = uyo 

Uzt = Uzo 

( 1 a) 

( 1 b) 

( 1 c) 

To evaluate contributions due to ex, note from figure A2A that 

cos a y/ OP and sin a = z/ OP ; thus 

X X 

0 

OP cos (a + e ) OP cos a y cos ex - z sin ex-y X X X 

uzrx OPx sin (a+ ex) OPx sin a z cos ex+ y sin ex- z 

Similarly, contributions due to ey' from figure A2~, are 

uxry X COS e + z sin e - X 

y y 

uyry 0 

uzry z cos e -x sin e - z y y 

(2a) 

(2b) 

(2c) 

(3a) 

(3b) 

(3c) 

and contributions due to ez, from figure A2f, are 

ux 

uy 

uz 

Uxrz x cos ez- y sin ez- x (4a) 

Uyrz Y COS ez + X Sin ez - y (4b) 

Uzrz 0 (4c) 

Adding these together, the displacement components of point P become 

uxo + X (cos e + cos e -2) - y sin e + z sin e y z z y 

uyo + X sin e + y (cos e + cos e -2) -z sin e z X z X 

uzo - X sin e + y sin e + z (cos e + cos e -2) y X X y 

(5a) 

(5b) 

(5c) 

It will be assumed that an instrument is positioned for each direction at point P. In 

equations 5, Ux, Uy, and Uz are the measured displacements and x, y, and z are the 

coordinates of the location of the instruments. The unknowns are rigid-body displacements 

Uxo• Uyo• and Uzo• and rigid-body rotations e , e , and e . The configuration of a rigid 

body in space can be described by three point~ th~t are n~t on a straight line. Thus, in 

order to determine six unknowns, six linearly independent equations need to be written in at 

least three locations. One way of obtaining these equations is to put six instruments at 

three locations in arbitrary directions as shown in figure A3!· The following conditions 

should be satisfied in order to have a nonsingular solution for the equations: 

z A B z c Q--r----r-1 ---- y 

P' -----~~X 

1-------r/---11_"71 p X 

/ 

I 

I 

/IJX 

~~-~~~---y IJX 

P' 

y ~; 

I ' I 

I 

I ', 

x----~~--L---~ 

IJY 

IJZ ', : 

X 

I 

I 

' ,, I 

_->f p 

z 

P' z 

FIGURE A2.--Rotational components of motion. !• ex. 

.£, ez. 

21 

~. e . y 
1. The three locations should not lie on a straight line. If they do, the rotations with 

respect to that line cannot be detected by the instruments. 

2. Directions of instruments should not be all parallel. Otherwise, the in-plane motions 

perpendicular to the direction cannot be measured. 

3. All directions should not intersect at one point, so that the rotations around a line 

through the intersection point, which is perpendicular to the plane formed by three 

instrumented points, can be detected. 

Records from each of these instruments would provide three equations in x, y, and z 

directions, thus giving us 18 equations for 6 unknowns. Note that these 18 equations are 

not linearly independent. There are only six independent equations. Any combination of six 

equations written at three or more locations can be used for a solution. Another scheme of 

instrumentation is shown in figure A3B. In this case there are only 6 equations and 

measurements, all in the principal directions. They also satisfy the conditions described 

above; however, these equations are nonlinear and require numerical methods for solution. 

Small Rotations 

Nonlinear equations of motion given in equations 5a, b, and c can be linearized if the 

rotations e , e , and e are small. This is usually the case for many engineering 

problems. xThetefore, zapproximating that sine= 0 and cos 8 = 1, equations 5 become 

ux uxo - y ez + z ey (6a) 

Uy = Uyo + X 8z - Z 8X (6b) 

u z = u - x e + y e ( 6c ) ZO y X 

Now it is possible to calculate the closed-form solution. Assume, as shown in figure A3~, 

that ux 1, uy 1, uz 1 were measured at point 1; uy 2 , uz 2 were measured at point 2; and uz 3 was 

measured at point 3. For each measurement we can write the corresponding equation from 6. 

Thus, we obtain six equations for six unknowns. With these equations, the rigid body 

translations and rotations can be calculated explicitly as given below: 

u xo 

u 

yo 

u zo 

e X 

e 

y 

e z 

C1 

y1Ux2 - y2Ux1 + (y2z1 - y1z2) · C2 

y1 - y2 

A 

X X 

FIGURE A3.--Measurements for motion in space. A, Arbitrary 

directions. ~' Cartesian coordinates. 

22 

(7a) 

(7b) 

( 7c) 

( 8c) 
where 

C1 

C2 

C3 

Uz1 (y3 - y2) + Uz2 (y1 - Y3) + Uz3 (y2 - Y1) 

x1 (y2- Y3) + x2 (y3- Y1) + x3 (y1- Y2) 

uz1 (x3 - x2) + uz2 ex, - x3) + uz3 (x2 - x,) 

Plane Motion 

Large Rotation 

(9a) 

(9b) 

( 9c) 

Sometimes the motion in one direction is zero or very small in comparison to the other 

two orthogonal directions because of the constraints on the structure or because of the 

loading and structural parameters. In this case, the motion takes place in a plane rather 

than in space. This simplifies the equations given above significantly. 

