---
title: "Staircase for public building (5.5 m × 6.5 m)"
subject: "Building Technology"
chapter: 5
key: "semester_6/building_technology:stair-public-5p5x6p5"
source: https://www.bctengineer.com/q/semester_6/building_technology/stair-public-5p5x6p5
site: BCT Engineering
---

# Staircase for public building (5.5 m × 6.5 m)

Building Technology · Chapter 5: Stair, lifts and escalators

- Asked in: 2077 Chaitra, 6 marks
- 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-5p5x6p5
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-5

## Question

Plan a suitable staircase for public building in which stair room measures 5.5m × 6.5m. The vertical clear distance between floors is 3.4m and slab thickness is 150mm.

## Short answer

U-shaped open-well stair with 24 risers of 147.9 mm, treads 300 mm, two flights of 12 risers each, flight width 1.8 m.

## Answer

Floor-to-floor height $= 3.4 + 0.15 = 3.55$ m (clear height plus slab). Stair room 6.5 m (length) x 5.5 m (width). The width is large, so a U-shaped open-well stair with two flights of 1.8 m width is adopted.

#### Given data and assumptions
- Stairwell (internal): 6.5 m x 5.5 m; floor-to-floor height $H = 3.55$ 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** $= H/R = 3.55/0.15 = 23.67$, adopt **24 risers**; exact riser $= 3.55/24 = 147.9$ mm.
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 $= 12 \times 0.148 = 1.77$ m.
6. **Length check:** $ 3.3 + 1.8 = 5.1$ m $\le 6.5$ m, so it fits.
7. **Check:** $2R + T = 2 \times 147.9 + 300 = 596$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 26.2^\circ$ (within 25-40 degrees).
8. **Waist slab:** effective span about 5.1 m, so depth $\approx$ span/20 $= 255$ mm; adopt 275 mm thick waist slab (reinforcement to be designed separately). Provide handrail 0.9 m high, 20-25 mm nosing and non-slip finish.

#### Plan and section
```text
 +------------------------+---------+
 | FLIGHT 2  <<<<<<       |         |
 |                        | LANDING |
 +- - - open well - - - - +         |
 | FLIGHT 1  >>>>>>  UP   |         |
 +------------------------+---------+
```
```text
 SECTION (not to scale)
                          +---- upper floor 3.55 m
                    _____|
               ____|   flight 2
        ______|  landing
   ____|  flight 1
 _|
 ground floor +0.00
```

#### Summary
| Item | Adopted |
|---|---|
| Riser | 147.9 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 147.9 mm and treads of 300 mm; 2 flights x 12 risers; flight width 1.8 m.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Clear height plus slab thickness gives the floor-to-floor height. Which is correct here?
   A. 3.55 m
   B. 3.4 m
   C. 3.25 m
   D. 4.0 m
   Answer: A. Floor-to-floor = clear distance + 0.15 m slab = 3.55 m. (Key point: Floor-to-floor height)

2. Why is a U-shaped open-well stair with 1.8 m flights chosen?
   A. The width of 5.5 m is large
   B. The hall is tiny
   C. No landing is wanted
   D. To avoid risers
   Answer: A. A wide room suits two wide parallel flights with an open well between. (Key point: Stair type)

3. With W = 5.5 m and two 1.8 m flights, the open well is
   A. 1.9 m
   B. 3.6 m
   C. 0.1 m
   D. 7.3 m
   Answer: A. 5.5 - 3.6 = 1.9 m, protected by a balustrade. (Key point: Open well)

4. 24 risers in two equal flights gives
   A. 10 risers
   B. 12 risers and 11 treads
   C. 24 risers
   D. 6 risers
   Answer: B. Each flight has 12 risers and 11 treads. (Key point: Risers per flight)

## Related in Building Technology

- [Stair for public hall, 5.5 m by 7.5 m](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-public-hall-5p5m) — Building Technology, chapter 5
- [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 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
- [RCC stair, 2.1 m by 4.5 m room](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Arcc-stair-room-2p1m) — Building Technology, chapter 5
- [Open-well stair, 5.5 m by 3.5 m](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aopen-well-stair-public-5p5m) — Building Technology, chapter 5

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- [Job layout for 1000 sqm public building](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Ajob-layout-public-building-2079) — Construction Management, chapter 6
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- [Two-room building: excavation, PCC, brick, plaster](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Abuilding-h-two-room-fig1) — Estimating and Costing, chapter 6