The Challenge
A dramatic cantilevered amenity floor that became a residential tower's signature feature
The developer wanted a column-free amenity level that cantilevered well beyond the tower footprint, extending out over the city as a signature feature. It had to read as effortless: no exposed structure, no visible transfer elements, nothing that revealed how it was being held up. A cantilever of this scale on a slender high-rise also had to perform dynamically, since long cantilevers can respond to wind in ways that are perceptible to occupants, and an amenity floor full of people needs to feel solid underfoot. The structural solution had to solve the strength problem and the comfort problem at the same time.
Our approach
We engineered a transfer structure within the floor depth below the amenity level that redistributed gravity and lateral loads so the cantilever slab above could extend without visible support columns. The slab was post-tensioned, which gave us the span we needed at a depth that fit within the architectural floor section. Dynamic analysis showed the cantilever's natural frequency was within the range that could feel uncomfortable underfoot, so we tuned the stiffness and mass of the system to push the frequency out of the perceptible range. The result is a floor that performs without any of its structural logic being visible to the people standing on it.
The outcome
The amenity floor became the building's primary marketing asset, and the cantilever is the first feature every prospective tenant asks about. Lease-up was faster than the developer's projections, and the amenity level was specifically cited in multiple tenant conversations as a deciding factor. The floor performs as designed: no perceptible movement, no serviceability issues, and no visible structural elements interrupting the views. The project has been published in two trade journals, and the structural approach has since been referenced in feasibility discussions for similar amenity concepts on other towers.
28 ft
Cantilever reach
0
Visible columns
Level 31
Amenity level
"The cantilever is the first thing every prospective tenant asks about. Johnson made the impossible part look effortless."
Development Principal, Confidential
