In IS 800, the lateral torsional buckling (LTB) of a beam is prevented if:
Choose the correct answer
Web is very thick
Effective lateral unsupported length is small enough (λLT ≤ 0.4) — i.e., adequate lateral restraint is provided to compression flange
The beam is designed in WSD
Stiffeners are provided at midspan
Correct Answer
B. Effective lateral unsupported length is small enough (λLT ≤ 0.4) — i.e., adequate lateral restraint is provided to compression flange
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LTB (out-of-plane buckling): I-beams with unrestrained compression flanges can buckle laterally. IS 800 Cl. 8.2.2: LTB is prevented if the effective lateral unsupported length (Llt) is small enough that λLT ≤ 0.4 (no reduction). Lateral supports (purlins, floor beams) reduce effective length. Fmd = fbd × Zp.
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