When initial discussions about the concrete spiral staircase began with the client, the objectives were to create a functional staircase that would take up as little space as possible, to open up the room to the 2nd floor for better air circulation to distribute heat more evenly through the home, and of course to create a one-of-a-kind piece of art to compliment the modern decor.
At first, we weren't sure how to successfully form the large post so it would appear to be a solid, seamless piece. Standard sonotubes have the cardboard seams that leave the corkscrew lines on the finished concrete. As it turns out, fate stepped in while we were attending the World of Concrete in Las Vegas where we discovered fiberglass-lined sonotubes. The 12" diameter lined sonotube was the perfect size to support the steel treads both structurally and visually.
Since this was a renovation to a finished home, it was quite obvious that we were going to have to cast the post in place. Once the hole was made between the 2 levels, we carefully cut a 12" circle into the polished concrete floor below and chipped out about an inch of concrete. We then drilled and pinned half inch rebar into the floor and set the sonotube in place.
The tube was braced using only a few 2x4s wedged between the opening of the upper floor to lock upper side of tube in place. The concrete was poured using a 5000 psi mix w/plasticizer and fiber. Concrete was brought to the second floor in buckets and simply dumped in and vibrated. Concrete was allowed to cure for about two weeks, and upon demolding the post was basically finished. It came out of the mold with a sheen, so it did not need to be ground or even sealed.
The overall tread design revolved around the massive concrete post. We know it would be a mistake to have thick or solid treads - we needed to use a material that would offset the posts visual grandiosity. The simplest solution was to use raw steel treads that would allow us to create the illusion that the treads were basically floating around the large post.
The treads were all hand-cut by a local artisan and mounted to raw steel welded brackets. Mapping out the treads was one of the most critical tasks of the entire project. We knew we had to get the rise and run exact because we could only drill into the concrete once. We nearly miscalculated the placement which would have left us with a staircase suited only for people under two feet tall. After checking and rechecking our figures, we finally nailed it. Holes for the tread bolts we drilled 4" deep and we used long expanding bolts to fasten steel treads tot the post.
A precast piece of concrete holds the top of the post central to the opening and also acts as the first step. To keep with the modern feel, the first step and the upper floor insert were precast on Plexiglas which allowed us to achieve the finest grind possible. The 5' x 5' insert piece was extremely delicate to flip and transport from the shop, because it was such a large piece but with very little concrete mass around the perimeter. We also fabricated the upper rails with tubular steel and 3-form panels, again to keep with the modern feel and to lighten the visual weight of the entire structure.
Today, the staircase is used regularly. Though it looks perilous, with a little bit of extra care while climbing or descending, everyone can enjoy both the functional aspect and sculptural beauty of this one-of--a-kind design.