Assume that the motion is only in the x-y plane. Making Uzo = 8 = 8 

= 0 in equations 5, the equations for a plane motion can be obtained ~s y 

u X 

u 

y 

u z 

U + x (cos 8 - 1)- y sin 8 xO z z 

UyO +X sin 8z + y (cos 8z- 1) 

0 

( 1 Oa) 

(1 Ob) 

( 1 Oc) 

These equations have only three unknowns, UxO' UyO' and 8 • The configuration of plane 

motion of a rigid body can be described by two points. Tftus, we need three measurements 

from at least two locations in the plane of the motion to determine the unknowns. These 

measurements can be in three arbitrary directions as shown in figure A4A or they can be in 

orthogonal directions as shown in figure A4B. The directions of instruments should satisfy 

the following conditions: they should not be parallel, and they should not intersect at one 

point. 

Equations 10 can be written for each direction of each point. Any combination of three 

equations is sufficient to solve the unknowns; however, the equations are lengthy. A simple 

solution can be obtained if four measurements instead of three are made. For instance, in 

figure A4B, if we measure the displacements at point 1 and point 2 in both directions, using 

equations-10, we can write 

ux1 

ux2 

uy, 

uy2 

Uxo + x, (cos 8 1) - y1 sin 8 z z 

Uxo 

uyo 

uyo 

+ x2 (cos 8 1) - y2 sin 8 z z 

+ x1 sin 8 + y1 z (cos 8 z 1) 

+ x2 sin 8 + y 2 (cos 8 1) z z 

A B 

y 

FIGURE A4.--Measurements for motion in plane. !' Arbitrary 

directions. ~' Cartesian coordinates. 

23 

(11 a) 

( 11 b) 

( 11 c) 

(11 d) 
Note that there are only three linearly independent equations in 11. The fourth one is used 

for simplicity. Subtracting 11a from 11b and 11c from 11d, then eliminating (cos e - 1) 

in the resulting equations, one can obtain z 

e z 

. -1 (x2- x1) (Uy2- uy1)- (y2- y1) (Ux2- Ux1) J 

s1n [ 2 2 (x2- x1) + (y2- y1) 

( 12a) 

Once e is obtained, Uxo and Uyo can easily be calculated from equations 11a (or 11b) and 

11 c (o~ 11 d). Thus 

ux1 - x1 (cos ez - 1) + y1 sin ez 

uy1 - x1 sin ez - y 1 (cos ez - 1) 

Small Rotation 

(12b) 

( 12c) 

When the rotation ez is small, the equations 10 can be linearized as given below: 

( 13a) 

(13b) 

We again need three equations in order to find three unknowns, UxO' UyO' and ez. Using the 

directions given in figure A4~, for example, one can obtain 

0xo ux1 

y1 <uy2 - uy1) + x

2 - x1 

( 14a) 

uyo 

x2 uy1 - x1 uy2 

X - 2 x1 ( 1 4b) 

uy2 - u y1 e z X - 2 x1 ( 14c) 

Flexible Structures 

When the structure is not rigid, the instrumentation scheme given above is not 

sufficient to describe the motion. Flexible structures are basically structures with 

infinite degrees of freedom. Thus, the more instruments that are used, the better the 

response approximation. One practical way of instrumenting flexible structures is suggested 

below. 

Assume that the flexible structures can be divided into imaginary small segments which 

are approximately rigid, as shown in figure A5A; that is, model the flexible structure as a 

sum of finite rigid segments. Then, instrument each segment by following the guidelines 

given earlier for rigid structures. In order to provide compatibility and to minimize the 

number of instruments, place all the instruments on the common boundaries of the segments as 

shown in figure A5A, so that one measurement can be used for as many segments as possible. 

Next, for each segment, calculate the rigid-body translations and rotations by using the 

equations derived earlier with respect to a coordinate system, which is located at the 

center of the segment and which is parallel to the global axis system. Therefore, the 

calculated UxO' U 0 , Uzo displacements and e , e , e rotations for each segment are the 

discrete values ot the overall motion at that pa~tic~lar point. Now, consider the structure 

as an ensemble of beam elements connecting the centers of the segments, as shown in figure 

A5B. Since we already know the values for the six degrees of freedom at the ends of the 

beams, we can calculate the values for any intermediate point through simple 

interpolation. A better but more expensive way of doing this would be to make a finiteelement model of the structure after the measurements of each segment are obtained. Then, 

24 
by using the measured values as the input, the displacements and rotations can be obtained 

at every location. 

Some structures might be flexible in certain directions but otherwise rigid. In this 

case all the additional instruments are put only in flexible directions. As an example, 

suppose that the structure given in figure A5A is rigid in the x-y plane, but flexible in 

the x-z and y-z planes. This means the motions in the x-y plane (UxO' UYO' and 8 ) are the 

same for all segments and can be determined by three measurements in that plane; z 

U20 , e , and e are different for each segment and require segmental instrumentation. The 

sketchxof a po~sible instrumentation for that case is given in figure A6. 

