Mainshock-aftershock Ground Motion Characteristics And Seismic Response Analysis Of Structures

The existing earthquake records indicated that many aftershocks were triggered by strong mainshock.The time interval between the mainshock and aftershock is so short that it is impossible for the mainshock-damaged structures to be repaired before the occurrence of the subsequent aftershocks.In this scenario,the strong aftershocks potentially aggravate the damage of the structures.This phenomenon,well known as the damage accumulation,has been confirmed in the post-earthquake field reconnaissance.Therefore,the overall effects of seismic sequences on the response of structures,rather than those of the sole mainshock,should be taken into account in the seismic design.Unfortunately,the current seismic design codes just consider the mainshock and ignore the danger of aftershocksIn order to understand the seismic performance of structures under mainshock-aftershock sequences,the inelastic responses of structrues under manishock-aftershock sequences and the nonlinear static analysis of structures are investigated.Along these lines,this paper initially investigate the generation of artificial mainshock aftershock sequences;next propose an approach for identifying the dynamic characteristis of structures;then study the effect of aftershock on the maximum displacement,residual displacement,input energy and hysteretic energy;finally provide a propose for energy-based nonlinear static analysis by considering mainshock-aftershock sequences.The main research contents of this paper are as following:(1)The state-of-art of methods for generation artificial mainshock-aftershock sequences have systematically reviewed and pointed out their limitations that enlarge the responses of structures compared with those caused by the real recorded mainshock-aftershock sequences.An attenuation equation for aftershock's intensity measure is also given.A new generation method that can simulate a similar structural response compared with that due to the real recor ded mainshock aftershock sequences is proposeed based on the attenuation equation.(2)The limitations of the conventional Fourier-based technique in predicting the elastic and inelastic period of single degree of freedom(SDOF)systems when performing both in the elastic and inelastic regime under both sinusoidal waves and recorded strong ground motions are investiated.The coupling between the frequency characteristics of both the base excitation and the time domain structural response of SDOF systems is revealed.An easy to use,though,improved approach for FFT,namely,the Decoupled Fourier Amplitude Spectrum(DFAS),that addresses the aforementioned limitations and enables quantifying period elongation by removing the influence driven by the base excitationpropose are proposed;In order to estimate both elastic and inelastic predominant periods,the accuracy of the proposed DFAS technique against the conventional FFT one is critically assessed.The Decoupled Fourier Amplitude Spectrum is adopted to identify the dynamic characteristics of structures under 218 mainshock-aftershock sequences.The influences of vibration period,ductility factor,hysteretic model,aftershock intensity and damping are studied.A prediction equation is proposed to calculate the inelastic period of structures under the earthquake sequences.The upper and lower bound of the period range in the spectral matching is modified for selecting earthquake sequences in performance-based seismic design.(3)The inelastic response of SDOF structures under mainshock-aftershock sequences is studied by evaluating the effect of aftershock on the structural maximum displacement,residual displacement,input energy and hysteretic energy.The indix that can represent the effect of aftershock is gi ven.(4)The demand curve and capacity curve are introduced for structures subjected to mainshock-aftershock sequences.The influences of vibration period,ductility factor,post-stiffness ratio,mainshock-aftershock sequences,and pulse-like ground motions on demand curve are discussed.A prediction equation of the demand curve for structures under earthquake sequences and pulse-like ground motion is given.In order to verify the validity of energy-based pushover analysis method,the results from pushover analysis are compared with the results from nonlinear dynamic analysis.