SEISMIC INSTRUMENTATION OF 

BUILDINGS 

By 

 Mehmet Çelebi1

Open-File Report 00-157 

This report is preliminary and has not been reviewed for conformity with U.S. Geological 

Survey editorial standards (or with the North American Stratigraphic Code). Any use of 

trade, product or firm names is for descriptive purposes only and does not imply 

endorsement by the U. S. Government

April 2000 

 

1

 United States Geological Survey (MS977), 345 Middlefield Road, Menlo Park, CA. 94025 

 ( [Tel: 650-329-5623, Fax: 650-329-5163], [e-mail: celebi@usgs.gov] 
 2 

TABLE OF CONTENTS 

ABSTRACT............................................................................................................................. 3

I. INTRODUCTION............................................................................................................ 4

I.1 Objectives for Seismic Instrumentation of Structures ....................................................................... 4

I.2 Scope................................................................................................................................................. 5

I.3 Code versus Extensive Instrumentation ............................................................................................ 5

I.4 Current Programs for Instrumentation of Structures ......................................................................... 7

II. STEPS IN INSTRUMENTING A STRUCTURE......................................................... 9

II.1 Selection of Structures to be Instrumented....................................................................................... 9

II. 2 Requisite Information........................................................................................................................ 9

II.3 Site Visit.......................................................................................................................................... 10

II.4 Importance of Building Specific Free-Field Station........................................................................ 10

II.5 Tests on Existing Structures to Determine Dynamic Characteristics .............................................. 11

II.6 Dynamic Analysis ........................................................................................................................... 11

III. SELECTION AND INSTALLATION OF INSTRUMENTS .................................... 12

IV. MAINTENANCE........................................................................................................... 13

V. DATA RETRIEVAL AND PROCESSING................................................................. 14

VI. UTILIZATION OF DATA FROM INSTRUMENTED STRUCTURES................. 15

VI.1 General ........................................................................................................................................ 15

VI.2 A Sample Case: Pacific Park Plaza (Emeryville, Ca.)................................................................. 16

VI.3 Summary of Sample Lessons from Studies of Recorded Structural Responses .......................... 21

VII. COST/BUDGET ISSUES....................................................................................... 25

VIII. CONCLUSIONS..................................................................................................... 26

REFERENCES...................................................................................................................... 27

APPENDIX A: DESCRIPTION OF INSTRUMENTS ..................................................... 32

A.1 TYPICAL SENSORS USED .............................................................................................32

A.1.1 Force-Balance Accelerometers.................................................................................................... 32

A.1.2 Episensors.................................................................................................................................... 32

A.2 SUMMARY OF OLDER VERSIONS OF ANALOG RECORDERS.............................33

A.2.1 Accelerographs............................................................................................................................ 33

A.2.1.1 Triaxial Strong-Motion Accelerograph (SMA-1):.................................................................... 33

A.2.1.2 Central Recording Accelerograph [CRA-1] ............................................................................. 34

A.2.2. SSA-1 and SSA-2 Series .............................................................................................................. 34

A.3 CABLES ..........................................................................................................................35

A.4 SUMMARY OF DIGITAL RECORDERS ......................................................................35

A.4.1 K-2 Digital Recorder.................................................................................................................... 35

A.4.2 Mt. Whitney Recording System ................................................................................................... 37
 3 

ABSTRACT 

The purpose of this report is to provide information on how and why we deploy seismic 

instruments in and around building structures. 

The recorded response data from buildings and other instrumented structures can be and are 

being primarily used to facilitate necessary studies to improve building codes and therefore 

reduce losses of life and property during damaging earthquakes. Other uses of such data can 

be in emergency response situations in large urban environments. 

The report discusses typical instrumentation schemes, existing instrumentation programs, the 

steps generally followed in instrumenting a structure, selection and type of instruments, 

installation and maintenance requirements and data retrieval and processing issues. In 

addition, a summary section on how recorded response data have been utilized is included. 

The benefits from instrumentation of structural systems are discussed. 
 4 

I. INTRODUCTION 

There are three main approaches to evaluate seismic behavior and performance of structural 

systems. 

1. Laboratory Testing: 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 timedependent testing scheme, such as a shaking table or hydraulically powered and 

electronically controlled loading systems; however, testing of structural systems 

under controlled simulated 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. 

2. Computerized Analyses: Using special purpose public-domain or private software, 

structures are analyzed for prescribed loads determined either by code provisions or 

postulated site-specific ground motions. 

3. Natural Laboratory of the Earth: The third main approach to evaluate the behavior and 

performance of structural systems is to use the natural laboratory of the Earth, by 

observing and studying the performance (and possibly the damage to structures) 

following 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 moderate-level 

motions would be experienced frequently. Integral to the “natural laboratory” 

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 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. 

I.1 Objectives for Seismic Instrumentation of Structures 

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. 
 5 

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 and those observed in laboratories, 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 free-field 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 the frequency and 

damping, 

(4) correlate the damage with inelastic behavior, 

(5) determine the ground-motion parameters that correlate well with building response 

damage, and 

(6) make recommendations eventually to improve seismic codes (Çelebi and others, 

1987). 

(7) facilitate decisions to retrofit/strengthen the structural system as well as securing the 

contents within the structures. 

I.2 Scope 

The scope of this report is intended to consider issues that are related to instrumentation of 

structures and a variety of structural instrumentation schemes including those at free-field 

near structures. Thus, we are concerned mainly with how real structures respond to damaging 

earthquakes. Ultimately, the data obtained should reveal the performance of the subject 

structure at a particular site. 

I.3 Code versus Extensive Instrumentation 

The most widely used code in the United States, the Uniform Building Code (UBC-1997 and 

prior editions), recommends, for seismic zones 3 and 4, a minimum of three accelerographs 

be placed in every building over six stories with an aggregate floor areas 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. UBC-Code type 

instrumentation is illustrated in Figure 1a. 

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 
 6 

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 required at the basement level. This type of 

instrumentation scheme is called the ideal extensive instrumentation scheme herein and is 

illustrated in Figure 1b. 

Figures 1c and 1d illustrate typical special purpose instrumentations. Diaphragm effects are 

best captured by adding sensors at the center of the diaphragm as well as the edges (Figure 

1c). 

Figure 1. Typical Instrumentation Schemes 

Performance of base-isolated systems and effectiveness of the isolators are best captured by 

measuring tri-axial motions at top and bottom of the isolators as well as the rest of the 

superstructure (Figure 1d). 

