INFLUENCE OF SEISMIC-WAVES SPATIAL VARIABILITY ON BRIDGES - A SENSITIVITY ANALYSIS

The sensitivity of bridges to spatial variations of seismic ground mot ions is studied within the framework of the multiple support response spectrum (MSRS) method developed by Der Kiureghian and Neuenhofer (Ear thquake eng. struct. dyn. 21, 713-740 (1992)). A simplified approximat e formula for small spatial variations of seismic motions sheds some l ight on the response of bridges: the pseudo-static contribution which is commonly introduced to account for static effects of relative groun d displacements may be neglected in case of lateral or vertical ground displacements but may contribute significantly to the response in cas e of longitudinal ground motions. Mode shapes affect the dynamic contr ibution in the following way: spatial variability of ground motion inc reases the response of antisymmetrical modes and decreases that of swa ying modes which lie on one side of the structure position at rest. Th e approximate formula indicates properly the trend of the response giv en by the MSRS rule for small ground motion spatial variability and ma y therefore be useful at a design stage. A criterion is given to estim ate the number of modes that need to be included in the analysis. For large and/or uncertain spatial variability, an efficient algorithm bas ed on the MSRS rule is proposed to obtain the maximum (conservative) r esponse.