Seismic Expansion Joint Pounding Prevention and Impact Attenuation

Bridge span pounding at expansion joints during earthquakes is a major cause of structural damage. When adjacent spans move out of phase during an earthquake, they can collide with high impact forces that damage the joint, the deck edge, and the supporting structure. Preventing pounding requires adequate seismic gap and, in some cases, impact attenuation devices. Pounding occurs when the relative displacement between adjacent spans exceeds the joint gap. The relative displacement depends on the seismic excitation, the structural properties of the adjacent spans, and the degree of correlation between their motions. Spans with different natural periods tend to move out of phase, creating larger relative displacements than spans with similar periods. Seismic gap requirements for pounding prevention are specified in bridge design standards such as AASHTO LRFD and Eurocode 8. The minimum gap is typically calculated as the sum of the seismic displacements of the two adjacent spans, reduced by a correlation factor that accounts for the tendency of adjacent spans to move in the same direction. For spans with very different natural periods, the correlation factor may be close to zero, requiring the full sum of displacements. Impact attenuation devices are used when the required seismic gap is too large to be accommodated by the joint design. These devices are installed in the joint gap and absorb the impact energy when the spans collide. Rubber buffer pads are the simplest impact attenuation device, absorbing energy through elastic deformation. More sophisticated devices using hydraulic dampers or lead-rubber bearings can absorb more energy and reduce the impact force. Seismic isolation of bridge spans can reduce the relative displacement at expansion joints by decoupling the spans from the ground motion. Seismic isolation bearings (lead-rubber bearings, friction pendulum bearings) lengthen the natural period of the bridge, reducing the seismic forces and displacements. The reduced displacements allow smaller seismic gaps, which may allow a simpler joint type to be used. Post-earthquake assessment of pounding damage should include measurement of the joint gap, inspection of the deck edge for impact damage, and inspection of the supporting structure for cracking or deformation. Pounding damage that is not repaired before the next earthquake can lead to progressive failure of the joint and the bridge structure.