--- title: "Importance of beam deflections; pure bending examples" subject: "Strength of Materials" chapter: 7 key: "semester_3/strength_of_materials:importance-of-beam-deflection" source: https://www.bctengineer.com/q/semester_3/strength_of_materials/importance-of-beam-deflection site: BCT Engineering --- # Importance of beam deflections; pure bending examples Strength of Materials · Chapter 7: Theory of Flexure - Asked in: 2069 Chaitra, 2+1 marks - Video: [Bending and BEAM DEFLECTION in 13 Minutes!](https://www.youtube.com/watch?v=IW9uvArmypg) — Less Boring Lectures - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/strength_of_materials/importance-of-beam-deflection - Whole chapter: https://www.bctengineer.com/md/semester_3/strength_of_materials/chapter-7 ## Question Describe the importance of computing deflections in beams. Also give two typical examples of pure bending of beam. ## Short answer Checking deflection ensures serviceability (no cracking, springy floors), protects finishes, alignment, and provides compatibility for indeterminate beams. Pure bending examples: a simply supported beam with two symmetric loads has constant moment between loads; a wagon axle with equal overhangs and end loads has constant moment between supports. ## Answer #### Importance of computing deflection A beam may be strong enough in bending and shear yet still fail to serve its purpose if it deflects too much. Deflection is computed because: 1. **Serviceability:** codes limit deflection (for example span/250 for total, span/350 for loads after finishes) so floors do not feel springy or unsafe. 2. **Protection of finishes:** large deflection cracks plaster, partitions, tiles and glazing. 3. **Appearance:** sagging beams and slabs look unsafe and spoil the structure. 4. **Machine alignment:** shafts and machine beds need small deflection for gears, bearings and rails to work properly. 5. **Drainage and ponding:** sagging roofs collect water and increase the load. 6. **Statically indeterminate beams:** deflection (compatibility) conditions are needed to find the extra reactions. 7. **Vibration control:** stiffness affects natural frequency and comfort. #### Two examples of pure bending 1. A simply supported beam with two equal loads placed symmetrically at equal distances from the supports: the portion between the loads has constant bending moment and zero shear force. 2. A simply supported beam with equal overhangs carrying equal point loads at both free ends (for example, a railway wagon axle, loaded at its ends outside the wheels): between the supports the bending moment is constant and the shear force is zero. A bar bent into an arc by equal and opposite couples at its ends is also in pure bending. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why must deflection be checked even if the beam is strong enough? A. It reduces load B. Codes ignore it C. Excess deflection impairs serviceability, even when strength is adequate D. It increases strength Answer: C. Springy floors, cracked finishes and misalignment occur despite adequate strength. (Key point: Serviceability) 2. How does large deflection damage a building? A. It increases Young's modulus B. It cracks plaster, partitions, tiles and glazing C. It improves drainage D. It lowers the bending moment Answer: B. Brittle finishes cannot follow large deflections. (Key point: Finishes) 3. Why are deflection conditions needed for statically indeterminate beams? A. Compatibility equations give the extra reactions B. They are not needed C. To find the self-weight D. To find E Answer: A. Equilibrium is not enough, so compatibility from deflection is used. (Key point: Indeterminate beams) 4. Which is an example of pure bending? A. A beam under UDL B. A column in compression C. Middle portion of a beam with two equal symmetric loads D. A cantilever with a tip load Answer: C. There M is constant and V is zero. 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