Seismic Expansion Joint for Bridges with Seismic Isolation Systems

Seismic isolation systems reduce the seismic forces and displacements transmitted to the bridge deck, potentially reducing the required seismic gap at the expansion joint. However, the interaction between the isolation system and the expansion joint must be carefully considered in the design. Seismic isolation works by lengthening the natural period of the bridge, moving it away from the dominant period of the ground motion. This period shift reduces the seismic acceleration and force demand on the bridge. The isolation system also dissipates energy through hysteretic damping, further reducing the seismic response. The seismic displacement at the expansion joint of an isolated bridge is the relative displacement between the two adjacent bridge spans. For a bridge with isolation bearings at all supports, the deck moves as a rigid body on the isolation bearings, and the relative displacement between adjacent spans is small. However, if the isolation bearings have different properties on either side of the joint, the relative displacement can be significant. Friction pendulum bearings are the most common isolation system for highway bridges. They provide multi-directional isolation and have a self-centering mechanism that returns the deck to its original position after the earthquake. The displacement capacity of the friction pendulum bearing determines the maximum seismic displacement of the deck. Lead rubber bearings are another common isolation system that provides both isolation and energy dissipation. The lead core in the bearing yields during the earthquake, dissipating energy and limiting the displacement. The displacement capacity of the lead rubber bearing must be sufficient to accommodate the seismic displacement without damage. Expansion joint design for isolated bridges must account for the displacement capacity of the isolation system. The seismic gap must be at least equal to the maximum displacement of the isolation system. The joint must also be designed to accommodate the thermal movement and other non-seismic movements.