Loads on plan
The waist slab is inclined, so its self-weight per square metre of plan is γc·t/cos θ. The steps add a triangular wedge, on average R/2 thick. Landings carry only γc·t. Finishes and live load act on plan. The factored load is 1.2D + 1.6L (ACI 318-19 5.3.1); stair live load is typically 4.8 kPa for public buildings and 2 kPa in houses (ASCE 7).
Moment with a landing
The flight and landing are treated as one beam spanning horizontally between supports, with different loads on each part. The tool finds the reactions, the point of zero shear and the maximum moment. With monolithic supports it takes 0.8 × the simply supported moment (about wL²/10) – provide top steel at the supports in that case.
Reinforcement
Main steel runs along the stair, at least 0.0018bt (7.6.1.1), with spacing not more than 3t or 450 mm (7.7.2.3). Distribution bars across the width provide 0.0018bt at up to 5t or 450 mm (24.4.3.2). Shear uses Table 22.5.5.1(c) with the size effect factor λs, since slabs have no minimum shear reinforcement.
FAQ
What about the kink at the landing?
At an inside (re-entrant) corner the bottom bars must not run straight through – they would push out the cover. Lap them crossed and anchor each bar into the compression zone.
Does this cover cantilever or dog-legged stairs?
No – it covers longitudinally spanning flights only. Treads cantilevering from a wall, or flights spanning across the width, are designed as one-way slabs or cantilevers per step.