Input Ground Motion for Long Linear Structures: The Case History of the Bart Transbay Tube

Abstract eng:
The seismic vulnerability studies for the San Francisco Transition Structure (SFTS) of the Bay Area Rapid Transit system (BART) involved global structural dynamic analyses which modeled the response of the rigid and massive SFTS to the west and the Transbay Tube (TBT) to the east, a 5.8 km immersed railway tunnel connecting the city of Oakland to San Francisco, in California. This paper presents the approach adopted to develop input for the seismic response of the long TBT tunnel in the global analyses. The required input was in the form of 1) displacement time histories at every joint location along the entire length of the TBT, and 2) properties for the nonlinear soil springs connecting the TBT to the free field. Adopting an approach for the development of input time histories along the TBT involved addressing a number of issues relevant to the seismic response of long linear structures. These include ground motion attenuation, seismic wave incoherence, input depth, two-dimensional effects, and selection of appropriate soil models. These issues are discussed here in the context of the BART SFTS project.

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