Open-well stair, 5.5 m by 3.5 m

Functional requirements of a good staircase

1. Location: easily accessible, centrally placed, well lit and ventilated, and near the main entrance; not reached through rooms.
2. Width: residential at least 0.9-1.0 m, public buildings 1.5 m or more (as per NBC and local building bye-laws), so that two persons can pass.
3. Easy and safe ascent: riser 150-175 mm (public buildings not more than 150 mm), tread 250-300 mm; $2R + T$ about 580-600 mm; all steps in one flight equal.
4. Pitch: between 25 and 40 degrees.
5. Number of steps per flight: not more than 12 and not less than 3.
6. Landings: provided at turns and between flights, with width not less than the stair width.
7. Headroom: at least 2.0-2.2 m clear above every step.
8. Strength and fire resistance: strong enough for the live load; fire-resistant material, with a separate fire-escape stair in tall buildings.
9. Handrails and balustrades: about 0.9 m high, strong and safe for children.
10. Surface: non-slip treads, good lighting and ventilation, durable and pleasing in appearance.

Types of staircase by shape

 Straight   Quarter turn   Dog-legged   Open well   Spiral
 | |        |__             |>>|<<|      |>>|  |     ( o )
 | |        |  |_           |__|__|      |__|<<|
 | |        |__|

Design of open-well staircase

Room 5.5 m x 3.5 m, floor height 3.6 m, public building.

Given data and assumptions

• Stairwell (internal): 5.5 m x 3.5 m; floor-to-floor height $H = 3.6$ m; stair for a public building, RCC.
• Assumed riser $R = 150$ mm and tread $T = 300$ mm; flight width $b = 1.5$ m.

Design

1. Number of risers $= H/R = 3.6/0.15 = 24$, adopt 24 risers; exact riser $= 3.6/24 = 150$ 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: $W - 2b = 3.5 - 3 = 0.5$ m wide open well is left between the flights and protected by a balustrade.
5. Landing: 1.5 m x 3.5 m at height $= 12 \times 0.15 = 1.8$ m.
6. Length check: $3.3 + 1.5 = 4.8$ m $\le 5.5$ m, so it fits; the spare 0.7 m is kept as entry/exit space.
7. Check: $2R + T = 2 \times 150 + 300 = 600$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 26.6^\circ$ (within 25-40 degrees).
8. Waist slab: effective span about 4.8 m, so depth $\approx$ span/20 $= 240$ mm; adopt a 250 mm thick waist slab (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   |         |
 |                        | LANDING |
 +- - - open well - - - - +         |
 | FLIGHT 1  >>>>>>  UP   |         |
 +------------------------+---------+
 (arrows point up the stair)

 SECTION (not to scale)
                          +---- upper floor 3.6 m
                    _____|
               ____|   flight 2
        ______|  landing +1.8 m
   ____|  flight 1
 _|
 ground floor +0.00

Summary

Answer: 24 risers of 150 mm and treads of 300 mm; 2 flights x 12 risers; flight width 1.5 m.