Seismic Expansion Joint for Curved and Skewed Bridges

Curved and skewed bridges experience multi-directional seismic movements that are more complex than the primarily longitudinal movements of straight bridges. The expansion joint must accommodate these multi-directional movements while preventing pounding and maintaining structural integrity. Curved bridges rotate about a vertical axis during earthquakes due to the eccentricity between the center of mass and the center of stiffness. This rotation creates lateral movements at the expansion joint in addition to the longitudinal movements. The joint must be designed to accommodate the vector sum of the longitudinal and lateral movements. Skewed bridges have their expansion joints oriented at an angle to the traffic direction. During an earthquake, the relative displacement between the two spans has components both perpendicular and parallel to the joint. The joint must be designed to accommodate the full displacement vector, not just the component perpendicular to the joint. Torsional response of curved bridges can amplify the seismic displacements at the expansion joint location. The torsional mode of vibration creates large displacements at the ends of the bridge, which is where the expansion joints are typically located. The seismic response analysis must include the torsional modes to correctly predict the joint displacements. Restraint systems for curved and skewed bridges must be designed to resist forces in multiple directions. Cable restrainers oriented in the longitudinal direction do not provide restraint against lateral displacement. Multi-directional restraint systems using cables or link beams in both the longitudinal and lateral directions are required for curved and skewed bridges. Seismic isolation for curved and skewed bridges requires careful design to ensure that the isolation system provides the required reduction in seismic forces in all directions. Friction pendulum bearings provide multi-directional isolation and are well suited for curved and skewed bridges. The isolation system must be designed to maintain stability under the combined effects of gravity, wind, and seismic loading.