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Thursday | 05 February, 2009 | 8:45 am

Shear genius

By John Loos

February 2009 - Usually, one equates thin walls with cheap motels. Not anymore. The ultra-thin steel plate shear walls of the $1 billion L.A. Live Hotel & Residences project in Los Angeles will be its standout feature. They'll also allow real estate space to be maximized in the heart of L.A. Live, the city's $2.5 billion downtown residential, entertainment and sports development area.

Having broken ground in November 2007, the L.A. Live Hotel & Residences will be Los Angeles' first steel plate shear-wall high-rise, topping off at 56 stories, with 2 million square feet of space, including 1,001 hotel rooms and 224 luxury condominiums. The 1/4-inch to 3/8-inch steel plate shear walls replace bulky 30-inch concrete walls, reducing the structure's weight by 35 percent and enhancing its seismic resistance potential.

"The benefits of this type of construction are almost too good to be true," says Nina Kristeva, an American Institute of Steel Construction engineer for the West Coast region. "The dramatically reduced wall thickness, going from several feet to fractions of an inch, not only results in more useable floor area but also leads to a significant reduction of the total building mass, which, in return, when accelerated by an earthquake, generates less lateral force to be resisted. Therefore, there's a lighter foundation system as well."

Shaking it up
The concept for a steel plate shear wall structure came in March 2006 from Nabih Youssef Associates, Los Angeles, a structural engineering firm with a special interest in earthquake codes and standards. The walls were seen as a way to simplify construction while trimming schedules and reducing costs. The idea was embraced by developer AEG, Los Angeles, project architects Gensler Architects, San Francisco, and steel fabricator The Herrick Corp., Stockton, Calif., whose joint efforts have the project successful so far.

A steel plate shear wall is designed to resist lateral forces via diagonal forces in the web plates, which have negligible compression strength and shear buckle at low levels of lateral loading. This post-buckling behavior is known as tension-field action. The web plates reach their yield stress across the entire plane, and dissipate energy through inelastic deformations when the structure is subjected to the expected seismic action.

Since its inception, the steel plate shear wall design has been implemented elsewhere with great success, including a retrofitting job at the Northridge Hospital in San Francisco, the U.S. courthouse in Seattle and in a 34-story office building under construction in San Diego. The walls are fabricated in a shop-controlled environment and swiftly assembled by the same crew erecting the rest of the framing, eliminating the need to sync schedules between the tasks.

Up close
In October 2008, the AISC co-sponsored a breakfast and site tour for architects, steel fabricators, general contractors, developers and building code officials, allowing an up-close look at the L.A. Live Hotel & Residences' progress. About 200 attendees saw firsthand how the steel plate shear-wall design was enhancing and expediting the construction process.

"The fact that the steel plate shear-wall design solution was credited with reducing the construction schedule by more than three months, and with gaining 20,000 square feet of extra real estate floor space, was definitely the talk in the room," says Kristeva.

Along with this event, the AISC has hosted a Web site, www.aisc.org/LA-Live, to provide interested parties with updated information on the project's progress, as well as a webcam for live footage of the construction. The L.A. Live Hotel & Residences is scheduled to open its doors in early 2010. MM

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