Tuesday | 08 May, 2012 | 8:47 am

Starring steel

By Lynn Stanley

Design firm marries steel with innovative techniques to achieve lightweight structures, cut costs and save time

May 2012 - Like a modern-day monolith, The JW Marriott Hotel and Ritz Carlton Hotel and Residences at L.A. Live, 54 stories of curved linear steel, marks the first tower to rise in downtown Los Angeles in 20 years. The multi-use structure also is the first building project of its kind to house two leading hospitality brands, nesting JW Marriott and The Ritz-Carlton together. The around-the-clock residential, sports and entertainment spot is the heart of Los Angeles’ urban revitalization efforts. Behind the building’s sleek façade, the real story is the creativity of Gensler principals Kap Malik and Warwick Wicksman. They combined steel with innovative fabrication for a design strategy that light-weighted yet earthquake-proofed the structure, added saleable square footage and cut project costs and time.

Gensler is a global design, planning and strategic consulting firm. Its focus on sustainable designs earned it the Leadership Award from the U.S. Green Building Council in 2005. “Gensler has considerable experience with large, complex projects,” says Malik, project design director. “We have a large hospitality component and have worked with these brands. We also had a good relationship with AEG, the project developer. They were familiar with other work we had done on projects of this scale.”

Unique system
According to Wicksman, technical director, the 1.7 million square foot building incorporates more than 18,000 tons of steel. L.A. Live originally was conceived with concrete shear walls, which for a structure of this size could be as thick as 30 inches. A thin, steel-plate shear wall system offered an option for light-weighting the structure, reclaiming usable space and fortifying the building against seismic activity.

“The structural engineer Nabih Yousseff and Associates had worked with such a system on a smaller project and felt it would be a good candidate for this structure,” says Wicksman. “The ability to transition from concrete to steel allowed us to go from a foundation 16 feet deep to one just 8 to 10 feet deep. Use of steel over concrete light-weighted the structure, giving us another 20,000 square feet of space and allowed for four additional floors beyond what soil-bearing conditions would have accommodated if this were concrete. Building weight was reduced by nearly 35 percent. By fast tracking the steel procurement process, we were able to shorten the construction schedule by three to four months.”

The shear wall system contains steel plates 3/8 inch to 1 inch thick with vertical boundary elements or box columns made of 4-inch-thick high-strength, low-alloy steel, which extends from the foundation to the roof. The steel for the vertical boundary elements was sourced and custom fabricated in Japan. The thin steel plate and vertical boundary elements have the ability to deform and yield, dispersing an earthquake’s energy instead of cracking under its pressure. Accommodating two brands with a mix of private residences and hotel suites atop entertainment attractions and amenities presented Gensler with challenges. “The mechanical systems for the residences and the hotel had to be isolated,” says Malik. “Each component—JW Marriott, The Ritz-Carlton Hotel and Ritz-Carlton residences—required its own drop-off, identity and vertical circulation core.”

Curvilinear shape
The original concept called for a staggered building with a vertical line. “That meant pulling the columns in fairly deep, which took usable space away from the condos and was aesthetically disruptive,” Malik adds. Gensler and Nabih Yousseff worked with the steel subcontractor to develop the curved columns creating the boundary for the steel plate shear walls that tapered at the top. The curvilinear shape allowed the team to optimize the building program, improve efficiency and create a more open view for the residences and two-story penthouses. To further support the building and earthquake-proof it, the design featured trusses and buckling restraining braces at the mid-level and at the top of the building to stiffen the structure. “You have a very tall, skinny building,” says Wicksman. “The trusses contain angled elements called buckling restraining braces that connect to the shear wall steel plates. These components make it possible to stiffen the building at its midsection and at the top.”

Wicksman adds the building structure was designed and constructed to a unique building code. “Rather than follow traditional prescriptive requirements in the building code we used the allowable alternate methods, creating a performance-based design, and using a peer review analysis process with the city,” says Wicksman. “This method gathers leading engineers from across the state to review the project’s structure during the design process and offer innovative feedback. For example, typically a building like this would have moment frames around the perimeter at every floor level. These beams can be 30 to 36 inches deep, reducing the view for the residential area. The peer review analysis allowed us to eliminate these perimeter moment frames, reduce the beam sizes and open those views up.”

Gensler gave the building a glass skin of alternating transparent, translucent or fritted glass. Sustainable yet elegant, the project was completed in 2010. Gensler earned a LEED Silver rating with its energy-efficient, lighter-weight structure. MM

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