Vulnerability assessment of steel box-girder bridge under near-field and far-field earthquakes

The impact of near-field earthquake on vulnerability of bridges is a challenging concern which is yet to be investigated. In the present study, probabilistic seismic damage models are developed using the fragility analysis for the steel box-girder continuous bridge under the influence of the near-field and the far-field earthquakes. The fragility curves are developed for the sets of near-field ground motions (which includes the directivity pulses and the fling step) and the far-field ground motions by considering the damage measures like, displacement ductility of a pier (MPD), rotational ductility (MPR), drift ratio (MDR) and deck displacement (MDD). The frequency content of ground motion has an impact on bridge vulnerability which is investigated using a variation in PGV/PGA in near-field directivity earthquakes. The incremental dynamic analysis (IDA) is performed to develop the fragility curves for different damage states considering two ensembles of near-fault directivity earthquake having the peak ground velocity (PGV) upon the peak ground acceleration (PGA) ratio greater than 150 cm/s/g and lesser than 150 cm/s/g and also one ensemble of the near-field consisting fling effect and one ensemble of the far-field. The probabilistic earthquake demand model proposed is expected to be a well-versed framework capable of dealing with the uncertainty in ground motions while assessing the seismic damage of steel box-girder bridges. The results demonstrate that the impact of the near-field earthquake (high frequency content) directivity pulses and the fling step pose a substantial hazard to the bridge even at low PGA values. The frequency ratios (PGV/PGA) pose a major impact on the damage probability of the bridge under near-field ground motions, with higher PGV/PGA ratios having a higher damage probability than lower PGV/PGA ratios.