Abstract: This paper describes the approach used to design seismic upgrades for an existing high rise, transitional masonry building 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. Defined by its art deco design, ornate terra cotta cladding covers the building exterior. Perimeter walls are of unreinforced brick masonry infilled within and supported by a steel frame. After purchasing the building in the summer of 2007, the current owners undertook a program to convert this office building to residential occupancy. The occupancy conversion, along with associated architectural modifications throughout the building, trigger a seismic upgrade under Section 3403 of the San Francisco Building Code (SFBC). This section requires structures to have a code-compliant lateral-force resisting system that provides no less than 75% of the strength specified by the code for new buildings of similar occupancy and system. Since the existing building does not have a lateral system recognized by current codes, making it compliant with even 75% of the code forces would have required expensive retrofits. These retrofits would most certainly have interfered with the building's historic fabric making it unacceptable to the approval agency. SGH employed a performance-based approach to resolve this issue using the existing masonry infilled steel frame to perform in unison with a new interior reinforced concrete shear wall to provide seismic resistance. Rather than design to achieve arbitrary 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 infills and steel frame was a key component of this approach. This paper documents the approach used and describes the details of the non linear analyses used to validate the retrofit design.
Advanced Performance-Based Analysis for Seismic Retrofit of a Historic High-Rise Building
December 30, 2009
Publication: SEAOC 2009 Convention Proceedings, San Diego, Structural Engineers Association of California p 19-34
Services: Structural Rehabilitation