| The stochastic lateral responses of the Golden Gate suspension bridge, which has a center span length of 4,200 feet and a side span length of 1,125 feet was investigated. A two dimensional finite element model of the bridge was used. A space-time earthquake ground motion model that accounts for both coherency decay and seismic wave propagation was used to specify the support motions. The double-filter spectrum fitted to an artificial accelerogram similar to the El Centro earthquake was used.; Linear stationary random vibration analysis was used to compute the bridge responses. Three models of excitations were considered at the supports: (1) correlated ground motion model accounting for both wave propagation and coherency decay; (2) identical support motion; (3) delayed excitation caused by wave propagation. Transient response analysis was also performed to determine whether the suspension bridge will attain its stationary response during typical durations of strong shaking (10 to 20 seconds). The effects of shear deformation on the natural frequencies and mode shapes, and their corresponding effect on the linear stationary random vibration responses was investigated. Inclusion of shear deformation drastically lowers the frequencies of a group of modes, resulting in smaller moment and shear responses, but slightly higher displacement responses.; Results indicate that the use of identical excitations significantly over-estimates the responses at some locations and under-estimates the responses at others, the relative deviation being more severe for the longer center span. The use of delayed excitations gives acceptable results for the side span, but shows greater deviations for the center span in which the moment and shear are sometimes significantly under-estimated. The increase in the apparent wave velocity causes progressively higher responses at some locations of the span and progressively lower responses at others. Results of transient analyses indicate that for common ground motion durations, the assumption of stationarity may grossly over-estimate the side span responses. The transient displacement response of the center span can overshoot the stationary response considerably, but the moment and shear responses gradually approach their stationary values in about 40 seconds. |
