What is the effective length of a simply supported beam of span length 7 m for the following restraint conditions at the support under normal loading conditions ? Torsional restraint: Fully restrained; Warping restraint: Both the flanges fully restrained
- A5·25 m
- B5·95 m
- C4·90 mCorrect
- D6·30 m
Solution
Concept
Effective length of simply supported beams (IS 800 Table 15)
Step-by-step solution
1. IS 800:2007 Table 15: torsional restraint fully restrained and warping restraint of both flanges fully restrained → LLT = 0.70 L. 2. LLT = 0.70 × 7 = 4.90 m.
Explanation
The effective length for lateral–torsional buckling falls as the end restraint increases. With full torsional restraint and both flanges restrained against warping, it is 0.7 times the span.
Why the other options are wrong
0.75 L = 5.25 m applies when warping is partially restrained; 0.85 L = 5.95 m and 0.9 L = 6.3 m apply for lesser restraint.
Shortcut
Fully restrained: 0.70 L; partially: 0.75 L, 0.80 L, 0.85 L ... up to 1.2 L.
Common mistake
Choosing 0.75 L.
Correct answer: C) 4·90 m