Study On Incremental Damage And Collapse Capacity On SDOF Systems Under Mainshock-Aftershock Sequences

The mainshock-damaged structure often suffers “secondary damage” due to aftershocks. The aftershock-induced incremental damage makes the damaged structure more vulnerable to collapse, and thus results in enormous casualties and economic loss. Therefore, it is necessary to conduct study on the incremental damage and collapse of structures under mainshock-aftershock sequences. In this thesis, the incremental damage and the collapse capacity are investigated based on the nonlinear single-degree-of-freedom(SDOF) systems. This study aims to illustrate the damage potential of aftershocks from the viewpoints of incremental damage and collapse capacity. The main contents are summarized as follows:(1) The approaches that are used to generate artifical mainshock-aftershock sequences are studied. Three frequently used approaches including repeated approach, randomized approach and attenuation approach, are employed to generate corresponding artifical sequences as inputs by using the same seed mainshock records. Earthquake intensity measures of artifical mainshock-aftershock sequences and real mainshock-aftershock sequences are calculated and compared. The influences of different simulation approaches of mainshock-aftershock sequences on characteristics of seismic ground motions are evaluated.(2) The damage and incremental damage of mainshock-aftershock sequences are studied based on different damage parameters, including m aximum displacement, ductility demand, residual displacement ratio, inelastic displacement ratio and modified Park-Ang damage index. The effect of strength reduction factor, approaches of generating artifical mainshock-aftershock sequences and hysteretic models of SDOF systems on structural damage and incremental damage are examined.(3) The collapse capacity of mainshock-aftershock sequences is conducted for SDOF systems by incremental dynamic analysis(IDA). The SDOF system is modelled with the modified I-K hysterestic behavior. Through the modification of the key parameters of the modified I-K hysterestic model, the effects of structural characteristics including post-capping stiffness, ductility capacity, cyclic deterioration, hysteretic pinching and P-Δ effect on the collapse capacity are studied. Moreover, the influences of near-field and far-field earthquake records and the approaches of generating artifical mainshock-aftershock sequences are also investigated. By comparing the collapse capacity to resist the mainshock and that to resist mainshock-aftershock sequences, the aftershock impacts on the collapse capacity are investigated from the viewpoint of reduction factor of the collapse capacity.The results indicate that the aftershock intensities of earthquke sequences generated by repeated approach and randomized approach are both larger than those of the real earthquake sequences. The repeated mainshock-aftershock sequences and the randomized mainshock-aftershock sequences tend to overestimate the incremental damage of SDOF systems, whereas the artifical sequences generated by the attenuation approach will underestimate the incremental damages of SDOF systems since the attenuation method commonly generate aftershocks of smaller intensities than the other approaches. Overall, the incremental damage due to aftershocks can not be neglected especially for the SDOF systems of short periods. The capacity of SDOF systems to resist collapse is significantly weakened because of the effect of aftershock. The collapse capacity of SDOF systems under mainshock-aftershock sequences has larger variance than that under mainshocks only.