Design of Beams MCQ with answer and solution

Concept

Lateral-torsional buckling in unrestrained beams is primarily caused by the instability of the compression flange, which undergoes lateral deflection and twisting.

Step-by-step solution

1. Identify the behavior of a laterally unrestrained beam under bending. 2. Note that the top flange (under sagging moment) is in compression. 3. Because the compression flange acts like an unbraced column, it tends to buckle laterally out of the plane of bending. 4. This lateral displacement coupled with twisting results in lateral-torsional buckling (LTB).

Explanation

In laterally unrestrained beams, the compression flange behaves like a column subjected to compressive stress. Since the compression flange is unsupported laterally, it tends to buckle sideways and twist about the longitudinal axis, leading to lateral-torsional buckling.

Why the other options are wrong

A: Tension flange is under tension and naturally stable against buckling. C: Tension flange does not buckle. D: Web experiences shear and local buckling, but LTB is fundamentally governed by compression flange instability.

Shortcut

Remember: Compression flanges buckle because they are in compression, much like a column.

Common mistake

Confusing tension flange buckling with compression flange buckling; tension flanges are stable under tension.