Multivariate probabilistic seismic demand analysis of steel-concrete composite bridges under near-fault pulse-like ground motions

In recent years, the short-to-medium span steel-concrete composite bridges have been used more and more worldwide. However, the seismic performance of this kind of bridges, especially under near-fault pulse-like ground motions, has not been well investigated to the knowledge of the authors. In this paper, a two-span steel-concrete composite bridge is taken as a case study, whose finite element model is established by OpenSEES.To comprehensively investigate the effects of near-fault pulse-like ground motions on the seismic demands of this kind of bridges, also to fully consider the uncertainties in the input seismic ground motions, 80 double-side pulse-like ground motion records are selected from the PEER NGA strong ground motion database based on the perk point method (PPM). For comparison, 80 far-field records are also selected following the opposite rules. The spectral acceleration (Sa) and the peak ground velocity (PGV) are chosen as the two intensity measures (IMs) for the near-fault pulse-like ground motions, while the displacement of the middle pier of the case-study bridge is selected as a global engineering demand parameter (EDP). Based on a series of nonlinear dynamic time-history analysis of the case-study bridge structure, a multiple regression relationship between the global EDP and two IMs are firstly established. According to the test of significance of regression coefficients, the coefficients of determination R² are calculated and the multivariate probabilistic seismic demand model is proved to be feasible. The corresponding seismic demand fragility surfaces are then developed with the two input IMs, Sa and PGV. The numerical results of the short-to-medium span steel-concrete composite bridges show that the seismic demand of near-fault pulse-like ground motions are much larger than those of far-field ground motions. It is suggested that, to take into account more characteristics of nearfault pulse-like ground motions, the multivariate probabilistic seismic demand models should be developed for assessment of seismic vulnerability of this kind of bridges.