A Sleek Skyscraper in San Francisco Raises the Profile of Performance-Based Design
The nearly complete tower demonstrates multiple benefits of a nonprescriptive approach
The full-tower-height perimeter columns, 2 feet 8 inches by 7 feet 6 inches, are reinforced concrete, except for six stories at each outrigger system. There, an embedded steel column transfers loads from BRBs into surrounding concrete. "In addition, the purpose of the steel column was to aid the contractor installing the gigantic steel connection plates the BRBs are attached to," says Ola J. Johansson, MKA's project manager. Plates were shop-welded to the embeds, he explains. On-site, the assembly was lifted in place inside the outrigger column. "This helped locate the plates with a high level of precision, important due the tolerance requirements of the BRBs."
The MKA scheme both simplified and complicated construction. The absence of beams and the repetition made flying forms the choice for typical floors. But the core, more congested with rebar at the base, made things tougher there. Even so, Chris Plue, a Webcor vice president, notes, "We have nonperformance-based designs that have worse congestion."
The BRBs also had their challenges. Erection tolerances at the connection plates were about 1â„16 inch. Alignment of embedded steel was critical to ensure proper fit. That meant careful coordination and cooperation among all structural trades. Continuous survey control was performed during concrete placement to ensure connection plates remained true and plumb, according to Bovis.
The key to the shorter cycle was careful coordination of the trades, separation of vertical and horizontal construction by keeping the core three levels ahead of the deck, and the PSD process, says the contractor.
The short cycle resulted in earlier occupancy by two months compared with Webcor's usual four-day cycle for a dual system. Webcor's competitors confirm they know of no other West Coast tall building that has gone up this fast. But they also say the tower's small floor plate made the shorter cycle easier to achieve.
The tower's first typical floor is level eight. Workers hit the three-day cycle at level 18. But the contractor did not perfect the sequence immediately. Double work shifts helped. So did the 1,000-square-foot, truss-supported flying table forms for the slabs. Each form took only 40 minutes to set up, according to Bovis. Webcor used two sets of truss-supported flyers-eight per floor. The flyers are hung from columns as opposed to bearing on floors. The system is more costly than conventional formwork, but it eliminates the need for shoring on the floors below. That kept them wide open for the follow-on trades to do their work.
Bovis says a big challenge with the aggressive schedule was keeping information flowing, especially to the design-build teams for the mechanical-electrical-plumbing and curtain-wall systems. Coordination of these was only a few floors ahead of the structure's construction. "Getting information out to all trades as soon as it was available was critical," says Dean.
After working out the shorter cycle at Rincon Hill, Webcor was able to achieve it on the taller of the two Infinity towers. The building's structure is complete, and about two thirds of its 43 stories are enclosed.
One Rincon Hill's phased occupancy, a first in the city for such a tall residential tower, also had to be planned from the start of construction. Bovis figured out a way to keep the external hoist operating so that workers could get materials to upper floors even with lower floors occupied. "We are not turning over the unit [adjacent to the hoist] on each floor until the hoist comes down," says Read. "The homeowners know what that is going to look like."
Condo owners also have been forewarned that construction will begin later this summer on the second tower. At 52 stories, it is not as tall as the first tower. But work will still kick up the usual dust and make a lot of noise.