Performance Evaluation Of Base-isolated RC Frame Buildings Subjected To Near-fault And Far-field Ground Motions Considering SSI Effects

Seismic isolation technology is widely used to protect structures against damage from strong ground motions by limiting the earthquake energy transmitted to the superstructure.A few researches were conducted to investigate the performance of LRB isolation systems in high-rise buildings.However,researches should be conducted to ensure that the base isolation can effectively improve the performance of high-rise buildings.A 12-story residential building with RC moment frames is selected and considered to be located in a high seismic region constructed on soft soil(site class D)designed using the modal response spectrum method,the frame structure is designed to satisfy design limits under the design-based earthquake(DBE)level according to IBC(2012)and ACI 318-14 provisions.Lead–rubber bearing(LRB)isolation system is used,the mechanical properties and the dimensions of the LRB isolation system are calculated.The nonlinear time history analysis is carried out using computer software SAP2000 to investigate the seismic responses of the base-isolated and base-fixed RC frame under various intensities of four earthquake events,every event has three records as near-fault with a pulse(NFP),near-fault without a pulse(NFNP)and far-fault record(FF),these records were scaled to match the design earthquake level(DEL).The inelastic behavior of the superstructure was modeled by assigning the plastic hinges at the end of frame members while the behavior of a lead rubber bearing modeled as a bilinear hysteretic element and defined as link element in SAP2000.The numerical model was developed to perform a linear time history analysis to investigate the effect of soil-structure interaction(SSI)on the performance of baseisolated frame and fixed frame as well.In this case,the LRB isolators were modeled as inelastic elements while the soil and superstructure were assumed to be elastic.The soft soil was considered and modeled as a plane strain element using finite element method with two degrees of freedom of translation.The direct half space method was utilized,and the boundary conditions were defined as artificial viscous spring-dashpot to represent the far field.The performances of the isolated structure were compared with those of the fixedbase structures in terms of peak roof acceleration,peak inter-story drift,base shear,maximum base displacement and number of the generated plastic hinges.The results of the nonlinear time history analysis showed a significant reduction of the structural response of the base-isolated frame compared to the fixed-base one under far-field and near-field without pulse-like other than near-fault with pulse-like ground motions.Also,the maximum base displacement is more considerable in the isolated frame subjected to near-fault with pulse-like than those far-fault ground motions while the results of linear time history analysis with considering SSI effects showed that the structural responses of the base-isolated and fixed frame increased very significantly and the efficiency of LRB isolation is reduced.