Staircases

A staircase is a sloping slab, the waist, with steps formed on top, spanning between floors, landings or beams. The commonest form in buildings is the dog-legged stair: two flights with a landing between them. Others include open-well stairs, single-flight stairs between beams, cantilever steps from a wall or a central spine, and helical stairs, which need a full three-dimensional analysis.

Structurally, most stairs are designed as one-way slabs spanning along the slope or across it. When the waist spans longitudinally, the load is worked out per square metre of horizontal plan: the self-weight of the inclined waist is increased by the ratio of its sloping length to its horizontal length, and the weight of the triangular steps is added at half the riser height times the unit weight. Finishes and the imposed load from IS 875 (Part 2) complete the load.

IS 456 gives rules for the effective span of stairs, including how much of a landing may be counted when the landing spans in the other direction, and for how load on areas common to two crossing systems may be shared. The depth is chosen from span-to-depth limits and checked in flexure, shear and deflection like any slab. Main bars run along the span at the bottom of the waist; distribution bars run across.

Detailing at the kink where the waist meets a landing is critical. Bottom bars that simply follow the re-entrant corner try to straighten under tension and can burst the cover, so they are crossed and anchored into the compression zone of the other member. Stairs are also escape routes, so fire cover and robust supports matter as much as strength.