Curved concrete staircases?

The concrete staircase is designed so that it appears to float up to an intermediate landing and curve into a turn to continue upward to the second floor. Doka engineered the custom stair formwork for Nashville-based contractor Charter Construction. The specially designed formwork made the construction of the architecturally demanding staircase relatively effortless.

Designed and engineered for fast assembly

The goal was to create high-quality formwork with easily assembled pieces. Doka’s engineering team wanted to enable in-house preassembly as much as possible to reduce the amount of manpower needed onsite to fit the forms together. Leveraging the latest modeling technology, the engineering team’s first step was to create a detailed 3D model according to drawings. With a solid foundation for the formwork design in place, the team could then divide the stair formwork system into five sections (forms) for easy transport. Today’s formwork is planned digitally, and formwork production is largely automated. For this project, the engineering team used AutoCAD computer-aided design software along with RSTAB 3D structural analysis software. The virtual designs were exported to files that a CNC cutting machine can read to create the interlocking shaping timbers used to support the curves and turns of the formwork shells.

Preassembly: Extraordinary teamwork and craftsmanship

Doka’s engineering and preassembly teams worked together to optimize formwork efficiency. When the first form’s shaping timbers went to preassembly, the team worked to cut the timber sheets, fit them together in a lattice pattern to hold the curved form’s shape, and label and sequence them for nesting and easy construction onsite. They sent the engineering team valuable feedback about the process that led to additional improvements in the formwork design.

The preassembly team also spent hours planning and milling each form’s plywood facing to achieve smooth, rounded shapes. Multiple layers of bending plywood were used for the facing, with each layer placed in a different pattern to strengthen the layer below and maintain the form shape. The forms for the bottom portion of the staircase were the most challenging to cover with plywood, as they had the most extreme curves. For each form, a team of two to three people assembled the latticed forming timber (called a forming timber box) and prepared it for the rounded plywood facing. Each formwork element was milled, smoothed, sealed with epoxy and then put together.

Results

The stair formwork system was delivered to the project site as five separate form pieces, transported via two truckloads. Onsite assembly work was completed quickly and efficiently. When the epoxy-coated forms were removed, they left behind a smooth, unblemished concrete surface with minimal rework needed to achieve an excellent architectural finish. For help with the bends, twists and turns of your concrete project, contact Doka!