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 of earth dams has been addressed by Fedock 

(1982). 

Within the last decade plus, system identification techniques have made it possible to identify 

structural characteristics (modal frequencies, modal damping) using recorded responses of 
 7 

structures (Ljung, 1987). These methods have evolved into single-input single-output and 

multi-input multi-output versions that enable construction of modal shapes. 

Like the superstructure, the foundation system needs to be instrumented to study its response. 

This is easily accommodated along 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 

the 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 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. 

Instrumentation needs of a structure have been addressed by Rojahn and Matthiesen (1977), 

Hart and Rojahn (1979) and Çelebi and others (1987). 

I.4 Current Programs for Instrumentation of Structures 

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, state and local governments and private 

organizations. 

2. State Programs. In California, the State Division of Mines and Geology (CDMG) has 

the responsibility to develop strong-motion arrays to instrument typical structures 

within the State of California (Shakal, 1984). Other states (for example, Alaska) have 

similar programs. 

3. Private Institutions. Some private institutions such as International Business 

Machines (IBM), Kaiser Permanente (Kaiser), Pacific Gas & Electric Company (San 

Francisco, CA.) and University of Southern California (USC) have developed their 

own instrumentation programs. 

Through these programs, more than 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 
 8 

coordination of efforts (National Research Council, 1982). Similar concerns were aired 

during a national strong-motion instrumentation workshop (Iwan, 1981). However, recently, 

a new organization, Consortium of Organizations for Strong-Motion Observation Systems 

(COSMOS, 1999) has been incorporated to fill this void. 
 9 

II. STEPS IN INSTRUMENTING A STRUCTURE 

II.1 Selection of Structures to be Instrumented 

In selecting structures for seismic instrumentation, unless other factors are considered and/or 

specific organizational choices are made a priority, the following general parameters can be 

followed to rank structures for instrumentation: 

1. Structural parameters: the construction material, structural system, geometry, 

discontinuity, and age, 

2. Site-related parameters: 

a. Severity-of-shaking factor to be assigned to each structure on the basis of its 

closeness to one or more of the main faults within the boundaries of the area 

considered (e.g. for the San Francisco Bay area, the San Andreas, Hayward, and 

Calaveras faults are considered). 

b. Probability of a large earthquake (M = 6.5 or 7 occurring on the fault(s) within the 

next 30 years was obtained. The purpose of this parameter is to consider the 

regions where there is strong chance of recording useful data within an 

approximately useful life of a structure. 

c. Expected value of strong shaking at the site, determined as the product of a and b. 

3. Building usage, functionality, occupancy and relevance to life safety requirements 

 following damaging earthquakes. 

4. Other parameters of interest to owners or public officials. 

The next step in ranking structures is to assign rational weighting factors for structural 

parameters and site-related parameters. A ranked list of structures emerges from this effort. 

As an example, the USGS, with input from an Instrumentation Advisory Committee for the 

San Francisco Bay Area, in 1983 developed a recommended list of structures for seismic 

instrumentation and rank them according to a rational set of parameters and criteria (Çelebi 

and others, 1984). 

Once the particular structure to be instrumented is identified, 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. 

II. 2 Requisite Information 

Once it is decided to instrument a particular structure and permit is obtained, it is imperative 

that a series of studies, deductions, and decisions be made. Furthermore, it is important to 

optimize the instrumentation schemes from the points of view of both cost and required data. 

This necessitates study of the expected dynamic behavior of the structure. The preliminary 

studies include the following steps: 
 10 

 (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, if feasible and necessary. 

In general, the following information, if available, will be required: 

(1) relevant blueprints and design calculations, 

(2) dynamic analysis (mode shapes and frequencies), 

(3) if available, forced-vibration test results, and ambient-vibration test results. 

Seldom is all this information available for any structure. In particular, for a structure that is 

yet to be constructed, blueprints, design calculations and if available, dynamic analyses may 

be all the information to design its instrumentation scheme so that part of installations of 

conduits and cables can be feasibly carried out during construction. 

The collected 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. 

After 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 is 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. 

These steps are described in more detail in the following section. 

II.3 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 (for existing buildings). 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. 

II.4 Importance of Building Specific Free-Field Station 

If physically feasible, it is advisable to include into the instrumentation scheme, a building 

specific free-field station. Such a free-field station is usually deployed at a distance greater 

than 1.5-2 times the height of the nearest/tallest building. This is due to the desire that 

motions recorded by a free-field station should not be influenced by the shaking of the 

buildings. As can be expected, in urban areas, this may be a problem due to the density of 

built facilities. 
 11 

In general, free-field and ground-level motions should be known in order to quantify the 

interaction of soil and structure. However, data recorded at building specific free-field 

stations can be used to augment data bases used for structural response studies as well as 

ground motion studies including development of attenuation relationships and quantification 

of site response transfer functions and characteristics. 

II.5 Tests on Existing Structures to Determine Dynamic Characteristics 

Although it is possible to obtain a satisfactory understanding of a structure's expected 

dynamic behavior by preliminary analytical studies, when feasible and necessary, an 

ambient-vibration and/or a forced vibration test on an existing structure can be performed to 

identify mode shapes and frequencies. Ambient vibration tests can be performed efficiently 

using portable recorders at three to five locations that are expected (from analytical studies or 

other information) to have maximum amplitudes during the first three to four vibrational 

modes. Thus, elastic properties of the structure can be determined. If the subject structure 

experiences nonlinear behavior during a strong shaking, it will be much easier to evaluate the 

nonlinear behavior once linear behavior is determines before the nonlinear behavior occurs 

during the strong shaking. 

Compared to ambient-vibration test, a forced-vibration test is more difficult to perform. 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 (Çelebi and others, 1987). 

 

II.6 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 could be developed to obtain the elastic 

dynamic characteristics. This is performed with any one of the several tested computer 

programs available (e.g. SAP2000, ANSYS, and STRUDL). 
 12 

III. SELECTION AND INSTALLATION OF INSTRUMENTS 

In selection and defining an instrumentation scheme, an optimum list of hardware is 

developed after careful consideration of cost and data requirements. Appendix A describes 

technical capabilities of commercially available sample sensors and recording systems. While 

developing the instrumentation scheme within the budgetary constraints, it is best to consider 

the maximum available channels for each recording system. Most recording systems have 

maximum of 12 or 18 channels of recording capability. 

