A high-rise office tower, 350 Mission Street, will be located catty-corner from the future Transbay Transit Center in downtown San Francisco. © Skimore, Owings & Merrill, LLP 2013
A nonprescriptive, 30-story office tower will replace a tired concrete structure near the site of the future Transbay Transit Center in San Francisco’s South of Market district.
March 26, 2013—When the Great Recession struck in 2008, construction projects across the nation stalled. But as the economy has begun to improve and demand for new buildings has increased, construction cranes are becoming familiar sights once again. One city that has experienced this shift is San Francisco, where construction will soon begin on the first high-rise office tower there in five years.
Known as 350 Mission Street, the new 30-story office tower will replace a typical four-story reinforced-concrete building at the corner of Mission and Fremont streets in the city’s South of Market district. The tower is expected to make better use of the site, which is catty-cornered from the future Transbay Transit Center, a highly anticipated regional transit hub that is being billed as the Grand Central Station of the West. (See “Multimodal Mass Transit Center to Connect California Bay Area Counties,” Civil Engineering, September 2010.)
Taller and more architecturally stunning than the existing building, the tower will also have several features, including an “urban living room,” that are meant to engage the city. The San Francisco office of Skidmore, Owings & Merrill (SOM), a design firm headquartered in Chicago, is both the architect and the structural engineer of the project.
SOM completed the tower’s architectural design several years ago at the request of GLL Properties, a San Francisco-based real estate investment firm that owned the 350 Mission Street site at the time. But GLL Properties eventually sold the property, its development rights, and SOM’s tower design to Kilroy Realty Corporation, a real estate investment firm headquartered in Los Angeles. After securing an anchor tenant for the building, Kilroy Realty decided to proceed with construction of the office tower as SOM had designed it. Salesforce, a San Francisco-based computer software company, will occupy the entire tower above the street-level lobby.
The 390,000 sq ft tower will have several unconventional features, not the least of which will be its core-only lateral load-resisting system. This nonprescriptive system will be outside of the building code because it will not have a perimeter moment-resisting frame, as is typically required for buildings of this height. “We wanted to create a space that was free of down-turned framing using concrete,” says Mark Sarkisian, P.E., S.E., LEED BD+C, M.ASCE, the director of structural engineering in SOM’s San Francisco office. Without a frame, the tower floor framing will be flat in all directions and have long spans of 45 ft, which are typically difficult to achieve using concrete. To make it work, the tower’s 11 in. thick flat-plate slabs will be posttensioned using a sophisticated system that requires banding and cambering until the desired deflections and strength are realized, Sarkisian says. “We are really trying to create a very efficient, open space,” he explains. “And using posttensioning in that flat slab system really does reduce the mass, it reduces the seismic demand, and it reduces the amount of material.” As a result of this approach, the design had to go through a rigorous peer review to ensure it would meet all of the requirements for a 43-year (service level) earthquake, a 475-year (design code-level) earthquake, and a 2,475-year (maximum considered) earthquake.
At street level, 350 Mission Street will have sliding glass doors
that will open as an invitation to passersby. On the levels above,
glass panels tilt forward and back to create movement and direct
light. © Skimore, Owings & Merrill, LLP 2013
The tower will be founded on a mat foundation from which four basement levels will rise to accommodate parking, mechanical systems, and bicycle racks—just one of the building’s many sustainable design features. The tower’s entry lobby, known as the urban living room, will cap the basements. This 50 ft high space will accommodate a cafe, restaurant, amphitheater-style seating, and lobby seating that will move ever so slowly on a horizontal axis to give occupants different views of the space and other people within the lobby. It will also house two levels of eat-in seating, a light-emitting-diode canvas for digital artwork and other displays, and room for special events. The urban living room will be encased in glass, with sliding glass panels that will open to welcome passersby into the building. “The 50-foot-high and 60-foot-long membrane of glass ... opens the great urban living room to hundreds of thousands of daily commuters and visitors arriving across the street when the Transbay terminal is complete,” said Masis Mesropian, the senior design architect for SOM, in a prepared statement.
The design team set out to make the urban living room as open as possible, blurring the lines between the tower perimeter and the social vibrancy of the street. To that end, the corner facing the Transbay Transit Center will not have a supporting column. Instead, the two posttensioned beams at that corner will form an unusual double cantilever, extending 30 ft from either side of the building. “The column on either side of that corner as you move along the face of the exterior wall is thirty feet back; so it’s thirty feet back in one direction, along the Fremont Street side, and thirty feet back along the Mission side,” Sarkisian explains. “So you’ve got a beam that’s now coming from either side and meeting at the corner, but it doesn’t have a column. That’s the double cantilever.”
The tower will be clad in high-performance glass panels, providing floor-to-ceiling windows in the offices to maximize views and daylight without a great deal of heat gain. But considering all of the building’s other unusual elements, perhaps it’s not surprising that the cladding will not be a conventional curtain wall. Rather, the glass panels will alternate, tipping in and out of the supporting mullions to create texture and reflect light. “The panels tipping in will reflect the sky brightness and the panels tipping out will reflect light from below,” Sarkisian says. “Because of the way the light strikes the surface throughout the day, the exterior will be animated.” The glass panels will extend above the roof line to conceal the rooftop mechanical system and crown the building. Those panels will be illuminated at night.
Demolition of the old concrete building is now complete, and crews will soon install soil retention systems in preparation for the foundation and basement work, which is scheduled to begin in June. Completion of the tower is anticipated for the summer of 2015. The project is expected to be certified at the platinum level, the highest rank, in the U.S. Green Building Council’s Leadership in Energy and Environmental Design system. Sarkisian says the 350 Mission Street project represents a renewed demand for quality commercial space in San Francisco as the city and the rest of the country begin to see economic improvement. “I believe this project represents signs of a strong comeback for development in San Francisco,” Sarkisian says. “It is one of several [projects] now being slated for construction.”