--- title: "Controlling deflection and cracking" subject: "Design of RCC Structure" chapter: 8 key: "semester_7/design_of_rcc_structure:controlling-deflection-and-cracking" source: https://www.bctengineer.com/q/semester_7/design_of_rcc_structure/controlling-deflection-and-cracking site: BCT Engineering --- # Controlling deflection and cracking Design of RCC Structure · Chapter 8: Limit States of Serviceability: Deflection and Cracking - Asked in: 2076 Chaitra, 2+2 marks - Video: [RC Serviceability Overview](https://www.youtube.com/watch?v=XtAH05RXiSI) — Mukai UWYO Structural Concrete - 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/controlling-deflection-and-cracking - Whole chapter: https://www.bctengineer.com/md/semester_7/design_of_rcc_structure/chapter-8 ## Question Describe the method of controlling deflection and cracking in RCC structure. ## Short answer Deflection controlled by span/depth ratio method, increasing depth/steel, adding compression steel, using quality concrete, limiting total deflection to span/250 and post-finish to span/350 or 20mm. Cracking controlled by limiting bar spacing (max 3d or 300mm in slabs), using small well-distributed bars, adequate cover, side-face steel (0.1% web area for beams >750mm deep), limiting steel stress, and crack width to 0.3mm (0.2mm in aggressive environments). ## Answer #### Controlling deflection - **Span/depth ratio method** (IS 456 cl. 23.2.1): keep $l/d$ within the basic value (7, 20 or 26 for a cantilever, simply supported or continuous member) multiplied by the modification factors for tension steel, compression steel and flanged section. - Increase the depth of the member, or provide more tension steel (this reduces the service stress), and add compression steel to reduce creep deflection. - Use better concrete (higher modulus), cure it properly to reduce shrinkage, and keep a low water-cement ratio. - Limit the deflection to $\text{span}/250$ (total) and $\text{span}/350$ or 20 mm (after finishes). - Camber (pre-camber) the member, and delay the fixing of partitions and finishes. #### Controlling cracking - Limit the **bar spacing** to the maximum allowed (IS 456 cl. 26.3.3, cl. 43 and Table 15), and use many small bars rather than a few large ones. - Provide sufficient **cover** (not more than 50 mm to the tension bars), and provide side-face steel (0.1% of the web area) in beams whose web depth exceeds 750 mm. - Limit the steel stress and the crack width to 0.3 mm (IS 456 cl. 35.3.2), checked by the calculation of Annex F when necessary. - Control shrinkage and thermal cracks by proper curing, minimum steel and contraction joints. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which measure reduces deflection? A. Using wider bar spacing B. Reducing curing C. Increasing depth or tension steel, and adding compression steel D. Reducing the depth Answer: C. These lower service stress and creep deflection. (Key point: Deflection control) 2. The deflection limits are A. Span/100 total B. Span/500 after finishes C. Span/50 total D. Span/250 total and span/350 or 20 mm after finishes Answer: D. IS 456 cl. 23.2 sets these limits. (Key point: Deflection limits) 3. Which measure helps control cracking? A. Many small bars well distributed within the maximum spacing B. No curing C. Cover above 100 mm D. Few large bars Answer: A. Close, small bars distribute cracks into finer ones. (Key point: Crack control) 4. The limit on surface crack width in normal conditions is A. 1 mm B. 0.03 mm C. 0.3 mm D. 3 mm Answer: C. IS 456 cl. 35.3.2 gives 0.3 mm (0.2 mm in aggressive environment). 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