The following general approach is followed to install seismic instruments: 

1. After an instrumentation scheme is developed and approximate sensor locations are 

chosen, USGS engineers and technicians and the owner's representative review the 

site to determine exact sensor locations and routing of cables and conduits, if 

required, satisfactory to both parties. This is important from viewpoint of long-term 

accessibility, potential interference with the occupant's space, placement of data cable 

runs, and aesthetic requirements of the owner. Figure 2 exhibits a sample schematic 

showing locations of sensors, routing of cables, location of junction boxes and 

recording units. 

2. Next the USGS technician inspects the entire structural scheme with an electrical 

contractor who will install the data cable, junction boxes at key locations and terminal 

boxes (if required) at each sensor site. The modern recording systems may not 

require terminal boxes (see Appendix A - e.g. Mt. Whitney2) as they have internal 

terminals. 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. 

3. The cable-termination box (if necessary) is prepared in the USGS shop and includes 

data circuits, batteries and battery charges. This box is normally mounted on the wall 

above the recorder. 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. 

4. 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. For modern digital 

systems, this information is entered into the recorder data section and is stored in a 

general database. Other documentation includes precise sensor location, period and 

damping of each unit, location of cable runs, access information, and circuit diagrams. 

 

2

 Quotation of brand names does not constitute endorsement of any particular product. 
 13 

IV. MAINTENANCE 

It is essential to have periodic and consistent maintenance of instruments in order to have a 

successful program. Unless maintenance arrangements are made, successful recording of 

data cannot be accomplished. Therefore, routine maintenance is conducted every 3-12 

months if circumstances and experience so allow. This maintenance includes the following: 

Figure 2. Schematic showing typical deployment of sensors and routing of cables to the 

recorder. 

1. Remote calibration of period and damping. 

2. Inspection of battery terminals, load voltage, and charge rate (batteries are replaced 

every 3 years). 

3. Measurement of threshold of triggering system and length of recording cycle. 

As a final maintenance procedure, a calibration record is obtained and then examined for the 

desired characteristics. All inspection procedures are recorded in the permanent station file at 

the laboratory. 
 14 

V. DATA RETRIEVAL AND PROCESSING 

Modern strong motion instruments now have capabilities to store and transmit digital data 

through telecommunications links and other media, including the WWW (Appendix A). This 

communication makes data retrieval easier and faster than with older analog systems, where 

data were retrieved only by site visits to collect the films and required digitizing by manual or 

automatic systems. Currently only digital systems are manufactured, so that as the older 

analog systems are replaced data retrieval and processing will become easier. However, data 

retrieval by telecommunications has additional hardware costs as well as monthly 

subscription costs. 

1. The data from digital recordings are passed through a correction algorithm that 

applies a high-frequency filter (typically 50 Hz), instrument corrections, if necessary, 

and decimation to 200 samples per second. A low-cut Butterworth filter (or another 

appropriate 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 signals, 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 each channel of 

recording are prepared. 

2. Response spectra are calculated for periods up to about half of the long-period limit. 

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

pseudo-velocity response are prepared. 

3. Fourier amplitude spectra, calculated by fast Fourier transform, are presented on 

linear axes and log-log axes. 

These sets of processed data are then provided to the user community for their evaluation, 

assessment of facilities and structures, and research. 
 15 

VI. UTILIZATION OF DATA FROM INSTRUMENTED STRUCTURES 

VI.1 General 

Seismic monitoring of structural systems constitutes an integral part of the National 

Earthquake Hazard Reduction Program in the United States and similar programs in other 

countries. Recordings of the acceleration response of structures have served the scientific and 

engineering community well and have been useful in assessing design/analysis procedures, 

improving code provisions and in correlating the system response with damage. Table 1 

summarizes some of the uses for the data from instrumented structures. Unfortunately, only 

a few damaged structures have been instrumented in advance to perform studies of the 

initation and progression of damage during strong shaking (e.g. Imperial County Services 

Building during the 1979 Imperial Valley earthquake, [Rojahn and Mork, 1981]). In the 

future, instrumentation programs should consider this deficiency. Jennings (1997) 

summarizes this view as follows: “As more records become available and understood, it 

seems inevitable that the process of earthquake resistant design will be increasingly, and 

quite appropriately, based more and more upon records and measured properties of materials, 

and less and less upon empiricism and qualitative assessments of earthquake performance. 

This process is well along now in the design of special structures”. 

The methods used in studying structural response records are quite diverse: (a) mathematical 

modeling (finite element models varying from crude to very detailed, subjected to timehistory, response spectrum or modal analyses). The procedure requires the blueprints of the 

structures which may not be readily accessible; (b) system identification techniques: single 

input/single output or multi input/multi output. In these procedures, the parameters of a 

model are adjusted for consistency with input and output data (Ljung, 1987); (c) spectral 

analyses: response spectra, Fourier amplitude spectra, autospectra, Sx or Sy, cross-spectral 

amplitudes Sxy, and coherence functions (γ) [using the equation : γ

2 

xy (f) = S2xy (f) / Sx (f)Sy (f) ] 

and associated phase angles (Bendat and Piersol, 1980); and (d) simple procedures based on 

principles of structural dynamics (e.g. recently Jennings (1997) analyzed data from two 

buildings within close proximity (<20 km) to the Northridge epicenter, calculated the base 

shear from the records as 8 and 17 % of the weights of the buildings, drift ratios as 0.8 and 

1.6 % (exceeding code limitations). Jennings (1997) states: “A difference between code 

design values and measured earthquake responses of this magnitude – approaching a factor of 

ten – is not a tenable situation.” 

Until recently, in general, only accelerometers (single, biaxial or triaxial) were used to 

instrument structures. However, observations of damages during the 1994 Northridge and 

1995 Kobe earthquakes, have forced engineers and scientists to focus on performance based 

seismic design methods and to find new techniques to control drift and displacements. To 

verify these developments, sensors directly measuring displacements or relative 

displacements (transducers, laser devices and GPS units) are now being considered. A recent 
 16 

development in using differential GPS measurements for monitoring long-period structures is 

summarized by Çelebi and others (1999). 

VI.2A Sample Case: Pacific Park Plaza (Emeryville, Ca.) 

