Structural Damage Evolution Under Mainshock-aftershock Seismic Sequences With Frequency Content

Strong aftershocks may worsen the damage state of structures and even lead to collapse, by reason of damage accumulation. At present, in the seismic design code at home and at abroad, there are not special consideration shown for the unfavorable role of aftershock. Therefore, it is necessary to propose an aftershock incremental damage model with structural damage state after mainshock attack considered, to evaluate the influence of aftershocks on structures quantitatively. Frequency content, one of the three parameters of ground motion, poses influences on structural damage evolution. Researching on the structural global damage evolution of under the action of mainshock-aftershock seismic sequences with different frequency content, could provide a basis for design method considering structural aftershock damage performance.In the beginning, on basis of the modified Park-Ang damage model, an aftershock-induced incremental damage model considering mainshock damage state of sections is proposed, where the weighted combination of section damages proposed by Park and Ang to represent the global damage is modified to obtain an incremental damage model that could reasonably depict RC structures under the action of aftershock. The proposed incremental section damage model for structures suffering from aftershock could basically reflect the damage process of a damaged RC section; The Park-Ang combination method modified herein could relatively reasonably reflect the damage evolution of a RC frame structure attacked by earthquake.Thereafter, natural ground motion records with complex frequency content are simplified as harmonic excitations with different periods, and then applied on objects with SDOF to study the damage evolution rule of SDOF structures. Besides, evaluations on several frequency content parameters of ground motion records are conducted. The study indicates that, compared to the hardening bi-linear system, the deteriorative bilinear system has its damage more sensitive to the long period components of an excitation while the increase of structural period as it yields and aggravation of damage state would worse the scenario; Among the frequency content parameters, extraordinary period of spectral acceleration TpA, characteristics period of ground motion Tg and mean period Tm, Tm presents the best correlation with structural damage, with Tg worse and TpA worst.Finally, with OpenSees, incremental dynamic analyses are conducted on multiple RC structures to study the effect of factors such as frequency content of aftershock, mainshock-induced structural damage state, and structural dynamic characteristics, on the damage evolution rule of structures under mainshock-aftershock seismic sequences. The research shows that, long Tm aftershocks tend to arouse structural incremental damage in contrast to short Tm aftershocks; if the structural period before and after aftershock is located in the vicinity of the mean period of ground motions, the aftershock would induce a severer damage and the role played by its frequency content would be magnified greatly. When the aftershock has small intensity, structural global damage states are mainly controlled by mainshock damage state, which effect with the increase of the intensity of aftershock would mitigate, the global damage index of a structure under mainshock-aftershock seismic sequences is then controlled by the intensity and the frequency content of aftershock.