Structural staircase design
A staircase is a sloping structural member that spans between floors and carries people, and increasingly it also has to be an open feature with hidden supports. We design concrete, steel and timber stairs, for clients across the UK.
Does a staircase need a structural engineer?
A standard timber stair from a joiner usually does not, but any concrete stair, steel stair, cantilever or floating stair, or a stair where the supports are hidden or unusual, needs designing. We work out how it spans, size the members and connections and check deflection and vibration so it is strong and feels solid.
How a staircase spans
Structurally a flight of stairs is an inclined slab or beam that spans between supports and carries the imposed load of the people on it, taken as a uniformly distributed load with a concentrated load check for a single heavy point. The way it spans decides everything. A flight can span longitudinally from the floor at the bottom to a landing or floor at the top, span transversely as treads cantilevering or spanning between two stringers at the sides, or, in a cantilever stair, project each tread out from a single wall or spine with no support at the free end.
The support conditions are where stairs are often misjudged. A half-landing that is itself supported only by the flights, a flight built into a wall at one side only, or a stair hung from the floor above all create load paths that have to be traced back to something solid. We identify how each flight and landing is really held, size the members for the bending and shear along the incline, and make sure the reactions are carried by beams, walls or hangers that can take them.
Concrete, steel and timber stairs
A reinforced concrete stair is designed as an inclined slab, with the main tension steel following the underside of the flight and continuing into the landings, and particular care at the kinks where the flight meets a landing, because the reinforcement has to be detailed so the bend does not try to burst outward. Steel stairs are built from stringers, channels or plates with treads spanning between them, designed for strength and, just as importantly, for the springiness a person feels, so the stair does not bounce underfoot.
Feature stairs are where the engineering gets interesting. Open-riser flights, glass balustrades acting as structure, floating treads cantilevering from a wall, and helical or spiral stairs all hide their supports, so the load has to be carried in ways that are not obvious: treads bolted deep into a concealed steel spine, a stringer buried in a wall, or a folded steel plate acting as a beam and finish at once. We design these so they are stiff and quiet as well as strong, and coordinate the fixings with the wall or floor that has to resist them.
Vibration, connections and compliance
Slender and open stairs can pass every strength check and still feel wrong, so we control deflection and vibration as much as capacity. A stair that deflects too much or has a natural frequency close to the rhythm of someone walking will feel lively and unnerving even though it is perfectly safe, so on feature and cantilever stairs we keep the stiffness high enough that the movement stays imperceptible. The balustrade is checked too, for the horizontal line load a crowd can apply along the handrail.
The connections carry the whole design. A cantilever tread is only as good as its fixing into the wall or spine, so we detail those anchorages, the base and head connections of steel flights, and the bearing of concrete flights onto their landings, with the tolerance a fabricator and site can actually achieve. The design is prepared to the Eurocodes and Approved Document A, coordinated with the geometry the architect and Part K set, and delivered to clients across the UK.
What we check.
The points our calculations resolve for a project like this.
- Span direction, longitudinal, transverse or cantilever, and its supports
- Imposed load as a distributed load with a concentrated load check
- Reinforcement detailing at the flight to landing kinks in concrete stairs
- Deflection and vibration so slender and feature stairs feel solid
- Cantilever tread and stringer anchorages into wall or spine
- Balustrade horizontal load and coordination with the stair geometry
From enquiry to sign-off.
Enquiry
Send drawings or describe the problem. We confirm the scope, the deliverables and a target timescale.
Information
We agree the survey, drawings or data we need and any site access required.
Engineering
Design, calculation or assessment to the relevant Eurocodes and UK National Annex.
Issue
A clear, defensible report or set of calculations, with assumptions and limitations stated.
Common questions
Does a staircase need a structural engineer?
A standard timber stair from a joiner usually does not, but any concrete stair, steel stair, cantilever or floating stair, or a stair where the supports are hidden or unusual, needs designing. We work out how it spans, size the members and connections and check deflection and vibration so it is strong and feels solid.
Why do open and floating stairs need special design?
Because their supports are hidden, the load has to be carried in ways that are not obvious, such as treads bolted into a concealed spine or a stringer buried in a wall. They also tend to be slender, so they can feel bouncy if only checked for strength. We design them for stiffness and vibration as well, and detail the concealed fixings.
Can you design a cantilever staircase?
Yes. A cantilever or floating stair projects each tread from a wall or spine with no support at the free end, so the whole load is carried by the fixing back into that support. We design the spine or the wall reinforcement, the tread anchorages and the stiffness needed to keep the free ends from feeling springy.
Do you set the going and riser dimensions?
The going, riser and headroom are set by Approved Document K and coordinated by your architect for comfort and compliance, and we work within that geometry. Our role is the structure: how the stair spans, what carries it, the member sizes, the reinforcement and the connections.
Related in structural design & elements.
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