Near-fault ground motion effects on pounding and unseating using an example of a three-span, simply supported bridge

A simple model of a three-span, simply supported bridge consisting of three rigid decks supported by two axially rigid piers and rigid abutments at two ends is analyzed by solving the dynamic differential equations. The response spectrum method is not considered, and it is assumed that there is no soil-structure interaction. The bridge is acted upon by the acceleration of gravity, g, and excited by differential horizontal-, vertical-, and point- and cord-rocking components of near-fault ground motion. The model's in-plane linear response shows that the vertical and rocking ground-motion components have no noticeable effect on the maximum pounding force between bridge segments when computed for horizontal ground motion only. For the model with system periods longer than 1s subjected to the strong motion pulse corresponding to magnitude M=7, the vertical ground-motion component contributes to the destabilizing effect of the gravity. For bridge periods longer than 0.5s, the simultaneous action of horizontal and vertical ground-motion components can noticeably increase the minimum gap size required to prevent pounding and the minimum seating length to avoid the unseating of bridge segments when computed for horizontal ground motion only. For system periods shorter than 1s, the time delay of input ground motion has a significant effect on the minimum gap size to prevent pounding as well as on the minimum seating length to avoid unseating of the deck. The response of the bridge subjected to differential horizontal, vertical, and point-rocking ground-motion components is almost the same as the response under differential horizontal and vertical components of the ground motion. The main contribution to changes in the response, which is computed for horizontal ground motion only, is caused by the vertical and cord rocking of the ground motion. The minimum seating length to avoid the unseating of bridge segments suggested in seismic Iranian Code, No: 463 (Road and railway bridges seismic-resistant design code, 2008), is conservative for pulses with magnitudes M=5 and 6, but not conservative for bridge periods longer than about 0.6s and a near-field pulse with M=7 magnitude.