Question

Design of laterally unsupported steel beams is governed by ......

• A. yield stress
• B. shear capacity
• C. lateral-torsional buckling
• D. bearing strength at supports

Hint:

When a beam is not laterally supported, always check for lateral-torsional buckling before checking for yield. Provide lateral restraints to increase its load capacity.

Answer 1

The Correct Option is C

Step 1: Understand the condition
- A steel beam is considered laterally unsupported if it is not braced against lateral movement and twisting (torsion) along its length.
Step 2: Identify the governing failure
- In such cases, the limiting factor is not yield stress or shear capacity but the beam’s tendency to buckle laterally and twist — a phenomenon called lateral-torsional buckling (LTB).
Step 3: Design implication
- For laterally unsupported beams, the moment capacity is reduced due to LTB. Design codes (like IS 800 or AISC) provide methods to compute reduced moment capacity.
Step 4: Final Answer
The design is governed by lateral-torsional buckling.