Stochastic response of bridges seismically isolated by friction pendulum system

Stochastic response of bridges seismically isolated by the friction pendulum system (FPS) is investigated. The earthquake excitation is modeled by a nonstationary random process (i.e., uniformly modulated broadband excitation). The stochastic response of the isolated bridge is obtained using the time dependent equivalent linearization technique due to nonlinear force-deformation behavior of the FPS. The nonstationary response of the isolated bridge is compared with the corresponding stationary response in order to study the influence of nonstationary characteristics of earthquake excitation. An optimum value of the friction coefficient of FPS for which the root mean square absolute acceleration of the bridge deck attains a minimum value was observed. The influence of system parameters such as isolation period of the FPS, frequency content, and intensity of an earthquake on the optimum friction coefficient of FPS is investigated. It was observed that the above parameters have significant effects on the optimum friction coefficient of FPS. A closed form expression for the optimum friction coefficient of FPS and corresponding response of the isolated bridge system are proposed. These expressions were derived based on the model of the bridge with rigid deck and pier conditions subjected to stationary white-noise excitation. It was concluded that there is a good comparison between the proposed closed form expressions and actual optimum parameters and the response of the isolated bridge system. The maximum difference observed was about 10-15% percent and these expressions may be used for initial optimal design of the FPS for bridges.