Tower 12

Owner: Continental Properties
2015 2nd Avenue, Seattle, Washington

Architect: MG2
Engineer: Cary Kopczyski & Company
General Contractor: PCL Construction Services, Inc.
Concrete Contractor: PCL Construction
Reinforcing Bar Fabricator: Harris
Total Project Cost: $120 Million
Total Project Size: 492,000 sf
Rebar Producer: Harris
Rebar Placer: Central
Building Height: 385 ft
Number of Floors: 34
Total Concrete Yards: 26,000
Reinforcing Tons: 3,200


The floor system consists of 8.5 inch thick two-way post-tensioned concrete slabs, with fourteen feet perimeter cantilevers and outriggers extending five feet from the core walls. The outriggers allowed overall slab spans of nearly forty feet. The long span slab design eliminated internal columns, which resulted in open and spacious interior spaces without transfer beams at the lobby, retail, and parking levels.

The seismic system used a Performance Based Design (PBD) approach. Working with structural and geotechnical peer reviewers, the structural engineer used non-linear time history analysis to cycle the building’s performance through a suite of input seismic ground motions. The resulting state-of-the-art PBD core minimized the shear wall area and enhanced the ratio of leasable to gross floor area. It also reduced wall reinforcing requirements, resulting in a more constructible building.


The tower architectural design required that the tower façade have multiple angles to augment the views in all units; this lead to a challenging slab edge, long cantilever, and ultra-long spans without internal columns. In addition, the tower floor plate was less than 11,000 square feet and extended 385 feet above the base. This Tower 12 utilized high strength concrete and rebar to improve performance and constructability. A column concrete compressive strength of 14,000 psi was specified up to Level 6 to reduce column sizes and improve layout efficiency. ASTM A706 Grade 80 reinforcement was used for tie steel in columns and shear wall boundary elements, boundary element vertical bars, and foundations. This synergistic combination of high-strength concrete and rebar created several benefits. First, it allowed smaller columns, compact shear wall boundaries, and a more efficient foundation. Second, it enhanced interior design flexibility and increased the leasable floor area. Finally, it allowed column sizes to be kept constant nearly full height, which maximized formwork productivity and helped maintain a rapid construction schedule.


The structural system selection of reinforced concrete was driven by the building type and the ability to mold the concrete material in the unique building shape without adding a premium on the slab construction. Another unique factor was the ability to create ultra-long spans with column free space using 8.5″ slabs with outriggers extending off the concrete core. The outriggers increased the slab spans without the addition of internal columns. It was determined early in design that concrete was the most appropriate material for achieving these objectives.