Application to Vibration of Buildings 

Vibrations of buildings under dynamic loads, such as earthquakes, winds, or blast loads 

can best be studied through measurements of the actual motion. These measurements not only 

allow engineers to determine the dynamic parameters of the building and of the excitation 

but also help them to estimate the complex interaction between the building vibration and 

the excitation. 

Rigid Floors 

For most practical cases, it can be assumed that buildings vibrate horizontally, and 

that the floors are rigid in their own planes. This means each floor has three degrees of 

X 

FIGURE A5.--Measurements for flexible structures. A, Placement 

of instruments assuming rigid segments. ~' Beam model 

connecting midpoints of segments in A. 

z 

X 

/ 

/ 

/ 

/ 

L/ // 

---- ~~~~---_T?~--- ---JL--- -,~~,-~~----------~----- / / 

/ / 

/ / 

/ / 

/ / 

FIGURE A6.--Measurements for structure flexible only in x-z and 

y-z planes. 

25 
freedom; two translations in the horizontal plane and a rotation with respect to the 

vertical axis. Therefore, we need three instruments at each floor that satisfy the 

conditions given above for "Plane Motion." A typical instrumentation plan for a floor is 

given in figure A7A. If the vertical vibrations are also important, which might be the case 

for earthquake excitations, we need six instruments, because of the six degrees of freedom 

at each floor. These instruments should satisfy the conditions given above under "Motion of 

a Rigid Body." Figure A7B shows a typical instrumentation plan for this case. 

It is recommended that these instruments be put close to the perimeter of the building, 

since the perimeter undergoes larger amplitude vibrations than the center. The center of 

the coordinate system can be arbitrarily placed as long as the locations of the instruments 

are known with respect to the center. However, if one chooses the center of rigidity as the 

coordinate center, the stiffness matrix of the analytical model becomes uncoupled, which 

simplifies the analytical calculations that might follow the measurements. It should be 

kept in mind that in order to be able to interpret the measured values for overall motion, 

the coordinate centers of each floor should be on a vertical straight line, even though the 

centers of rigidity might not be. 

Ideally, every floor would be instrumented with the minimum required number of sensors, 

but this is impractical for most buildings. Thus, the question is: given a certain number 

of instruments, where are the best locations to place them? For buildings, a general rule 

is that it is much better to have complete sets of instruments at a few floors than to have 

incomplete sets at many floors. The complete set is six instruments for three-directional 

motion and three instruments for horizontal motion only. 

It is recommended that for placing the instrument sets in buildings the order of 

priorities given below be followed: 

1. For base excitations, such as earthquakes, the first set should be put at foundation 

level. The foundation level is not top priority for wind-excited motion. 

2. The next set should be at top, since all modes have non-zero amplitudes here. 

3. Subsequent sets should be put at structural discontinuity points, such as stories where 

there is a sudden change in story height, story mass, story stiffness, or geometry. 

4. The remaining sets should be put at levels where the individual mode shapes are 

expected to reach their maxima. 

Flexible Floors 

Some buildings may not have rigid floors; this is usually the case if one in-plane 

dimension is much larger than the other one. In this case, the floors are divided into 

approximately rigid segments, as outlined above. Instruments are placed on the common 

boundaries of the segments, and the measurements for each segment are evaluated as described 

earlier. The locations of the instruments are selected as those for the rigid floors. As 

an example, an instrumentation plan is given in figure A8 for a floor that is assumed rigid 

for UY and ey directions and flexible for the remaining directions. 

A B 

1 

X 

FIGURE A7.--Typical instrumentation plan for a rigid floor of a 

building. A, Horizontal motion only. ~, Horizontal and 

vertical motion. 
X 

FIGURE A8.--Instrumentation plan for a floor rigid in y 

and e directions, and flexible in remaining directions. 

y 

APPENDIX B--ANALYSIS OF AMBIENT-VIBRATION DATA 

The method of analysis presented here for ambient-vibration data is based on the stochastic response of a multiple-degree-of-freedom system to white-noise excitation. The major 

assumption of the method is that the forcing function specified at discrete times is a zeromean, independently and identically distributed, Gaussian white-noise process. The study of 

numerous ambient-vibration data validates this assumption. Detailed information on the 

justification of the assumptions and the theory and limitations of the methodology are 

provided by Sotoudeh (1986). Here, only the outlines of the steps in calculating structural 

parameters are given. 

Natura~ Frequencies and Damping 

The natural frequencies and damping are calculated from the location of the poles of the 

Z transform of the unit impulse response functions, H(z), on a complex plane. First, H(z) is 

calculated as a sum of the ratios of the Z transforms of modal responses to the forcing 

function. The difference equation corresponding to H(z) can then be determined. Using the 

above assumptions for the forcing function, an estimate of the coefficients of the difference 

equation can be made through multiple linear regression analysis. Once the coefficients are 

known, the poles of H(z) can be calculated by finding the roots of the characteristic 

equation. The location of the poles in the complex plane gives the necessary information for 

determining the natural frequencies and damping. 

