Are Bridges Safe Under Near-Fault Pulse-Type Ground Motions Considering the Vertical Component?

This study aims to investigate the effect of the vertical component of earthquake excitation on the seismic safety of bridges. Two types of three-dimensional bridges, including an ordinary beam bridge, and a suspension bridge, are modelled, and their seismic performance is evaluated under a set of seismic records. Particular emphasis is placed on near-fault pulse-type ground motions including not only the horizontal component but also the vertical component that is particularly critical for this kind of records. The axial load and bending moment are the critical response indicators checked to assess the design safety requirements of the bridges. In the numerical analysis, bridges are considered to be safe only if the moment induced by the ground motion is lower than the corresponding bending resistance. For the considered bridges and for the given set of near-fault pulse-type ground motions including the vertical component, fragility curves are constructed to show the probability of exceeding the limit state threshold representative of the safety of the bridges. The peak ground velocity is considered to be the intensity measure for developing the fragility curves. A critical comparison of fragility curves accounting for and neglecting the vertical component of the seismic excitation is illustrated. It is observed that the probability of crossing the limit state condition is significantly affected by the consideration of the vertical component. In particular, this effect is much higher in ordinary beam bridges. The analyses highlight that the seismic vulnerability of bridges is increased by the vertical component of the excitation, which requires further investigation to develop novel technological solutions and appropriate structural control strategies.