BNL is investigating the ranges of validity of the analytical methods used to predict the behavior of nuclear safety related structures under accidental and extreme environmental loadings. During FY 85, the investigations were concentrated on special problems that can significantly influence the outcome of the soil structure interaction evaluation process. Specially, limitations and applicability of the standard interaction methods when dealing with lift-off, layering and water table effects, were investigated. This paper describes the work and the results obtained during FY 85 from the studies on lift-off, layering and water-table effects in soil-structure interaction

Costantino, C.J.

Miller, C.A.

Philippacopoulos, A.J.

UNT Digital Library

:PT> FEB 1 2 1SBNL-NUFEG-37589STANDARD PROBLEMS FOR STRUCTURAL COMPUTER CODESA.J. Philippacopoulos, C.A. Mille~ and C.J. CostantinoBrookhaven National LaboratoryUpton, New York 11973 B N L - N U R E G — 3 7 5 8 9TI86 006247ABSTRACTVarious numerical approaches with diffarent degrees of approximationshave been developed and utilized for the evaluation of the structuralresponse of nuclear containments and other Class I nuclear structures.These approaches however, inherently rely on various degrees ofapproximations in order to simplify the mathematical equations' associatedwith the analysis methods. Thus, they may not necessarily represent theactual response behavior of the structure in question. This is especiallytrue for operating or accident conditions that involve seismic and dynamicloads. Under this program BNL is investigating the ranges of validity ofthe analytical methods used to predict the behavior of nuclear safetyrelated structures under accidental and extreme environmental loadings.During FY 85, the investigations were concentrated on special problemsthat can significantly influence the outcome of the soil structureinteraction evaluation process. Specially, limitations and applicabilityof the standard interaction methods when dealing with lift-off, layeringand water table effects, were investigated. This paper describes the workand the results obtained during FY 85 from the studies on lift-off,layering and water-table effects in soil-structure interaction.1. INTRODUCTIONThis paper describes work performed at Brookhaven National Laboratory(BNL) on the program "Standard Problems for Structural Computer Codes" (FINNo. A-3242) during FY 85. The work reported here is concerned with threetasks related to the soil-structure interaction area. There tasks are: a)lift-off effects, b) layering effects and c) water table effects. Theoverall objective of these tasks is to evaluate these effects for nuclearplant structures and utilize experimental data, when available, to quantifythe uncertainties which exist in the mathematical models.In the lift-off area, the capability of the SIM code which wasdeveloped at BNL was extended in order to treat the lift-off process.Mathematical laws pertaining to the structure-foundation interface weredeveloped to account for local •onlinearities associated with the lift-offphenomenon. The SIMQUAKE experiment was utilized to compare analyticalpredictions with the SIM code versus recorded data. Furthermore, theWork performed under the auspices of the U.S. Nuclear RegulatoryCommission.WSTHI3UTT0N OF THIS 0OCUSIENT IS UNWjHIJElconditions under which lift-off can occur in nuclear plant structures wereinvestigated.The influence of the foundation layering on the response of struc-ture-foundation system was studied. Several foundation configurations wereused with different geometrical and wave propagation characteristics inorder to quantify the effect on the system transfer functions. Based onthe numerical data generated for the system transfer functions, a methodwas developed which gives an approximation to the layering problem. Theprocedure was applied to a set of foundation configurations in order todemonstrate its applicability and limitations.Water table effects were studied for nuclear facilities located onsites characterized by high water tables. In conventional SSIt evaluationsthe foundation impedances are computed without taking into account theeffect of the water table. The influence of the latter on the foundationimpedances studied using a version of the SLAM code. A capability oftreating the water phase in SSI evaluations was developed and incorporatedinto the SLAM computer program. Numerical results for foundationimpedances were obtained with the without the presence of water in thefoundation.Details pertaining to the lift-off, layering and water table studiesare described in the following sections.2. LIFT-OFF EFFECTSA study was performed to evaluate the extent to which lift-off(separation of foundation and soil) may be important in evaluating theseismic response of nuclear power plant structures. The standard lumpedparameter analysis method wes modified by representing the lumped soil/structure interaction horizontal and rocking springs and dampers withdisturbed (over the foundation area) springs and dampers are than modifiedso that they can only transmit compressive stresses. Additional inter-action damping is included to account for the impact which occurs when aportion of the foundation which has separated comes back into contact withthe soil.The validity of the model is evaluated by comparing predictions madewith it to data measured during the SIMQUAKE II experiment. Thepredictions were found to correlate quite weli with the measured dataexcept for some discrepancies at the higher frequency (>10 cps) range.This discrepancy was attributed to the relatively crude model used impacteffects.Data is presented which identify the peak accumulation required tocause lift-off. For parameters typical of nuclear power plant structureslift-off was found to occur when the peak accelerations are in the range of0.3 - 0.6 G's. Studies were then performed to evaluate the consequence ofneglecting lift-off when it occurs. A typical result is shown as Fig. 1.Spectra were compiled for the rocking motions both including and neglectinglift-off. This was done for three inputs having peak accelerations of2 - 0 PERLhllF I j f i l iPIMGrTTTTTTn-|-rri i |~ i | i | i I iI ,i i i i 1 1 ' i i i |_ I I I I I M I I I I I l | I 1 | I t l_ J _ l _ | | |_ l-LLJ.a_LUj4~L-l-l 1 I —I—I—L-.4 .6 I . 2. 1. 6. ID.FREQUENCY ( C P S )20. 