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February 6, 2023

National Finalist: Gold Award  
Structural Systems 

Photo from Mortenson
The historic 160,000-square-foot arena roof was supported for construction to happen underneath it.

Thornton Tomasetti
Project: Climate Pledge Arena  
Client: Populous  

Seattle’s historic Key Arena, dating back to the 1962 World’s Fair, was transformed into Climate Pledge Arena during a $930 million renovation in 2021. The new world-class arena is double the size of the original building, featuring an 800,000-square-foot, mostly below-grade venue with a capacity for 17,000 fans attending hockey, basketball, concerts, and other events.  

This extensive renovation project presented a unique set of complex issues for the engineers at Thornton Tomasetti. The required preservation of the historically landmarked, 44 million-pound roof structure and exterior curtain wall and near-total demolition of the structure below was much like building a ship in a bottle.  

How do you support a roof the size of three football fields, weighing more than eight 737 jets, in a high seismic zone, during construction work below? With plenty of expertise and ingenuity. The team designed a temporary support system using 3,700 tons of steel framing to uphold most of the roof’s gravity load and resist wind and lateral seismic forces during the 20 months of construction that would undermine nearly all existing roof supports.  

Thornton Tomasetti performed a seismic retrofit of the existing roof to ensure it would resist the seismic demands of modern codes. The advanced performance-based design process relied on realistic ground motions based on site-specific seismicity and accounted for the structure’s nonlinear behavior. This technique significantly reduced the number of steel roof members requiring retrofitting. 

Evaluation, design, and construction planning for the temporary lateral force-resisting system required extensive collaboration among project team members. Thornton Tomasetti designed a perimeter shoring system of soldier piles, tiebacks, lagging, and shotcrete to support the excavation and removal of 680,000 cubic yards of soil and to support permanent building and soil loads. This dual-purpose system was cost-efficient compared to a more traditional shoring system used for temporary conditions. The system configuration changed as excavation progressed. As excavation areas proceeded in alternate quadrants, the contractor installed bracing trusses in a top-down sequence as soon as soil removal allowed.  

An integrated approach to solving challenges is critical to overall success on complex projects like this. The construction engineering and structural design teams at Thornton Tomasetti worked in parallel with the steel contractor to provide a full Tekla model for the 8,700 tons of permanent steel for the arena and parking garage. Complete shop drawings for the 3,700 tons of temporary roof shoring structural steel and this design approach allowed the team to accelerate the steel procurement and detailing process while enabling fabrication to begin much earlier, shaving months off the schedule. The project team worked together seamlessly to address these challenges and transform a historic venue into a first-class sports and event facility, which opened in time for the NHL’s 2021-2022 season.

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