Abstract
The authors employed high-fidelity computer modeling to analyze the probable seismic performance of an ex- isting high-rise, steel frame building with unreinforced masonry infill walls in downtown San Francisco. The Pacific Telephone and Telegraph Co. building at 138 New Montgomery is a 26-story, historic office building. Constructed in 1926 this building was the tallest building in the western United States for a long period of time. The building's structural system comprises perimeter unreinforced brick masonry walls infilled within and supported by a steel frame. We con- ducted our analysis in support of the building's conversion to residential occupancy. As a result of the occupancy conver- sion, along with associated architectural modifications throughout the building, Section 3403 of the San Francisco Build- ing Code (SFBC) requires a seismic upgrade such that the building has a code-compliant lateral-force resisting system with no less than 75% of the strength specified by the code for new buildings of similar occupancy and structural system. Since the existing building does not have a lateral force-resisting system recognized by current U.S. codes, compliance with the prescriptive requirements would have required expensive retrofits. Instead, the authors employed a performance- based approach using the existing masonry infilled steel frame for a substantive portion of the building's seismic resis- tance, together with a new supplemental interior reinforced concrete shear wall. Rather than design to achieve code- specified strength limits, we used nonlinear response history analysis to demonstrate acceptable behavior under Maximum Considered Earthquake (MCE) shaking. Detailed modeling of the interaction between the masonry infill and steel frame is a key component of this approach.
Highlights
For many years, the tallest structure in San Francisco, the 138 New Montgomery Building located at the corner of New Montgomery and Minna Streets has been designated by the City of San Francisco as a Class A landmark structure
Since the required reconfiguration of most interior spaces, constitutes “Substantial Change” under Section 3403 of the San Francisco Building Code (SFBC) [1], the code requires seismic upgrade such that the building contains a code-conforming lateral force-resisting system having at least 75% of the strength required for new buildings of similar size and occupancy
For buildings of ordinary occupancy, this performance is stated as a goal to provide an acceptably small risk of collapse given that the structure experiences Maximum Considered Earthquake (MCE) shaking
Summary
The tallest structure in San Francisco, the 138 New Montgomery Building located at the corner of New Montgomery and Minna Streets has been designated by the City of San Francisco as a Class A landmark structure. The building has 26 above-grade stories, a basement and subbasement It was constructed in 1926 as the headquarters for the Pacific Telephone and Telegraph Company and in fitting with this use the exterior façade features extensive terra cotta finishes and ornamentation. Construction consists of reinforced concrete floor and roof slabs supported by a complete vertical load carrying steel frame. Perimeter walls are unreinforced brick masonry infill within and supported by the steel frame. As the SFBC does not recognize steel frames with unreinforced masonry brick infill as a lateral force resisting system compliance would require introduction of a new, compliant, lateral forceresisting system. Since the perimeter masonry walls are quite rigid, and somewhat fragile the new system would require substantial stiffness, to demonstrate compatibility with the walls
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