The Characteristics Of Near-fault Sequential Ground Motions And Its Impacts On Seismic Fragility Of Continuous Girder Bridges

Particular characteristics of the tectonic plate movements in Southwest China have been constantly producing a complex and unprecedented development of large and deep active faults.The continuous plate thrusting and collisions have caused strong seismic activities on the eastern margin of the Qinghai-Tibet Plateau,resulting in frequent large earthquakes.Usually,numerous aftershocks may occur following the mainshocks,prompting the ground to experience sequential movements.Since the damage of bridge structures caused by mainshocks might not get rehabilitated in time,accumulated severer damage or even collapse might occur subjected to the action of sequential ground motions.Previous studies mainly focused on separately studying near-fault ground motions or mainshock-aftershock sequences,with few studies combining the two issues together for analyzing the seismic performance of bridge structures.As a crucial lifeline project,bridge structures play an important role in the implementation of emergency rescue as well as post-disaster recovery and reconstruction.Under such a context,this study intends to investigate the characteristics of near-fault sequential ground motions as well as its impacts on the seismic performance of continuous girder bridges.The main contents are as follows:(1)A quantitative algorithm based on the moving average(MA)method is proposed for identifying near-fault pulse-like ground motions.To obtain a smooth velocity,MA is adopted to filter out high-frequency content of the original records.The pulse-starting and-ending time points as well as the pulse index of the velocity-pulse are then determined by the peakpoint method and the energy method.Based on statistical results,the energy threshold level for identification of the velocity pulses is derived,and a near-fault mainshock database for dynamic analysis of bridge structures is compiled.Finally,the proposed algorithm is applied to analyzing the 2010 Darfield earthquake data of New Zealand;strong correlations are found between the velocity pulses and the forward-directivity effects.(2)Near-fault mainshock-aftershock sequences are constructed based on the spectramatching method.The software of Seismo Match is utilized to achieve spectral matching of the near-fault ground motions,and then a near-fault mainshock-aftershock database consisted of different intensities of aftershocks and their corresponding mainshocks is built.In addition,based on the Bouc-Wen hysteresis model,the inelastic responses of single-degree-of-freedom systems subjected to near-fault sequential ground motions are analyzed.(3)The seismic performance of a typical continuous girder bridge subjected to near-fault records is investigated.By means of the incremental dynamic analysis and seismic fragility analysis,the seismic damage rules of continuous girder bridges along the transverse and longitudinal directions are studied.Furthermore,the mathematical distribution characteristics of the curvature of the bridge pier bottom and the displacement of the support are analyzed,and then the correlations between the failure states of bridges and the intensity measure of ground motions are confirmed.