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January 26, 2015

Structural systems
National finalist: Gold award
CKC Structural Engineers

Photo courtesy of CKC Structural Engineers [enlarge]
Seattle’s Viktoria apartments were built with a unique shear wall core designed to eliminate interior columns.

Project: Viktoria
Client: Weber Thompson

Located in the heart of downtown Seattle near Pike Place Market, Viktoria has 249 apartments ranging from studio to two-bedroom units, six levels of parking above grade and 3,700 square feet of street-level retail.

The 25th floor has panoramic views and is devoted to socially focused amenities. An evergreen tree pierces through an oculus opening, and a dramatic butterfly roof cantilevers more than 20 feet from the sky lounge, adding unique charm to the modern high-rise building.

Viktoria’s structural system consists of cast-in-place concrete with post-tensioned floor slabs and a shear wall core for seismic and wind resistance. The column sizes and total concrete volume were effectively decreased by optimized concrete strength in the lower level core walls.

CKC proposed a unique shear wall core design which provided a highly advantageous way to eliminate interior columns, increase architectural design flexibility, and improve structural efficiency. The structure has no internal columns from the core to the exterior glass line, providing completely open interior space with enhanced views from each unit while increasing architectural design flexibility.

By thickening the slab from 8.5 inches to 16 inches for a distance of 6 feet around the core perimeter and creating a unique “drophead,” all internal columns became unnecessary. Slab spans of nearly 40 feet from the central core to the exterior glass line were made possible, providing completely open living units and parking layout without structural obstructions.

This approach eliminated the need for transfer beams that would otherwise have been required to shift interior column locations at levels where occupancies change.

Another benefit of this structural system was reduction in reinforcing steel requirements. With fewer interior columns, the majority of the building’s dead load was transferred to the core, minimizing net tension in the walls under overturning from wind and seismic forces. Additional costs for thickening the slab were therefore canceled by savings in core wall reinforcing.

The increased thickness around the core also created a more uniform and consistent deflection distribution, which would remain stable with time. Thus, the slab is less susceptible to serviceability problems.

Through careful coordination among the design and construction teams, CKC’s unique solution to the design challenge resulted in a clean economical structural layout without compromising other design features of the building.

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