Seismic resilience of novel rocking bridge foundation including soil structure interaction.

Abstract

The conventional design philosophy of bridges allows damage in the pier through yielding. A fuse-like action is achieved if the bridge piers are designed to develop substantial inelastic deformations when subjected to earthquake excitations. Such a design can avoid collapse of the bridge but not damage. The damage is the plastic hinge formation at location of maximum forces and stresses that can lead to permanent lateral displacement. This can impair traffic flow and cause time consuming repairs or in some cases even complete demolition of the bridge. Rocking can act as a form of isolation by not fixing the foundation to the ground but to allow it to uplift and thus act as a mechanical fuse, limiting the forces transferred to the base of the structure. Rocking isolation enhances the seismic resistance of the structure and their post-earthquake serviceability. In this context, this research proposes a novel rocking isolation technique which uses elastomeric pads incorporated beneath the footing of the bridge piers and external restrainer in the form of shape memory alloy bar (SMA). The rocking mechanism is achieved by restricting the horizontal movement of footing by providing stoppers at all sides of footing. The pads are designed to remain elastic without allowing their shearing. The pier, the footing and the elastomeric pads are supported on concrete sub base which rests on firm strata such as stiff soil, hard rock or on pile cap. By performing nonlinear dynamic time history analysis and nonlinear pushover analysis, the proposed bridge with the novel resilient pier foundation is compared against an existing conventional bridge on spread foundation as well as on pile foundation. The proposed pier rocking on elastomeric pads and external restrainer has been found to have good re-centering capability and negligible residual drifts during earthquakes. It is also found that by allowing the foundation to uplift, the forces at base of pier are effectively reduced but the horizontal displacements at pier top are increased. However, these excessive pier displacements can be controlled by the sacrificial external SMA bars attached from the footing to the base slab of the foundation.