Open-well staircase (6.0 m × 3.5 m hall)

A staircase is a set of steps (a series of risers and treads) that connects floors at different levels, giving access from one storey to another, usually with landings and a balustrade.

Ideal requirements of a stair

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 stair by shape in plan

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

Design of open-well staircase

Floor-to-floor height $= 3.0 + 0.12 = 3.12$ m (ceiling height plus slab thickness). Hall 6.0 m x 3.5 m. For a public building the riser must not exceed 150 mm, so $3.12/0.15 = 20.8$ risers are needed; an even number, 22, is taken so that both flights are equal.

Given data and assumptions

• Stairwell (internal): 6 m x 3.5 m; floor-to-floor height $H = 3.12$ m; stair for a public building, RCC.
• Assumed riser $R \le 150$ mm and tread $T = 300$ mm; flight width $b = 1.5$ m.

Design

1. Number of risers $= H/R = 3.12/0.15 = 20.8$, adopt 22 risers; exact riser $= 3.12/22 = 141.8$ mm ($\le 150$ mm, OK).
2. Flights: open-well (U-shaped) stair with two equal parallel flights joined by a landing at one end; each flight has 11 risers and 10 treads.
3. Going of one flight $= 10 \times 0.3 = 3.0$ 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 $= 11 \times 0.1418 = 1.56$ m.
6. Length check: $3.0 + 1.5 = 4.5$ m $\le 6$ m, so it fits; the spare 1.5 m is kept as entry/exit space.
7. Check: $2R + T = 2 \times 141.8 + 300 = 583.6$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 25.3^\circ$ (within 25-40 degrees).
8. Waist slab: effective span about 4.5 m, so depth $\approx$ span/20 $= 225$ mm; adopt a 225 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   |         |
 | 10 treads              | LANDING |
 +- - - open well - - - - +         |
 | FLIGHT 1  >>>>>>  UP   |         |
 | 10 treads              |         |
 +------------------------+---------+
 (arrows point up the stair)

 SECTION (not to scale)
                          +---- upper floor 3.12 m
                    _____|
               ____|   flight 2
        ______|  landing +1.56 m
   ____|  flight 1
 _|
 ground floor +0.00

Summary

Answer: 22 risers of 141.8 mm and treads of 300 mm; 2 flights x 11 risers; flight width 1.5 m; open well 0.5 m.