Effect of support conditions on seismic response of cable-stayed bridges

Abstract

The seismic response of a cable-stayed bridge is the result of the interaction between the input earthquake characteristics and the bridge properties like mass, stiffness, and support conditions. The support type used for the deck at the towers of cable-stayed bridges is one of the important parameters that greatly influences the seismic response of the bridge. This is demonstrated in the present paper by study of the change in modal characteristics and seismic response of two cable-stayed bridges for different types of the support conditions. The first bridge has concrete towers and a steel deck with a 371 m main span, and the second bridge also has concrete towers, but a composite deck with a 465 m main span. The study shows that elastic supports for the deck at the towers give very low deck displacements and deck bending moments compared to pinned or fixed connections. Roller supports produce low tower base shear and base moment but not necessarily a low response for the bridge as a whole. Roller supports also cause the bridge to have very low first longitudinal direction mode frequency; this indicates that the bridge is very flexible in the longitudinal direction. A study of the effect of variation of the longitudinal stiffness of the elastic deck supports shows that it strongly influences the first few longitudinal direction modes, but not the symmetric or higher modes. The study also shows that the seismic response of the bridge is highly influenced by the variation of the support stiffness for some earthquakes like 1940 El Centro earthquake, but to a lesser degree for others. Wave propagation effects were also studied for different support conditions of the bridge. It is observed that the seismic response shows a higher degree of variation with the changing wave velocities when the bridge deck has pinned or fixed supports, and that the change in the bridge response with the wave velocities is not very pronounced when the bridge deck has elastic or roller supports. For the first bridge, the critical responses were observed at very low wave velocities, and for the second bridge, the uniform motion case caused the critical response of the bridge at many locations.Key words: cable-stayed bridges, support conditions, seismic response, wave propagations.