Affluent baby boomers becoming empty-nesters are fueling a demand for luxury urban living. The Californian, a new Los Angeles condominium built to meet the demands of this demographic, breaks new ground in two areas of concrete design and construction: First, it uses new engineering research to minimize interior columns and beams despite its location in one of the most seismically active regions of the country. And second, the building is believed to be the first high-rise structure to use high reactivity metakaolin, a pozzolanic supplementary mineral admixture that boosts concrete strengths to over 12,000 psi.
Developed by the Fifield Companies, the structure is 23 stories high over three levels of subterranean parking and contains 80 opulent units ranging from 2700 to 8000 square feet. Despite price tags that ran into the multimillions of dollars for a penthouse with views of the Santa Monica mountains, the downtown Los Angeles skyline, and the Pacific Ocean, all units were sold before the structure was topped out.
According to Dale Yonkin, executive vice president of Nadel Architects, the project's designer, “The client's goal was to create the most luxurious high-rise condo in Los Angeles, with an architecture that would appeal to more mature buyers who tend to appreciate the warmth of traditional residential design.”
This criterion guided all design decisions. For example, the firm selected an architectural precast concrete cladding, fabricated by Clark Pacific Precast, because it offered a rich texture and visual appeal and permitted extensive use of molding and ornamentation.
Yonkin says that one primary reason for selecting a concrete structure was the superior floor-to-floor sound attenuation provided by a concrete slab compared with steel decking, a feature expected by buyers of the units who, for the most part, were coming from the relative quiet of single-family homes. Also, he explains, “We wanted to offer dramatic, 10-foot-high rooms in the units. Concrete allowed us to use the underside of slabs as a ceiling and reduce overall floor-to-floor height for the project.”
Other features intended to appeal to buyers also affected selection of a structural system: Floor-to-ceiling windows that make the most of the high ceilings and spectacular views required a solution that did not rely on beams around the perimeter of floor slabs or running through units. In addition, the client wanted wide-open floors unobstructed by columns or shear walls in order to maximize net sellable floor area and allow buyers to customize the layout of their units.
To meet these criteria, structural engineer Englekirk Partners drew upon recent research it had conducted, along with the University of Southern California and the University of California at Irvine, under the sponsorship of Southern California's Carpenter/Contractor Cooperation Committee. This “C4” study investigated heavily reinforced moment frames designed to be rigid at beam-column connections and flexible at the midspan of beams and columns. Each midspan acts as a hinge, within the elastic limits of the steel reinforcing, to allow a structure to flex without damage during an earthquake.
According to Lawrence Ho, a principal at Englekirk, his firm relied on the C4 findings to develop a structural core of moment-resisting columns and beams capable of resisting seismic forces. Ho says, “Pulling a moment frame to the core of the structure is not the most traditional design. However, we compared it to shear wall and a dual moment framed/shear wall systems and found it was the most effective approach to meet the architectural design criteria.” The approach allowed the size of perimeter columns to be reduced to allow for large areas of fenestration. Beams supporting the post-tensioned floor were formed within the 8-inch depth of the floor slab to maintain an uninterrupted ceiling plane and the floor-to-ceiling window height.
For moment-frame columns below the seventh floor, Englekirk specified concrete that would develop compressive strength of 10,000 psi after 56 days and 12,000 psi after one year. In addition to resisting seismic forces, the high-strength concrete allowed column sizes to be reduced up to 50 percent, helping to maximize floor space. On upper floors, diminishing seismic loads allowed for the use of more conventional concrete mixtures, and 6000-psi concrete was used in decks. The 12,000-psi concrete was required to have greater than ordinary ductility to accommodate the flexing action of the moment frames; for similar reason, its compressive strength was not allowed to exceed 14,500 psi. The project is believed to be the first to use a moment-frame system based on the C4 research and the first high-rise in Southern California to use such high-strength concrete.
Developing the mixture for the 12,000-psi concrete was a team effort that included Englekirk, general contractor Webcor Builders (a CC100 firm), ready-mix provider Catalina Pacific Concrete, and Twining Laboratories, the quality assurance and materials consultant retained for the project. Inspectors for the City of Los Angeles were also brought into the approval process early in the project so they could witness testing of mock-ups and gain confidence in the proposed construction methods.