The SR 99–Spokane Street over-crossing is located in Seattle, Washington, and was built in the late 1950s. Its construction is mostly of reinforced concrete; the reinforcement detailing is poor by modern seismic standards, particularly with respect to confinement, and the underlying soils are also weak. Furthermore, the structure contains many “outrigger bents,” in which columns are displaced from their ideal locations to accommodate passage of railroad tracks beneath the structure. These outrigger bents cause significant asymmetry in the structure that could lead to unusual and undesirable seismic response.
In this study, the seismic response of the structure was evaluated analytically, and seismic retrofit strategies were developed and evaluated. In a companion experimental study by Washington State University, the as-existing structure was investigated by testing scale models of representative bents, and one retrofit strategy was also tested.
In this report, modeling approaches are discussed in detail, and the results of evaluations of the as existing structure as well as the retrofit strategies are presented. Some components of the structure (e.g., the deck, the already jacketed columns, and the soil-foundation-structure system) were found to be critical to an accurate determination of response and were therefore modeled in some detail. Site-specific ground motions at three different hazard levels were used.
The structure was found to be vulnerable, especially to 72- and 475-year ground motions. The knee-joints that connect the outrigger beams and columns were found to be the most critical components, and their vulnerability was shown to be influenced by the type of retrofit performed on the outrigger column. Retrofit strategies included some designed to increase the strength and ductility of the components, and some intended to reduce the demands on them.