Towards a seismic capacity design of caisson foundations supporting bridge piers

The present work investigates, by means of finite element analysis in 3D, the nonlinear response under lateral monotonic and slow-cyclic loading of caisson foundations supporting bridge piers in cohesive soils. The study is performed with respect to the combined moment (M)-horizontal load (Q) on the foundation, and involves similar rigid cubic caissons carrying a column-mass superstructure of varying height (H) and pier-to-deck joint rigidity. The latter is simulated by a rotational spring at the deck level with parametrically varying stiffness, relating to the rigidity of the connection. The lateral load is imposed at the deck level, and the resulting M-Q load path on the caisson is mapped within the bounds of the respective failure envelope. The analysis revealed that due to pier-to-deck joint stiffness and the nonlinear coupling between the rotational and translational degrees of freedom at the foundation, the loading at the caisson head follows a nonlinear path characterized by the mobilization of a specific failure mechanism (identified as the “inverted” pendulum failure mode), triggered by the “negative effective height” effect. Interestingly, all load-paths display certain characteristics, such as a particular “overstrength” in bearing capacity that is mobilized by the foundation, irrespective of the stiffness properties and constraints at the superstructure system. Regarding the response under cyclic loading, the M-Q loops are shown to be well enveloped by the monotonic “backbone” curve for all examined cases. Finally, from the numerical results, a closed-form expression for the load-path is formulated, and a methodology to be used towards the seismic design of caisson foundations is proposed.