Staircase for public building (5.5 m × 6.5 m)

Floor-to-floor height $= 3.4 + 0.15 = 3.55$ m (clear height plus slab). Stair room 6.5 m (length) x 5.5 m (width). The width is large, so a U-shaped open-well stair with two flights of 1.8 m width is adopted.

Given data and assumptions

• Stairwell (internal): 6.5 m x 5.5 m; floor-to-floor height $H = 3.55$ m; stair for a public building, RCC.
• Assumed riser $R = 150$ mm and tread $T = 300$ mm; flight width $b = 1.8$ m.

Design

1. Number of risers $= H/R = 3.55/0.15 = 23.67$, adopt 24 risers; exact riser $= 3.55/24 = 147.9$ mm.
2. Flights: open-well (U-shaped) stair with two equal parallel flights joined by a landing at one end; each flight has 12 risers and 11 treads.
3. Going of one flight $= 11 \times 0.3 = 3.3$ m.
4. Open well: $W - 2b = 5.5 - 3.6 = 1.9$ m wide open well is left between the flights and protected by a balustrade.
5. Landing: 1.8 m x 5.5 m at height $= 12 \times 0.148 = 1.77$ m.
6. Length check: $3.3 + 1.8 = 5.1$ m $\le 6.5$ m, so it fits.
7. Check: $2R + T = 2 \times 147.9 + 300 = 596$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 26.2^\circ$ (within 25-40 degrees).
8. Waist slab: effective span about 5.1 m, so depth $\approx$ span/20 $= 255$ mm; adopt 275 mm thick waist slab (reinforcement to be designed separately). Provide handrail 0.9 m high, 20-25 mm nosing and non-slip finish.

Plan and section

 +------------------------+---------+
 | FLIGHT 2  <<<<<<       |         |
 |                        | LANDING |
 +- - - open well - - - - +         |
 | FLIGHT 1  >>>>>>  UP   |         |
 +------------------------+---------+

 SECTION (not to scale)
                          +---- upper floor 3.55 m
                    _____|
               ____|   flight 2
        ______|  landing
   ____|  flight 1
 _|
 ground floor +0.00

Summary

Answer: 24 risers of 147.9 mm and treads of 300 mm; 2 flights x 12 risers; flight width 1.8 m.