Effects of spatially varying ground motions on short bridges

The effects of non-uniform excitation on short bridges were studied by computing the response of an idealized bridge to a suite of earthquake ground motions. The ground motions for each support were developed from seven measured ground motions and from idealizations of wave passage and coherency loss effects. For each set of support motions and for a range of bridge lengths and periods, the maximum support reactions were compared with the reactions calculated for coherent motions. For variable support motion, the contribution of the antisymmetric modes tended to increase, whereas the contribution of the symmetric modes generally decreased. Consequently, the dynamic component of the end support reaction computed using coherency loss excitation exceeded the response to coherent excitation for 62% of the bridges considered, varying from 75-180% of the coherent response. In contrast, the dynamic component of the central support reaction, which is affected only by symmetric modes, was unconservatively predicted in only 20% of the cases. Based on these observations, a method that relies on modifying the modal participation factor was developed for incorporating the effects of multisupport excitations into coherent response calculations.