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Stair for public hall, 5.5 m by 7.5 m

Short answer

U-shaped open-well stair with 24 risers of 152.1 mm, treads 300 mm, two flights of 12 risers each, flight width 1.8 m.

Design a suitable staircase for public building in a hall of size 5.50 m * 7.50 m. The vertical clear distance between floor is 3.5 m and the RCC slab thickness is 150 mm. (assume any necessary criteria, if required).

Floor-to-floor height =3.5+0.15=3.65= 3.5 + 0.15 = 3.65 m (clear distance plus slab). Hall 7.5 m (length) x 5.5 m (width). A wide U-shaped open-well stair with 1.8 m wide flights is adopted.

Given data and assumptions

  • Stairwell (internal): 7.5 m x 5.5 m; floor-to-floor height H=3.65H = 3.65 m; stair for a public building, RCC.
  • Assumed riser R=150R = 150 mm and tread T=300T = 300 mm; flight width b=1.8b = 1.8 m.

Design

  1. Number of risers =H/R=3.65/0.15=24.33= H/R = 3.65/0.15 = 24.33, adopt 24 risers; exact riser =3.65/24=152.1= 3.65/24 = 152.1 mm. (This is only 2 mm above the 150 mm usually preferred for public buildings and is acceptable; 26 risers would need flights of 13 steps, more than the usual maximum of 12.)
  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 =11×0.3=3.3= 11 \times 0.3 = 3.3 m.
  4. Open well: W−2b=5.5−3.6=1.9W - 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 =12×0.1521=1.83= 12 \times 0.1521 = 1.83 m.
  6. Length check: 3.3+1.8=5.13.3 + 1.8 = 5.1 m ≤7.5\le 7.5 m, so it fits.
  7. Check: 2R+T=2×152.1+300=6042R + T = 2 \times 152.1 + 300 = 604 mm (within 550-650 mm); pitch =tan⁡−1(R/T)=26.9∘= \tan^{-1}(R/T) = 26.9^\circ (within 25-40 degrees).
  8. Waist slab: effective span about 5.1 m, so depth ≈\approx span/20 =255= 255 mm; adopt a 275 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  <<<<<<       |         |
 |                        | LANDING |
 +- - - open well - - - - +         |
 | FLIGHT 1  >>>>>>  UP   |         |
 +------------------------+---------+

 SECTION (not to scale)
                          +---- upper floor 3.65 m
                    _____|
               ____|   flight 2
        ______|  landing
   ____|  flight 1
 _|
 ground floor +0.00

Summary

ItemAdopted
Riser152.1 mm (24 nos.)
Tread300 mm
Flights2 flights x 12 risers
Width of flight1.8 m
Waist slab275 mm

Answer: 24 risers of 152.1 mm and treads of 300 mm; 2 flights x 12 risers; flight width 1.8 m.

✓ Answer checked by Claude Opus 5.5 (high effort)

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