--- title: "Dog-legged stair, 4.5 m by 3.0 m lobby" subject: "Building Technology" chapter: 5 key: "semester_6/building_technology:dog-legged-stair-lobby-4p5m" source: https://www.bctengineer.com/q/semester_6/building_technology/dog-legged-stair-lobby-4p5m site: BCT Engineering --- # Dog-legged stair, 4.5 m by 3.0 m lobby Building Technology · Chapter 5: Stair, lifts and escalators - Asked in: 2062 Baisakh (old course), 16 marks - Video: [Double Landing staircase Numerical Problem | Double landing numerical solution in Doglegged Stair](https://www.youtube.com/watch?v=Z70-0K7z4cA) — Civil Guys - 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-lobby-4p5m - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-5 ## Question Define staircase. Mention ideal requirements of the stair. Describe the different types of stair by shape with neat sketches. Design a dog-legged staircase for a residential building in a lobby of 4.5m×3.0m and floor height 3.0m assume necessary data. Give neat and clean sketch to support your design. ## Short answer Designed dog-legged stair: floor height 3m, 20 risers at 150mm, tread 250mm, flight width 1.45m, going 2.25m, half-space landing 1.45m×3m. ## Answer A **staircase** is a set of steps (a series of risers and treads) that connects floors at different levels, giving access from one storey to another, usually with landings and a balustrade. #### Ideal requirements of a stair 1. **Location:** easily accessible, centrally placed, well lit and ventilated, and near the main entrance; not reached through rooms. 2. **Width:** residential at least 0.9-1.0 m, public buildings 1.5 m or more (as per NBC and local building bye-laws), so that two persons can pass. 3. **Easy and safe ascent:** riser 150-175 mm (public buildings not more than 150 mm), tread 250-300 mm; $2R + T$ about 580-600 mm; all steps in one flight equal. 4. **Pitch:** between 25 and 40 degrees. 5. **Number of steps per flight:** not more than 12 and not less than 3. 6. **Landings:** provided at turns and between flights, with width not less than the stair width. 7. **Headroom:** at least 2.0-2.2 m clear above every step. 8. **Strength and fire resistance:** strong enough for the live load; fire-resistant material, with a separate fire-escape stair in tall buildings. 9. **Handrails and balustrades:** about 0.9 m high, strong and safe for children. 10. **Surface:** non-slip treads, good lighting and ventilation, durable and pleasing in appearance. #### Types of stair by shape | Type | Description | |---|---| | Straight (single flight) | One continuous flight between two floors, no turn | | Quarter-turn | Direction changes by 90 degrees at a quarter landing | | Dog-legged | Two flights in opposite directions with a half-space landing, no well between flights | | Open-newel / open-well | Two or more flights with a clear well between them | | Half-turn (two-flight) | Turn of 180 degrees, either dog-legged or open-well | | Three-quarter turn | Three flights turning through 270 degrees around a well | | Bifurcated (branching) | One wide flight splits into two narrower flights at a landing | | Geometrical | Curved flights (circular or elliptical in plan) around an open well, without newel posts | | Circular / spiral (helical) | Wedge-shaped steps (winders) radiating from a central post; used where space is limited | ```text Straight Quarter turn Dog-legged Open well Spiral | | |__ |>>|<<| |>>| | ( o ) | | | |_ |__|__| |__|<<| | | |__| ``` #### Design of dog-legged stair Lobby 4.5 m x 3.0 m, floor height 3.0 m, residential. A dog-legged stair has (almost) no well, so the flight width is taken as 1.45 m, leaving a gap of only 3.0 - 2 x 1.45 = 0.1 m between the flights. #### Given data and assumptions - Stairwell (internal): 4.5 m x 3 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 $b = 1.45$ 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 1.45 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 $= 10 \times 0.15 = 1.5$ m. 6. **Length check:** going + landing $= 2.25 + 1.45 = 3.7$ m $\le 4.5$ m, so it fits; the spare 0.8 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 3.7 m, so depth $\approx$ span/20 $= 185$ mm; adopt a 200 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 W) | | FLIGHT 1 >>>>>> UP | | | 9 treads | | +------------------------+---------+ <-------- going -------> <-- b --> (entry at left end of flight 1; arrows point up the stair) ``` ```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 | 1.45 m | | Waist slab | 200 mm | **Answer: 20 risers of 150 mm and treads of 250 mm; 2 flights x 10 risers; flight width 1.45 m.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a dog-legged stair differ from an open-well stair? A. It has a large open well between flights B. It turns by 90 degrees using a quarter landing C. It consists of a single straight flight D. Its two flights run in opposite directions with a half-space landing and no well between them Answer: D. A dog-legged stair has two opposite flights joined by a half-space landing, with no well. (Key point: Dog-legged stair definition) 2. For H = 3 m, R = 150 mm, how many risers and treads in each flight? A. 20 risers, 19 treads B. 10 risers, 10 treads C. 10 risers, 9 treads D. 9 risers, 10 treads Answer: C. 20 risers split into two flights of 10; the upper floor acts as the last tread so 9 treads per flight. (Key point: Risers and treads per flight) 3. In a 3.0 m wide lobby with b = 1.2 m and T = 250 mm, what are the well gap and going of one flight? A. 0.6 m gap and 2.25 m going B. 0.6 m gap and 2.5 m going C. 1.8 m gap and 2.25 m going D. 0.3 m gap and 2.25 m going Answer: A. Gap = 3 - 2.4 = 0.6 m; going = 9 x 0.25 = 2.25 m. (Key point: Well gap and going) 4. What does the design check and adopt for waist slab? A. 2R + T = 400 mm; slab 100 mm B. 2R + T = 550 mm; going + landing 3.45 m fits in 4.5 m; waist slab 175 mm from span/20 C. Pitch 15 degrees; slab 300 mm D. 2R + T = 550 mm; going + landing 6.9 m; slab 175 mm Answer: B. 2(150) + 250 = 550 mm, 2.25 + 1.2 = 3.45 m <= 4.5 m, and 3450/20 about 173 mm so 175 mm is adopted. (Key point: Checks and waist slab) ## 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 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 - [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 - [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 - [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 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×