Stair for hospital (6.0 m × 4.5 m lobby)

Hospital stairs need a wide flight for stretchers and a low riser (not more than 150 mm). A height of 4.2 m with not more than 12 steps per flight gives three flights.

Given data and assumptions

• Stairwell (internal): 6 m x 4.5 m; floor-to-floor height $H = 4.2$ m; stair for a hospital building, RCC.
• Assumed riser $R = 140$ mm and tread $T = 300$ mm; flight width $b = 1.5$ m. The riser is reduced to 140 mm for patients and elderly persons.

Design

1. Number of risers $= H/R = 4.2/0.14 = 30$, adopt 30 risers; exact riser $= 4.2/30 = 140$ mm.
2. Flights: at most 12 risers per flight are allowed, so the 30 risers are divided into three equal flights of 10 risers (9 treads each), arranged side by side in a zig-zag with two half-space landings (one at each end).
3. Going of one flight $= 9 \times 0.3 = 2.7$ m.
4. Width: three bays of 1.5 m give $3b = 4.5$ m $\le 4.5$ m.
5. Landings: two landings, each 1.5 m x 3 m (covering two bays), at heights 1.4 m and 2.8 m.
6. Length check: landing L1 + going + landing L2 $= 1.5 + 2.7 + 1.5 = 5.7$ m $\le 6$ m, so it fits; the spare 0.3 m is kept at the entry.
7. Check: $2R + T = 2 \times 140 + 300 = 580$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 25^\circ$ (within 25-40 degrees).
8. Waist slab: effective span about 4.2 m (going + one landing), so depth $\approx$ span/20 $= 210$ 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

 far end
 +-------------------+---------+
 |    LANDING L1     | arrival |
 |    (+1.40 m)      | (upper  |
 |                   |  floor) |
 +---------+---------+---------+
 |   F1    |   F2    |   F3    |
 |   ^     |   v     |   ^     |
 |   ^     |   v     |   ^     |
 +---------+---------+---------+
 |  start  |    LANDING L2     |
 |         |    (+2.80 m)      |
 +---------+-------------------+
 near end   (arrows point up the stair)

 SECTION (not to scale)
                                  +---- upper floor +4.20
                            _____|  flight 3
                      _____|  landing L2 +2.80
                _____|  flight 2
          _____|  landing L1 +1.40
     ____|  flight 1
 ___|
 ground floor +0.00

Summary

Answer: 30 risers of 140 mm and treads of 300 mm; 3 flights x 10 risers; flight width 1.5 m.