350 Mission

Owner: Kilroy Realty Corp
San Francisco, CA

Architect: Skidmore, Owings & Merrill LLP
Engineer: Skidmore, Owings & Merrill LLP
Construction Management: Kilroy Realty Corp.
General Contractor: Webcor Builders
Concrete Contractor: Webcor Concrete
Reinforcing Bar Fabricator: Gerdau
Total Project Cost: $100M
Total Concrete Usage: 27,000 CY
Reinforcing Tons: 3,100 Tons


350 Mission uses a traditional concrete shear wall core and post tensioned pre-cambered flat slabs without a redundant perimeter moment frame system. The structure was designed according to performance-based seismic design principles. Every structural element was analyzed for seismic behavior and vetted by municipal and independent third-party reviewers. This design approach is what removed the need for any perimeter moment frames, and led to significant reductions in rebar/PT quantities when compared to traditional designs that would have included the perimeter moment frame system. The result was reduced costs and increased aesthetic potential.


The primary corner of the tower was cantilevered primarily to achieve the unique lobby condition with fully retractable doors. This lead to an architectural intent to expose the flat structural soffit at each floor. The corner cantilever with flat exposed concrete achieved the desired minimal urban aesthetic.

Spanning nearly 45 feet, the post-tensioned flat slab floor system features a sophisticated camber that is digitally mapped to shoring post locations and engineered to flatten out within 90 days, prior to installation of the raise flooring. The camber was incorporated to achieve material quantities similar to conventional flat slab construction in residential buildings with 30-foot spans. Slab and beam elevations were surveyed at casting, and at 30, 60, and 90 days after casting to verify slab defections. Surveyed elevations reveal that the detailed deflection analysis based on American Concrete Institute (ACI) 435 methods were highly accurate. Results were presented to ACI in 2015. This achieved what designers and contractors often fail to achieve in cambered decks: flat slabs.


There were three key influences that lead to reinforced concrete construction. First, the San Francisco market has trended toward reinforced concrete due to the number of tall residential projects and numerous construction efficiencies have been developed on those projects. These projects typically have a 30 foot span and are post-tensioned, flat-plate construction. 350 Mission is the first new office building constructed in over a decade in San Francisco and SOM wanted to combine local construction efficiencies with the longer 45 foot spans needed for Class A office towers. Second, an urban/raw appearance is popular with many current technology companies. By creating a flat-plate construction spanning 45 feet, both local construction efficiencies and tenant desires could be satisfied. Third, early cost studies revealed that if a long-span flat plate slab could be achieved that it would be less expensive than traditional steel framing.