Modal Amplitudes 

Modal amplitudes are estimated from the autocovariance function of the response. For a 

lightly damped system the modal responses can be assumed to be statistically independent. 

This allows the system autocovariance function to be written as a linear combination of modal 

autocovariance functions. The coefficients of the modal autocovariance functions are the 

squares of the corresponding modal amplitudes. Using the Gaussian white noise excitation 

assumption, a matrix equation for the modal amplitudes is derived. Again, using linear 

multiple regression an estimate of the modal amplitudes can be obtained from this equation. 

Phases 

Estimation of phases can be made using the cross-covariance functions of the response. 

The cross covariances are calculated between two responses of which one is due to the actual 

excitation and the other is due to a time delayed excitation. The estimation of phases is, as 

before, done through multiple linear regression analysis. 

Example 

An acceleration time-history sample of ambient-vibration data is shown for the site of 

the Great Western Savings Building (fig. 81) along with the autocovariance function of the 

unsealed acceleration time history (fig. 82). From the autocovariance function, the power 

27 
4b 

30 

(/) 

1-

z 20 :;) 

>-

a: 

< a: 10 1-

iii 

a: 

<{ 

~ 0 

z 

! 0 

i= 

<{ 10 a: 

w 

...J 

w 

u 20 u 

<{ 

30 

40 0 100 200 300 400 

TIME, IN SECONDS 

FIGURE B1.--Acceleration versus time history of ambientvibration data (units of acceleration not adjusted in 

accordance with recorder dynamic range, amplifier gain, and 

sensor sensitivity). 

200 

z 100 

0 

i= 

u 

z 

:;) 

LL 

w 

u 

~ 0 

a: 

~ 0 

u 

0 

1-

::> 

<{ 100 

200 oL-------------~s--------------1~o------------~15------------~2o 

TIME LAG, IN SECONDS 

FIGURE B2.--Autocovariance of function of unsealed acceleration 

time history. 

28 
spectral density plot of the same data is obtained by fast Fourier transform (fig. B3). From 

this figure, qualitatively the frequencies of interest can be determined. 

Reference 

Sotoudeh, V., 1986, Measurement and analysis of structural vibrations: Stanford, California, 

Stanford University, Ph.D. dissertation. 

~ (j) 

4 

~ 3 

c 

..J 

< a: 

~ 

u 

w 

a.. 

en 

a: 

w 

~ 0 2 a.. 

(!) 

0 

..J 

0 2 4 6 8 10 

FREQUENCY, IN HERTZ 

FIGURE B3.--Power spectral density of acceleration time history 

(obtained through fast Fourier transform of autocovariance 

function). 

APPENDIX C--FORCED-VIBRATION TESTING 

Forced-vibration testing of many types of structural systems has been performed by 

utilizing a vibration generator in conjunction with accelerometers and data-acquisition 

units. The vibration generators essentially provide horizontal unidirectional sinusoidal 

motions with controlled frequency and amplitude. This section presents a very general 

description of the method and related references. 

Early work on forced vibration of structural systems using a vibration generator dates 

from 1935 and is credited to Blume (1935). Hudson (1962) further developed the synchronized 

vibration generator. Other early work on vibration generators was discussed by Hudson (1970) 

and Nielsen (1964). 

Some unique and novel structural systems have been evaluated by means of forced vibration 

using vibration generators. It should be emphasized that the majority of the testing 

performed was nondestructive. Structures have included a pyramid-shaped building (Stephen and 

others, 1973), a triangular building (Petrovski and others, 1976), a high-rise diagonally 

braced building (Rea and others, 1971), school buildings (Rea and others, 1968), a pedestal 

base multi-story building (Stephen and others, 1978), a tall steel-frame building (Jennings 

and Matthiesen, 1972), a prefabricated apartment building (Bouwkamp and Stephen, 1980), and an 

intake tower of a dam (Keightley and others, 1961). Jurokovski (1980) tested box-type 

29 
structural systems, and several low-rise structures have been tested by Celebi and others 

(1977a, b). A summary of tests on box-type structures and comparative evaluation was 

presented by Celebi and Erdik (1982). Using a vibration generator for forced-vibration 

testing of structures is therefore an established method of nondestructive testing of 

structural systems. 

Vibration generators employ counterrotating eccentric weights. The main generator unit 

is anchored to the system to be tested. A control unit is used to adjust the frequency of the 

counterrotating masses, which are placed in a basket at a defined eccentricity. The 

counterrotating masses generate sinusoidal unidirectional force since the lateral components 

cancel out. The magnitude of the force generated is equal to 

F = MRw2 sin wt 

where M is eccentric mass in the baskets of the vibration generator, R is radial distance of 

eccentricity (in inches or centimeters), w is circular frequency (rad/s) at which the 

eccentric masses are rotated, and t is time (seconds). 

