Stairs And Landings Structural Requirements

A stair looks like a secondary element and it is designed to a higher load than the floor at either end of it.

The reason is crowding. People on a stair bunch up, and a crowd on a flight is a heavier and more dynamic load than the same crowd spread over a floor.

What a stair has to resist

A uniform live load per square metre, generally higher than the equivalent floor in the same building.

A concentrated load on a single tread, which governs the tread design and is the check that catches a thin precast tread or a cantilevered stone one.

Horizontal load on the balustrade, applied at handrail height, which produces a large moment at the base of the post. This is the condition described in [balustrades and handrails, structural requirements](/blog/balustrades-and-handrails-structural-requirements), and the fixing is the hardest part of it.

Dynamic response, because people move rhythmically on stairs and a flight whose natural frequency falls near the pace of a footstep feels alive underfoot.

Vibration is the complaint you get

A stair that satisfies every strength and deflection check can still be unacceptable, and the symptom is a flight that bounces or a landing that shakes when somebody walks on it above.

Which happens on long, slender, lightly loaded stairs, particularly steel stringers with open treads, and on cantilevered flights. Mass and stiffness are the levers, and the easiest fix at design stage is an additional support or a stiffer stringer. Retrofitting it later means a new support beneath a finished stair.

The same mechanism as [floor vibration and footfall in apartments](/blog/floor-vibration-and-footfall-in-apartments), with less mass to damp it.

How a stair spans

The support arrangement is the main design decision and it drives everything else.

Spanning between landings, where the flight is a one way member supported at top and bottom. The simplest and most common, and it means the landings are carrying substantial reactions.

Cantilevered treads, fixed into a wall or a spine beam with no support at the free end. Visually the most striking and structurally the most demanding, because each tread is a cantilever with a moment connection and the supporting wall or beam is carrying torsion. The torsion part is the mechanism in [torsion in concrete beams](/blog/torsion-in-concrete-beams).

Spine or stringer supported, where one or two members carry the treads. Fine, and the stringers are deep members with their own deflection and vibration behaviour.

Spiral and helical, which are three dimensional structures carrying bending and torsion together and are not detailable from a standard section.

The landings are where the loads concentrate

A landing takes the reaction from the flight above and the flight below, and those are point loads at its edges rather than a spread load across it.

Which means the landing is frequently the critical element, and the connection of the landing to the surrounding structure is where the design attention belongs. On a concrete stair, that connection involves reinforcement continuity into the wall or slab; on a steel stair, a bolted or welded connection with real forces in it.

The landing is also where the stair interacts with the building's movement. A stair fixed rigidly to two structures that move relative to each other becomes an unintended structural link, which is the issue in [expansion and control joints in concrete](/blog/expansion-and-control-joints-in-concrete) and matters for the seismic separation described in [earthquake design requirements in Australia](/blog/earthquake-design-requirements-in-australia).

In a fire the stair is the exit

Which puts a structural requirement on it that is easy to overlook: it has to remain usable while the building is being evacuated.

Fire rating of the stair structure, protection of any steel supporting it, and continuity of the enclosure around it are all part of the design rather than additions to it. The coordination is the subject of [fire engineering integration in structural design](/blog/fire-engineering-integration-structural-design).

A stair that is also the construction access carries a different load again during the build, which is a temporary works consideration alongside [backpropping and temporary works](/blog/backpropping-and-temporary-works).

Adding a stair to an existing building

A new stair means a new opening in a floor, and the opening is usually the larger structural issue.

Cutting a void through a slab removes reinforcement, interrupts the floor's ability to act as a diaphragm, and changes the span of the surrounding slab. That is the assessment in [core drilling and penetrations in existing slabs](/blog/core-drilling-and-penetrations-in-existing-slabs) and [diaphragm action and how floors brace a building](/blog/diaphragm-action-and-how-floors-brace-a-building), and it is the same class of work as [adding a lift to an existing building](/blog/adding-a-lift-to-an-existing-building).

Then the new stair's reactions have to land on structure that can take them, which frequently means a new beam under each landing rather than bearing on whatever is convenient.

Tolerances and the finish

A stair is one of the few structural elements where dimensional accuracy is visible and regulated.

Riser heights have to be consistent within a tight tolerance across a flight, and that tolerance has to survive the structural tolerance plus the thickness of the finish. Which means the structural levels are set with the finish allowance already in them, and a stair poured to the structural level and then tiled produces a first or last riser out of tolerance.

That is the coordination described in [tolerances in concrete construction](/blog/tolerances-in-concrete-construction), and on a stair it is the difference between a compliant flight and one that has to be rebuilt.

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