Saiful Bouquet is the structural engineer for the seismic retrofit of the 29-story steel structure constructed in the 1960’s. Floors 1-12 are office space and apartments are located on floors 14 through 29. The building has a very unique and unconventional lateral force resisting system consisting of steel moment resisting frames having haunched beam assemblies with Pre-Northridge welded moment connections to both the strong and weak axis of built-up columns in a space-frame configuration. The tower structure rests upon a large subterranean two-level parking structure that also supports two other buildings that make up the Fox Plaza development. Reinforced concrete shear walls are located at the subterranean levels and are typically 12” thick. The foundation system consists of cast-in-place concrete pile caps located at the base of the steel columns with 11-1/4” diameter step-taper Raymond piles that are 56 ft. long.
Saiful Bouquet performed a state-of-the-art performance based analysis of this tower and developed seismic retrofit schemes to safeguard the building from major regional earthquakes. A seismic retrofit scheme was developed for the building that is specifically tailored to address the concerns with excessive building drift and potential for collapse for both the upper and lower portions of the building. At the upper residential levels, a retrofit scheme that increases the ductility of the column weak-axis Pre-Northridge moment connections is proposed with connections to be retrofitted being located at Floors 17 to 21. The seismic retrofit scheme at the lower office floors employs the use of exterior braced frames with fluid viscous dampers in the longitudinal direction of the building located on the east and west sides of the building at the first seven levels above the ground level. The addition of viscous dampers reduces the earthquake induced displacements by dissipating energy without increasing the overall stiffness of the existing structure.
The seismic retrofit scheme and earthquake ground motion development went through a rigorous peer review process as required by the City of San Francisco.