Ground-structure Interaction Assessment

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Structural Response AspectsInstrumental Systems for Diagnostics ofSeismic Response of Bridges and Dams

4.3 Ground/Structure Interaction Assessment

W.-S. TsengInternational Civil Engineering Consultants, Inc.

INTRODUCTION

All instrument-recorded data obtained during an earthquake are response data for the instrumented loca-tions. To use such recorded data as the bases for diagnosis of the behavior and/or condition of a structureduring the earthquake, one must understand the process by which the earthquake response of the structureis inferred from such data. Without such an understanding, the results of the diagnosis could be mislead-ing and, in some cases, could even lead to erroneous conclusions.

Among many factors that influence the earthquake response of a structure during an earthquake, themost important factor is a realistic and accurate measure of the earthquake input to the structure duringthe earthquake. In all cases of structures with instrument recording of earthquake response motions, theactual earthquake input to the structure is not directly measurable. Instead, it is inferred from responsemotions recorded at so-called “free-field ground stations” located at some distances away from, but adja-cent to, the structure. The use of motions recorded at such locations to infer the actual earthquake inputto the structure during an earthquake involves assumptions of how the earthquake input to the structure isrelated to the ground motions recorded at the free-field ground stations. It is the objective of the discus-sions in this session to (1) examine the various implicit assumptions embedded in using the data recordedat the free-field ground stations to infer the earthquake input to the structure and (2) discuss instrumenta-tion need for obtaining recording data that can be used for a more accurate inference of the earthquakeinput to the instrumented structure during an earthquake.

GROUND/STRUCTURE INTERACTION ASSESSMENT

In its most fundamental sense, the earthquake input to a structure during an earthquake must be defined interms of the traction time-functions or time-histories acting on all ground/structure or, more precisely,ground/foundation interface. While it is realistically not possible to measure such forcing time-functionsdirectly by instruments, the objective is then to obtain sufficient instrumental data that can be used to infersuch input functions. In order to characterize such forcing functions during an earthquake, it is necessary tocharacterize the following parameters of the ground medium that surrounds and supports each foundationof the structure:

� Free-field ground-motion time histories at all points on the ground/structure interfaceboundaries

� Traction-versus-displacement relationships of the ground medium at all points on theground/structure interface boundaries

It is clear that measurement of the free-field motions in Item (1) above by instruments is practicallyimpossible. Instruments can, however, be placed to measure the ground/structure-interaction responsemotions of each foundation of the structure, even though such measurements are rarely made in actual

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situation.

The characterization of traction-displacement relationships in Item (2) mentioned above involvescomplete characterization of the constitutive relations of the soil/rock materials of the ground mediumadjacent to each structural foundation under the seismic loading condition. This subject by itself is rathercomplex and is a subject currently still under intense research. It involves characterization of the nonlinearinelastic near-field and linear elastic far-field behavior of the soil/rock ground medium. Within the scope ofthis workshop, it will not be further discussed.

In the current common practice, the inference of seismic input to an instrumented structure is usuallymade, as mentioned previously, using the ground response motions obtained at some free-field groundinstrument stations, which are located at some distances away from but in the vicinity of the structure. Toutilize the motions recorded at such locations to infer the free-filed seismic input motions to the structure asdefined in Item (1) above involves assumptions on the seismic input parameters as follows:

� Seismic input wave field in the free-field ground medium within the dimensions ofthe structures

� Influence of ground topography on the seismic wave field in the free-field groundregion of the structure

� Influence of soil/rock dynamic response characteristics on the free-field seismic groundmotions

� Spatial variations (due to both wave-passage and scattering effects) of free-field groundmotions within each ground/structure interface region and among all such regionsof the structure

� Scattering of free-field seismic waves due to presence of the structural foundations,i.e., foundation scattering of free-field seismic motions

Because of the necessity of employing these assumptions in inferring the seismic input to the struc-ture based on earthquake motions recorded at free-field ground stations, it would be very desirable to haveadditional instrument recordings that will provide additional earthquake response data, which can be usedto validate and/or modify such assumptions and to quantify their effects.

FIELD EVIDENCE OF GROUND/STRUCTURE INTERACTION

In-depth analyses of instrument-recorded earthquake response motions of a few instrumented structuressupported on soil sites in the past have provided valuable evidences that, in order to assess the earthquakeresponse behavior of these structures, the effects of ground/structure interaction must be properly taken intoconsideration. Two of such examples, in which the author has a direct experience, are briefly describedbelow.

Lotung, Taiwan, Soil-Structure Interaction Field ExperimentA comprehensive field experimental program, called the large-scale seismic test (LSST) program,

which involved a large ¼-scale cylindrical-shell structure simulating a prototypical nuclear power plantreactor-containment structure, erected at a relatively soft soil site in Lotung, Taiwan, was conducted under

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the joint sponsorship of the U. S. Nuclear Regulatory Commission (NRC), Electric Power Research Institute(EPRI), and Taiwan Power Company (TPC) in the 1980s. The objective of the program was to investigateand quantify the soil-structure interaction effects of structures during earthquakes. Extensive instrumentationwas provided to record the earthquake response motions of the containment model structure, internal struc-ture and piping system, free-field ground-surface as well as soil response motions at depth, and spatial varia-tions of the free-field ground motions in the vicinity of the experimental structure. In addition, since theexperimental site was located within the Lotung, Taiwan, SMART-1 instrument array, the extensive free-fieldinstruments of the array provided further recorded regional ground-surface response data, which furthersupplemented the data recorded in the LSST program. The field experimental facility was completed in1986. During the following year in 1987, a total of 16 earthquakes occurred, producing a vast amount ofinstrument-recorded data for the experimental facility. The free-field horizontal ground surface accelerationsrecorded at the site varied from a low value of a few percent g to a high value of 0.23g [1].

