---
title: "RCC dog-legged stair for office building"
subject: "Building Technology"
chapter: 5
key: "semester_6/building_technology:rcc-stair-office-building"
source: https://www.bctengineer.com/q/semester_6/building_technology/rcc-stair-office-building
site: BCT Engineering
---

# RCC dog-legged stair for office building

Building Technology · Chapter 5: Stair, lifts and escalators

- Asked in: 2066 Jestha (old course), 10 marks
- Video: [How to draw RCC Doglegged Staircase in AutoCAD using Design Details.|| Reinforcement Details part 1](https://www.youtube.com/watch?v=7MY6inRClmw) — Save Nature - Civil Engg
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/rcc-stair-office-building
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-5

## Question

Draw complete details of an R.C.C. dog-legged staircase for an office building leading from the ground floor to first floor. State reasons for size of the risers and treads adopted by you. The size of stair is 4.8m×3.0m×3.6m (height).

## Short answer

Riser 150 mm for easy climbing, tread 280 mm for safe footing (2R+T=580 mm). For 3.6 m height in 4.8×3.0 m: 24 risers at 150 mm, two flights of 12 risers, 11 treads; flight width 1.45 m, waist slab 250 mm.

## Answer

#### Reasons for the size of riser and tread
- **Riser 150 mm:** an office building has heavy traffic of people of all ages, so the riser is kept at the lower limit (150 mm) for comfortable climbing; the floor height of 3.6 m divides exactly into 24 equal risers.
- **Tread 280 mm:** a wide tread (250-300 mm) gives safe foot placing; $2R + T = 2 \times 150 + 280 = 580$ mm, which is within the usual comfort range of 550-650 mm.
- The riser and tread are kept equal throughout so that people do not trip.

#### Given data and assumptions
- Stairwell (internal): 4.8 m x 3 m; floor-to-floor height $H = 3.6$ m; stair for an office building, RCC.
- Assumed riser $R = 150$ mm and tread $T = 280$ mm; flight width $b = 1.45$ m. Stair size 4.8 m x 3.0 m x 3.6 m (length x width x height). Gap between flights = 3.0 - 2 x 1.45 = 0.1 m.

#### Design
1. **Number of risers** $= H/R = 3.6/0.15 = 24$, adopt **24 risers**; exact riser $= 3.6/24 = 150$ mm.
2. **Flights:** a dog-legged stair has two equal flights, so each flight has 24/2 = 12 risers and 12-1 = 11 treads (the landing or upper floor acts as the last tread).
3. **Going of one flight** $= 11 \times 0.28 = 3.08$ m.
4. **Width:** two flights of 1.45 m side by side need $2b = 2.9$ m; the remaining $W - 2b = 3 - 2.9 = 0.1$ m is the gap between the flights.
5. **Landing:** a half-space landing 1.45 m x 3 m (full stairwell width) at height $= 12 \times 0.15 = 1.8$ m.
6. **Length check:** going + landing $= 3.08 + 1.45 = 4.53$ m $\le 4.8$ m, so it fits; the spare 0.27 m is kept as entry/exit space.
7. **Check:** $2R + T = 2 \times 150 + 280 = 580$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 28.2^\circ$ (within 25-40 degrees).
8. **Waist slab:** effective span about 4.53 m, so depth $\approx$ span/20 $= 226$ mm; adopt a 250 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   |         |
 | 11 treads              | LANDING |
 +------------------------+ (b x W) |
 | FLIGHT 1  >>>>>>  UP   |         |
 | 11 treads              |         |
 +------------------------+---------+
 <-------- going -------> <-- b -->
 (entry at left end of flight 1; arrows point up the stair)
```
```text
 SECTION (not to scale)
                          +---- upper floor 3.6 m
                    _____|
               ____|   flight 2
        ______|  landing
   ____|  flight 1
 _|
 ground floor +0.00
```

#### Summary
| Item | Adopted |
|---|---|
| Riser | 150 mm (24 nos.) |
| Tread | 280 mm |
| Flights | 2 flights x 12 risers |
| Width of flight | 1.45 m |
| Waist slab | 250 mm |

**Answer: 24 risers of 150 mm and treads of 280 mm; 2 flights x 12 risers; flight width 1.45 m.**

#### Details to be drawn
- Plans at ground floor and first floor with UP/DN arrows, flight width and landing.
- Section showing the 150 mm riser, 280 mm tread, waist slab, landing slab, nosing (25 mm) and balustrade (0.9 m high).
- Reinforcement: main bars along the slope (e.g. 12 mm dia at 150 c/c), distribution bars 8 mm at 200 c/c, concrete M20, steel Fe415 (to be designed separately).

## Check yourself

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

1. Why is a riser of 150 mm adopted for the office building stair?
   A. Heavy traffic of all ages needs the lower-limit riser for comfortable climbing
   B. It makes the waist slab thinner
   C. It reduces the number of risers so the stair is shorter
   D. It is the only riser allowed for RCC stairs
   Answer: A. An office has heavy traffic of people of all ages, so the riser is kept at the lower limit (150 mm) for comfort. (Key point: Choice of riser and tread)

2. For H = 3.6 m and R = 150 mm in a dog-legged stair, how many risers are in each flight and how many treads?
   A. 24 risers and 24 treads per flight
   B. 12 risers and 11 treads per flight
   C. 12 risers and 12 treads per flight
   D. 24 risers and 23 treads per flight
   Answer: B. 24 risers total split into two equal flights of 12 risers; the upper floor acts as the last tread, so each flight has 11 treads. (Key point: Number of risers and flights)

3. The stairwell is 3.0 m wide and each flight is 1.45 m wide. What is the gap between the flights?
   A. 1.55 m
   B. 0.27 m
   C. 0.1 m
   D. 0.45 m
   Answer: C. Gap = W - 2b = 3.0 - 2 x 1.45 = 0.1 m. (Key point: Width and well gap)

4. Which pair of checks confirms the adopted stair fits and is comfortable?
   A. Going + landing = 4.53 m is not more than 4.8 m, and 2R + T = 580 mm is within 550-650 mm
   B. Going + landing must be exactly 4.8 m, and 2R + T must be 300 mm
   C. Going alone must exceed 4.8 m, and pitch must be 45 degrees
   D. Landing width must be 3.0 m, and 2R + T must be 150 mm
   Answer: A. Going 3.08 m + landing 1.45 m = 4.53 m <= 4.8 m, and 2(150) + 280 = 580 mm lies in the 550-650 mm range. (Key point: Length check and comfort check)

## Related in Building Technology

- [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×
- [Dog-legged RCC stair, 4.6 m by 2.6 m](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adog-legged-stair-residential-4p6m) — Building Technology, chapter 5, asked 3×
- [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
- [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, 4.5 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
- [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