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January 22, 2013

ACEC National finalist: Platinum award
Structural systems


Project: City Creek Center redevelopment, Salt Lake City

Client: The Church of Jesus Christ of Latter-day Saints (operating as City Creek Reserve) and its equity development partner, Taubman.

Photo by Allen Macbean/Photographic Solutions [enlarge]
MKA designed the 446-foot-long retractable glass roof and an all-glass skybridge in Salt Lake’s City Creek Center redevelopment. The firm also did the center’s seismic engineering.

Magnusson Klemencic Associates provided structural engineering expertise for the redevelopment of City Creek Center in Salt Lake City.

The Church of Jesus Christ of Latter-day Saints, operating as City Creek Reserve, and its equity development partner, Taubman, wanted to halt urban decline and reinvigorate the heart of downtown Salt Lake City. They wanted a walkable and sustainable community where tourism would flourish and residents could live, work, shop and play.

MKA was the sole structural engineering firm for the 23-acre, 5.5 million-square-foot project. The firm worked with three architectural teams to renovate and preserve some older buildings and to build several new buildings, then tie them all together with pedestrian and transportation options.

What made this project particularly challenging was the fact that downtown Salt Lake City is built on a sensitive seismic area — 25 percent worse than San Francisco.

The structural challenges included building three new high-rise towers and excavating basements as deep as five stories straight down the face of existing 20-story buildings. To stabilize this seismic environment, MKA developed an unprecedented "super deck" that had to fit around existing buildings, transferring column loads, delivering seismic loads to the ground, and supporting the entire park. MKA also created gigantic buckling-restrained braces that dissipate seismic energy and provide significant architectural flexibility.

MKA's "performance-based seismic design," a fairly new and rarely used methodology, created two new firsts in the project involving nonlinear seismic analysis: First, it eliminated 6 inches of concrete wall thickness in the basement walls of City Creek Center, and second, it eliminated the need for a redundant system at the perimeter of the new 33-story high-rise residential tower, opening up views and increasing layout flexibility for residents.

MKA engineered two of the most distinctive and complex elements of City Creek Center, its 446-foot-long, first-of-its-kind retractable glass roof, and a 140-foot-long all-glass pedestrian skybridge that connects the two center blocks, spanning a street and light-rail line.

The glass roof is in two sections, each of which consists of three sets of glass-covered panels that arch across a 56-foot-wide retail concourse. To open, the panels from each side part in the middle and retract, then bow down out of sight of the people below.

The key to the roof's success is its seven whalebone-shaped ribs that support the glass roof. A front wheel and back set of rollers support each of the 10.5-ton whalebones, with the front wheel following one geometric path and the rollers riding an incline at the rear.

As the back rollers go up the incline, the roof's cantilevered front edge dips down, causing the roof to bow. The whole process takes just seven minutes.

The pedestrian skybridge relies on rods and hidden connections to create its design transparency. The 160-ton structure was constructed off-site and then erected in one piece. The entire event was filmed for a National Geographic special on “World’s Toughest Fixes.”

City Creek Center has 750,000 square feet of retail space, 76,000 square feet of office space, 800 residential units and 2.2 million square feet of below-grade parking. It incorporates nine existing structures, including a hotel tower and multiple office towers, five concrete residential towers, a seven-story steel office tower, as well as the pedestrian skybridge and movable roof.

The center also has nearly 6-acre public park with a 1,200-foot replica of historic City Creek winding through it, three waterfalls, extensive landscaping and live trout, all supported on the below-grade, long-span parking structure.

Utah Gov. Gary Herbert said, "This is going to be an asset to our entire state, throughout the intermountain West and throughout the country."

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