Cast-in-Place Structures

Cosmopolitan Tower

Location: Seattle

Owner/developer: 9th and Virginia LLC

Project team: M.A. Mortenson Co., general and concrete contractor; Mithun, architect; Cary Kopczynski & Co., structural engineer; Stoneway Concrete, ready-mix supplier

Cosmopolitan Tower is 33 stories of condos, retail and parking in downtown Seattle.

Cast-in-place concrete was used because of its advantages for multifamily buildings. One of the most distinctive features of the project is its shear wall drop head system, which includes a central reinforced concrete shear wall core with a 6-inch-wide thickened slab around the core’s perimeter. The system allowed the building’s 8-inch post-tensioned slabs to span uninterrupted nearly 40 feet from the central core — eliminating interior columns and opening up the condos’ floor plans and parking layouts.

Exterior building columns and walls were used as part of the fascia, which eliminated the need for cladding.

Exterior building columns and walls were used as part of the fascia, which eliminated the need for exterior column and wall cladding. That reduced building weight and saved on fascia costs.

High-strength 10,000-psi concrete was used in the shear walls, allowing a reduction in wall sizes and eliminating the need for ductile frames. By framing the building without perimeter ductile frames, views were maximized.

Cast-in-place concrete allowed for exterior balconies without any reduction in ceiling height. Also, concrete allowed the building to meet fire-resistance ratings without any fireproofing.

One of the project’s biggest challenges was the design of its main lateral force resisting system, which is classified as an undefined system in the design code. To meet the requirement of the code, a nonlinear model was developed and a pushover analysis was completed in each direction.