--- title: "Staircase design concept and detailing" subject: "Design of RCC Structure" chapter: 9 key: "semester_7/design_of_rcc_structure:staircase-design-concept-and-detailing" source: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/staircase-design-concept-and-detailing site: BCT Engineering --- # Staircase design concept and detailing Design of RCC Structure · Chapter 9: Design of slabs and staircase - Asked in: 2066 Bhadra (old course), 5 marks - Video: [Design of Reinforced Concrete Stairs](https://www.youtube.com/watch?v=KB2FXHb5Vck) — The Efficient Civil Engineer (by Dr. S. El-Gamal) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/staircase-design-concept-and-detailing - Whole chapter: https://www.bctengineer.com/md/semester_7/design_of_rcc_structure/chapter-9 ## Question Explain the concept of design of a staircase. Show the detailing of reinforcement of straight flight in plan and section. ## Short answer Design as inclined waist slab spanning horizontally; effective span per IS 456, thickness l/20 to l/25, loads on horizontal projection include waist slab, steps, finishes and live load. Main bars along slope, distribution bars across, top bars at supports, bars at landing junction anchored separately. Straight flight section and plan show main bars at bottom along slope and distribution bars across. ## Answer #### Concept of design of a staircase A staircase is an inclined slab (the **waist slab**) with steps built on it. The common types are dog-legged and straight flights. The design steps are: 1. **Fix the geometry**: tread (going) $T$ about 250-300 mm, riser $R$ about 150-175 mm, width of flight (usually 1.0-1.5 m), number of steps and landing width. 2. **Effective span** $l$ as per IS 456 cl. 33.1 (horizontal distance between supports; where the flight is supported by a landing spanning across it, the going plus half the landing width or 1 m, whichever is smaller, at each end). 3. **Assume the thickness** of the waist slab $= l/20$ to $l/25$ (simply supported) or $l/26$ (continuous), and check it later by the span/depth ratio. 4. **Loads** (per m² of horizontal plan): - self-weight of waist slab $= 25 \times t \times \sqrt{R^2+T^2}/T$, - weight of steps $= 25 \times \dfrac{R}{2}$, - finishes, and live load (3 kN/m² for residential stairs and 4-5 kN/m² for public buildings, as per IS 875 Part 2). The loads act vertically on the horizontal projection, so the span used is the horizontal span. 5. **Bending moment**: $M_u = \dfrac{w_u l^2}{8}$ per metre width for a simply supported flight (use $w_u l^2/10$ where the ends are continuous or partly fixed). 6. **Reinforcement**: find $A_{st}$ from $M_u$ with $d$ measured perpendicular to the slope; main bars run along the slope; distribution steel $= 0.12\%$ of the gross area. 7. **Check** shear ($\tau_v \le k\tau_c$), deflection and development length. #### Detailing of a straight flight ```text SECTION PLAN ___ +---------------+ ___| top landing | o o o o o o o | distribution ___| | | | | | | | | | bars 8 mm _| / main bars along slope | | | | | | | | | | / waist slab (bottom) | | main bars | | main bars | / | | | | | | | | | at bottom, |/___ support (beam/wall) +---------------+ along slope ``` - Main bars (say 12 mm @ 125 c/c) at the bottom along the slope, extended into the supports by $L_d$. - Distribution bars (8 mm @ 200 c/c) across the main bars. - Top bars at the supports (about 50% of the mid-span steel) where the stair is built into walls. - Bars at the junction with a landing are anchored separately (they are not bent round the re-entrant corner). - Cover 20 mm; the steps need no reinforcement. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How are stair loads taken for design of the span? A. As acting vertically on the horizontal projection, using the horizontal span B. Along the slope on the inclined length C. Only as live load D. As half the dead load Answer: A. Loads are per m2 of plan and the span is the horizontal span. (Key point: Load on horizontal plan) 2. What is the weight of steps per m2 of plan? A. 25 x R/2 B. 25 x R x T C. 25 x T/2 D. 25 x sqrt(R^2+T^2) Answer: A. Triangular steps average R/2 thickness over the plan, so weight = 25 x R/2 kN/m2. (Key point: Weight of steps) 3. What bending moment is used for a simply supported stair per metre width? A. wu l^2/2 B. wu l^2/8 C. wu l^2/24 D. wu l/4 Answer: B. Mu = wu l^2/8 for simply supported; wu l^2/10 for continuous with fixity. (Key point: Bending moment) 4. What is the starting assumption for waist slab thickness for a simply supported stair? A. l/20 to l/25 B. l/5 C. l/50 D. l/2 Answer: A. Thickness is taken as l/20 to l/25 (l/26 for continuous) and checked by span/depth. (Key point: Waist slab thickness) 5. Do the steps need reinforcement? A. Yes, 12 mm bars B. No, steps need no reinforcement; main steel is in the waist slab C. Yes, stirrups D. Only at the landing Answer: B. Steps are just a load on the waist slab. 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