Abstract
The sensitivity of bridges to spatial variations of seismic ground motions is studied within the framework of the multiple support response spectrum (MSRS) method developed by Der Kiureghian and Neuenhofer (Earthquake eng. struct. dyn. 21, 713-740 (1992)). A simplified approximate formula for small spatial variations of seismic motions sheds some light on the response of bridges: the pseudo-static contribution which is commonly introduced to account for static effects of relative ground displacements may be neglected in case of lateral or vertical ground displacements but may contribute significantly to the response in case of longitudinal ground motions. Mode shapes affect the dynamic contribution in the following way: spatial variability of ground motion increases the response of antisymmetrical modes and decreases that of swaying modes which lie on one side of the structure position at rest. The approximate formula indicates properly the trend of the response given by the MSRS rule for small ground motion spatial variability and may therefore be useful at a design stage. A criterion is given to estimate the number of modes that need to be included in the analysis. For large and/or uncertain spatial variability, an efficient algorithm based on the MSRS rule is proposed to obtain the maximum (conservative) response.
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