Stair design, 1.8 m by 3.9 m

Data: stair room 3.9 m x 1.8 m, floor height 2.4 m, waist slab thickness 150 mm. A single straight flight would need 16 risers in one flight (more than the usual maximum of 12) and would fill the whole 3.9 m, leaving no landing for entry and exit. The 1.8 m width suits a dog-legged stair with two 0.9 m flights side by side, so this is adopted.

Given data and assumptions

• Stairwell (internal): 3.9 m x 1.8 m; floor-to-floor height $H = 2.4$ m; stair for a residential building, RCC; waist slab 150 mm (given).
• Assumed riser $R = 150$ mm and tread $T = 280$ mm; flight width $b = 0.9$ m.

Design

1. Number of risers $= H/R = 2.4/0.15 = 16$, adopt 16 risers; exact riser $= 2.4/16 = 150$ mm.
2. Flights: a dog-legged stair has two equal flights, so each flight has 16/2 = 8 risers and 8-1 = 7 treads (the landing or upper floor acts as the last tread).
3. Going of one flight $= 7 \times 0.28 = 1.96$ m.
4. Width: two flights of 0.9 m side by side need $2b = 1.8$ m $= 1.8$ m, so there is no well between the flights (a true dog-legged stair).
5. Landing: a half-space landing 0.9 m x 1.8 m (full stairwell width) at height $= 8 \times 0.15 = 1.2$ m.
6. Length check: going + landing $= 1.96 + 0.9 = 2.86$ m $\le 3.9$ m, so it fits; the spare 1.04 m is kept as entry/exit space.
7. Check: $2R + T = 2 \times 150 + 280 = 580$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 28.2^\circ$ (within 25-40 degrees). Headroom over the first step of flight 1 (below the upper floor slab) $= 2.4 - 0.15 - 0.15 = 2.1$ m, which meets the usual minimum of 2.0-2.1 m.
8. Waist slab: effective span about 2.86 m, so depth $\approx$ span/20 $= 143$ mm $\le 150$ mm, so the given 150 mm waist slab is adequate (reinforcement to be designed separately). Provide a handrail 0.9 m high, 20-25 mm nosing and a non-slip finish.

Plan and section

 +------------------------+---------+
 | FLIGHT 2  <<<<<<  UP   |         |
 | 7 treads               | LANDING |
 +------------------------+ (b x W) |
 | FLIGHT 1  >>>>>>  UP   |         |
 | 7 treads               |         |
 +------------------------+---------+
 <-------- going -------> <-- b -->
 (entry at left end of flight 1; arrows point up the stair)

 SECTION (not to scale)
                          +---- upper floor 2.4 m
                    _____|
               ____|   flight 2
        ______|  landing +1.2 m
   ____|  flight 1
 _|
 ground floor +0.00

Summary

Answer: dog-legged stair with 16 risers of 150 mm and treads of 280 mm; 2 flights x 8 risers; flight width 0.9 m; half-space landing at 1.2 m.