3-D Dynamic analysis of bridges with rocking isolation under extreme earthquakes

Rocking response of spread foundation has been found to be a unique isolation system in bridge structures. Through the extension of structural periods due to rocking mechanism, the induced seismic force can be decreased. In addition, the underlying soil may go through plastic behavior and dissipate more energy under extreme earthquakes. However, the stability of the bridges with rocking response is generally concerned by practical engineers. This study is aimed to analyze the ultimate state of bridges with rocking response subjected to extreme ground motions by using 3D Vector Form Intrinsic Finite Element (VFIFE). The VFIFE, a new computational method, is adopted in this study because it has the superior in managing the engineering problems with material nonlinearity, discontinuity, large deformation and arbitrary rigid body motions of deformable bodies. Nonlinear behavior of soil is taken account in the interaction between soil and foundation. Nonlinear soil springs are developed in the VFIFE including vertical soil springs, lateral passive soil springs and friction force soil springs. All developed soil springs are verified to be feasible and accurate through numerical simulation. Finally, a five-span continuous bridge is analyzed to investigate the effectiveness of rocking isolation of bridges with spread foundation. The simulation results show that the induced seismic force decreases as the rocking response increases. Also the unstable response of the whole bridge does not occur under the extreme earthquakes.