--- title: "Factors affecting ductility, IS 13920 beam" subject: "Design of RCC Structure" chapter: 12 key: "semester_7/design_of_rcc_structure:factors-affecting-ductility-is-13920-beam" source: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/factors-affecting-ductility-is-13920-beam site: BCT Engineering --- # Factors affecting ductility, IS 13920 beam Design of RCC Structure · Chapter 12: Introduction to Earthquake Resistant Design and Provisions for Ductile Detailing - Asked in: 2078 Bhadra, 6 marks - Video: [Beam Lapping Zone as per IS 13920 - Beam Reinforcement Lap Length Formula & Lapzone](https://www.youtube.com/watch?v=kyFx8eCQtRw) — Factual Civil Engineering - 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/factors-affecting-ductility-is-13920-beam - Whole chapter: https://www.bctengineer.com/md/semester_7/design_of_rcc_structure/chapter-12 ## Question What are the factors affecting the ductility? Explain the ductility requirements of R.C.C. beam as per IS 13920. ## Short answer Tension steel ratio, compression steel, material grades, confinement, axial load, shear/bond, and detailing affect ductility. Beam ductility per IS 13920: b≥200mm, b/D≥0.3, D≤L/4; ρ between 0.24√fck/fy and 2.5%; two continuous top and bottom bars; bottom steel ≥50% top at joint; mid‑half splices with hoops ≤150mm; closed hoops 135° hooks, spacing ≤min(d/4,8φ,100) mm within 2d; shear design for hinge moments. ## Answer #### Factors affecting ductility 1. **Tension steel ratio:** a higher $\rho$ gives lower ductility; under-reinforced sections are ductile. 2. **Compression steel:** reduces the neutral axis depth and increases ductility. 3. **Grade of steel:** a higher yield strength (and smaller elongation) reduces ductility; Fe 415/500 with good elongation is preferred. 4. **Grade of concrete:** for the same steel ratio, higher-strength concrete reduces the neutral axis depth and so increases section ductility, although very high-strength concrete is itself more brittle (it has a smaller strain capacity after the peak). 5. **Confinement:** closely spaced stirrups/hoops increase the strain capacity of the concrete. 6. **Axial load:** high axial compression reduces the ductility of columns. 7. **Shear and bond:** a shear or bond failure prevents flexural ductility from developing. 8. **Detailing of anchorages and laps:** poor detailing causes bar slip. #### Ductility requirements of beams (IS 13920) - $b \ge 200$ mm, $b/D \ge 0.3$, $D \le L/4$. - $\rho_{min} = 0.24\sqrt{f_{ck}}/f_y$, $\rho_{max} = 2.5\%$. - At least two 12 mm bars continuous at the top and bottom. - Bottom steel at a joint face $\ge 50\%$ of the top steel; capacity anywhere along the span $\ge 25\%$ of the maximum end capacity. - Splices in the middle half only, with hoops at $\le 150$ mm over the lap; none in the joint or within $2d$ of it. - Closed stirrups with $135^\circ$ hooks and a $10\phi$ tail; diameter $\ge 6$ mm (8 mm for spans > 5 m). - Spacing: $\min(d/4,\ 8\phi,\ 100)$ mm within $2d$ of supports, with the first stirrup at 50 mm; $\le d/2$ elsewhere. - Shear design based on the hinge (flexural) capacity. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a higher tension steel ratio affect ductility of a beam? A. Increases it B. Lowers it; under-reinforced sections are ductile C. No effect D. Makes it infinite Answer: B. Over-reinforced sections fail by brittle concrete crushing. (Key point: Tension steel ratio) 2. Which pair increases ductility? A. Compression steel and closely spaced stirrups B. High axial load and higher concrete grade C. Poor anchorage and high rho D. Compression steel and widely spaced stirrups Answer: A. Compression steel lowers neutral axis depth; confinement raises strain capacity. (Key point: Compression steel and confinement) 3. IS 13920 beam geometry limits are: A. b >= 200 mm, b/D >= 0.3, D <= L/4 B. b >= 100 mm, b/D >= 0.1, D <= L/2 C. b >= 300 mm, b/D >= 0.5, D <= L/10 D. b >= 200 mm, b/D <= 0.3, D >= L/4 Answer: A. These are the stated limits. (Key point: Beam dimension limits) 4. Which is correct for beam longitudinal steel? A. Max 4%, no continuous bars B. rho_max = 2.5%, at least two 12 mm bars continuous top and bottom, bottom steel at joint face >= 50% of top C. rho_min = 2.5% D. Bottom steel at joint face >= 150% of top Answer: B. These are the IS 13920 limits given. (Key point: Beam steel limits) 5. Beam stirrups near supports: first stirrup position and hook? A. At the face, 90 degree hook B. 50 mm from the joint face, closed with 135 degree hooks and 10 phi tail C. 200 mm from the face, open D. At d from the face, 180 degree hook Answer: B. Closed stirrups with 135 degree hooks, first at 50 mm. (Key point: Stirrup details) ## Related in Design of RCC Structure - [Ductile design per IS 13920, beam and column](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aductile-design-per-is-13920-beam) — Design of RCC Structure, chapter 12 - [Ductility requirements of RC beam with sketch](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aductility-requirements-of-rc-beam-with) — Design of RCC Structure, chapter 12, asked 4× - [Ductility requirement for beam, column, joints](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aductility-requirement-for-beam-column-joints) — Design of RCC Structure, chapter 12 - [Define ductility; ductility in joints](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adefine-ductility-ductility-in-joints) — Design of RCC Structure, chapter 12 - [Define ductility, joint detailing sketch](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adefine-ductility-joint-detailing-sketch) — Design of RCC Structure, chapter 12, asked 2× ## Related in other subjects - [Factors affecting water demand; variation](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Afactors-affecting-demand) — Water Supply Engineering, chapter 3, asked 5× - [Factors affecting bearing capacity; water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abearing-capacity-factors-water-table) — Foundation Engineering, chapter 6, asked 5× - [Incentives; factors affecting wage/salary](https://www.bctengineer.com/question.md?key=semester_7%2Forganization_and_management%3Aincentives-wage-factors) — Organization and Management, chapter 7, asked 3× - [Forms of chlorination and affecting factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aforms-of-chlorination-factors) — Water Supply Engineering, chapter 6, asked 3×