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In uence of foundation exibility on the seismic response · PDF fileframe; Nonlinear analysis; Design code. Abstract. Recent studies have shown that the e ects of Soil-Structure Interaction

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  • Scientia Iranica A (2017) 24(3), 979{992

    Sharif University of TechnologyScientia Iranica

    Transactions A: Civil Engineeringwww.scientiairanica.com

    Inuence of foundation exibility on the seismicresponse of low-to-mid-rise moment-resisting framebuildings

    H. Tahghighi and M. RabieeDepartment of Civil Engineering, University of Kashan, Kashan, P.O. Box 87317-51167, Iran.

    Received 24 October 2015; received in revised form 25 January 2016; accepted 13 February 2016

    KEYWORDSSeismic response;Soil-structureinteraction;Moment-resistingframe;Nonlinear analysis;Design code.

    Abstract. Recent studies have shown that the eects of Soil-Structure Interaction(SSI) may be detrimental to the seismic response of structure, and the neglect of SSIin analysis may lead to an un-conservative design. The objective of this study is tosimulate the performance of multi-storey building-foundation systems through a Winkler-based approach. Four typical steel Moment-Resisting Frame (MRF) buildings on threesoil types with shear wave velocities of less than 600 m/s subjected to actual groundmotion records of varied hazard levels are modeled with and without SSI. It is observedthat the performance level of models supported by exible foundation, particularly in anintense earthquake event, may alter signicantly in comparison to xed-based structures.Moreover, for MRFs on soft soil, the nonlinear foundation is found to have a signicanteect on the force and displacement demands. This indicates the necessity for considerationof exible foundation behavior in the modern design codes in order to accomplish a moreeconomic yet safe structural design. 2017 Sharif University of Technology. All rights reserved.

    1. Introduction

    The dynamic response of a structure to earthquakeexcitation can signicantly be aected by interactionsbetween three linked systems: the structure, thefoundation, and the soil underlying and surroundingthe foundation. In reality, the supporting soil some-what inuences the structural response by permittingmovement due to its natural ability to deform. Soil-Structure Interaction (SSI) analysis evaluates the col-lective response of these systems to a specied free-eld ground motion. When the ground is sti enough,the dynamic response of the structure will not beinuenced signicantly by the soil properties during

    *. Corresponding author. Tel.: +98 31 55912430;Fax.: +98 31 55912424E-mail addresses: [email protected] (H. Tahghighi);[email protected] (M. Rabiee)

    the earthquake, and the structure can be analyzedunder the xed-base condition. But, for the structureresting on a exible medium, the dynamic response ofthe structure will be dierent from that of the xed-base condition, owing to the interaction between thesoil and the structure [1]. In the case of a exible-basestructure, in addition to the added rocking componentto the horizontal motion of the structure, a part ofthe structure's vibrating energy will be transmittedto the soil layer, and it can be dissipated due toradiation damping resulting from the wave propagationand hysteresis damping of the soil materials.

    In general, SSI eects can be summarized asfollows: reduction of the natural frequency of thesystem, increase in damping, increase in the rate of thelateral displacement, and change in the force demandsof the structure [2,3]. For sti structural systems,such as shear wall and braced frame founded on soil,ignoring the inuence of foundation movements could

  • 980 H. Tahghighi and M. Rabiee/Scientia Iranica, Transactions A: Civil Engineering 24 (2017) 979{992

    lead to signicant misestimation of the fundamentalfrequency in the system [4-6]. Analyses conductedunder various soil and structure conditions showedthat this inuence depends mainly on the soil-structurerelative rigidity. The increased period and dampingof the soil-structure system largely lead to the conse-quences and the benecial eects the SSI has, whichare mostly ignored in seismic design of buildings. Thisconclusion could be misleading, since ignoring base

    exibility may over-or under-predict seismic responseof the structure depending primarily on the charac-teristics of the ground motions. It was shown thatthe SSI can play a detrimental role in producing suchinuences, and neglecting its inuence could lead to anunsafe design [7,8]. According to more recent ndings,neglecting the eects of SSI on seismic performanceof rocking structures with shallow foundations maydrastically bias the estimation of force demands againstnear-fault pulse, such as ground motions [9]. Therefore,it is recommended to take the nonlinear SSI eectsinto consideration in order to avoid rough estimationof structural seismic demands.

