Bridge Expansion Joint for Bridges with Seismic Isolation and Dampers

Bridges with combined seismic isolation and damping systems have complex movement characteristics that affect the design of expansion joints. The isolation system reduces the seismic forces and displacements, while the dampers dissipate energy and limit the displacement. The expansion joint must accommodate the residual displacement after the isolation and damping systems have performed their functions. Fluid viscous dampers are commonly used in combination with seismic isolation bearings to limit the displacement of the isolation system. The damper force is proportional to the velocity of the displacement, providing maximum force at the peak velocity and zero force at the peak displacement. This velocity-dependent behavior means that the damper does not affect the static thermal movement of the bridge. Displacement capacity of the combined isolation and damping system determines the maximum seismic displacement at the expansion joint. The isolation system provides the primary displacement capacity, while the dampers limit the displacement to a fraction of the isolation system capacity. The expansion joint must be designed for the maximum displacement that can occur with the isolation and damping systems in place. Thermal movement of bridges with isolation systems is not affected by the isolation bearings, which are designed to allow free movement under slow (quasi-static) loading. The expansion joint must accommodate the full thermal movement of the bridge deck, as if the isolation bearings were not present. Maintenance of bridges with isolation and damping systems requires periodic inspection of the isolation bearings and dampers in addition to the expansion joints. The isolation bearings must be inspected for correct position and condition. The dampers must be inspected for leaks and correct operation. Any problems with the isolation or damping system must be repaired promptly to maintain the seismic performance of the bridge. Post-earthquake inspection of bridges with isolation and damping systems must verify that the isolation bearings have returned to their original position and that the dampers have not been damaged. Any permanent displacement of the isolation bearings indicates that the seismic displacement exceeded the design limit, and the bridge may have sustained additional damage.