Spiral stairs are special staircase kits with all components combined as a single unit. They are designed to fit in a four-square (2050x2050mm) unit space. The key component of Utilihab spiral stairs is a special 150mm wide octagonal Primary frame profile serving as a central support post. This post will usually feature a cast foundation plate over spacer plates at a floor frame cross-point that attaches by t-nuts to the four upper slots of the frame cross point and by threaded rod to its own pier footing below the floor -allowing standard floor panels to still be slipped under the foundation plate. Treads are formed of 50x100-150mm beams supporting a pre-finished channeled wood tread plate attached by counter-sunk bolts. Each tread unit is supported by two beams, one at the leading edge and one at the trailing edge. Each trailing edge is overlapped by the leading edge of the next-highest stair. Tread beams attach by the usual tension plugs but with the addition of threaded spacers in the slot to insure that the tension plugs are securely supported at the exact vertical spacing. (optionally, triangular gusset plates plates with milled T-rails may be used)
For open stairs, the treads feature a rounded edge and all are of uniform shape. The outer corners of the tread -at the ends of the diagonal beams- feature mounts for a rounded profile post. A short post sits on the trailing edge corner to support and rigidly connect to the leading edge of the next higher tread. On the leading edge a long post is used to support the hand rail which is pre-formed in sections forming a helical shape and may be based on tubular profile or a formed pre-finished wood profile. Working from the bottom-up, each tread is installed and rigidly attached to the one beneath. The helical handrail is then attached to the top of the posts, usually using ball swivel joints.
For enclosed stairs, The tread beams are lengthened to extend to connect to Secondary framing members in a surrounding stair well. The wood tread plates are then formed to fit flush to the surrounding wall. The beams at 90 degrees attach right to secondary posts. Those are 45 degrees attach to the corner framing using overlapping alloy plates that ‘wrap’ the Secondary corner profile while connecting to the tread beam sides. (A more specialized interleaved overlapping corner connector piece interfacing to an end tension plug may also be used) The helical hand rail is then side-mounted to the Secondary frame posts at the 90 degree angle points. If riser plates are used, they are simply fitted in slots in the wood tread plate where trailing and leading ends overlap.