The maximum force generated therefore equals 

Fmax = MRw2 = (2 n) 2 ~ R f 2 

g 

0.102 (WR) f 2 (lbs) 

0.0472 (WR) f 2 (kg) 

where W is weight used in the baskets of the vibration generators, g is acceleration of 

gravity, and f is frequency (cycles/s). 

Generally, the currently available vibration generators are rated for maximum 5,000 lbs 

(22.4 kN) force. It then remains to record responses of the building at different 

frequencies. The plot of response against the frequency reveals those frequencies that cause 

resonance of the structure, which are therefore the natural frequencies of the structure. 

References Cited 

Blume, J.A., 1935, A machine for setting structures and ground into forced vibration: 

Seismological Society of America Bulletin, v. 25, no. 4 p. 

Bouwkamp, S.G., and Stephen, R.M., 1980, Dynamic properties of prefabricated apartment 

buildings, in, Structural aspects, Part I: World Conference on Earthquake Engineering, 

7th, Istanbul, Turkey, Proceedings, v. 4, p. 233-240. 

Celebi, M., and Erdik, M., 1982, Consideration of vibration test data in earthquake structural 

engineering, in International Earthquake Microzonation Conference, 3d, Seattle, Wash., 

Proceedings: Volume II, p. 875-882. 

Celebi, M., Erdik, M., and Yuzugullu, 0., 1977a, Forced vibration testing of a ten-story 

reinforced concrete building: Earthquake Engineering Research Institute of Middle East 

Technical University (METU-EERI) Report 77-01. 

_____ 1977b, Forced vibration testing of a reinforced concrete building without infill walls: 

Earthquake Engineering Research Institute of Middle East Technical University (METU-EERI) 

Report 77-03. 

Hudson, D.E., 1962, Synchronized vibration generators for dynamic tests of full-scale 

structures: Pasadena, California Institute of Technology, Earthquake Engineering 

Research Laboratory (EERL) Report. 

Hudson, D.E., 1970, Dynamic tests of full-scale structures, in Wiegel, R., ed., Earthquake 

Engineering: Englewood Cliffs, N.J., Prentice-Hall, chapter 7. 

Jennings, P.C., Matthiesen, R.B., and Hoerner, J.B., 1972, Forced vibration of a tall steelframe building: International Journal of Earthquake Engineering and Structural Dynamics, 

v. 1, p. 107-132. 

Jurokowski, D., 1980, Full scale forced vibration studies: Research Conference of Earthquake 

Engineering, Skopje, Yugoslavia. 

Keightley, W.O., Housner, G.W., and Hudson, D.E., 1961, Vibration tests of the Encino Dam 

intake tower: Pasadena, California Institute of Technology, Earthquake Engineering 

Research Laboratory (EERL) Report. 

30 
Nielsen, N. N., 1964, Dynamic response of multi-story buildings: Pasadena, California 

Institute of Technology, Earthquake Engineering Research Laboratory Report. 

Petrovski, J., Stephen, R.M., Gartenbaum, E., and Bouwkamp, J.G., 1976, Dynamic behavior of a 

multistory triangular-shaped building: Berkeley, Earthquake Engineering Research Center, 

Report EERC 76-3, 126 p. 

Rea, D., Shah, A.A., and Bouwkamp, J.G., 1971, Dynamic behavior of a high-rise diagonally 

braced steel building: Berkeley, Earthquake Engineering Research Center Report, EERC 71-

5, 101 P• 

Rea, D., Bouwkamp, J.G., and Clough, R.W., 1968, Dynamic properties of Mckinley School 

buildings: Berkeley, Earthquake Engineering Research Center EERC 68-4, 134 P• 

Stephen, R.M., Hollings, J.P., and Bouwkamp, J.G., 1973, Dynamic behavior of a multistory 

pyramid shaped building, U.C. Berkeley, Earthquake Engineering Research Center Report 

EERC 73-17, 97 p. 

Stephen, R.M., Wilson, E.L., Bouwkamp, J.G., and Button, M., 1978, Dynamic behavior of a 

pedestal-base multistory building: Berkeley, Earthquake Engineering Research Center, 

Report UCB/EERC 78-13, 145 p. 

APPENDIX D--DRAFT SCHEME FOR INSTRUMENTING THE BAY BRIDGE 

Introduction 

An overall layout and elevation of the San Francisco-Oakland Bay Bridge is provided in 

figure 01. The Bay Bridge has two typical and identical suspension bridge spans (west span 

and east span) between San Francisco and Yerba Buena Island (fig. D2A); on the other side, 

between Yerba Buena Island and Oakland, there is a 1,400-ft main truss-bridge span (the 

cantilevered section) extending to Yerba Buena Island on the west and connected to five 

smaller (approximately 500 ft) truss-bridge spans on the east (fig. D2B). 

In instrumenting structures of large span like the Bay Bridge, it-would ideally be of 

interest to develop a scheme to cover all spans. However, the return of response data from 

such heavy instrumentation may not necessarily justify the expenditure. This appendix 

provides a preliminary study; a detailed analysis and testing project is not complete, therefore the described scheme does not follow the usual routine of analysis and testing. 