The 30-story reinforced concrete Pacific Park Plaza Building (PPP) is an equally spaced 

three-winged, cast-in-place, ductile, moment-resistant framed structure. Constructed in 1983 

and instrumented in 1985, it is the tallest reinforced concrete building in northern California. 

A general view, a plan view, a three-dimensional schematic, and its instrumentation are 

shown in Figure 3 (Çelebi, 1992, 1996). The building with a 5-ft mat foundation rests on 828 

(14-inch-square) prestressed concrete friction piles, each 20-25 m in length, in a primarily softsoil environment, with an average shear-wave velocity between 250 and 300 m/s and a depth of 

approximately 150 m to harder soil. 

Figure 3. Plan layout and three-dimensional schematic and instrumentation scheme of 

Pacific Park Plaza (PPP), Emeryville, CA. 

The planning for the instrumentation scheme of PPP was achieved after study of the 

blueprints and after giving careful consideration to the available dynamic analyses and lowamplitude test results. The building 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 4 (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. 3) includes 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 
 17 

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, a triaxial strong-motion accelerograph is deployed at a 

free-field site on the south side of the building (SFF or EMV3,4). Thus, a total of 24 sensors 

(FBA-11) linked to two central recorders (now updated to two digital 12-channel K-2 

recording systems) provide a practical instrumentation scheme for this building. 

Figure 4. Translational and torsional mode shapes of Pacific Park Plaza (Stephen, 1984) 

During the Ms=7.1 Loma Prieta earthquake of October 17, 1989, a complete set of records 

were obtained from the Pacific Park Plaza (PPP). The building, at 100 km from the epicenter 

of the earthquake, had considerably amplified input motions but was not damaged during the 

earthquake. The east-west components of acceleration recorded at the roof and the ground floor 

of the structure, at the associated free-field station (SFF in Fig. 3) and, for comparison, the 

motion at Yerba Buena Island (YBI), the closest rock site with a peak acceleration of 0.06 g, are 

 

3

 In most studies, the site of south free-field (SFF) is referred to as the Emeryville site (EMV). 

3

 In 1997, the analog recording instruments at Emeryville were upgrade to digital. A downhole accelerograph 

was 

 installed at the same location as the surface free-field station, SFF. 
 18 

shown in Figure 5. The response spectra also shown in Figure 5 clearly demonstrate that the 

motions at EMV were amplified by as much as five times when compared with YBI. 

Amplification is also indicated by the amplitude of the peak accelerations (0.26 g for EMV 

and 0.06 g for YBI). The differences in peak acceleration at the free-field station (0.26 g) and at 

the ground floor of the building (0.21 g) (Fig. 5a) suggest that there was soil-structure 

interaction (SSI). 

In the design of the building, site-specific design response spectra (based on three probabilistic 

earthquakes based on expected levels of performance) were used: (a) the maximum probable 

earthquake (50 % probability of being exceeded in 50 years with 5 % damping) anchored at 

zero period acceleration (ZPA) of 0.32g, and two maximum credible earthquakes both with 10 

% damping but 10 % probability of being exceeded in (b) 50 years [ZPA of 0.53 g] and (c) 100 

years [ZPA of 0.63 g]. The design response spectra and the spectrum of the EW component of 

recorded motion at the SFF are shown in Figure 6. The ZPA of the recorded EW acceleration at 

SFF (0.26 g) (at 100 km from the epicenter) is close to that of the postulated maximum 

probable earthquake (0.32 g). Furthermore, the spectral accelerations of the EW component of 

SFF is considerably higher than the maximum probable earthquake for periods >0.6 seconds – 

that is, practically for the first three modes of the building. Therefore, one important conclusion 

derived from the records is that improvements are necessary in establishing site-specific design 

response spectra to account for realistic shaking at a specific site taking into account expected 

future closer earthquakes likely to produce larger peak accelerations. 

Figure 5. Recorded (EW components) of accelerations and corresponding response spectra at 

the free-field, ground floor and roof of Pacific Park Plaza (PPP), and at Yerba Buena Island 

(YBI), at approximately the same distance as PPP, depict the level of amplification. 

0 10 20 30 40

0

0.5

1

TIME (S)

ACCEL. [G]

LPE (EW): PPP & YBI

ROOF

GR.FL.

SFF

YBI

PEAK=0.38 G

PEAK=0.21 G

PEAK=0.26 G

PEAK=0.06 G

10−1 100 101

0

0.5

1

PERIOD (S)

SPEC. ACCEL. [G]

LPE (EW): PPP & YBI

YBI

ROOF

SFF

GR.FL.

.06

.38
 19 

Table 1. A Preliminary List of Data Utilization & Sample References 

GENERIC UTILIZATION 

Verification of mathematical models (usually routinely performed) (e.g.Boroschek et al, 1990) 

Comparison of design criteria vs. actual response (usually routinely performed) 

Verification of new guidelines and code provisions (e.g.Hamburger, 1997) 

Identification of structural characteristics (Period. Damping, Mode Shapes) 

Verification of maximum drift ratio (e.g. Astaneh, 1991, Çelebi, 1993) 

Torsional response/Accidental torsional response (e.g. Chopra, 1991, DeLalera, 1995) 

Identification of repair & retrofit needs & techniques (Crosby, 1994) 

SPECIFIC UTILIZATION 

Identification of damage and/or inelastic behavior (e.g. Rojahn & Mork, 1981) 

Soil-Structure Interaction Including Rocking and Radiation Damping (Çelebi, 1996, 1997) 

Response of Unsymmetric Structures to Directivity of Ground Motions (e.g. Porter, 1996) 

Responses of Structures with Emerging Technologies (base-isolation, visco-elastic dampers, 

and combination (Kelly and Aiken, 1991, Kelly, 1993, Çelebi, 1995) 

Structure specific behavior (e.g. diaphragm effects, Boroschek and Mahin,1991, Çelebi, 1994) 

Development of new methods of instrumentation/hardware (Çelebi, 1997, Straser, 1997) 

Improvement of site-specific design response spectra 

Associated free-field records (if available) to assess site amplification, SSI and attenuation 

curves 

Verification of Repair/Retrofit Methods (Crosby et al, 1994, Çelebi and Liu, 1997) 

Identification of Site Frequency from Building Records (more work needed) 

RECENT TRENDS TO ADVANCE UTILIZATION

Studies of response of structures to long period motions (e.g. Hall et al, 1996) 

Need for new techniques to acquire/disseminate data (Straser, 1997, Çelebi, 1997, 1998) 

Verification of Performance Based Design Criteria (future essential instrumentation work) 

Near Fault Factor (more free-field stations associated with structures needed) 

Comparison of strong vs. weak response (Marshall, Long and Çelebi, 1992) 

Functionality (Needs additional specific instrumentation planning) 

Health Monitoring and other Special Purpose Verification (Heo et al, 1997) 

Figure 6. Design response spectra and response spectra of recorded motions at the ground 

floor and SFF of Pacific Park Plaza. Also shown is the 1979 UBC response spectrum for 

comparison. 

