San Francisco Public Utilities Commission Headquarters plans originally called for a steel-frame building, but plans were changed to a concrete structure to reduce costs by $10M and add an additional floor within the same zoning envelope. The 13-story, 277,500 square foot structure incorporated six different concrete mixtures using slag cement or combinations of slag cement and fly ash to reduce CO2 emissions by 7.4 million pounds (nearly 50% better than traditional mixes) in pursuit of a LEED Platinum rating.
A ternary mixture of 40% slag cement and 30% fly ash was used for mat-slab, column, and core-wall concrete mixtures, which had a specified compressive strength of 8,000 psi at 90 days, The elevated post-tension floor slabs used a 65% slag cement mixture with no fly ash to achieve 4,500 psi in 3 days to allow for stressing and stripping. This mixture met the 56-day, 6,000-psi specified compressive strength criteria for post-tension slabs at earlier ages, enabling a rapid speed of construction. Innovative structural design, concrete mixture development, and construction techniques all support the goal of achieving a LEED Platinum rating on this project.
Project credits: City & County of San Francisco, Owner; San Francisco Department of Public Works, Project Manager/Developer; KMD Architects and Stevens + Associates JV, Architects; SOHA Engineers and Tipping Mar Structural Engineering, Engineers; Webcor Builders/Webcor Concrete, Contractor; Central Concrete Supply Co., Inc., Concrete Supplier; Lehigh Southwest Cement Company, Slag Cement Supplier