On this 277,000 gsf building, our high-performance design contributions saved $10 million and reduced CO2 emissions by 7 million pounds.
This thirteen-story building, with a performance mandate of immediate occupancy after a design basis earthquake (DBE), had two previous lateral systems designed by other seismic consultants. The first was a base-isolation system; the second, value-engineered design was a structural-steel moment frame with supplementary viscous dampers. Despite the value engineering, estimated construction costs were still far too high. The contractor, Webcor, had learned about Tipping Mar’s innovative post-tensioned lateral system that returns a building to plumb after a seismic event and challenged us to devise a third system for SFPUC. We chose to redesign the building in concrete with our post-tensioned concrete cores, thus delivering immediate-occupancy performance at a much lower cost.
Our design solution (1) added a thirteenth story owing to lowered floor-to-ceiling heights; (2) trimmed the construction schedule owing to our optimized link-beam design that, in conjunction with our PT lateral system, reduced steel reinforcement by 50 percent; (3) saved the project $10 million; and (4) decreased the project’s carbon footprint by 7.4 million pounds of carbon dioxide emissions.
Our optimization of the link-beam design is a clear example of integrated set-based design. We studied five different configurations of link beams (the structural header within the concrete core); these elements need to be tough, energy absorbing, damage resistant, and compatible with the self-centering cores. The five configurations reflected the input of the steel fabricator, the rebar supplier, and the general contractor. The measure of best fit was a balance of erection speed and fabrication cost that could be determined only by the construction team’s evaluating the price and schedule impacts of all five configurations.
The June 2011 issue of Architectural Record features an article, Green at Its Core, that discusses in detail the design and construction of the SFPUC headquarters, highlighting especially Tipping Mar's seismic design.