A Sleek Skyscraper in San Francisco Raises the Profile of Performance-Based Design

The nearly complete tower demonstrates multiple benefits of a nonprescriptive approach
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From Architectural Record
Nadine M. Post

Stronger structures

Owners like PSD because it costs less. Architects like it because it offers more design freedom. Contractors like it because the process produces buildings that are easier to construct. And engineers like it because it can result in higher-quality structures.

 

Rebar in the core of a PSD building is typically more dense than that of a prescriptive tower, especially at the base. Two sets of BRB outriggers span from levels 26 to 32 and 51 to 55, and tie the core to full-tower-height outrigger columns. Each system of eight BRBs consists of two pairs of diagonals that form mirror-image Ks.

Photo courtesy Webcor (bottom); diagrams: MKA Associates (top two)

 

For example, computer earthquake simulations revealed significant stress demands halfway up Rincon Hill-a bowing effect rather than the anticipated swaying. MKA added mid-height shear and confinement rebar to give the frame additional strength and robustness. "This shows the value of PSD," says Klemencic. "A prescriptive design would have never identified this. [PSD] allows us to study the building with more rigor and put the strength and detailing where it does the most good or is required."

There were other surprises during engineering. Results of wind-tunnel tests prompted the addition of the rooftop damper midway through design. "The building is very lively," says Klemencic. It sits by itself on a hill with no shielding from wind, and one face of the building curves. As wind whips around the building from south to north, it creates a cross-wind effect. "Like the wing of an airplane, it has lift," causing the building to sway from east to west, says Klemencic.

The damper consists of four concrete, water-filled tanks equipped with baffles to absorb energy. To determine optimal water levels in the tanks, which have a total capacity of 50,000 gallons, the structural engineer monitored the building sway with electronic sensors. Contractors began filling the tanks in early May. The water will double as a supply for firefighters.

Engineers also fine-tuned the design based on tests of slab-to-core-wall connections conducted at the University of California, Berkeley. In one configuration, post-tensioning tendon anchors were located one slab-depth away from the face of the core wall. The engineer also increased slab-bottom reinforcing to match top reinforcing. The tests showed smaller and better distributed slab cracks, and the connection demonstrated more ductile behavior. "This configuration demonstrated substantially improved behavior," says Klemencic. The tower uses the improved detail.
MKA made another adjustment to the design, this time involving reinforced link beams that span openings in core walls. Tests at the University of California, Los Angeles showed that a detail with a simpler reinforcing steel configuration could perform as well as the more rebar-intensive, but nearly unconstructable, approach outlined by the prescriptive code. The alternate detailing has been adopted into the American Concrete Institute's model code released earlier this year.

Unlike the adjustments, BRBs were part of the original concept. The engineer chose BRBs over the more typical concrete outrigger walls because BRBs absorb a lot of energy in a more predictable fashion. Another option would have been a system made of standard steel sections. However, in order to comply with the provisions of the code targeted at preventing failure due to buckling, these elements would have had to be very large, making the structural frame costly and difficult to erect, says Klemencic.

The tower, with a 90-by-110-foot footprint and 50-by-33-foot core, is slender for its height. The core's aspect ratio-the relationship between its height and width-is 12:1 in the long direction and 18:1 in the short direction. But the BRBs, which consist of steel sleeves containing steel cores coated with a debonding agent surrounded by concrete, effectively reduce the aspect ratio to a more manageable 8:1, helping reduce building sway by tying the core to the outrigger columns, explains Klemencic.

Outrigger locations are based on the desired residential unit mix, the architect's design, and structural considerations. One system spans from level 26 to 32; the other from 51 to 55. Each system of eight BRBs consists of two pairs of outrigger diagonals that form mirror image Ks. Each BRB diagonal, 18 to 20 feet long, spans two levels. The capacity of each of the 16 BRBs, fabricated by Star Seismic, based in Park City, Utah, is 1,200 kips. At the time, it was the largest tested BRB on the market.

 

One Rincon Hill is the first project in the country to use BRBs (bottom diagram and photo) as outriggers. The structural efficiencies realized through the PSD process saved money and allowed more to be invested in architecturally important components, such as the unitized curtain wall that cloaks nearly all of the building, including the rooftop mechanical space (top).

Photo courtesy MKA Associates (top); Bovis Lend Lease (bottom); diagram: MKA Associates

 

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Originally published in Architectural Record
Originally published in June 2008

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