0 0.5 1 1.5 2 2.5 3 3.5 4

0

0.2

0.4

0.6

0.8

1

1.2

PERIOD (S)

SPEC. ACCEL. [G]

PPP: EMERYVILLE: RESPONSE SPECTRA [DESIGN VS. RECORDED]

A: MAX PROBABLE EQ. [5 % DAMP.]

 [10% PROB. OF EXCEEDENCE IN 100 YEARS]

 [50% PROB. OF EXCEEDENCE IN 50 YEARS]

B: MAX CREDIBLE EQ. [10 % DAMP.]

A

B

UBC[1979]

SFF [EW−5 % DAMP.]

Gr.FL.[EW−5 % DAMP.]
 20 

Using different methods, the building has been studied in detail by Anderson and Bertero 

(1994), Anderson and others (1991), Kagawa and others (1993), Kagawa and Al-Khatib 

(1993), Aktan and others (1992), Kambhatla and others (1992) and Çelebi and Safak (1992). 

All investigators agree that the predominant three response modes of the building and the 

associated frequencies (periods) are 0.38 Hz (2.63 s), 0.95 Hz (1.05 s), and 1.95 Hz (0.51 s). 

These three modes of the building are torsionally-translationally coupled (Çelebi, 1996) and 

are depicted in the cross-spectra (Sxy) of the orthogonal records obtained from the roof, ground 

floor and SFF (the south free-field site) (Figure 7) and the normalized cross-spectra of the 

orthogonal records (bottom right in Figure 7). The frequency at 0.7 Hz (1.43 s) observed in the 

spectra is this site frequency (Çelebi, 1996). 

System identification techniques, when applied to the records of this building, yielded unusually 

large damping ratios corresponding to the 0.38-Hz first-mode frequency [11.6 % (NS) and 15.5 

% (EW)] [Table 2] (Çelebi, 1996a). Such unusually high damping ratios attributed to a 

conventionally designed/constructed building with its large mat foundation in a relatively soft 

geotechnical environment is due to radiation (or foundation) or material damping. This is one of 

two cases where large damping percentages implied by the recorded responses of buildings have 

been attributed to radiation damping; the other case is from the Olive View Hospital in Sylmar, 

Ca. – data from the Northridge earthquake (Çelebi, 1997). 

Figure 7. Cross-spectra of orthogonal accelerations (A350 & A260) at the roof, ground floor, 

free-field of PPP. Also shown (bottom right) is the normalized cross-spectrum depicting 

structural and site frequency peaks. (350 & 260 depict degrees clockwise from true north). 

0 1 2 3 4

0

1

2

3

4

5

x 104

FREQ (HZ)

Sxy

PPP:ROOF

A350 & A260

0.7

0.95

0.38

1.95

0 1 2 3 4

0

500

1000

1500

2000

2500

FREQ (HZ)

Sxy

PPP:SOUTH FF

0.7

A350 & A260

0 1 2 3 4

0

5000

10000

15000

FREQ (HZ)

Sxy

PPP:GROUND FL.

A350 & A260

0.7

1.95

0 1 2 3 4

0

0.2

0.4

0.6

0.8

1

FREQ (HZ)

Sxy

PPP:ROOF,GR.FL.,SFF

a:ROOF

b:GR.FL.

c:SFF

a

b

c

.7 .95
 21 

Table 2. Summary of dynamic characteristics for Pacific Park Plaza 

Frequencies (Hz) Damping (%) 

Mode Mode 

1 2 3 1 2 3 

1989 (LPE) STRONG-MOTION DATA (from Çelebi, 1996) 

N-S 0.38 0.95 1.95 11.6 

E-W 0.38 0.95 1.95 15.5 

MODAL ANALYSES (from Stephen and others, 1985) 

N-S 0.60 1.67 3.10 

E-W 0.60 1.67 3.10 

TORSION 0.57 1.70 3.25 

The dynamic characteristics determined from Loma Prieta response records of Pacific Park 

Plaza as well as those determined from modal analyses (Stephen and others, 1985) are 

summarized in Table 2. Also, it is noted in Table 2 that although flexibility of the foundation 

was considered in the 1985 analyses, the structural frequency remained the same as the 

frequency determined with fixed base assumption. Clearly, the mathematical models developed 

at that time needed improvements. This conclusion could only be reached because we have 

recorded on scale motions. Most recent studies indicate that the frequencies from recorded 

motions can be matched when soil-structure interaction (SSI) is incorporated into the 

mathematical models (Kagawa and others, 1993; Aktan and others, 1992; Kambhatla and 

others, 1992). Furthermore, a study of the building for dynamic-pile-group interaction by 

(Kagawa and Al-Khatib, 1993; Kagawa and others, 1993) indicates that there is significant 

interaction. Their studies show that computed responses of the building using state-of-the-art 

techniques for dynamic-pile-group interaction compares well with the recorded responses. 

On the other hand, Anderson and others (1991) and Anderson and Bertero (1994) concluded 

that soil-structure interaction was insignificant for Pacific Park Plaza during the earthquake. 

They compared the design criteria, code requirements, and the elastic and nonlinear dynamic 

response of this building due to the earthquake using both simplified and detailed analytical 

models. 

VI.3 Summary of Sample Lessons from Studies of Recorded Structural Responses 

Instrumentation of structures as part of hazard reduction programs is very beneficial, as 

studies of this type will help to better predict the performance of structures in future 

earthquakes. 

(1) Studies of recorded responses of buildings help researches and practicing 

professionals to better understand the cumulative structural and site characteristics 

that affect the response of buildings and other structures. Such studies impact 

mitigation efforts. 
 22 

(2) In turn, the behavior that may be expected from buildings during future earthquakes 

with large input motions (either due to larger magnitude earthquakes or 

earthquakes at closer distances to the building) can be forecast. 

