The structural engineer for One Rincon Hill's two residential towers (top left) relied on a performance-based approach for seismic design. Site plan and rendering courtesy Solomon Cordwell Buenz

 

There's more. PSD saved $5 per square foot and allowed Webcor Concrete, of Hayward, California, to achieve a three-day pour cycle-the norm is five-on typical tower floors. That's a West Coast speed record for high-rise concrete work. The general contractor is especially proud that the short cycle was also achieved on the upper system of the tower's two sets of BRBs. (The lower system met a five-day cycle, as planned.) "It's pretty amazing," says Tim Dean, structural project manager in the local office of Bovis Lend Lease.

The single-frame PSD project, engineered using sophisticated computer simulations and analysis, consists of a ductile concrete core with the supplemental outriggers and post-tensioned flat slabs. It has no spandrel beams because it has no backup perimeter moment frame, as required under prescriptive code provisions.

For the 362-unit condo's developer, that's a selling point. "Being able to remove the moment frame and use floor-to-ceiling glass was a key issue to differentiate the building in the marketplace," says Jeffrey S. Sell, vice president of the local Project Management Advisors. The firm represents the developer, Urban West Associates, of San Diego.

For the architect, elimination of the traditional perimeter moment frame allowed more flexibility with apartment layouts. "We weren't fighting with the structure," says Chris Pemberton, a vice president in the local office of architect Solomon Cordwell Buenz. In addition, the savings realized by using a more efficient system meant there was more to spend on important architectural components, like the building's unitized curtain-wall system, he adds.

Risks and rewards

But adopting the PSD approach was not without uncertainties. "The process had never been approved in San Francisco," says Sell. Not only that, Klemencic made sure Urban West knew all about the ongoing excruciating peer-review process for the 350-foot and 400-foot Infinity towers. Urban West "had no assurances about whether the Rincon Hill project would be delayed in the review, and if so, at what cost," says Sell.

One Rincon Hill is the first project in the country to use BRBs (bottom diagram and photo) as outriggers. It has no perimeter moment frame, as is standard for buildings taller than 240 feet designed with a prescriptive approach. Flying forms (bottom) kept floors free of shoring and helped achieve a West Coast speed record for high-rise concrete work. Photo courtesy Webcor (bottom); diagrams: MKA Associates (top two)

 

After weighing benefits and risks, the developer decided to commit to the performance-based approach at a project kickoff meeting in July 2004. But it had a strategy for success. MKA knew the structural design would get picked to pieces on all fronts during the peer review. So it was critical to keep the structural system very simple in order to keep the architecture simple. Repetition and consistency ruled to minimize reviewers' questions. "The design team as a whole had to get religion about the core, not because we couldn't change it, but because we wanted to fight only one big battle in the peer-review process," says Klemencic.

That had ramifications for the architects and their subconsultants. The core and its contents, including elevators, stairs, electric rooms and service areas, the typical floor plate, and the floor-to-floor height, had to be placed, sized, and frozen four months earlier than usual. Lobby design had to be locked in eight months early. "The concrete shown in the architect's plans never changed from August 15, 2004," says Pemberton.

The strategy worked. Peer review went "incredibly smoothly" with "technical issues resolved quickly to everyone's satisfaction," says Ronald O. Hamburger, senior principal in the San Francisco office of Simpson Gumpertz & Heger, and the chair of the three-person panel.

Klemencic gives the Infinity some credit for the ease with which One Rincon Hill moved through its review. "Infinity took the lumps in terms of technical arguments," he says. "Rincon Hill benefited from that."

In the end, the Rincon Hill team met its schedule goals. Construction of typical tower floors 8 through 60 started in July 2006. The building has already received a temporary certificate of occupancy for the first 34 floors and full completion is set for later this summer.