Seismic Expansion Joint Design for Bridges in High Seismic Zones

Bridges in high seismic zones require expansion joints that can accommodate large seismic displacements while maintaining structural integrity. The seismic gap must be large enough to prevent pounding between adjacent bridge spans, and the restraint system must prevent excessive displacement that could cause the deck to fall off its supports. Seismic gap calculation is the first step in seismic joint design. The seismic gap must be at least equal to the maximum expected relative displacement between the two bridge spans at the joint location. This displacement is calculated from the seismic response analysis of the bridge, considering the ground motion at the site and the dynamic properties of the bridge. For bridges with different span lengths or different support conditions on either side of the joint, the relative displacement can be significantly larger than the displacement of either span alone. Out-of-phase vibration of the two spans can cause relative displacements that are larger than the sum of the individual span displacements. Restraint systems prevent excessive displacement that could cause the deck to fall off its supports. Cable restrainers connect the two spans at the joint location and limit the relative displacement. The restrainer must be designed to resist the maximum seismic force without failure, while allowing the required seismic gap to open. Isolation systems can be used to reduce the seismic forces on the bridge and the required seismic gap. Seismic isolation bearings at the pier and abutment locations reduce the seismic forces transmitted to the bridge deck, reducing the required seismic gap. The isolation system must be designed to provide adequate reduction in seismic forces while maintaining stability under wind and traffic loading. Post-earthquake inspection of expansion joints is required after any significant seismic event. The joint gap must be measured to verify that the bridge has returned to its pre-earthquake position. Any permanent displacement or damage to the joint must be repaired before the bridge is reopened to traffic.