Staircase design concept and detailing

Concept of design of a staircase

A staircase is an inclined slab (the waist slab) with steps built on it. The common types are dog-legged and straight flights. The design steps are:

1. Fix the geometry: tread (going) $T$ about 250-300 mm, riser $R$ about 150-175 mm, width of flight (usually 1.0-1.5 m), number of steps and landing width.
2. Effective span $l$ as per IS 456 cl. 33.1 (horizontal distance between supports; where the flight is supported by a landing spanning across it, the going plus half the landing width or 1 m, whichever is smaller, at each end).
3. Assume the thickness of the waist slab $= l/20$ to $l/25$ (simply supported) or $l/26$ (continuous), and check it later by the span/depth ratio.
4. Loads (per m² of horizontal plan):
   • self-weight of waist slab $= 25 \times t \times \sqrt{R^2+T^2}/T$,
   • weight of steps $= 25 \times \dfrac{R}{2}$,
   • finishes, and live load (3 kN/m² for residential stairs and 4-5 kN/m² for public buildings, as per IS 875 Part 2). The loads act vertically on the horizontal projection, so the span used is the horizontal span.
5. Bending moment: $M_u = \dfrac{w_u l^2}{8}$ per metre width for a simply supported flight (use $w_u l^2/10$ where the ends are continuous or partly fixed).
6. Reinforcement: find $A_{st}$ from $M_u$ with $d$ measured perpendicular to the slope; main bars run along the slope; distribution steel $= 0.12\%$ of the gross area.
7. Check shear ($\tau_v \le k\tau_c$), deflection and development length.

Detailing of a straight flight

SECTION                           PLAN
          ___                     +---------------+
       ___|   top landing         | o o o o o o o |  distribution
    ___|                          | | | | | | | | |  bars 8 mm
  _|  /  main bars along slope    | | | | | | | | |
 |  /  waist slab  (bottom)       | | main bars | |  main bars
 | /                              | | | | | | | | |  at bottom,
 |/___ support (beam/wall)        +---------------+  along slope

• Main bars (say 12 mm @ 125 c/c) at the bottom along the slope, extended into the supports by $L_d$.
• Distribution bars (8 mm @ 200 c/c) across the main bars.
• Top bars at the supports (about 50% of the mid-span steel) where the stair is built into walls.
• Bars at the junction with a landing are anchored separately (they are not bent round the re-entrant corner).
• Cover 20 mm; the steps need no reinforcement.