Concrete Innovation

Novel research and a high-strength admixture reduce the need for beams and supports in a new L.A. condo tower.
August 11, 2010

The Californian, a 23-story, $200 million condominium tower that opened last September in Los Angeles, is located in the heart of a seismic zone. Yet with 327,034 sf of residential space, its luxury units (ranging in size from 2,700 sf to as much as 8,000 sf) have few visible beams and supports. How can the Californian offer such huge unobstructed residential spaces and still claim to have a seismic capacity 33% stronger than any other building on Wilshire Boulevard, as the design firm, Nadel Architects, claims?

The answer is new engineering research combined with the use of high-reactivity metakaolin, a mineral admixture that enhances concrete strength to more than 12,000 psi. Chicago-based developer Fifield Cos. wanted to create the most luxurious high-rise in the city, with an architecture that would appeal to affluent empty nesters. Floor-to-ceiling windows, sound-deadening floor slabs, and wide-open floor plans were a must for the buyers Fifield wanted to reach.

"When you reduce the size of the square columns, you gain billable square footage," said Edward Kirk, preconstruction project director for San Mateo-based Webcor Builders, the general contractor. "These things are billed at $900-1,000/sf. The end result is it takes a little more work to build, but it's more profitable in the end."

Structural engineer Englekirk Partners of Los Angeles used research it had conducted with the University of Southern California and the University of California, Irvine, for the state's Carpenter/Contractor Cooperation Committee (C4). The C4 study investigated reinforced moment frames, which are designed to be rigid at beam-column connections and flexible in a beam or column's midspan. The midspans act as hinges within the limits of the steel reinforcement, to allow the structure to move a little during an earthquake. Englekirk used the C4 findings to develop a structural core of moment-resisting columns and beams capable of resisting seismic forces.

"It has no conventional core," said Lawrence Ho, an Englekirk principal. "It's a moment frame pulled to the core of the structure, which is not a very traditional design. But once we compared it to shear wall and dual-moment framed/shear wall systems, we found it was the most effective approach."

This pseudo-core system allowed the perimeter columns to be reduced in size. Beams supporting the post-tensioned floor were formed within the eight-inch depth of the floor slab to maintain an uninterrupted ceiling plane and floor-to-ceiling window height. Ho said Englekirk began testing a variety of high-strength concrete products shortly after deciding to use the moment-frame system. City inspectors were brought in early to witness the testing and gain confidence in the construction methods.

For moment-frame columns below the seventh floor, Englekirk specified concrete that would achieve a compressive strength of 10,000 psi after 56 days and 12,000 psi after one year. The 12,000-psi concrete also needed to be ductile to allow the moment frames to flex in an earthquake. Lesser seismic loads in the upper floors allowed Englekirk to specify more conventional concrete mixtures.

Silica fume vs. metakaolin

To find the strongest concrete formulation, Englekirk and Webcor built a set of mockups and tested a number of supplementary cementitious mixtures six months before the start of construction. Two finalists emerged: a silica fume product and high-reactivity metakaolin, an amorphous aluminate silicate refined from kaolin clays and formed into a pozzolanic admixture. Looking at the data from the mockups, both strengthened well soon after being poured, but metakaolin had a higher strength at 56, 90, and 180 days.

A metakaolin mix with Engelhard Corporation's MetaMax HRM was used in the project and reached the target 12,000 psi at 365 days. The MetaMax HRM also made the exposed beams a bright shade of ivory, which matched the travertine marble and concrete cladding of the other levels.

Webcor saved the expense of covering the exposed concrete in the building's 114,695-sf, three-floor underground parking garage, due to the white color.

Webcor then faced numerous challenges in the construction phase.

The high-reactivity metakaolin had a set time of six hours. The forms had to be watertight. The proprietary mixture from Catalina Pacific of Catalina, Calif., had to be batched in the trucks; each load took up to 50 minutes to batch. The locally available stone was too fine to make a suitable aggregate for the high-strength mixture, so 3/8-inch-diameter crushed stone had to be shipped in from British Columbia.

"On the constructability side, the high-strength mix had to be pooled where the beams made the column connections," said Kirk.

The Californian was completed on schedule with all units pre-sold at prices ranging from $2 million to 6 million. It's the first high-rise in Southern California to use a C4 moment-frame system with such high-strength concrete.

"Any project of this magnitude is going to be challenging," said Ho. "In the end, we think we pushed technology forward and made some real advances in California construction."

         
 

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