In-depth post-earthquake analyses of the data recorded at the free-field ground surface as well as down-hole recording stations, and in-structure response locations have indicated that: (1) spatial variation of free-field ground motions at he site was significant, although it has a relatively small effect on the dynamicresponse of the model structure, which had a relatively small (15m-diameter) foundation foot-print; (2) free-field ground motions varied with depth below the ground surface, and such variations can be approximatelymodeled locally in the vicinity of the model structure by vertically propagating plane shear and compressionseismic waves for the horizontal and vertical motions, respectively; (3) scattering of free-field ground mo-tions due to the presence of the structural foundation was important and its effect must be taken into accountin assessing the dynamic response of the model structure; and (4) soil-structure interaction was significant forthis model structure, which was supported on the relatively soft soils at the site; and, its effect must beappropriately considered in assessing the seismic responses of the structure [1].

Response of BART Hayward Elevated Structures during the Loma Prieta EarthquakeA three-span segment of the Bay Area Rapid Transit (BART) elevated structures located near the

BART Hayward Station has been instrumented with strong-motion accelerometers by the California StateDepartment of Mines and Geology (CDMG) under the California Strong Motion Instrumentation Pro-gram (CSMIP). The instrumentation is capable of recording both the free-field ground-surface and the in-structure earthquake response motions. During the 1989 Loma Prieta earthquake, both free-field ground-surface and in-structure dynamic response data were recorded. The peak horizontal free-field ground-surface acceleration recorded was 0.16g.

Post-earthquake analyses conducted to study the structural responses during the earthquake usingthe motions recorded at the free-field ground-surface station as the input and comparing the predictedseismic response motions of the structure with the corresponding recorded earthquake response motionsindicated that the ground/structure interaction effect was important and it must be taken into account inthe response prediction in order to obtain a reasonable correlation between the predicted structural re-sponse based on the free-field recorded motions as the seismic input and the recorded in-structure responsemotions [2]. Without accounting for the ground/structure interaction effect, the predicted structural re-sponse amplitudes could be underestimated by as much as 30 to 50%.

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INSTRUMENTATION NEEDS

From the foregoing discussions, one can deduce that, with the exception of structures supported directly onhard rock, the use of instrument-recorded earthquake response motions in the free-field ground and in thestructure for structural condition diagnosis purposes must also properly consider ground/structure interac-tion effects occurring during the earthquake. As direct measurement to provide data for characterizingground/structure interaction effects is difficult in practice, one must carefully design the instrumentationprogram such that the recorded data can be used to infer the various effects of the ground/structure interac-tion. It is the author’s opinion that, in addition to the free-field ground surface instruments normallyprovided, as a minimum additional instruments should be installed for measuring earthquake responsemotions of each foundation of the structure directly and for measuring free-field ground motions that canbe used to characterize (1) spatial variations of ground motions within the ground region of the structure,(2) variations of free-field ground response motions with depth below the ground surface within the depthof the structural foundations, and (3) influence of global as well as local nonlinear soil responses on theoverall structural response. Furthermore, in order to use the instrument-recorded data effectively to inferground/structure interaction effects, extensive pre-earthquake assessment of the effects of ground/structureinteraction on the structural response must be carried beforehand.

References

1) Electric Power Research Institute (1989). Proceedings: EPRI/NRC/TPC workshop on seismic soil-struc-ture interaction analysis techniques using data from Lotung, Taiwan, EPRI NP-6154, Palo Alto, Calif.

2) Tseng, W. S., Yang, M.S., and Penzien, J. (1992). Seismic performance investigation of the HaywardBART elevated section, California Strong Motion Instrumentation Program, Data Utilization Report CAMIP/92-02, Report by International Civil Engineering Consultants, Inc.

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4.3 Discussion

J. KaoGolden Gate Bridge and Transportation District

Presentation

It was emphasized that in order to properly evaluate and assess damage of a structure, validation of themethods of interepretation of the measurements is critical; only then can appropriate emergency responseto an earthquake be addressed. The main objective is to examine the implicit assumptions and discussinstrumentation needs.

To characterize proper response spectra the following characterization is necessary:

� ground motion history of ground/structure interaface.

� traction-displacement relationships of ground medium at interface

� integration of traction force all around the boundary

For appropriate interaction assessment we need:

� seismic input of ground topography

� influence of soil/rock dynamic response

� spatial variations of ground motion

� scattering of seismic input motions due to founcation

Field evidence of ground/structure interaction from various earthquakes is critical: i.e., Lotung,Taiwan and the BART elevated structures of importance were different responses due to com-puter models at Lotung and the additional 1.5 factor from pile cap interaction.

Experience suggests the following instrumentation goals:

� earthquake response motions of foundation

� spatial variations of free-field ground motions

� variations of free field ground motion with depth of foundation

� influence of global as well as local nonlinear soil response

Discussion

A number of comments and questions followed the presentation. Correlation with strong-motion mea-surements will reduce the conservativity of the design and improve the owner’s level of confidence. Deter-mination of actual fundamantal modes will validate models.

Suggestions included:

� Additional instruments at Lotung. Pore pressures were primarily measured and free-fieldmeasurements provided a useful validation of site ground motions

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� Measurement data can be input to models to determine damage

� Measure the relative displacements for stress calculations, although it is difficult to read atpile caps

� Discussion of how Caltrans places recorders along the depth of pile during construction.Fiber optic strain gages might be used for such measurements

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Ground-structure Interaction Assessment