40- bO. li'ij.Fig. 1 Ratio Rotational Lift-off/No Liftoff Spectra;hat required to cause lift-off. The ratio of theI lift-off to that neglecting lift-off is shown as:hat the spectra including lift-off effects area factor of 3 when the peak acceleration is 2mse lift-off) than those neglecting lift-off.methods are available today for the; stiffness of foundations resting on the uniformi. It is desirable, however, to have simplifiedresonable approximation to the foundationare very important in the computation of theiing-foundation systems. Approximate methods forlance functions can be used in preliminary design:he overall results from computer codes. Suchproposed for the uniform halfspace and the case ofse. It 5s generally more difficult to derive." impedance functions associated with layered} the fact that the wave propagation is dispersivespeed of v.he propagation depends on the wavelength.a simplified procedure to evaluate the resnon~e ofyered foundations was developed. This procedure isadiation damping which was evaluated for different3. Comprehensive data to demonstrate the applica-he method were generated. Response comparisonsathod and a vigorous method were made for masslessith mass and finally for flexible structures,parison of the harmonic response of a foundationequal to the layer depth. The ratio of the S-wavematerial and the material of the halfspace is equalthat the response obtained by the approximategreement with the rigorously computed response.plication of the method in computing the transientke inputs was investigated. In these investi-ition was represented by a synthetic accelerationacy was examined at the floor spectra level. Fig.ween approximate and rigorously computed floorquipment damping. The agreement is very good,tained for a set of different foundation systems.oil-structure interaction process which has notin the soil-structure interaction process is theor pore water) on the response of typical nuclearocated at soil sites where the ground water tableoCMO—. a•—'CMOXccSoo- 1 - CM- . c oUJu_COHo9S•/ \+ ExncTA flPPRO•VXIMRTE0-00 1-50 3-00 4.50DIMENSIONLESS FREQUENCY6.00Fig. 2 Steady-State Response ComparisonDISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi-bility for the accuracy, completeness, or usefulness of any information, apparatus, product, orprocess disclosed, or represents that its use would not infringe privately owned rights. Refer-snce herein to any specific commercial product, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by ihe United States Government or any agency thereof. The viewsand opini jns of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.2.0 PERCENT DAMPINGO>;COLi I- JII II 1I .<i!I I I I D i l i I 1 I I 1 • I I II I I I I 111| I I I I |—i [ i |"i I i i I i I i I i n| I I I I |—I—| I | I | IEXACTAPPROXIMATEi i i i i i 1111 i i i i | i.4 .6 I . 2 . 4 . 6 -FREQUENCY (CPS)ID. ?0. 40. 60-Fig. 3 Floor Spectra Comparison3-rv-FlexibleStructure/ \Dr<j3oSaturaSoil :NIs/\tec11pcWTTvo DimensionalTvo PhaseTransmitting BoundaryFig. 4 Finite Element RepresentationtoCOLOa0- 4-8-12•16Saturated—KSoil \••• i i i iDry Soil•\Y/vt i l l1 2 3 4DIMENSIONLESS FREQUENCY A = A Q / V ,Q_T.QICALetcLU15105 •1 2 3 4DIMENSIONLESS FREQUENCY A = A Q / V rFig. 5 Companson of Dry and Fully Saturated Computations UsingMesh 1, k = 0.1 cm/secis reasonably close to the structural foundations. There are very goodreasons for the current state of affairs, however, not the least of whichis the difficulty of incorporating this aspect into the SSI analysis.At one extreme of the problem at which soil strains approach thefailure strain condition, the analysis typically focuses on the potentialfailure under the structure, and the development of liquefaction conditionsin the soil. The current state of the art in this area is to a greatextent based upon on empirical methods of analysis. The difficulties inthis area stem primarily from a lack of knowledge of soil constitutive dataat large strains.At the small strain end of the spectrum, the analytic approaches thatcan be used to study the impact of pore water are more tenable, and in facta relatively long history, extending back some 40 years, is available toguide the development. To be sure, difficulties still exist in this area,and these again are primarily associated with constitutive properties ofreal soils: However, with the availability of computer power, realisticproblems can be investigated to allow engineers to assess the potentialimpact" as seismic structural analysis.The objective of this phase of the study was to generate a finiteelement computer program to treat the seismic response of a soil-structuresystem in which a typical linear structure is situated near the surface ofthe ground. The soil is represented as a linear medium in which allpc^rt"1"?! nonlinearities are at most lumped into an equivalent hystereticdamping modulus. However the soil pores is saturated with compressiblefluid (water) to some depth close to the structure (Fig. 4). A numericalfinite element model is developed to treat this two-phased linear media,this model being based upon the analytic developments extending back to thework of Biot. In keeping with typical SSI analyses, in order to make thefinite element approach yield reasonable results, a comparable two-phasetransmitting boundary formulation was developed to adequately take care ofrelative damping effects.The code was developed and made operational with several parametervariations performed to assess impact of pore water on response. A typicalresult is shown in Fig. 5, in which a comparison of frequency dependentinteractive coefficients (stiffness and damping) is p -ented for verticalmotions of a rigid footing of half-width A. As may be moted, the porewater has a major impact on the character and magnitude of thesecoefficients! with effective radiation damping increasing from the dry tothe fully satuarated case.ACKNOWLEDGEMENTThis work is being performed under the auspices of the Division ofEngineering Technology, Office of Nuclear Regulatory Research, U.S. NuclearRegulatory Commission. The authors wish to express their special thanks toNRC Project Manager, for his continued support and guidance throughout thecourse of this program.

Standard problems for structural computer codes

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Standard problems for structural computer codes

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