San Francisco’s 590-ft Skyscraper Lifts Seismic Design’s Stature

Nadine M. Post / ENR Webcor’s Plue, left, MKA’s Klemencic and Johansson, and Bovis senior project manager Nori Mizushima (front) teamed to achieve a three-day-per-floor cycle.

Structural engineer Ron Klemencic had extra reasons for gratitude during the 2005 Thanksgiving season. After hitting his head against the wall on and off for more than three  years, he finally received stamps of approval for the first two performance-based seismic design high-rises in earthquake prone San Francisco. PSD can cost less, improve design and ease construction.

Word about the 38- and 43-story Infinity towers came the last week of November, followed by news about the 64-story One Rincon Hill (ORH) in early December. “I was elated both times,” says Klemencic, president of Magnusson Klemencic Associates, Seattle. “It was two years of blood sweat and tears on [the Infinity] before we even initiated Rincon Hill.” That review took a year, twice the norm for San Francisco towers.

The city’s approval marked the beginning of the end of a logjam in big California cities for performance seismic design (PSD) of buildings taller than 240 ft. “People are going to be doing this left-handed at 100 mph in five years thanks to Ron...,” predicts one prominent San Francisco architect who declines to be identified.

Bovis Lend Lease Hoist will remain to feed upper floors during phased move-in starting next month.

The approach is a way to meet the intent of the code’s prescriptive provisions by using a single framing system instead of a costlier and “clunkier” dual system to resist lateral loads. But some cities are cautious in allowing PSD because it requires extreme engineering.

The 590-ft-tall ORH is the higher profile of the two cutting-edge projects, and not just because it is taller and sits on a hill nudged up against the Bay Bridge approach. The nod from the city unlocked the door not simply to PSD in California but to the tallest “performance” skyscraper in the U.S. ORH is also the tallest high-rise to contain buckling-restrained braces and the first to use BRBs as outriggers. The slender tower is the first residential building in the U.S. to have a liquid tuned mass damper to reduce sway to acceptable comfort levels, says the engineer.

There’s more. PSD saved $5 per sq ft and allowed Webcor Concrete, Hayward, Calif., to achieve a three-day 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 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 Inc.

Webcor Spandrel-beam-free perimeter eases flying form construction, saves money.

The single-frame PSD, 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 back-up 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 Inc. PMA represents ORH’s developer, Urban West Associates, San Diego.

But there was a hitch. “There were risks associated with the design because 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-ft and 400-ft Infinity towers. Urban West “had no assurances about whether ORH project would be delayed in the review, and if so, at what cost, says Sell.

After weighing benefits and risks,  the developer decided to commit to MKA’s scheme 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.

Magnusson Klemencic Associates

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 architect and its subconsultants. The core and it 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. At ORH, “the concrete shown in the architect’s plans never changed from Aug. 15, 2004,” says Chris Pemberton, a vice president in the local office of architect Solomon Cordwell Buenz.

Magnusson Klemencic Associates

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 that. “Infinity took the lumps in terms of technical arguments,” Klemencic says. “Rincon Hill definitely benefited from that.”

Webcor

In the end, the ORH team met its schedule goals. Construction of typical tower floors 8-60 started in July 2006. The building received a temporary certificate of occupancy for the first 20 floors on Jan. 4. Bovis expects a TCO for the next seven in a couple weeks. Condo owners are scheduled to begin a phased move-in next month and full completion is set for the summer. “We are happy with the schedule,” says Peter Read, Bovis project director.

Owners like PSDs because they cost less. Architects like them because they offer more design freedom. Contractors like them because they are easier to build. And engineers like them because they can result in higher-quality structures.

For example, computer earthquake simulations revealed significant stress demands half way up the tower—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.”

Webcor In performance core, rebar is thicker at the base, where it does the most work.

There were other surprises during engineering. Results of wind tunnel tests prompted the  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 curves. As  wind whips around the building from south to north, it creates a cross wind effect, causing the building to sway from east to west.

The damper consists of four concrete tanks equipped with baffles to absorb energy. The tanks have a total 50,000-gallon capacity. Water doubles as a supply for firefighters. MKA expects to finish determining the best water level by May.

Webcor

Webcor

MKA also fine-tuned the design based on tests of slab-to-core-wall connections 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.

Slab cracks were smaller and  better distributed 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 congested link beam rebar above openings in core walls. Tests at the University of California, Los Angeles, showed that if required sets of diagonal bars are placed free of direct confinement and the overall beam section is confined similar to a column, the beams perform equally as well as the more traditional but nearly unconstructible approach (ENR 5/21/07 p. 10). The alternate detailing has been adopted into the American Concrete Institute’s model code, which is being released this week.

Unlike the adjustments, BRBs were part of the original concept. MKA chose BRBs over the more typical concrete outrigger walls because BRBs absorb a lot of energy in a more predictable fashion.

The tower, with a 90-ft x 110-ft footprint and 50-ft x 33-ft core, is slender for its height. The core’s aspect ratio is 18:1 in the long direction and 12:1 in the short direction. BRBs, which consist of steel sleeves containing steel cores coated with a debonding agent surrounded by concrete, reduce the aspect ratio to 8:1.

Webcor Trussed flyers bear on columns.

Outrigger locations are based on the developer’s desired residential unit mix, the architect’s design and structural considerations. One system spans from level 26-32; the other from 51-55. Each system of 8 BRBs consists of two pairs of  outrigger diagonals that form mirror image Ks. Each BRB diagonal, 18  to 20 ft long, spans two levels, The capacity of each of the 16 BRBs, design and fabricated by Star Seismic, Park City, Utah, is 1,200 kips. At the time, that was the largest tested BRB on the market.

Full tower height perimeter outrigger columns, 2 x 8 ft in one direction and 7 x 6 ft in the other, are reinforced concrete except for six stories at each outrigger system. There, MKA embedded a steel column that 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 to locate the plates with...

... a high level of precision, important due the tolerance requirements of the BRBs,” says Johansson.

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At the top and bottom of each system, composite drag elements consisting of steel-plate beams embedded in reinforced concrete connect outrigger columns to embedded steel in the core.

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. That said, “We have nonperformance-based designs that have worse congestion,” says Chris Plue, a Webcor vice president.

Magnusson Klemencic Associates

Magnusson Klemencic Associates

BRBs, installed by TC Steel, Petaluma, Calif., also had their challenges. Erection tolerances at the connection plates were about 1⁄16 in. 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, says Bovis.

Bovis says 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 MKA’s  PSD.

“MKA values their role in the speed of construction,” says Plue.

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 ORH’s small floor plate helped.

Webcor took 10 floors to perfect the sequence. The tower’s first typical floor is level eight. Workers hit the three-day cycle at level 18.

Magnusson Klemencic Associates Tuned Liquid-Mass Damper

Double work shifts helped. So did the 1,000-sq-ft, truss-supported flying table forms for the slabs. Each form took only 40 minutes to set, says Bovis. Webcor used two sets of truss-supported flyers—eight per floor. The flyers are hung off columns as opposed to bearing on floors. The system is more costly but eliminates reshores 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 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.

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Magnusson Klemencic Associates One Rincon Hill is first U.S. tower to have a tuned liquid-mass damper.

Having worked out the shorter cycle at ORH, Webcor now is achieving it on the taller tower of the Infinity. The building is currently up 35 of 43 floors.

ORH’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 is set to begin late next month on the second tower. At 52 stories, it is not as tall as ORH. But work will still kick up the usual dust and make a lot of noise.