This stair concept was conceived by the modern luxury design and architecture firm, Ingrao Inc. for a private residence in Manhattan, New York. The half oval-shaped room the stair was to live in drove both the design and structure of the stair. The curved outside wall was envisioned with L-shaped cantilever solid stone treads extending into the center of the room. As each tread extended from the wall it organically turned upwards along the opening to provide an integrated handrail illuminated from the interior. Wall cladding in matching stone curved down from the floor above transitioning the eye from the staircase to the stone-clad balcony which rounded the rest of the room. The stair spanning from one floor to the next also included one landing which turned the stair 90 degrees and became the crux of the engineering. While ascending the staircase the open risers coupled with the soft choice of material and custom stone finish ensured an elegant light look to the overall design.
The material to be used on the staircase was Montpellier Gris from Marble City, Oklahoma. Efforts were made to meet the architect’s aesthetic preference for the project by selecting a specific bench of the quarry based on the color and vein structure. Once the stone was selected for aesthetic qualities extensive material testing was performed to understand the specific properties of the natural material. Ultimately, the engineering of the stairs was developed around the properties of the stone while minimizing changes to the design set forth.
The first 4 L-shaped treads were designed to be reinforced together and secured to the quarter landing. The quarter landing was a stone-clad steel structure that spanned from the corner walls and supported the main run of stairs. After the quarter landing the upper portion of the stairs consisted of 18 L-shaped treads that were post-tensioned through the balustrade locking the sections together. A steel box stringer was utilized along the outside edge of the upper portion of the stair to hold the treads in place and ensure the stability of the stair while also stiffening the balustrade. The wall panels were anchored to a structural wall and self-supported so they did not bear on the treads below.
Several unique challenges arose during the engineering studies which included a large inter-story drift and the need to bear as little weight on the upper floor deck as possible. These two problems would normally have been a reason to shift the design away from natural stone toward a compromised design aesthetic. However, by combining several engineering technologies and bespoke engineering the staircase was made to work. One clever solution for the inter-story drift was the introduction of hidden movement joints and a large steel swing arm that controlled the top connection of the stairs. These technologies allowed the stone to be utilized for its inherent properties and become both aesthetic and structure. This ultimately reduced coordination between trades and simplified the installation process to something which could be completed in a minimal amount of time.