Proposed Scheme 

Regarding the longitudinal spacing of the instrumentation packages, it makes sense to 

instrument only the east suspension span and the longest truss span, located immediately west 

and east, respectively, of Yerba Buena Island. The central recorders could be located on Yerba 

Buena Island and be activated by the first of·two triggers, located at the two extremities of 

the instrumented sections, or by one trigger only. This would also optimize the cable 

layout. The only disadvantage is that the access·to data will be available only at Yerba 

Buena Island; however, that should not constitute a major problem. 

East Suspension Span 

Given the fact that for the suspension bridge (east span) the two towers, the Yerba Buena 

anchorage, and the center anchorage (fig. D2C) clearly have to be instrumented, one·of the 

remaining longitudinal locations of instruments is the center of the main span. Other 

locations include both third points of the main span, although if a choice is made to 

instrument only one third point, then it is preferable to instrument the one closer to the 

center anchorage as a larger response is expected there (see fig. D2C). 

Each of these longitudinal locations will have a minimum of three sensors. The two 

towers and center anchorage will have sufficient sensors to obtain data on the-response of 

caissons, the deck level, and the top of the towers. Two horizontal and a vertical sensor will 

be required at the caisson and deck level. Additional sensors will be needed to record 

torsion of the double deck cross section about a longitudinal axis as well as torsion of the 

tower structure about a vertical axis. The main-span center locations will have sensors to 

monitor horizontal, vertical, and torsional motion of the deck. 

31 
w 

~ 

1>--------------------- 43,163 feet or 8.18 miles from the east line of Fofth Street to the center line of Market Street-----------------------------------ol 

~EAST SUSPENSION SPAN-<\ !--MAIN TRUSS SPAN--! 

rii71'123IO'III60'll"60'12310'1"60'l Err'400'~~-·@291'll3@292'--m--488.ToToi/Pim~ 

I I I I I I l?l;lil il I' I! I! I I IiI i 1111

1

1il "" 1'1 ~·- 1

1

1 I 4iJi ::12: ;;1?9 71=: 4?: <==:- ~~ ! I I b :a I I • I I • I 6 d I I a I 

TREASURE 

ISLAND 

ELEVATION 

E3 E4 E5 E6 E7 

East Bay Crossing 

San Francisco Bay 

/ 0 2500 FEET 

PLAN VIEW 

OAKLAND 

FIGURE 01.--General layout of San Francisco-Oakland Bay Bridge. Modified from plan by Department of Public Works, State 

of California. 
w 

w 

A 

:;; i E ::::t:::= ;;T;fNTERANCHOR======ASTSPAN:::J?z?:~ND 

ANCHORAGE I 

-~ 

==-====~ 2310' 1160' a ~ 1160' 2310' -----~ ~ANCHORAGE 14------------- EAST SUSPENSION SPAN--------~ 

0 400 800 FEET 

8 ~==== FEET 

+500 

~ 

~ ~ 

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io-508'-6" 1400' 512'--+-510'-6"--+- 509'-+-- 509'--+-509'---oj 

-300 

0 400 800 FEET I I I I I I I I 

it----- MAIN TRUSS SPAN -----oi 

z z :0 w <{w (!) 

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j 

I l I 

I 

I 

I 

I 

~ 

1160' --------------~--------------------------- 2310' 1160' 

0 400 890 FEET 

FIGURE 02.--Elevations of San Francisco-Oakland Bay Bridge used to determine instrumentation scheme. Modified from plan 

by Department of Public Works, State of California. A, General dimensions of two suspension spans of West Bay 

Crossing. ~, Cantilevered main truss span. £, Longitudinal location of instruments on east suspension plan. 
Main Truss Span 

Caisson and deck-level instrumentation of the truss span will be as in the suspension 

span. Abutment instruments will be located near the support on Yerba Buena Island. At 

midspan and third point of the truss span, horizontal and vertical sensors, as well as sensors 

to measure torsion of the double about the longitudinal axis motion, will be placed. 

Total Instrumentation 

Tables D1 and D2 show the tentative required instrumentation for suspension and truss 

spans, respectively. A total of approximately 41 instruments for the suspension bridge, and 

30 for the cantilevered section, will be required. This means six sets of CR-1 's with 12 

sensors to each set. 

Additional instrumentation could be placed within some of the caissons or additional 

spans of the truss part of the bridge between Yerba Buena Island and toll plaza depending on 

the resources. 

Ambient-Vibration tests could be performed before (using temporary instrumentation) or 

after permanent instrumentation is in place. 

