Recent awareness of the spatial variation of earthquake ground motion has caused concern about the safety of large engineered structures under seismic excitation. The effects of spatially varying ground motion on the response of a few grossly simplified structures have been analyzed in the past few years. This paper summarizes the results obtained from studies of the stochastic response of three actual long-span bridges excited by spatially varying ground motion. Studies were conducted on: (a) the lateral response of the Golden Gate suspension bridge (GGB) in California with a 4,200 ft center span and 1,125 ft side spans; and (b) the longitudinal and lateral responses of the 1,700 ft New River Gorge arch bridge (NRGB) in West Virginia, and the 700 ft Cold Spring Canyon arch bridge (CSCB) in California (Hawwari 1992, Sweidan 1990).
Two-dimensional finite element models were used for all the bridges. For the suspension bridge, the model developed by Abdel-Ghaffar and Rubin (1983) was used, with the corrections made by Castellani and Felloti (1986). For the arch bridges, the models developed by Dusseau and Wen (1989) were used.
Linear stationary random vibration analysis was performed. Total mean-square displacement and force responses were computed from the dynamic and static variances, and the (possibly negative) covariances between the dynamic and static responses (Harichandran and Wang 1990). Due to the flexibility of the bridges, the dynamic variances were the most dominant, typically contributing between 80% and 110% to the arch bridge member forces, and about 100% to the suspension bridge member forces.