--- title: "Curtailment of tension steel in beams" subject: "Design of RCC Structure" chapter: 7 key: "semester_7/design_of_rcc_structure:curtailment-of-tension-steel-in-beams" source: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/curtailment-of-tension-steel-in-beams site: BCT Engineering --- # Curtailment of tension steel in beams Design of RCC Structure · Chapter 7: Reinforcement detailing: Codal Provisions - Asked in: 2079 Baisakh, 4 marks; 2067 Asar (old course), 8 marks (asked 2 times) - Video: [What is Curtailment of Bars in Beam](https://www.youtube.com/watch?v=5Fux2GyZMRs) — 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/curtailment-of-tension-steel-in-beams - Whole chapter: https://www.bctengineer.com/md/semester_7/design_of_rcc_structure/chapter-7 ## Question What do you understand by curtailment of tension steel in simple beams? Discuss the actual and theoretical point of curtailment of re-bar, showing by illustrating a neat sketch. ## Short answer Curtailment stops tension bars where bending moment reduces. Theoretical cut-off is where remaining bars' moment resistance equals applied moment. Actual cut-off extends beyond theoretical point by at least effective depth d or 12φ (whichever is greater) per IS 456 cl. 26.2.3.1. Conditions include development length, shear limits, and minimum steel continuation into supports. ## Answer #### Meaning of curtailment **Curtailment** means stopping (cutting off) some of the tension bars of a beam where the bending moment has fallen so much that fewer bars are needed. The main bars are designed for the maximum bending moment at mid-span, but near the supports the moment is small, so the same amount of steel is not needed there. Curtailing the extra bars saves steel and reduces congestion at the supports, but it must be done so that the remaining bars can still carry the moment and the shear, and so that every bar has enough bond length. #### Theoretical and actual cut-off points - **Theoretical cut-off point**: the section where the bending moment diagram shows that the bar is no longer required, i.e. where the moment of resistance of the remaining bars equals the applied moment $M_u$. - **Actual cut-off point**: the bar cannot be stopped exactly at the theoretical point, because the stress in the bar is still being developed there and the real moment diagram may differ from the assumed one (shift due to diagonal cracking, load variations). IS 456 cl. 26.2.3.1 therefore requires every bar to be extended beyond the theoretical cut-off point (towards the support) by a distance not less than the **effective depth $d$ or $12\phi$, whichever is greater** (except at a simple support or the end of a cantilever). ```text Bending moment diagram, left half of a simply supported beam M ^ | ______ Mu,max (mid-span): | ___---- all bars needed | __-- |- - - - - - - -*- - - - - - - - - - MR of continuing bars | _-' : | _-' : | _-' : : | _-' : : +--'---------+----+-----------------> x support Q P mid-span |<-->| PQ >= d or 12 phi (whichever is greater) P = theoretical cut-off point (M = MR of continuing bars) Q = actual cut-off point (bar is stopped here) ``` At the theoretical point P the moment of resistance of the remaining bars is just equal to the applied moment. The curtailed bar is carried on to Q, so that it is fully effective up to P. #### Conditions to be satisfied (IS 456 cl. 26.2.3) 1. The bar must also have a development length $L_d$ measured from the point of maximum stress, i.e. the length of bar beyond the section of maximum moment must be at least $L_d$. 2. Flexural bars must not be stopped in a tension zone unless one of the following is satisfied at the cut-off point: (a) the shear there does not exceed two-thirds of the permitted shear; or (b) extra stirrups are provided over a distance of $3d/4$ from the cut-off point; or (c) for bars of 36 mm or less, the continuing bars provide double the area needed for flexure and the shear does not exceed three-quarters of the permitted value. 3. At least one-third of the positive moment steel (one-fourth in continuous members) must be carried into the support. 4. At a support, at least one-third of the negative moment steel must extend beyond the point of inflection by at least $d$, $12\phi$ or $l/16$, whichever is greater. 5. Bars are curtailed symmetrically and in stages, never all at one section. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is curtailment of tension steel? A. Increasing bar diameter near supports B. Bending bars into stirrups C. Cutting off some tension bars where the bending moment has fallen and fewer are needed D. Adding extra bars at the supports Answer: C. Curtailment saves steel and reduces congestion where moment is small. (Key point: Meaning of curtailment) 2. Theoretical cut-off point is the section where A. The moment of resistance of the remaining bars equals the applied moment Mu B. The beam is cracked C. The shear is zero D. The bar has full development length Answer: A. Beyond this section the bar is no longer required for flexure. (Key point: Theoretical cut-off) 3. How far must a bar extend beyond the theoretical cut-off point? A. Exactly Ld B. Not less than the greater of d and 12φ C. Not more than 4φ D. The lesser of d and 12φ Answer: B. IS 456 cl. 26.2.3.2 requires extension of max(d, 12φ) as stress is still developing. (Key point: Actual cut-off) 4. How much positive moment steel must be carried into a simple support? A. One-tenth of it B. At least one-third of it C. All of it D. None of it Answer: B. At least one-third of the positive moment steel (one-fourth in continuous members) must extend into the support. (Key point: Support bars) ## Related in Design of RCC Structure - [Two-way slab on RCC beams, 4x6 m](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Atwo-way-slab-on-rcc-beams) — Design of RCC Structure, chapter 9, asked 2× - [WSM moment of resistance, 230x350 beam](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Awsm-moment-of-resistance-230x350-beam) — Design of RCC Structure, chapter 2 - [Idealized stress-strain, characteristic strength](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aidealized-stress-strain-characteristic-strength) — Design of RCC Structure, chapter 3 - [Slab continuous over three edges, 5x4 m (225 mm beam)](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aslab-continuous-over-three-edges-5x4-2) — Design of RCC Structure, chapter 9 - [Shear at bent-up section, 8 mm stirrups](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Ashear-at-bent-up-section-8) — Design of RCC Structure, chapter 5 ## Related in other subjects - [Simple program illustrating OOP concept](https://www.bctengineer.com/question.md?key=semester_3%2Fobject_oriented_programming%3Aoop-simple-program) — Object Oriented Programming, chapter 1 - [Lateral-torsional buckling; flange curtailment](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Altb-curtailment-plate-girder) — Design of Steel and Timber Structure, chapter 7 - [Cable with four point loads, max tension in segments](https://www.bctengineer.com/question.md?key=semester_4%2Ftheory_of_structures_i%3Acable-25m-different-levels-four-loads) — Theory of Structures I, chapter 7 - [Rigid bar ABCD with brass and steel rods](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Arigid-bar-abcd-brass-steel-15kn) — Strength of Materials, chapter 3