Strength Reduction Factor Based On Main Shock-aftershock Sequence-type Ground Motions

The historical earthquakes have shown that many aftershocks would happen in theshort intervals of time after the mainshock. Because of the short interval of timebetween the aftershock and msinshock, it is impossible for the damaged structureinduced by mainshock to be repaired before the aftershock. Thus for the structuresunder the mainshock-aftershock sequence, the damage of structure will be increased byaftershock for the damage accumulation. However, the current seismic codes in theworld are based on the isolated ‘design earthquake’, ignoring the additional damageinduced by aftershock. The strength reduction factor plays the very important role in theseismic design of structures, and the previous models of strength reduction factor cannotaccount for the disadvantage of aftershock. Therefore, it is significant to investigate thestrength reduction factor based on the mainshock-aftershock sequence-type groundmotions, in order to incorporate the aftershock in the seismic design. This thesis studiesthe characteristics of the strength reduction factor based on the recorded and simulatedmainshock-aftershock sequence-type ground motions. The main contents of this thesisare as follows:(1) The characteristics of the constant ductility strength reduction factor are studiedstatistically based on the recorded and simulated mainshock-aftershock sequence-typeground motions. The influences of period of vibration, ductility factor, site class, theintensity of aftershock ground motion, hysteretic model and damping ratio on theconstant ductility strength reduction factor are analyzed. A simplified expression ofconstant ductility strength reduction factor is recommended.(2) The characteristics of the constant damage strength reduction factor areinvestigated with the modified Park-Ang damage index. The effects of period ofvibration, damage index, site class, the intensity of aftershock ground motion, hysteretic model and damping ratio on the constant damage strength reduction factor are studied.A simplified equation of constant damage strength reduction factor is suggested.(3) The influence of P-Δ effect on the constant ductility and constant damagestrength reduction factors are researched, and the corresponding modified factors ofsimplified expressions for strength reduction factors are proposed.