TABLE Dl.--Suggested instrumentation for suspension span between Yerba 

Buena Island and San Francisco 

Location 

Center anchorage 

At base of caisson 

At deck level 

Yerba Buena anchorage 

Pier 5 

At caisson base 

At deck level 

At tower top 

Pier 6 (same as Pier 5) 

2 

1 

3 

2 

1 

2 

1 

3 

4 

2 

Instruments required 

horizontal (1 long., 

vertical 

horizontal (1 long., 

horizontal (1 long., 

vertical 

horizontal (1 long., 

vertical 

horizontal (1 long., 

horizontal (2 long., 

vertical 

Center of main suspension span 

1 transverse) 

2 transverse) 

1 transverse) 

1 transverse) 

2 transverse) 

2 transverse) 

1 horizontal (transverse only) 

2 vertical 

Third point of main suspension span 

TOTAL 

Additional instrumentation: 

1 horizontal (transverse only) 

2 vertical 

Additional third point of main suspension span 

Midpoint of side span between Piers 4 and 5 

Midpoint of side span between Pier 6 and Yerba 

Buena anchorage 

TOTAL 

34 

Total 

3 

3 

3 

3 

3 

6 

12 

3 

3 

41 

(3) 

(3) 

(3) 

(50) 
TABLE D2.--Suggested instrumentation for cantilevered truss span 

between Yerba Buena Island and Oakland 

Location Instruments required 

Pier El 

At base 

At deck level 

Pier E2 

At base of caissons 

At deck level 

At top of truss 

Pier E3 (same as Pier E2) 

Pier E4 

At base of caisson 

At deck 

Midspan of truss 

Third point of truss 

TOTAL 

2 horizontal 

1 vertical 

3 horizontal 

2 horizontal 

1 vertical 

2 horizontal 

1 horizontal 

2 horizontal 

1 vertical 

3 horizontal 

2 vertical 

1 horizontal 

2 vertical 

1 horizontal 

(Additional third point of truss) 

TOTAL 

(1 long., 1 transverse) 

(2 transverse, 1 long.) 

(1 long., 1 transverse) 

(both transverse) 

(transverse) 

(1 long., 1 transverse) 

(2 transverse, 1 long.) 

(transverse) 

( transverse) 

Total 

3 

3 

3 

2 

1 

6 

3 

3 

3 

3 

30 

(3) 

(33) 

APPENDIX E--INSTRUMENTATION RELATED TO SOIL-STRUCTURE INTERACTION 

It is well established that the response of a structure can best be estimated by including 

the influence of the surrounding soil around the structure and the characteristics of the input 

ground motion. If it is convenient and economical, engineers make the assumption that for rock 

and very stiff soils, the motion of the foundation of a structure is the same as the motion in 

the free field or nearby ground level. However, for softer soils, significant coupling takes 

place between soil and foundation. In analytical models this coupling is often represented by 

foundation springs which alter the frequency characteristics and the dynamic response of the 

overall structure. The foundation soil spring is frequency-dependent, and possibly straindependent. 

In a soil-structure interaction analysis, definition of the free-field ground motion, 

knowledge of the depth to bedrock, characteristics of the underlying soil strata, and the 

particulars of the foundation are required. It is therefore essential to follow this path in 

any integrated instrumentation scheme of a structure. 

Of engineering importance today is the confirmation and revision of the various numerical 

and closed-form computational techniques by the use of data yet to be measured, during the next 

large earthquake, through the use of integrated instrumentation schemes. Most of the current 

methodologies are based on the general assumption of parallel layered strata and a linearly 

effective shear modulus. Generally a free-field or ground level motion is deconvoluted to the 

bedrock to determine the motion there and convoluted back to the lowest foundation level to 

estimate the motion for input to the structure. This is performed assuming that either a 

previous earthquake record is applicable as the ground-level motion or a synthetic free-field 

motion is obtained by kinematic models that use previous data or analytical techniques. This 

35 
experiment will provide measurements of free-field, bedrock, and foundation motions at a single 

site to confirm or show inadequacies in present analyses. 

The importance of developing horizontal and vertical spatial arrays to determine and 

confirm the change in characteristics of the media and the motion in underlying strata of a 

foundation has been repeatedly emphasized (Lee and others, 1978; Lysmer, 1978; and Clough, 

1983). Researchers have recommended installation of arrays that will provide data on horizontal 

and vertical spatial variation of motion in underlying soil strata around foundations (Johnson, 

1981). Instrumentation related to soil-structure interaction was a main recommendation of a 

recent U.S. National Workshop on Strong Motion Instrumentation (Iwan, 1981). Yet, to date, 

there is no such array in the United States. The only relevant arrays are the USGS Imperial 

Valley liquefaction array, installed to look into liquefaction problems in an area of high 

seismicity (Bennett and others, 1984), the USGS array near Menlo Park (Joyner and others, 1976), 

and some arrays in Japan (Omote and others, 1980, 1984). Ideally, the following studies will be 

required to answer some of the questions that have been raised: 

1. Study by instrumentation with boreholes the effect on ground level motion of varying depths 

of different underlying soil layers under near-source and far-source conditions. Sites 

should be found where there are distinct layers of the following foundation soils: 

granular or alluvial 

silty clay 

soft clay 

hard soil (stiff soil) 

rock 

The sites should be near a fault with high probability of occurrence of earthquakes. 

Preferably, there should be a structure within 5-10 km of the fault (near-source studies) 

and another within 50 km of the fault. 

