The Challenge
A thirty-story mixed-use tower engineered onto a quarter-acre infill lot most thought too tight to build on
The site was a quarter-acre infill lot in Seattle, hemmed in on three sides with no room for a wide structural footprint. The program stacked retail at grade, residential above, and parking below, each with different structural demands and column grids that could easily conflict. A slender tower on a tight site leaves no room for generous structure; every inch of the floor plate that went to columns or walls was a square foot the developer couldn't sell or lease. The height limit was fixed, which meant floor-to-floor efficiency was the primary lever for getting the unit count the program required.
Our approach
We paired a concrete core with an efficient post-tensioned floor system, which kept structural depth thin and the column grid tight to the perimeter. Post-tensioned slabs span further than conventional concrete with less depth, which translated directly into lower floor-to-floor heights and additional floors within the building height limit. The core was positioned to serve both the lateral system and the vertical circulation, minimizing the structural footprint on the residential floors and keeping the floor plates as open as possible. We coordinated the structural grid with the parking layout below early in design to avoid conflicts that typically emerge late and cost time to resolve.
The outcome
Thirty stories of mixed program went up on a quarter-acre lot that most thought too small to build on at this scale. The post-tensioned floor system delivered the floor-to-floor efficiency needed to hit the unit count within the height limit. Column-free residential floors gave the architect flexibility on layout, and leasable area on every floor was as high as the site would allow. The structure's slim profile also reduced impact on adjacent properties during construction, which mattered on a site with neighbors on three sides.
30
Stories
0.25
Acre site
22
Floors of mixed program
"Everyone told us the site was too tight for a tower. Johnson gave us thirty stories and made every floor count."
Sarah Nguyen, Principal, Pacific Urban Partners
