Effects of Incoherence, Wave Passage and Spatially Varying Site Conditions on Bridge Response

Recent earthquakes in California and Japan have clearly demonstrated that seismic ground motions can vary significantly over distances that are of the same order of magnitude as the span lengths of extended structures, such as bridges. Such incoherent support motions may induce forces in multiply-supported structures, commonly known as pseudo-static forces, that are absent in structures uniformly excited at their base. At the same time, the forces generated by dynamic inertia effects tend to be smaller in the case of incoherent support motions due to the effect of random cancellations. The total effect, i.e., the sum of dynamic and pseudo-static forces, can be larger or smaller than the forces generated by uniform base excitation, depending on the characteristics of the structure and the ground motion field.