    Although not widely used in practice, engineeringguidelines are available for simple evaluation of SSIeects. Recent code-compliant seismic designs for SSIsystems, such as NEHRP [10] and ASCE 7 [11], arebased on the approximation to which the predominantperiod and associated damping of the correspondingxed-base system are modied. On the other hand,ATC 40 [12] and FEMA 356 [13] partially address theeects of exible foundation through inclusion of thestiness and strength of the soil components of thefoundation (Winkler-based model) on the structuralanalysis. However, none of these procedures addressesthe shaking demand on the structure relative to thefree-eld motion caused by kinematic interaction or thefoundation damping eect. Guidelines on the inclusionof the eects of kinematic interaction are given inFEMA 440 [14] and ASCE 41 [15]. The Ratio ofResponse Spectra (RRS) factor is used to representkinematic interaction eects in these codes. RRS issimply the ratio of the response spectral ordinatesimposed on the foundation (i.e., the foundation in-put motion) to the free-eld spectral ordinates. Itis noteworthy to mention that the kinematic eectsare usually most pronounced in short periods, andhence are unlikely to signicantly aect responses ofbuildings within a fundamental period greater thanapproximately 1 sec [6,16].

    SSI, particularly for superstructures resting onrelatively soft soils, may signicantly amplify thelateral displacements and inter-storey drifts [17,18].The amplication of lateral deformations may changethe performance level of the building frames whilesubjected to earthquakes. Tabatabaiefar et al. [18]investigated the accuracy of nonlinear method against

    equivalent linear method for dynamic analysis of SSI,using shaking table tests. They concluded that theresults obtained from the fully nonlinear method ofanalysis t the experimental results reasonably well,but the equivalent linear method underestimates theinelastic seismic response of mid-rise moment-resistingbuilding frames resting on soft soils. Therefore, thefully nonlinear method was recommended by practicingengineers to be used.

    Various numerical methods were used to modelthe behaviour of structures on shallow foundations [19-22]. However, the application of simple methods,such as the Winkler approach, is preferred in practicalSSI problems. In this context, Beam-on-Nonlinear-Winkler-Foundation (BNWF) method, proposed byHarden et al. [23], Harden and Hutchinson [24], andlater by Gajan et al. [25], has been widely appliedin recent studies due to its relative simplicity andminimal computational eort (e.g. [9,26,27]). Thepresent article attempts to cover a wide range of inter-action problems in terms of the superstructure, the soilcharacteristics, and the seismic excitation intensities inwhich the BNWF method is used to model the behaviorof shallow foundation. To do so, an extensive para-metric study, including dierent approaches of analysisalong with various soil and structure conditions, iscarried out to evaluate seismic response of low-to-mid-rise steel Moment Resisting Frame (MRF) buildingssubjected to earthquake motions of dierent hazardlevels. The numerical results indicate that the SSIwhich is signicantly important to the performance-based design of structures can alter the force and driftdemands.

    2. Structural and geotechnical description ofthe models

    A set of 4-, 8-, 12-, and 16-storey steel MRFs locatedon hypothetically soft, medium, and hard sites isconsidered. The building models have a oor planof 18:0 18:0 m, three bays each having a horizontaldirection at intervals of 6.0 m, a uniform mass distri-bution over their height, and a non-uniform stinessdistribution (Figure 1). The storey height of the modelswas considered as 3.3 m; a normal height for residentialbuildings. These models represent a large number ofconventional low-to-mid-rise buildings in mega citiesat relatively high-risk earthquake-prone countries, suchas Iran. Buildings were designed as special framesbased on the requirement of Iranian national buildingcodes [28,29] and Iranian code of practice for seismicresistant design of buildings (Standard No. 2800) [30].The dead and live loads of 600 and 200 kgf/m2, respec-tively, were used for gravity loads. Consequently, theself-weight of 306720, 643960, 982530, and 1334880 kgwere obtained for 4-, 8-, 12-, and 16-storey frames,

  • H. Tahghighi and M. Rabiee/Scientia Iranica, Transactions A: Civil Engineering 24 (2017) 979{992 981

    Figure 1. Conguration of building models: (a) Plan and (b) elevation.

    respectively. In regard to the frames' design, theimportance factor of I = 1, response modicationfactor of R = 7:5, and seismic zone factor of A = 0:30were considered. In this paper, the steel type St-37was used for the structural beam and column members.Table 1 lists the details of utilized I-beam sections forthe MRF models shown in Figure 1.

    Columns are supported by strip footings whichrest on dierent soil conditions including types II(375 < Vs < 750 m/s), III (175 < Vs < 375 m/s),and IV (Vs < 175 m/s) according to the classicationof the Standard No. 2800. It is worth

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