Switchback and egress stairs

What it is

A switchback stair (also called a dog-leg or U-return stair) rises in two or more flights that turn 180° at intermediate landings. It reaches the same height as a straight stair in roughly half the length, which makes it the standard layout for egress stairs in buildings, external fire escapes, multi-level platforms and free-standing stair towers serving tanks, silos, conveyors and equipment structures.

Egress stairs form part of the means of escape of a building, so their geometry, width, rails and fire protection are set by the building code rather than by plant-access rules. World Steel details and fabricates the steel – stringers, landings, posts, rails and stair tower frames – to the code basis and fire strategy given by the project designer.

Types and options

Treads can be grating treads for external and industrial stairs, or concrete-filled pan treads and landings for internal egress stairs, which are quieter and accept floor finishes. Rails range from industrial pipe rails to architectural balustrades with picket or glass infill.

Design and compliance

• United States (IBC 2021): riser 4–7 in, tread ≥ 11 in, width ≥ 44 in (36 in where the occupant load is below 50), headroom ≥ 80 in, and a flight may not rise more than 12 ft (3,658 mm) between landings. Handrails 34–38 in with a circular Type I profile of 1¼–2 in diameter; guards ≥ 42 in with openings that reject a 4 in sphere (21 in in industrial areas not open to the public). Egress stairs are nonstructural components under ASCE 7 Chapter 13 and must accommodate storey drift where the building is seismic.
• US workplaces (OSHA 1910.25): angle 30°–50°, riser ≤ 9.5 in, tread ≥ 9.5 in, width ≥ 22 in; stair rails installed since 17 January 2017 need a top rail of at least 42 in.
• Australia (NCC 2022): riser 115–190 mm, going 250–355 mm, 2R + G 550–700 mm, not more than 18 or fewer than 2 risers per flight; barriers ≥ 1000 mm (≥ 865 mm along the stair) with openings that reject a 125 mm sphere; handrails ≥ 865 mm. Plant-access stairs follow AS 1657.
• Europe: stairs in buildings follow national building regulations, with barrier loads from EN 1991-1-1 Table 6.12 and seismic actions from EN 1998 where relevant; machinery-access stairs follow EN ISO 14122-3 (angle 20°–45°, 600 mm ≤ g + 2h ≤ 660 mm, guard-rail ≥ 1100 mm).

Fire protection of egress stair steel, where required, is part of the fire strategy – see structural fire protection. Seismic detailing of slip connections and landing supports is coordinated with the building designer through seismic design inputs.

Fabrication and finishing at World Steel

Flights and landings are fabricated as separate bolted units so that a multi-storey stair can be packed into containers and erected floor by floor. Stringers are CNC-drilled for treads and connections; landing frames, posts and rail returns are welded in jigs so that the handrail runs continuously from flight to landing to flight. Welding follows AWS D1.1, AS/NZS 1554.1 or EN 1090-2 / EN ISO 3834-2 procedures. Where specified, a complete flight-landing-flight set is trial-assembled to check rise, rail continuity and the slip connections. Finishes are hot-dip galvanizing by qualified galvanizers to ASTM A123, AS/NZS 4680 or EN ISO 1461, or paint to the project system.

Installation and specifying

Erect the landing supports first, then lift each flight with its stringers braced, bolt to the landings and install rails as each level is completed so the stair can be used safely during construction. Leave slotted seismic connections un-tightened as detailed. For a quotation, send floor levels, available footprint, code basis, occupant load or required width, tread and rail type, fire and seismic requirements and finish – see the quotation process.