---
title: "Dog-legged RCC stair, 4.6 m by 2.6 m"
subject: "Building Technology"
chapter: 5
key: "semester_6/building_technology:dog-legged-stair-residential-4p6m"
source: https://www.bctengineer.com/q/semester_6/building_technology/dog-legged-stair-residential-4p6m
site: BCT Engineering
---

# Dog-legged RCC stair, 4.6 m by 2.6 m

Building Technology · Chapter 5: Stair, lifts and escalators

- Asked in: 2078 Poush, 6 marks; 2073 Bhadra (asked 2 times)
- Video: [Design of RCC Dog-Legged Stair | Staircase | Design of Stair | Building design | Structural Design](https://www.youtube.com/watch?v=ZSurkcbC7GA) — Visual Civil
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/dog-legged-stair-residential-4p6m
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-5

## Question

Design a dog-legged RCC staircase for a residential building with internal dimension of 4.6 m × 2.6 m and 3 m floor to floor height. Draw plan and section to support your answer.

## Short answer

Assume riser 150 mm, tread 250 mm. Number of risers = 3000/150 = 20, so two flights of 10 risers each. Going = 9×250 = 2250 mm, flight width 1.25 m, landing 1.25 m; total length 3.5 m fits within 4.6 m. Result: 20 risers of 150 mm, tread 250 mm, 2 flights × 10 risers, flight width 1.25 m.

## Answer

#### Given data and assumptions
- Stairwell (internal): 4.6 m x 2.6 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 $b = 1.25$ m.

#### Design
1. **Number of risers** $= H/R = 3/0.15 = 20$; adopt **20 risers**; exact riser $= 3/20 = 150$ mm.
2. **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).
3. **Going of one flight** $= 9 \times 0.25 = 2.25$ m.
4. **Width:** two flights of 1.25 m side by side need $2b = 2.5$ m; the remaining $W - 2b = 2.6 - 2.5 = 0.1$ m is the gap between the flights.
5. **Landing:** a half-space landing 1.25 m x 2.6 m (full stairwell width) at a height of $10 \times 0.15 = 1.5$ m.
6. **Length check:** going + landing $= 2.25 + 1.25 = 3.5$ m $\le 4.6$ m, so it fits; the spare length of 1.1 m is kept as entry/exit space.
7. **Check:** $2R + T = 2 \times 150 + 250 = 550$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 31^\circ$ (within 25-40 degrees).
8. **Waist slab:** effective span about 3.5 m, so depth $\approx$ span/20 $= 175$ 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
```text
 +------------------------+---------+
 | FLIGHT 2  <<<<<<  UP   |         |
 | 9 treads               | LANDING |
 +------------------------+  (b x   |
 | FLIGHT 1  >>>>>>  UP   |  2b+g)  |
 | 9 treads               |         |
 +------------------------+---------+
 <------- going --------> <-- b -->
 (entry at left end of flight 1)
```
```text
 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 | 1.25 m |
| Waist slab | 175 mm |

**Answer: 20 risers of 150 mm and treads of 250 mm; 2 flights x 10 risers; flight width 1.25 m.**

## Check yourself

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

1. With floor height 3 m and riser 150 mm, how many risers are needed?
   A. 20
   B. 15
   C. 25
   D. 30
   Answer: A. 3000/150 = 20 risers. (Key point: Number of risers)

2. Each flight has 10 risers. How many treads does it have?
   A. 10
   B. 9
   C. 11
   D. 20
   Answer: B. The upper floor acts as the last tread, so treads = risers - 1 = 9. (Key point: Treads per flight)

3. What is the going of one flight with 9 treads of 250 mm?
   A. 2.5 m
   B. 1.5 m
   C. 2.25 m
   D. 3.5 m
   Answer: C. 9 x 0.25 = 2.25 m. (Key point: Going)

4. Two 1.25 m flights in a 2.6 m wide stairwell leave what gap?
   A. 0.35 m
   B. 1.35 m
   C. 0
   D. 0.1 m
   Answer: D. 2.6 - 2 x 1.25 = 0.1 m. (Key point: Well gap)

## Close variants set in other exams

- [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×

## Related in Building Technology

- [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
- [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
- [Dog-legged stair, 4.5 m by 3.0 m lobby](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adog-legged-stair-lobby-4p5m) — Building Technology, chapter 5
- [Dog-legged stair design with elevators and ramp](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-078c) — Building Technology, chapter 5
- [Dog-legged stair, 5.0 m by 4.0 m](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adog-legged-stair-public-5m) — Building Technology, chapter 5
- [Dog-legged stair, 4.8 m by 2.25 m](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adog-legged-stair-4p8m-room) — Building Technology, chapter 5