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In this paper, some paramters affecting the dynamic behavior of cantilevered truss bridges under earthquake loadings are studied. For this purpose, Three bridges, each with three spans of 360, 552 and 744 meters have been designed according to AASHTO. The effects of uniform and nonuniform ground motions at all supports, wave propogation velocity, seismic input wave travel and coherency on the longitudenal direction are evaluated. The effect of spatial correlation in the transvers direction is not very important because of relatively smaller width of bridges. Therefore, in the transverse direction, uniform ground motions are considered at all supports. Three dimentional linear dynamic analysis of bridges under earthquakes are carried out using artifitial spatial incoherent gaussian seismic
ground motions. The simulated ground motions satisfy a prescribed, or target local
By power spectrum and a target coherency function. Nonstationary characteristics are introduced by superimposing a time dependent
of envelop function. The simulated ground motions in the longitudinal and two perpendicular transvers directions have been considered simultaneously.