Spiral steel stairs

What it is

A spiral stair winds around a central column, with wedge-shaped treads cantilevering from the column and supported at the outer edge by balusters and a helical handrail. It needs a circular footprint only slightly larger than its diameter, so it is used where a straight or switchback stair will not fit: access to small mezzanines, roof hatches, crane walkways, tank tops, plant rooms, offices inside warehouses and feature stairs in commercial interiors.

Because the treads narrow towards the column, a spiral stair is steeper and harder to use while carrying loads than a conventional stair, and most codes limit where it may be used. It should be chosen for occasional access by people who know the building, not as the main route for materials or as a general exit, unless the code specifically allows it.

Types and options

The top landing is usually a quarter-circle or square platform that connects to the floor edge and closes the opening with a guardrail or balustrade.

Design and compliance

• US workplaces (OSHA 1910.25(d)): spiral stairs only where a conventional stair is not feasible; clear width ≥ 26 in, riser ≤ 9.5 in, tread depth ≥ 7.5 in measured 12 in from the narrow edge, headroom ≥ 78 in. Handrails and stair rails follow OSHA 1910.29(f).
• US buildings (IBC 2021, 1011.10): width ≥ 26 in, tread ≥ 6¾ in at the walkline, riser ≤ 9½ in, headroom ≥ 78 in, with all treads identical; spiral stairs are permitted as means of egress only in the limited situations listed in the code. Guards ≥ 42 in and rail loads to ASCE 7 (50 lb/ft and 200 lb concentrated).
• Australia: the NCC restricts spiral stairs as required exits; where used, barriers (≥ 1000 mm, or ≥ 865 mm along the stair, 125 mm sphere rule) and handrails (≥ 865 mm) still apply. For plant access the designer confirms suitability against AS 1657, including the 2000 mm headroom.
• Europe: EN ISO 14122-1 guides the choice of means of access for machinery, and stairs in buildings follow national regulations. Guard-rails for machinery access are ≥ 1100 mm with knee-rail and toe-plate (EN ISO 14122-3).

The centre column carries the full stair load and must be fixed at the base and, normally, restrained at the top landing; the designer checks column buckling, tread cantilever bending and the deflection of the outer tread edge.

Fabrication and finishing at World Steel

The column is cut to the floor-to-floor height and marked with the angular position of every tread. Treads are welded in a jig so they are identical, then either welded to the column or fitted with sleeves or brackets for a bolted kit. The handrail is rolled to the helix and trial-fitted on a full stair assembly in the shop, so that balusters and rail joints line up on site. Welding follows AWS D1.1, AS/NZS 1554.1 or EN ISO 3834-2 procedures. Finishes are hot-dip galvanizing by qualified galvanizers to ASTM A123, AS/NZS 4680 or EN ISO 1461, or paint and powder coat for interiors – see paint and coating systems.

Installation and specifying

Set the column base plate on a level slab, stand and plumb the column, fix it at the top landing, then install treads from the bottom up, followed by balusters and handrail. To quote, send the floor-to-floor height, available diameter, rotation direction with entry and exit positions, code basis and intended use, tread and rail type and finish – together with the mezzanine or platform layout.