(3) There is a large inventory of buildings within 0-10 km of the many major faults 

capable of generating M>7 earthquakes. This is particularly important because, very 

recently, the Structural Engineers Association of California (SEAOC) issued the 

1996 edition of the Recommended Lateral Force Requirements and Commentary

which has provisions for increasing the design base shear by 0-100 % depending on 

the 0-10 km distance of the building from the fault. This implies that the forecasting 

of performance of buildings within 0-10 km of major faults must be done more 

informatively. This requisite information can be achieved only through acquiring 

and studying response data from buildings during earthquakes. 

(4) There is an acute need to better evaluate structural and site characteristics in 

developing earthquake-resisting designs of building structures. Studies of recorded 

responses of structures to date show that designs of buildings with low structural 

damping, resonation and beating effects caused by closely-coupled translational 

and torsional modes must be avoided. 

(5) As expected in most tall buildings, higher modes are excited. Higher modes play an 

important role in the response of building structures and therefore must be carefully 

evaluated to assess their future performances. 

(6) Some of the building response data are from tall buildings on soft soils. The motions 

at the soft-soil sites of some of the important tall buildings are amplified by 3-5 

times within the periods of engineering interest when compared with the motions at 

rock site approximately the same distance away from the epicenter of the 

earthquake. Responses of tall buildings on soft soils due to motions originating at 

considerable distances must be accounted for. 

(7) Drift ratios calculated from observed data in certain cases exceed code drift 

limitations for part or all of the structural systems. Assessing the drift exposure of 

structural systems are ever more important since the design/analyses of buildings 

are recently being shifted towards a performance based design procedure. 

(8) One of the most important aspects of structural behavior that is least understood is the 

soil-structure interaction (SSI). Such interaction alters the dynamic characteristics 

of structures and consequently may be beneficial or detrimental to the performance 

of the structure. In Mexico City (1985), SSI was detrimental. In other cases, it was 

beneficial. Recorded response data provides much needed information to identify as 

to when SSI is beneficial and when detrimental. During Northridge, for example, 

one feature of SSI, radiation damping was found to be helpful in the behavior of the 

New Olive View Hospital – even though the building experienced resonance. 

Specific studies are underway to determine improved methods for incorporating SSI 

into design procedures. 

 SSI stands to be prominent in the behavior of several instrumented buildings (e.g. 

Pacific Park Plaza Building [PPP], Santa Clara County Office Building [SCCOB], 

Transamerica Building [TRA] etc) as assessed from studies of the recorded 

responses of buildings during the October 17, 1989 Loma-Prieta earthquake 

(Ms=7.1). Therefore, two specific issues are that (1) identification of beneficial and 

adverse effects of soil-structure interaction is a necessity, and (2) design offices must 
 23 

be informed and trained in consideration of the effect of soil-structure interaction in 

estimation of fundamental period and damping of a building – as this is not yet the 

case. 

(9) Development of design response spectra deserves more intensive consideration by 

geotechnical engineers since site effects play an important role in the response of 

building structures. There are significant discrepancies in the comparison of the 

response spectra derived from recorded motions with the actual design response 

spectra. Amplified motions due to soft soil conditions, site-specific resonating 

frequency content (e.g. basin effects) must be kept in mind in the development of 

design response spectra. 

(10) The propagation direction of surface waves arriving at the buildings affect 

particularly unsymmetrical buildings or buildings. 

(11) Design of basements of tall buildings has been done in the past with input motions 

at the ground level. Measurements (records) from tall buildings with 2-4 floors of 

basements indicate that the motions at the basemat are different that that at the 

ground level. Also the rotations of the basemat are different than the rotations of the 

sidewalls of the basements. Design considerations for basements therefore need to 

be reviewed. The implication is that the current practice, which assumes that the 

inertial forces at the ground level and basemat level to be the same, is not correct. 

Some suggested changes in design procedures have been made. 

(12) Low-amplitude tests have been conducted on five buildings that recorded the Loma 

Prieta earthquake. Results indicate, as expected, that the first-mode periods extracted 

from strong motion response records are longer than those associated with the ambient 

vibration records. Similarly, the percentages of critical damping for the first mode for 

the ambient data are significantly smaller than those from the strong-motion data. 

These differences may be caused by several factors including: (a) possible 

soil-structure interaction which is more pronounced during strong-motion events than 

during ambient excitations, (and similarly, in buildings with pile-foundations, possible 

pile-foundation interaction which may not occur during ambient excitation), (b) 

non-linear behavior of the structure (such as micro-cracking of the concrete at the 

foundation or superstructure), (c) slip of steel connections, and (d) interaction of 

structural and non-structural elements. Changes in damping values and fundamental 

period values commensurate with inferred strong-motion values should be considered 

to improve design and analyses results. 

(13) Specific instrumentation schemes of some of the already instrumented buildings and 

of those buildings yet to be instrumented must be improved and/or implemented so 

that the response characteristics expected of that building can be captured (e.g. SSI, 

pounding, variation of drift due to abrupt changes in stiffness). When applicable, 

specific buildings should be specially instrumented extensively to better capture 

their behavior in response to actions such as pounding and SSI. 

(14) Because the energy of the ground motions can be azimuthally variable, structures 

with wings or unsymmetrical structures can be significantly affected by it. 

(15) Instrumented structures that have been built or retrofitted with innovative 

technologies such as base isolated systems indicate which types of base-isolation 

systems are more effective than others. During the Northridge earthquake, the USC 

hospital built on rubber bearings performed well but another system did not perform 
 24 

as expected. Therefore, instrumentation of structures helps us to identify which 

innovative emerging isolation technologies perform as expected during strongshaking. 

(16) Methods are being developed to identify initiation and propagation of damage by 

studying the response data from instrumented (and damaged) structures. 

Unfortunately there were only 2 buildings that were damaged and had seismic 

instrumentation. This aspect of acquiring information from instrumented structures 

is gaining importance to develop methods for assessing whether structures are 

damaged or not. 

(17) Response data is providing to be a key to the analyses of few buildings that showed 

higher shaking at mid-floor levels that their respective top floors. This phenomenon 

is not yet understood and is being investigated. 

