--- title: "RCC stair for 6.0 m by 4.2 m lobby" subject: "Building Technology" chapter: 5 key: "semester_6/building_technology:rcc-stair-public-lobby-6m" source: https://www.bctengineer.com/q/semester_6/building_technology/rcc-stair-public-lobby-6m site: BCT Engineering --- # RCC stair for 6.0 m by 4.2 m lobby Building Technology · Chapter 5: Stair, lifts and escalators - Asked in: 2078 Baisakh, 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/rcc-stair-public-lobby-6m - Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-5 ## Question Design a proper layout of RCC stair for public building with following data: Lobby size: 6.0m × 4.2m; Floor height: 4.5m; RCC slab thickness: 150mm; Adopt Riser = 150mm; Number of steps per flight <= 12. Justify your design with neat sketches. ## Short answer 30 risers of 150 mm and treads of 300 mm; 3 flights x 10 risers; flight width 1.4 m. ## Answer Data: lobby 6.0 m x 4.2 m, floor height 4.5 m, slab thickness 150 mm, riser 150 mm, not more than 12 steps per flight. #### Given data and assumptions - Stairwell (internal): 6 m x 4.2 m; floor-to-floor height $H = 4.5$ m; stair for a public building, RCC. - Assumed riser $R = 150$ mm and tread $T = 300$ mm; flight width $b = 1.4$ m. Since 30 risers cannot be put in flights of at most 12, a three-flight (zig-zag) stair with two landings is adopted. #### Design 1. **Number of risers** $= H/R = 4.5/0.15 = 30$, adopt **30 risers**; exact riser $= 4.5/30 = 150$ mm. 2. **Flights:** at most 12 risers per flight are allowed, so the 30 risers are divided into three equal flights of 10 risers (9 treads each), arranged side by side in a zig-zag with two half-space landings (one at each end). 3. **Going of one flight** $= 9 \times 0.3 = 2.7$ m. 4. **Width:** three bays of 1.4 m give $3b = 4.2$ m $\le 4.2$ m. 5. **Landings:** two landings, each 1.4 m x 2.8 m (covering two bays), at heights 1.5 m and 3 m. 6. **Length check:** landing L1 + going + landing L2 $= 1.4 + 2.7 + 1.4 = 5.5$ m $\le 6$ m, so it fits; the spare 0.5 m is kept at the entry. 7. **Check:** $2R + T = 2 \times 150 + 300 = 600$ mm (within 550-650 mm); pitch $= \tan^{-1}(R/T) = 26.6^\circ$ (within 25-40 degrees). Headroom above landing L1 $= 4.5 - 1.5 - 0.15$ (slab) $= 2.85$ m $> 2.1$ m, OK. 8. **Waist slab:** effective span about 4.1 m (going + one landing), so depth $\approx$ span/20 $= 205$ mm; adopt a 225 mm thick waist slab, with the given 150 mm slab for the floors (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 far end +-------------------+---------+ | LANDING L1 | arrival | | (+1.50 m) | (upper | | | floor) | +---------+---------+---------+ | F1 | F2 | F3 | | ^ | v | ^ | | ^ | v | ^ | +---------+---------+---------+ | start | LANDING L2 | | | (+3.00 m) | +---------+-------------------+ near end (arrows point up the stair) ``` ```text SECTION (not to scale) +---- upper floor +4.50 _____| flight 3 _____| landing L2 +3.00 _____| flight 2 _____| landing L1 +1.50 ____| flight 1 ___| ground floor +0.00 ``` #### Summary | Item | Adopted | |---|---| | Riser | 150 mm (30 nos.) | | Tread | 300 mm | | Flights | 3 flights x 10 risers | | Width of flight | 1.4 m | | Waist slab | 225 mm | **Answer: 30 risers of 150 mm and treads of 300 mm; 3 flights x 10 risers; flight width 1.4 m.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is a three-flight stair needed? A. Floor is 4.5 m with 30 risers; max 12 per flight B. Lobby is too small for two C. Riser is 300 mm D. Tread is small Answer: A. 30 risers cannot go in flights of at most 12, so 3 flights of 10 are used. (Key point: Why three flights) 2. H = 4.5 m, R = 150 mm gives A. 20 B. 25 C. 30 D. 36 Answer: C. 4.5/0.15 = 30 risers. (Key point: Risers) 3. Three bays of 1.4 m side by side need A. 2.8 m B. 4.2 m C. 1.4 m D. 5.6 m Answer: B. 3 x 1.4 = 4.2 m, equal to the lobby width. (Key point: Width) 4. 2R + T = 600 mm. This is A. Outside 550-650 mm B. Within the 550-650 mm range C. Equal to the pitch D. The slab depth Answer: B. 2 x 150 + 300 = 600 mm, acceptable. (Key point: Comfort rule) ## Related in Building Technology - [Stair for hospital (6.0 m × 4.5 m lobby)](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Astair-hospital) — 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× - [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 - [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 - [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 - [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 ## Related in other subjects - [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 - [RCC floor joist design, 15 m x 6 m hall](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Afloor-joist-15x6-rcc) — Design of Steel and Timber Structure, chapter 7 - [Building: PCC, brick, sal wood frame, RCC slab](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Abuilding-r-four-items) — Estimating and Costing, chapter 6 - [Column slab base: sketch and design steps](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Aslab-base-procedure-sketch) — Design of Steel and Timber Structure, chapter 6, asked 3×