Pounding damages of bridge girders have been observed in almost all major earthquakes, like the 1989 Loma Prieta, the 1994 Northridge and the 1995 Kobe events. Even if the damages do not cause a collapse of the bridge girder, they can significantly affect the safety and serviceability of the bridge. Bridge integrity is essential, especially, just after the earthquake for the rescue works. Serious problem may also occur when a regional economy depends strongly on the lifeline.
In this study two long-span bridge girders bridging a valley with soft soil layers at the bottom are considered. It is assumed that the motions at the bedrock below the soil layers have a simple Ricker wavelet form with the low dominant frequency of a near-source earthquake. In the analysis of the bridge response, the effect of soil-structure interaction is considered, since bridge structures located at soft subsoil can behave different from the one with an assumed fixed base. This paper addresses the influence of the spatial variation of ground motions induced by soft local soil profile on the pounding response between two adjacent bridge girders.
The ground motions at the bridge foundations are determined in the time domain. The soft soil layers at the valley bottom and the surrounding subsoil are modeled by finite elements and boundary elements, respectively. The pounding responses of the bridge girders to the obtained ground motions are determined subsequently in the Laplace and time domain. The bridge structure and the subsoil are described by a finite element method and a boundary element method, respectively.
The investigation reveals that even if the ground accelerations hardly excite the bridge structures due to the large difference between the natural frequencies of the bridge structures and the dominant frequencies of the ground accelerations, the large non-uniform ground displacements amplified by the soft subsoil can induce strong poundings between the girders, and consequently increase the damage potential of the bridge girders.
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