Empirical models of bridge seismic fragility surface considering the vertical effect of near-fault ground motions

Abstract

Near-fault vertical ground motions (VGMs) caused great damage to bridge structures, but investigations of the effect on probabilistic seismic performance and structural vulnerability are insufficient. For this purpose, a novel model of bridge seismic fragility surface conditioned on the horizontal peak ground velocity (Vx) and the αVH (ratio of vertical to horizontal ground motion) is proposed. The quantitative formulation for probabilistic seismic demand model (PSDM) and probabilistic seismic capacity model (PSCM) of bridge piers considering the effect of VGM are established, and then the empirical model of fragility surface with the parameters of Vx and αVH is obtained by convolution. A simply supported bridge model is simulated in OpenSees as an illustrated example to establish the seismic fragility surface under the combined actions of horizontal ground motion and VGM. Furthermore, the damage probability (Fr) influenced by VGM is analyzed compared with the damage probability (Fr0) without VGM, and the sensitivity intervals of the difference between Fr and Fr0 for varying damage states are identified. The mechanism of fragility variation under VGM is explored, indicating that the variation of seismic demand caused by VGM is the main reason. Finally, the seismic fragility surface model considering VGM is applied to a set of variable piers, and the sensitivity interval associated with the fundamental period is investigated under different Vx.