Rocking of Large Bridge Caissons Due To Seismic Excitation

Caisson foundations are massive structures that respond to seismic loads in a primarily rocking mode. This paper presents a case study on an existing bridge caisson where the capacity spectrum approach was used to evaluate the performance for a specific seismic event. The deformation capacity curve was evaluated from nonlinear static pushover of a three dimensional finite element model of the caisson embedded in the surrounding soil. The model included interface elements to capture gapping, sliding, and rocking of the caisson during the analysis. The nonlinear behavior of the soil was represented by a cap plasticity model, which is based on the well known Drucker–Prager yield condition and a non-associated flow rule. Alternatively, a theoretical approach was developed, using basic principles of structural mechanics and the half-space theory, to determine the capacity curve. Results of nonlinear static pushover methods compared favorably to nonlinear time history analyses.