Modal Analysis of Isolated Bridges with Transverse Restrains at the End Abutments

摘要

This paper examines the eigenvalues of multi-span seismically isolated bridges in which the transverse displacement of the deck at the end-abutments is restricted. For moderate long bridges the first natural period of the bridge is the first longitudinal period, while the first transverse period is the second period, given that the flexural rigidity of the deck along the transverse direction shortens the isolation period offered by the bearings in that direction. This paper shows that for isolated bridges longer than a certain critical length, the first transverse period becomes longer than the first longitudinal period despite the presence of the flexural rigidity of the deck. This critical length depends on whether the bridge is isolated on elastomeric bearings or on spherical sliding bearings. On the other hand this result can not be captured with the limiting idealization of a beam on continuous distributed springs (beam on Wrinkler foundation) -a finding that has practical significance in design and system identification studies. The paper discusses the implications of this finding in design.