Framing the moment

New projects show progress of precast hybrid moment-resistant frame technology
August 11, 2010

New design and construction technologies are seldom used unless they can be economically justified. Private-sector building projects, in particular, have little room for technological innovation purely for the sake of advancing the building process.

The precast hybrid moment-resistant frame (PHMRF) is a concrete structural system that incorporates a column/beam connection that provides both elastic and inelastic action at the joint to enable a structure to withstand earthquakes. It is an example of a recently developed technology that is proving to be both economically and structurally viable.

In the first commercial building applications since its development by an industry consortium in the mid 1990s, three PHMRF projects either have been built or are nearing completion. The first phase of the Westside Media Center, a 160,000-sq.-ft. office building in Los Angeles, was completed in September 2000. (A second phase which also uses the PHMRF is in construction.) In San Francisco, tenants soon are expected to move into the 400,000-sq.-ft. Pacific Plaza at Daly Center office building, and the 547,000-sq.-ft. Paramount apartment tower is to be completed in October.

The buildings' design/builder is Charles Pankow Builders Ltd., Altadena, Calif., a member of the consortium that developed the system. Pankow says the system was selected for the projects because it offered the most cost-effective structural solutions.

PHMRF milestone

The Paramount apartment tower represents a milestone for concrete technology. The 39-story tower, which includes two floors of retail and one floor of office space, will be the world's tallest concrete building located in a seismic Zone 4 region.

"It will open up a whole new niche for us," says Brian Goodmiller, national marketing director for the Chicago-based Precast/Prestressed Concrete Institute, also a member of the development consortium. While concrete is a popular building material for high-rise residential structures elsewhere in the United States, high-rise residential structures are new to high-seismic areas such as San Francisco. "The use of precast concrete as lateral members in seismic zones has not been done a lot. Codes discourage it the way they require you to design precast," says Brett Kaufman, project director for Los Angeles-based Robert Englekirk Consulting Structural Engineers Inc., the project's structural engineer.

PHMRF was selected as the structural system for the Paramount because of its economic feasibility and structural soundness, according to Joseph Sanders, Pankow's director of engineering. "Our goal with owners and developers [in this instance, The Related Companies of California, Irvine] is to make their projects work based on their economic pro forma. There is incentive for us to find more economical ways to build and to create the structure they want."

A number of factors pointed to PHMRF as the most appropriate system for the project. Though the project includes mixed-use facilities at its lower levels, the building is principally residential. Use of a concrete rather than a steel structural system allowed lower floor heights.

"The hybrid-frame system saved several million dollars compared to a structural steel building. We were able to reduce the height of the building and still accommodate the same square footage in the apartments. This results in less skin and less interior buildout," says Sanders.

In addition, because of San Francisco's moderate climate, air-conditioning requirements were reduced, resulting in less ductwork.

Compared with cast-in-place concrete, factory-produced PHMRF provided a higher level of quality control, reduced labor requirements and provided a higher degree of finish, according to Sanders.

The structural system comprises nearly the entire building skin on the Paramount project, as well as on the 10-story Pacific Plaza office building. "On the perimeter of the Paramount, the precast component is the finish component. It doesn't require the separate finish that is applied over steel," says Jim Fong, the Paramount's project architect with San Francisco-based Kwan Henmi Architecture/Planning Inc., the project's executive architect. Both the Paramount and Pacific Plaza buildings used colored precast concrete to add to the aesthetic of the building skin. The beams and columns of the Paramount were cast using a special white mix and a grayish-brown mix, while Pacific Plaza incorporated a pink color.

The structural system of the four-story Westside Media Center office building in Los Angeles does not also serve as the building's skin. But its ability to project the high-tech industrial image desired by the project's owner Kilroy Realty was important in the selection of PHMRF for the project, which was designed by Dallas-based HKS Architects. The high-tech appearance also was key in the choice of the system for the Pacific Plaza, developed by Summit Commercial Properties of Los Angeles and designed by Kaplan McLaughlin Diaz of San Francisco. "Most of the high-tech tenants wanted exposed structure," says Sanders.

While the structural performance of the PHMRF may not be the overriding reason for its selection on the Paramount or the other two buildings, they are important, says Sanders. Kaufman concurs. "The system is proven to be ductile and constructable and is anticipated to perform well in earthquakes based on the testing that has been done," he says.

Rising to a high-rise challenge

Of the three PHMRF building projects, the Paramount is the most challenging from a construction standpoint, according to Sanders. "The Paramount is a significant step up in terms of the challenges of the detailing, dimensioning and precasting and erection associated with the building," says Sanders.

Twenty-two of the 29 beams on each floor are of different dimensions. One side of the building requires curved pieces. The building employs three basic architectural shapes that include ledges, bullnoses and reveals.

Getting it right the first time was emphasized on the project for reasons of economy and scheduling. Significant time was devoted to the preplanning and detailing phases. In some instances, pieces were cast six or eight months prior to being erected. All of the structural pieces of the PHMRF frame — precast concrete, post-tensioning strands, mild reinforcing steel and PVC ducts — were tailored to exacting dimensions (see "Inside the precast hybrid moment-resistant frame," page 33).

The labor-intensive operation of grouting the reinforcing steel into the PVC ducts was improved on the project to the point that "it was a routine operation," says Sanders.

The structural design is a full-frame perimeter system, according to Kaufman. "In essence it's a perimeter tube system," he says. According to Sanders, with the tube system, the frame has no stop or start, which creates corner conditions. This resulted in the reinforcing steel coming from two orthogonal directions into the corner columns. As a result, the frames were designed to interlock the reinforcing steel and the steel-filled PVC ducts so that the steel and ducts on one side of the corner are below the steel and ducts on the opposite side.

At the corners, a system was created that enabled the two frames to be stressed in one stressing operation. The post-tensioning strands that channel through the column/beam connections wrap around the 90-degree corner of the two frames. In doing so, the strands were stressed around the corner instead of being stressed in one direction.

Because the tower was on a pour cycle of one floor every five days, high-rise sequencing techniques were essential.

Is it time for PHMRF technology to take its place alongside structural steel and cast-in-place systems? Its future looks bright, according to Kaufman. "The technology has a future in the marketplace. There's a speed and a quality to the construction. I'm sure at some point that it will spark interest from other contractors."