2068 Baisakh (old course) · 2+6 marks

Vertical circulation elements; RCC dog-legged stair

Short answer

Elements: stair, ramp, lift, escalator, ladder. For 3 m height in 4.5 m × 2.3 m room: 20 risers of 150 mm, treads 250 mm. Two flights of 10 risers each, 9 treads; flight width 1.1 m, going 2.25 m, waist slab 175 mm.

What are different elements for vertical circulation in a building? Design a RCC doglegged staircase for a residential building in a room of 4.5m × 2.3m and floor height of 3m. Assume necessary data. Give neat sketches of the plan and section.

Elements for vertical circulation

Stair: the most common, with flights and landings.
Ramp: a sloping surface (about 1:12) for wheelchairs and trolleys.
Lift (elevator): a mechanical car in a shaft that carries persons or goods.
Escalator: a moving stair for heavy traffic.
Ladder: steep rung access to lofts and roofs.
Paternoster, chute and fire escape: other special devices.

Design of RCC dog-legged stair

Room 4.5 m x 2.3 m, floor height 3.0 m.

Given data and assumptions

Stairwell (internal): 4.5 m x 2.3 m; floor-to-floor height $H = 3$ m; stair for a residential building, RCC.
Assumed riser $R = 150$ mm and tread $T = 250$ mm; flight width $b = 1.1$ m.

Design

Number of risers $= H/R = 3/0.15 = 20$ , adopt 20 risers; exact riser $= 3/20 = 150$ mm.
Flights: a dog-legged stair has two equal flights, so each flight has 20/2 = 10 risers and 10-1 = 9 treads (the landing or upper floor acts as the last tread).
Going of one flight $= 9 \times 0.25 = 2.25$ m.
Width: two flights of 1.1 m side by side need $2b = 2.2$ m; the remaining $W - 2b = 2.3 - 2.2 = 0.1$ m is the gap between the flights.
Landing: a half-space landing 1.1 m x 2.3 m (full stairwell width) at height $= 10 \times 0.15 = 1.5$ m.
Length check: going + landing $= 2.25 + 1.1 = 3.35$ m $\le 4.5$ m, so it fits; the spare 1.15 m is kept as entry/exit space.
Check: $2R + T = 2 \times 150 + 250 = 550$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 31^\circ$ (within 25-40 degrees).
Waist slab: effective span about 3.35 m, so depth $\approx$ span/20 $= 168$ mm; adopt a 175 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   |         |
 | 9 treads               | LANDING |
 +------------------------+ (b x W) |
 | FLIGHT 1  >>>>>>  UP   |         |
 | 9 treads               |         |
 +------------------------+---------+
 <-------- going -------> <-- b -->
 (entry at left end of flight 1; arrows point up the stair)

 SECTION (not to scale)
                          +---- upper floor 3 m
                    _____|
               ____|   flight 2
        ______|  landing
   ____|  flight 1
 _|
 ground floor +0.00

Summary

Item	Adopted
Riser	150 mm (20 nos.)
Tread	250 mm
Flights	2 flights x 10 risers
Width of flight	1.1 m
Waist slab	175 mm

Answer: 20 risers of 150 mm and treads of 250 mm; 2 flights x 10 risers; flight width 1.1 m.

✓ Answer checked by Claude Opus 5.5 (high effort)

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