3-D NONLINEAR SEISMIC BEHAVIOR OF CABLE-STAYED BRIDGES

The dynamic behavior of three-dimensional (3-D) long-span cable-stayed bridges under seismic loadings is studied. The cases of synchronous and nonsynchronous support motions due to seismic excitations of these flexible structures are considered; furthermore, effects of the nondispersive traveling seismic wave on the bridge response are studied. Different sources of nonlinearity for such bridges are included in the analysis. Nonlinearities can be due to: (1) Changes of geometry of the whole bridge due to its large deformations, including changes in the geometry of the cables due to tension changes (known as the sag effect); and (2) axial force and bending moment interaction in the bridge tower as well as the girder elements. A tangent stiffness iterative procedure is used in the analysis to capture the nonlinear seismic response. Numerical examples are presented in which a comparison is made between a linear earthquake-response analysis (based on the utilization of the tangent stiffness matrix of the bridge at the dead-load deformed state) and a nonlinear earthquake-response analysis using the step-by-step integration procedure. In these examples, two models having center (or effective) spans of 1.100 ft (335.5 m) and of 2,200 ft (671 m) are studied; this range covers both present and future designs. The study sheds some light on the salient features of the seismic analysis and design of these long contemporary bridges.