BART Transbay Tube & Transition Structure

BART initiated the Earthquake Safety Program (ESP) to upgrade vulnerable portions of the original BART system, completed between 1972 and 1976, to ensure public and BART employee safety and to minimize operational downtime following a major earthquake. The Oakland to San Francisco segment of the BART system, which carries trains under the San Francisco Bay via the immersed Transbay Tube (TBT), was included in the Program. Transition from the TBT to the twin bored tunnel in San Francisco is handled by the San Francisco Transition Structure (SFTS) – a very large concrete structure partially embedded in surrounding soil – with complex seismic joints on the sides to accommodate significant seismic movements. Similar seismic joints were used to connect the TBT to the Oakland Transition Structure (OTS).

SC Solutions was tasked with evaluating the structural integrity and stability of the SFTS, the immersed TBT, and all seismic joints for both the SFTS and OTS. To ensure an accurate and reliable evaluation, SC Solutions employed an advanced analytical strategy. A 3D nonlinear finite element model integrating the full shore-to-shore system (SFTS, TBT, OTS, and portions of bored tunnels) was developed with enough fidelity to evaluate both global response and local component behavior, and analyzed for multiple time history records. This analysis strategy allowed SC Solutions to overcome unique project challenges related to the coupled soil-structure interaction (SSI) response, namely:

• The importance of capturing interaction between two coupled structures whose behaviors are controlled by different seismic phenomena: (1) the heavy and partially embedded SFTS controlled by its own inertia; and (2) the immersed TBT controlled by the deflections of the surrounding soil.
• The importance of the complex nonlinear behavior of the seismic joints between different structures (e.g. SFTS and TBT). This nonlinearity severely limits the effectiveness of traditional frequency-domain SSI analysis approaches which limit the solution to a linear elastic domain.

SC Solutions overcame these challenges by developing unique procedures that use frequency dependent soil parameters in time domain SSI analyses. A thorough validation and alignment of the results was carried out and successfully presented to several expert review panels. The resulting analyses captured accurate and realistic behavior of the total system, allowing the design team to focus retrofit effort on maximum risk reduction. The design team developed a very efficient and effective retrofit of this complex system made possible by the detailed SSI analysis from SC Solutions.