Question
There is a phenomenon where seismic forces in the longitudinal direction of a short-span bridge are significantly high at the bridge axis.
I understand that longitudinal reaction forces due to lateral seismic forces occur, but it seems these reaction forces are calculated to be even greater than the maximum lateral seismic forces.
Answer
In response spectrum analysis, maximum and minimum values are output for each location and component of member forces. Therefore, the reaction force results are not simultaneous occurrences but represent the maximum values at each location.
In short-span bridges, lateral vibrations due to longitudinal seismic forces cause rotation at the ends, leading to longitudinal reaction forces at the bearings constrained in that direction.
Hence, longitudinal reaction forces can occur due to longitudinal seismic forces.
The magnitude of these forces depends on factors such as the rotation angle at the ends and the lateral distance between bearings.
In cases with short spans where the rotation angle is significant or the bearings are closely spaced, the reaction forces can be quite large.
In typical bridge bearing arrangements, only one bearing in the same row is constrained in the longitudinal direction, preventing such phenomena.