--- title: "Ductility requirements of RC beam with sketch" subject: "Design of RCC Structure" chapter: 12 key: "semester_7/design_of_rcc_structure:ductility-requirements-of-rc-beam-with" source: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/ductility-requirements-of-rc-beam-with site: BCT Engineering --- # Ductility requirements of RC beam with sketch Design of RCC Structure · Chapter 12: Introduction to Earthquake Resistant Design and Provisions for Ductile Detailing - Asked in: 2082 Baisakh, 3 marks; 2080 Bhadra, 2 marks; 2075 Chaitra, 6 marks; 2072 Kartik, 6 marks (asked 4 times) - Video: [Ductile detailing of Beams as per IS-13920-2016 | detailing of beams |Beam detailing as per IS-13920](https://www.youtube.com/watch?v=OtGZM99vFLg) — Civil Engineering Mastery - 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/ductility-requirements-of-rc-beam-with - Whole chapter: https://www.bctengineer.com/md/semester_7/design_of_rcc_structure/chapter-12 ## Question Explain the ductility requirements/ductile detailing for the reinforced concrete beam with neat sketches (requirements on reinforcement detailing in beams to ensure sufficient ductility). ## Short answer Beam: b>=200 mm, b/D>=0.3, D<=L/4, two continuous bars, steel ratio 0.24sqrt(fck)/fy to 2.5%, bottom steel >=50% of top at joints. Closed stirrups with 135-degree hooks: min(d/4,8phi,100 mm) over 2d from supports, else d/2; splices only in middle half with hoops at 150 mm. ## Answer Ductility is the ability of a member to undergo large inelastic deformation without significant loss of strength. In a beam it is obtained by making the tension steel yield before the concrete crushes and by preventing early shear failure. IS 13920 gives the following detailing rules. #### Geometry - Width $b \ge 200$ mm and $b/D \ge 0.3$. - Depth $D \le \tfrac14$ of clear span. #### Longitudinal steel - At least two bars (12 mm or more) continuous at top and at bottom. - $\rho_{min} = 0.24\sqrt{f_{ck}}/f_y$ and $\rho_{max} = 2.5\%$ on any face. - Positive steel at a joint face is not less than 50% of the negative steel there. - Along the span, neither positive nor negative capacity is less than 25% of the maximum capacity at either joint face. - Lap splices only in the middle half of the span, never within the joint or within $2d$ of a joint face. Not more than 50% of bars are spliced at one section and the splice is enclosed by closely spaced hoops (spacing $\le 150$ mm). #### Transverse steel (stirrups) - Closed hoops with $135^\circ$ hooks and a tail of $10\phi$ (not less than 75 mm) so that they stay effective after the cover spalls. - Diameter $\ge 6$ mm (8 mm if clear span exceeds 5 m). - Over a length $2d$ from each joint face the spacing is the least of $d/4$, $8\phi_{min}$ and 100 mm, with the first hoop at 50 mm. Elsewhere the spacing is $\le d/2$. - Stirrups are designed for the shear that develops when plastic hinges form at both ends (capacity design). ```text Column |<-- 2d -->|<------ L/2 ------>|<- 2d ->| Column -------+-+-+-+-+-+---------------------+-+-+-+-+------- | | | | | | hoops at d/2 | | | | | | | | | | | | | | | | | | -------+-+-+-+-+-+---------------------+-+-+-+-+------- 50 mm, then s <= min(d/4, 8phi, 100) lap splice only in middle half ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Over 2d from each joint face, what is the maximum hoop spacing for ductile beams? A. d/2 B. The least of d/4, 8 times the smallest bar diameter, and 100 mm C. 150 mm D. 300 mm Answer: B. Close hoops (first at 50 mm) confine the concrete in the likely hinge zone. (Key point: Stirrup spacing near joints) 2. Where may lap splices be made in a ductile beam? A. Within the joint B. Within 2d of the joint C. Anywhere D. Only in the middle half of the span, with closely spaced hoops (<= 150 mm) Answer: D. Splices must be kept away from hinge zones, since they would be unreliable under inelastic cycles. (Key point: Lap splice location) 3. Why are 135 degree hooks and capacity-design shear used? A. Hooks stay anchored after cover spalls, and shear is designed for forces when hinges form so shear failure cannot precede yielding B. To make bending easier C. Hooks are only for anchorage of top bars D. To reduce steel cost Answer: A. 135 degree hooks remain effective after cover spalls, and designing for hinge shear ensures ductile flexural yielding governs. (Key point: Hoop hooks and shear design) ## Related in Design of RCC Structure - [Importance of ductile detailing, sketches](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aimportance-of-ductile-detailing-sketches) — Design of RCC Structure, chapter 12 - [Ductile detailing of beams and columns](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aductile-detailing-of-beams-and-columns) — Design of RCC Structure, chapter 12, asked 2× - [Column 350x350, 1440 kN, ductile detailing](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Acolumn-350x350-1440-kn-ductile-detailing) — Design of RCC Structure, chapter 10, asked 2× - [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× - [Ductile detailing of columns, confining steel](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aductile-detailing-of-columns-confining-steel) — Design of RCC Structure, chapter 12, asked 4× - [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 ## Related in other subjects - [Ductile behaviour of reinforced and unreinforced masonry](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Aductility-reinforced-unreinforced-masonry) — Design of Timber and Masonry Structures, chapter 6 - [Reinforced concrete column stresses in steel and concrete](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Arc-column-8-bars) — Strength of Materials, chapter 3 - [Pipe requirements; concrete, CI, PPR pipes](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Apipe-requirements-concrete-ci-ppr) — Water Supply Engineering, chapter 8