Seismic Retrofit of Vincent Thomas Suspension Bridge

The Vincent Thomas Bridge in Los Angeles, California, is a major suspension bridge with a total length of 1848 m (6,060 ft), including approaches, and a main span bridge 766 m (2,513 ft) long, whose center span is 458 m (1,500 ft). The bridge, built in the 1960s, used the seismic codes of the time. The site conditions and tall, slender towers and bents have resulted in vulnerabilities that are particular to suspended bridges. This paper focuses on the suspended spans. The bridge is located within the Port of Los Angeles in a vicinity that has a legacy of earthworks associated with such projects as dock expansion, fuel lines, and rail lines. Geotechnical conditions include an active fault running between the main towers with a potential for liquefaction, fault rupture, and lateral spreading. A nonlinear time history analysis of the main suspended bridge was performed, using ADINA. The model included the effects of geometric stiffness of the cables and of multiple-support excitation of the bridge. The critical vulnerabilities detected in this study, arising from large predicted longitudinal motions, indicated distress in the towers, in the cable bents near the anchorages, in the short midspan suspenders, and in the stiffening trusses. During the retrofit study the nonlinear behavior was further developed, including tower rocking and uplift, the postyield deformations of the tower, and the optimization of the damper properties. Retrofit measures include releasing of the side-span trusses at the cable bents, creating fused hinges in the stiffening trusses, the installation of dampers at these locations and between the stiffening trusses and the towers of the bridge, stiffening of the bridge towers to prevent plate buckling, and cable and suspender modifications.