Course · lesson 5 of 7

Staircases

How reinforced concrete stairs are designed with the STATIX staircase calculator on the SANS 10100-1 route: waist flights spanning along the stair or across it, the effective span with landings, loads on plan, the landing reactions, deflection, and the reinforcement and its schedule shapes.

1. Learning outcomes

After this lesson you can:

set the effective span of a flight with its landings, along or across;
work the loads on plan from the waist, steps and finishes;
check bending, shear and deflection, and give the reactions to the supports;
lay out the main, distribution and top bars;
design a flight cantilevered from a wall, and know what the page leaves out.

2. Before you start

RC Design 1 (slabs and span/d) and the worked example RC staircase to SANS 10100-1 (a dog-leg on landings across).
The staircase calculator.
A pocket calculator and about 50 minutes.

3. Stairs and their spans

Waist-slab stairs. Most concrete stairs are a sloping slab, the waist, with the steps cast on it. The steps are load, not strength: the section is the waist measured square to the soffit. A stepped (sawtooth) soffit with no waist is not modelled by the calculator; it designs waist flights.

Spanning along the stair. A flight with its landings in line spans from support to support: centre-lines for simple supports, or the clear span plus half of each supporting element (at most 900 mm) where it is built into members spanning at right angles (4.9.1.3.1). The landings in line are part of the same span.

Spanning across. A flight can instead span across its width: cantilevered from a wall along one side, or between two stringer beams. The calculator designs both on the SANS route (top bars at the wall, or bottom bars between the stringers); individual cantilever steps and helical stairs are NOT CHECKED.

Loads on plan. SANS 10100-1 takes the stair loads on the plan area. The waist weighs more per plan metre than its thickness because it slopes (divide by cosθ), and the steps add half a riser on average. The imposed load is the class value of SANS 10160-2.

Landings across and shared areas. Where a landing spans across a dog-leg or an open well, the area common to two spans at right angles may be shared equally. The calculator designs a landing across as its own one-way slab carrying the flight reaction; the existing Learn staircase page works that case.

4. Reactions and deflection

The reactions. The ends of the span push down on the walls or beams that carry them: the calculator gives each reaction per metre of width and for the whole stair, ultimate and characteristic, ready for the take-down. A cantilever flight gives its wall a moment as well.

Deflection. Stairs are checked by span/d like slabs, with table 10: 16 for truly simply supported ends, 20 where the walls restrain them. Where the flight takes up at least 60 % of the span, 4.9.2.3 allows 15 % more; long landings take that away.

5. Reinforcement and limits

The main bars. The bottom bars follow the soffit and must turn at the kinks between landing and flight; a bar on the inside of a re-entrant corner would pull out of the concrete, so the bars are lapped past each kink. The calculator lays them out and the calc sheet schedules them (SANS 282 shape 62 for the cranked bars, 20 for the straight ones).

Top steel and anchorage. A flight built into beams or walls at its ends gets some fixity whether you want it or not, so it needs top bars there. At simple supports the bottom bars must reach past the centre-line of the support (4.11.7.1.2): a narrow wall can be too short for them.

A cantilever flight. Spanning across from a wall, the flight is a cantilever slab: top bars at the wall, anchored in it (usually bent down at its far face), and the wall must take the moment and shear the calculator reports. Section through the flight, at right angles to the stair.

What the page does. This lesson uses the SANS 10100-1 route. The Eurocode routes design straight and dog-leg flights the same way; the cantilever and stringer flights are SANS only.