2. Study by subsurface instrumentation with boreholes the horizontal and vertical spatial 

variation of seismic waves for varying underlying soil strata. This scheme should include 

(1) instrumentation on a vertical array directly under a foundation; (2) instrumentation 

around the perimeter of a foundation to look into separation of the foundation from soil; 

and (3) instrumentation on a horizontal array near a building to capture the horizontal 

variation of free-field motion. Preferably the last one can be done in two orthogonal 

directions to study the idealization of three-dimensional motion into two-dimensional 

motion. 

3. Study by instrumentation the effect of different types of foundation. Possible foundation 

types to be looked into include deeply embedded mat foundation, mat foundation, mat or 

footing foundations with piles, and footings and spread footings. 

4. Study dynamic properties of soil layers within the above instrumentation schemes. 

5. Study by instrumentation the influence of strain softening of soil by liquefaction or 

weakening of sensitive clay on structural response. One area where this phenomenon has 

occurred is Anchorage, Alaska. 

6. Development of specific instrumentation if necessary and instrumentation schemes to detect 

the motion of layered strata, as well as separation between foundation and soil. 

A general schematic of the "integrated" array systems is shown in figures E1 and E2. 

References Cited 

Bennett, M.J., McLaughlin, P.V., Sarmiento, J.S., and Youd, T.L., 1984, Geotechnical 

investigation of liquefaction sites, Imperial Valley, California: U.S. Geological Survey 

Open-File Report 84-252. 

Clough, G.W., editor, 1983, A report of the Workshop on Research Needs in Experimental Soil 

Engineering: Blacksburg, Virginia Polytechnic Institute. 

Iwan, W.D., editor, 1981, U.S. strong-motion earthquake instrumentation:, U.S. National Workshop 

on Strong-Motion Earthquake Instrumentation, April 1981, Santa Barbara, California, 

Proceedings, California Institute of Technology, 69 p. 

36 
Johnson, J.J., 1981, Soil structure interaction: The status of current analysis methods and 

research: Washington, D.C., Nuclear Regulatory Commission, Report NUREG-CR-1780. 

Joyner, W.B., Warrick, R.E., and Oliver, A.A., 1976, Analysis of seismograms from a downhole 

array in sediments near San Francisco Bay: Seismological Society of America Bulletin, v. 

66, no. 3, p. 957-958. 

Lee, K.L., and others, 1978, Workshop on Research needs and priorities for Geotechnical 

Earthquake Engineering Applications: Arlington, University of Texas. 

Lysmer, J., 1978, Analytical procedures in soil dynamics: Geotechnical Engineering Specialty 

Conference on Earthquake Engineering and Soil Dynamics, Pasadena, Calif., Report PB-298455. 

Omote, S., Ohmatsuzawa, K., and Ohta, T., 1980, Recently developed strong motion earthquake 

instruments array in Japan, in Geoscience aspects, Part II: World Conference on Earthquake 

Engineering, 7th, Istanbul, Turkey Proceedings, v. 2, p. 41-48. 

Omote, S., and others, 1984, Observation of earthquake strong-motion with deep boreholes--An 

introductory note for Iwaki and Tomioka observation station in Japan, in Ground motion and 

seismicity: World Conference on Earthquake Engineering, 8th, San Francisco, Calif., 

Proceedings. v. II, p. 247-254. 

~~~~~~~~~~0-is_~_n_c_e_t_o_e_P_~_e_n_~_r~~~~~~~~~~ BUILDINGINSTRUMEN~TION 

Instrumentation nth floor 

A 

Strong-motion 

around perimeter r-r..-../-... (translation, torsion) 

of foundation 

Horizontal free- \ 

field arrays 

Mid-floors 

(translation, torsion) 

Foundation 

dense arrays ) 

_~5!._0 .2.0_50~------~~ 

~-cWnf.c 0 0 0 I 

0 

I 

(translation, torsion, rocking) 

Ground level 

1::<: /~~ Source of 

earthquake 

* 

B 

Strata thickness 

and sensors 

for vertical 

spatial variation 

Horizontal 

array 

Horizontal array 

,,_,-_ __...,......_..._ _____ \ 

0 0 0 0 

I 

0 

I 

I 

0 

~ ~ Planof 

foundation 

Height 

FIGURE E1 .--Qualitative description of an integrated instrumentation scheme. A' Vertical 

cross section. ~' Plan view of horizontal free-field array. 

37 
Free field 

• 

Strata thickness ---- ----- ----- and sensors 

for vertical 

spatial variation __ _ 

0 

• 0 

Building 

with 

specific 

instrumentation 

• • 0 

• 

• 0 

EXPLANATION 

• 

0 

Vertical 

accelerometer 

Three-dimensional 

accelerometer 

Pressure gauge 

-- Horizontal 

accelerometer 

Ground level 

,0-- ~ 

~ --45 --·----~4 Height 

• h3 -------f • h2 -------f 

• 

• 

y // ""' ---"77" "'-"' / Bedrock ~ ""' "" 

FIGURE E2.--Qualitative description of instrumentation around 

boundary and below foundation of a building. 

38 

GPO 785-048/48040