(18) Determination of dynamic characteristics of structural systems is important in 

assessing their vulnerability and in developing procedures to analyze and design 

future structures. The data recorded from instrumented structures is used to 

determine structural characteristics. One additional aspect of this is determination 

of the effect of accidental torsion of symmetric structures. The code provisions that 

require accidental torsion provisions are now under scrutiny. Following such 

earthquakes, the formulas in the codes for determination of fundamental periods of 

structures and design coefficients for assessing the strength requirements have been 

continuously revised over the years. 

(19) Performance of flexible diaphragms of industrial buildings has been improved as a 

result of studies of recorded motions of such structures. 

(20) It has been noted that azimuthal propagation of incoming earthquake motions from 

different events of different epicentral locations affects the performance of 

structures that are not symmetrical and/or structures with wings. This issue needs to 

be studied further to assess how design requirements can be affected by this action. 

(21) Studying response records from instrumented structures provides a means to 

identify the performance problems of the structures and therefore devise and select 

the best possible retrofit and/or upgrade system. An example of this is the Santa 

Clara County Office Building in San Jose, California. Records from 3 earthquakes 

showed that the building resonated and had very low damping. Consequently, the 

retrofit system selected and implemented is the viscous elastic damper system that 

increases both the stiffness and damping of the structure. 
 25 

VII. COST/BUDGET ISSUES 

The cost of instrumenting a structure in general is based on the number of channels of sensors 

that will be deployed following decision making process. However, it seems feasible to 

provide a standardized 12-18 channel instrumentation scheme as most commercially 

available recording systems have maximum 12 or 18 channels (e.g. K2 and Mt. Whitney, 

respectively). Experience of a decade and more indicates that the cost can be approximately 

estimated as follows: 

• Each channel of recording system ~ $1K 

• Each channel of sensor ~ $1K 

• Installation per channel (labor, cabling etc) ~$2K. 

Thus, a 12-channel system would cost approximately $48-50K. This normally will include a 

triaxial free-field station in the immediate vicinity of the building, if physically possible. 

Furthermore, the cost can go up depending upon the difficulty with which cables that connect 

the various sensors at different locations of a building can be pulled to connect the sensors to 

the recording system. 

Instrumentation costs of $50 K for a building and its contents is a small investment when 

compared with the actual worth of a building (and its contents). Naturally, larger and more 

complex building systems will require more than 12 channels to define its vibrational 

behavior. In such cases, combinations of 12 and 18 channels are used. 
 26 

VIII. 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. 
 27 

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Safak, E., and Çelebi, M., 1992, Recorded seismic response of Pacific Park Plaza: - II, 

System identification: Journal of Structural Engineering, v. 18, no. 6, p. 1566-1589 

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 

pyramid-shaped building: Berkeley, Earthquake Engineering Research Center Report EERC 

73-17, 97 p. 

Straser, E., 1997, Toward wireless, modular monitoring systems for civil structures, in the 

John A. Blume Earthquake Engineering Center Newsletter, Issue No. 2. 

Uniform Building Code, International Conference of Building Officials, Whittier, CA, 1970, 

1976, 1979, 1982, 1985, 1988, 1991, 1994, 1997 editions. 
 32 

APPENDIX A: DESCRIPTION OF INSTRUMENTS 

A.1 TYPICAL SENSORS USED 

A.1.1 Force-Balance Accelerometers 

Sensors usually installed in the free-field and structures are 50-Hz, 0.7 critically damped 

Kinemetrics5

 force-balance accelerometer models FBA-11, and FBA-13 (respectively 

uniaxial, and triaxial transducers). Biaxial versions are either ordered specially or mounted 

on a plate by customers. For special projects where triaxial motions below the ground 

surface needs to be recorded, tri-axial downhole accelerometers are used. Figure A-1 shows 

uniaxial, triaxial and downhole accelerometer used. The accelerometers are bolted to the 

building frame or floor, and sensed data are transmitted to the central recording location by 

shielded cable. 

Figure A-1 (from left to right), FBA-11 (uniaxial), FBA-13 (triaxial) and FBA-13DH 

(downhole) 

A.1.2 Episensors 

The latest version of triaxial accelerometer is the Episensor (Figure A-2). Episensor force 

balance accelerometers are also available in uniaxial (the FBA ES-U) and borehole (the FBA 

ES-SB shallow and FBA ES-DH deep) packages. The triaxial unit consists of three forcebalance accelerometer modules mounted orthogonally in one package. Recording ranges of ± 

0.25 to ± 4g are user selectable. The recording bandwidth ranges from DC to 200 Hz. These 

sensors also have four types of outputs that can be field-selected by the user: ± 2.5V single-

 

5

 Commercial names cited in this report is for demonstration only and does not constitute endorsement of the 

product. User should investigate all available products. 
 33 

ended output for use with traditional Kinemetrics earthquake recording instruments; ± 10V 

single-ended or ± 20V differential output for use with Kinemetrics new Everest 24-bit digital 

recorder and other 24-bit digital recorders currently on the market. 

Figure A-2. Triaxial Episensor (13.3 cm diameter, 6.2 cm height) 

A.2 SUMMARY OF OLDER VERSIONS OF ANALOG RECORDERS 

A.2.1 Accelerographs 

It can be stated that tri-axial accelerographs, whether analog or digital, are the most widely 

used instruments by many national and international organizations to record ground motion 

as well as structural response. This is because they are versatile enough to be deployed on 

ground as well as at different locations of structures. That is why perhaps the code-writers 

(UBC) adopted the tri-axial instruments as the code-requirement. A triaxial accelerograph 

contains the tri-axial accelerometers within the unit that also houses the recording 

components. Therefore, it is capable of recording triaxial motions (two orthogonal horizontal 

components and 1 vertical) at a point on a surface (free-field or structure). 

The different types of accelerographs used are: 

A.2.1.1Triaxial Strong-Motion Accelerograph (SMA-1): 

Triaxial self-contained accelerographs are often needed and deployed in conjunction with the 

remote-sensor system either to record free-field ground motions or to supplement the 

structural instrumentation when more than 12 channels are required. 

Although deployment of digital units in Japan, Taiwan and the United States is progressing 

speedily, Kinemetrics Model SMA-1 is the most widely deployed triaxial, self-contained, 

analog unit at the present (Figure A-3). This instrument records data optically onto 70-mm 

photographic film from 25-Hz flexure-type accelerometers. The SMA-1 has the same trigger 

system, recording capability, and real time options as described for the CRA-1 below. 
 34 

However, with new digital units that are competitively priced, SMA-1 models are no longer 

manufactured and the existing inventory is slowly being replaced by digital units. In fact, the 

manufacturer is now offering kits to convert analog SMA-1 units to digitally record. Because, 

in the future, the analog units will not be manufactured and used, detailed descriptions are not 

provided herein but are available elsewhere (Hudson, 1979). 

