May 9, 2017

Cast-in-place structures

Photo courtesy of WACA

The 34-story Tower 12 in Seattle uses high-strength concrete and rebar, allowing for smaller columns and more leasable floor area.

Tower 12

Location: Seattle

Tower 12, named after the Seattle Seahawks’ 12th Man, is a 34-story, 492,000-square-foot residential tower at Second Avenue and Virginia Street in downtown Seattle.

The building has 314 apartment units, 7,000 square feet of ground-level retail, four levels of parking above grade and five levels below. The project broke ground in early 2015 and was slated to finish in April.

Tower 12’s structural system has four main components: high-strength concrete and reinforcing bar, an innovative seismic core, performance-based seismic and wind design, and an optimized post-tensioned slab design.

Tower 12 utilized high-strength concrete and reinforcing bar to improve performance and constructability. A specified concrete compressive strength of 14,000 psi was used in columns up to the sixth level to reduce column sizes and improve the interior layout.

This synergistic use of high-strength concrete and rebar resulted in smaller columns and boundary elements, which enhanced interior design flexibility and increased the leasable floor area. It also allowed column sizes to be kept constant nearly full height, which maximized formwork productivity and helped maintain a rapid construction schedule.

The seismic system consisted of an innovative combination of coupled shear walls and ductile moment frames at the core, and included multiple link beams to effectively dissipate seismic energy. The unique core design enhanced seismic system ductility without the need for wall hinging. Seismic performance was improved without compromising constructability.

The structural engineer used a modified performance-based design with a nonlinear time history analysis to cycle the building’s seismic performance through a suite of input earthquake ground motions. The state-of-the-art core minimized the floor area required for wall structure and reduced the wall-reinforcing requirements, resulting in a more constructible system.

Due to the tower slenderness and shape, a wind tunnel test was performed to provide wind pressures for cladding design and to mitigate wind accelerations. The core stiffness and reinforcement were tuned to resist wind tunnel forces and provide acceptable levels of building acceleration.

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