---
title: "Stair for public hall, 5.5 m by 7.5 m"
subject: "Building Technology"
chapter: 5
key: "semester_6/building_technology:stair-public-hall-5p5m"
source: https://www.bctengineer.com/q/semester_6/building_technology/stair-public-hall-5p5m
site: BCT Engineering
---

# Stair for public hall, 5.5 m by 7.5 m

Building Technology · Chapter 5: Stair, lifts and escalators

- Asked in: 2074 Bhadra, 8 marks
- Video: [Staircase Part: 1 Design criteria and guidelines](https://www.youtube.com/watch?v=ZE0Jt_E_seQ) — My Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/stair-public-hall-5p5m
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-5

## Question

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).

## 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.

## Answer

Floor-to-floor height $= 3.5 + 0.15 = 3.65$ 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 $H = 3.65$ m; stair for a public building, RCC.
- Assumed riser $R = 150$ mm and tread $T = 300$ mm; flight width $b = 1.8$ m.

#### Design
1. **Number of risers** $= H/R = 3.65/0.15 = 24.33$, adopt **24 risers**; exact riser $= 3.65/24 = 152.1$ 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** $= 11 \times 0.3 = 3.3$ m.
4. **Open well:** $W - 2b = 5.5 - 3.6 = 1.9$ 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 $= 12 \times 0.1521 = 1.83$ m.
6. **Length check:** $3.3 + 1.8 = 5.1$ m $\le 7.5$ m, so it fits.
7. **Check:** $2R + T = 2 \times 152.1 + 300 = 604$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 26.9^\circ$ (within 25-40 degrees).
8. **Waist slab:** effective span about 5.1 m, so depth $\approx$ span/20 $= 255$ 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
```text
 +------------------------+---------+
 | FLIGHT 2  <<<<<<       |         |
 |                        | LANDING |
 +- - - open well - - - - +         |
 | FLIGHT 1  >>>>>>  UP   |         |
 +------------------------+---------+
```
```text
 SECTION (not to scale)
                          +---- upper floor 3.65 m
                    _____|
               ____|   flight 2
        ______|  landing
   ____|  flight 1
 _|
 ground floor +0.00
```

#### Summary
| Item | Adopted |
|---|---|
| Riser | 152.1 mm (24 nos.) |
| Tread | 300 mm |
| Flights | 2 flights x 12 risers |
| Width of flight | 1.8 m |
| Waist slab | 275 mm |

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

## Related in Building Technology

- [Staircase for public building (5.5 m × 6.5 m)](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-public-5p5x6p5) — Building Technology, chapter 5
- [Dog-legged RCC stair design for 3 m floor height](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-dog-3m-assume) — Building Technology, chapter 5, asked 3×
- [Open-well staircase (6.0 m × 3.5 m hall)](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-064j) — Building Technology, chapter 5
- [Vertical circulation elements; RCC dog-legged stair](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-circulation-068ba) — Building Technology, chapter 5
- [RCC dog-legged stair for office building](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arcc-stair-office-building) — Building Technology, chapter 5
- [RCC stair for 6.0 m by 4.2 m lobby](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arcc-stair-public-lobby-6m) — Building Technology, chapter 5

## Related in other subjects

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- [Design T-beam with 150 mm slab](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adesign-t-beam-with-150-mm) — Design of RCC Structure, chapter 4
- [Interior cross wall design: three-storey, 150 mm slab](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Ainterior-cross-wall-three-storey-150mm) — Concrete Technology and Masonry Structure, chapter 8, asked 2×
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