 

Figure A-3 shows a picture of an SMA-1 and the SMA-1 after digital conversion 

A.2.1.2Central Recording Accelerograph [CRA-1] 

Central Recording Accelerograph (CRA) is a 12-channel, analog recording system that 

records photographically on a single 7 in film. This analog recorder has been usually used for 

structural-response studies. It can be modified to have a 13th channel. 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 

as nominal threshold of 0.01 g between 1 and 10 Hz, has a total recording time of 25 minutes. 

The recording 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 half-second time marks on the edge 

of the film. A 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. Float-charged batteries located in a nearby battery box supply power. A 

rotary key switch provides for periodic testing of natural frequency and damping of the 

remotely located sensors. 

The sensors are connected externally to the CRA unit via a junction box. It is mainly used 

for structural instrumentation. As with SMA-1, the manufacturer is no longer supplying these 

units and therefore is not further deliberated herein. 

A.2.2.SSA-1 and SSA-2 Series 

These earlier version, 12-bit, solid-state, digital accelerographs record motions at 100-200 

samples per second per channel. Newer, 18-24 bit versions of digital systems are now being 

competitively priced. These earlier versions are no longer being manufactured. 
 35 

A.3 CABLES 

When hooking up accelerometers to Altus instruments (K-2 or Mt. Whitney) maximum 6 

accelerometers can be hooked up with one cable. It is preferable to hook up 3 accelerometers 

on one, 6-pair 18-gage cable (Belden part # 9774 or the equivalent) or to hook up 6 

accelerometers to one, 9-pair 18-gage cable (Belden part # 9775 or the equivalent). One pair 

is needed for each accelerometer and 3 pairs are used for power and commands to each 

accelerometer. Up to 6 accelerometers can share the power and command wires. In other 

words, you need at least one 4 pair cable for one accelerometer on up to one 9 pair cable for 6 

accelerometers. 

There should be a junction box at each location where the main cable breaks out to pick up 

more accelerometers. The junction box should be sized according to the number and size of 

the cables which will be connected in and, or passing through it. The individual 

accelerometers may be attached to the main cable using insulated Butt Splices such as 

Panduit part # BSN18. Wherever the main cable has a break in it, it should be spliced using a 

tubular, double screw barrier strip with wire protector of the appropriate size (Buchanan part 

numbers TSB23012DS, TSB5001231DS, and TSB100012DS). 

The recorder should be located such that it is the shortest possible run to an outside wall in 

order to facilitate the installation of 25 meter Bullet GPS antenna. The internal GPS option 

can be used if you can locate the recorder within 25 meters of the antenna. If this is not 

possible, then the external GPS option has to be ordered and the external GPS receiver has to 

be installed within 25 meters of the GPS antenna. If the external GPS option is used, then it 

will be necessary to install a power and data cable between the external GPS receiver and the 

recorder. An assigned vendor supplies a custom cable for this application or you can use a 

different cable using the appropriate guidelines as published in their operations manual. 

A.4 SUMMARY OF DIGITAL RECORDERS 

A.4.1 K-2 Digital Recorder 

Figure A-4. Modern digital K-2 recording system can download data to a laptop or transfer 

by modem or other communications media. 

One of the state-of-the-art digital recording systems is the K-2 recorder that has sensor 

interface, choice of sampling rates (20-250 sps), multi-recording modes, communication 
 36 

interfaces (RS232cable and/or modem) and recording as a data-logger with storage on local 

flash card hard disks of any size. These capabilities facilitate recording recording with shortperiod and broadband seismometers. The data can be transferred by site visit to manually 

retrieve the data or by dialing up the system. The system also works as a digitizer and can 

communicate via radio, telephone or satellites links, sending a serial data stream to another 

recording media such as a desktop computer. 

The sensor interface has a pre-amplifier board that can take input of 40 volts peak-to-peak 

(+/- 10 volts differential) to provide a direct connection for broadband seismometers. Shortperiod seismometers can also be used with gain of X1, X3, X10, X30 or X100. The units can 

also be modified by installation of damping resistors. 

For most structural instrumentation applications, high-frequency transducers such as 

accelerometers and velocity transducers are used. These transducers in general require high 

sampling rates (e.g. 100-200sps). However, the K-2 units are capable of recording with 

broadband seismometers at low sampling rates also. Data recorded by a K-2 unit can be 

retrieved by several methods: 

1. Remove the flash card or hard disk and place it in a PC card slot on the analysis 

computer. Analysis software can then open the files as it would any other program 

file. By this way, data transfer is not necessary. 

2. Direct connection with the RS-232C cable. Data transfer or interrogation can be 

accomplished over the serial port without having to first stop data acquisition. Finally, 

the data can be transferred to a remote-site computer via telephone, radio or satellite. 

3. Using an internal high-speed telephone (14.4 KB, 28.8 KB, or 56KB), cellular or 

ISDN modem installed on the second PC card slot and runs off the same power 

supply as the K2 system. 

4. Using an external telephone (traditional or cellular) or radio modem connected to the 

RS-232C port. Usually for radio connection, spread spectrum radio modems (902-928 

MHz) are used. 

5. Specific software can be used to automatically transfer from a network of K2s to a 

PC. 

Data from K2 can also be transferred continuously and in real-time to a central data 

acquisition system using radio, telephone or satellite communication links. 

Dynamic range of K2 series is greater than 114dB. The data acquisition can be accomplished 

with 24-bit DSP (digital signal processing). 
 37 

A.4.2 Mt. Whitney Recording System 

Figure A-5. Mt. Whitney – an up to 18-channel digital recording system specifically used for 

structural monitoring 

Mostly used for structural monitoring purposes, the Mt. Whitney is an 18-channel central 

recording (requiring only one master board), high dynamic range (>110dB) accelerograph 

system. The 18 channels of acceleration data are acquired in a single file at 19 bits of 

resolution at 200 sps. 

As with K-2, the data is stored on flash card type hard disk and is equipped with a GPS 

receiver for synchronization to absolute time and an internal PCMCIA modem for remote 

alerting and data transfer. It has remote alerting capability for both event and alarm 

incidence.