Jerry Jaksha, owner of Mesa Vista Design, Rio Rancho, N.M., was commissioned to create a concrete and foam house in San Carlos, Sonora, Mexico, designed around a 24-step, full 360-degree concrete circular staircase.
Jeksha designed the stairs in SketchUp, a 3D modeling program marketed by Google. He planned for the stairs to be self-supporting based on a simple cantilever design. Without a center support, the stair loads were designed to be carried by the landing and the wall. The calculations from the design software made it possible to preset the cantilever rebar in the correct position as the wall was built, long before the ramp and step pour.
This was the first circular concrete staircase that Jeksha and his son, Adam, have constructed. The stairwell was created using 5-inch-wide ICF forms. Each form was hotwired every 5 inches on one side to make the 5-foot-diameter radius bend. Packing tape held the forms together while 2-foot lifts were poured.
A 5 1/2-inch step rise dictates a full 360-degree stair. The stair landing and adjacent 8-inch concrete floor was supported with a 10-foot diameter cantilevered reinforced concrete ring beam and cast with the wall and the floor. The Jekshas formed the ring beam half on a couple of plywood sheets and held it up on scaffolding for the pour with the rest of the beam.
A temporary, vertical 20-foot 2-inch steel tube was installed at the center of the stairwell and the wall forms were placed against a rotating 5-foot positioning stick, all the way to the brick dome roof. Horizontal and vertical rebar in the foam wall formed a 3-inch diagonal concrete tube grid in the structural wall. Two cantilever rebars were tied into the wall at each stair step, as they worked their way up the wall.
Forming the ramp and risers was the next challenge. Jeksha came up with the idea to stack riser forms held in place with U-bolts to pivot around the center pole.
The bottom of the ramp is a hyperbolic, paraboloid shape with a sharp twist at the center. To create this shape, a 1-inch foam bead board was twisted and stretched over several step risers. A fiberglass skin was laid on the top, removed, and placed on the bottom to hold the double-curved shape. Two of these forms allowed them to pour the 3-inch-thick ramp in three sections.
The ramp begins at the fourth step, so the first three steps were cast on the floor. They used PL 100 glue to bond between pours. When the stair ramp was complete, the ramp measured 3 inches thick and cured for a week. All of the concrete was mixed onsite, using a superplasticizer to reduce the w/c ratio to 0.4 with a 1-inch slump.
To create the steps, the ramp’s support forms were rotated and reinforced to become step forms. U-shaped rebar was left exposed at the steps to tie them in, as well as the adhesive. Stacked 3/4-inch plywood for the stair form made creating even steps much easier. Originally, the plan was to tile the steps, but ultimately they were finished with a textured concrete overlay.
The stairs were a challenge for the Jekshas from start to finish, but the final product looks great and will be around for years to come.