Seismic Responses of an Isolated Long-span Bridge using Frequency Domain and Time Dependent Procedures

Abstract

Seismic behavior of an isolated bridge is analyzed in the frequency domain under the effects of nonstationary ground motions. For dynamic solutions, different ground environments are considered by simulating nonstationary quakes that can be represented from bedrock to soft ground level. In the simulations, power spectral functions and filtered white noise model are adopted for spectral densities of the earthquake excitations. Various computer algorithms have been developed for earthquake simulations, establishing the bridge finite element model and stochastic solutions. Twenty simulated ground motions are used for each soil profile and the parameters of Rayleigh dispersion are estimated by evaluating the system responses for each ensemble. A number of peak response factors dependent on soil conditions are presented for seismic responses. In addition, extreme value distributions of the responses are shown with the probability of exceeding functions and tables. The responses are discussed for the specific exceedance level of probabilities used in probabilistic design process. The stochastic analyses generally yielded responses consistent with time domain solutions. Exceedance probability functions of the peak responses were obtained in a close relationship. However, the probability distributions of the responses decomposed for the soft soil case and they displayed a wider dispersion even for low exceedance levels. The peak responses are expressed with some exceedance probabilities. In the estimation of response variations, this study showed the practicality of the frequency domain method and the results revealed higher peak response factors and variances for softer soil conditions. Furthermore, this study indicated that the frequency domain procedure is an effective tool in the obtaining of nonstationary seismic responses.