Stochastic response analysis of suspension bridges for wind and earthquake loads

Abstract

In this dissertation, dynamic response characteristics of suspension bridges are investigated for two important dynamic loads; i.e., wind and earthquake loads. For wind load, the effect of the geometric nonlinear stiffness of suspension bridges under random wind conditions is studied. For earthquake load, the nonstationary response characteristics of the bridges under random earthquake excitations at multiple supports are analyzed. For the investigation of the nonlinear effect of suspension bridges under random wind loads, a frequency domain method based on a stochastic linearization technique is presented. The equation of motion is formulated by including the coupling between the vertical and the torsional motions. In the linearization procedure, the nonlinear stiffness terms are approximated as a sum of linear and constant terms in order to take into account the non-zero mean components as well as the fluctuating components of structural responses. Example analyses have been carried out on two structures for various wind speeds and wind force parameters. Numerical results indicate that the nonlinear effect becomes quite significant, particularly in the vertical direction, as the wind speed increases. For the nonstationary response analysis of suspension bridges subjected to earthquake ground excitations at multiple supports, an efficient method to compute time dependent variance functions of the bridge response is developed. The equation of motion is formulated considering the vertical motions of the bridge only. The nonstationarity of the earthquake excitation is modelled by using a time varying envelope function. The correlation effects between ground excitations at different supports are investigated, utilizing particular cases of travelling seismic waves for various wave propagation velocities. The responses are obtained in terms of time dependent variance functions. Expected peak responses are also evaluated, thereafter. Numerical results from the exam...