Design of Beams MCQ with answer and solution

Concept

Lateral-torsional buckling in unsupported beams

Formula

NONE

Step-by-step solution

1. For laterally unsupported beams, lateral instability affects flexural capacity under bending loads. Answer: C

Explanation

Lateral-torsional buckling occurs when an unbraced beam subject to flexure undergoes both lateral displacement and twisting before reaching its ultimate plastic bending capacity. Without lateral restraints, the compression flange becomes unstable under high bending moments. Therefore, lateral-torsional buckling is the primary factor limiting the moment capacity of laterally unsupported beams, making option C correct.

Why the other options are wrong

B) Shear capacity limits shear resistance rather than bending moment capacity. D) Deflection control is a serviceability requirement rather than a flexural strength limit.

Shortcut

Unsupported beam capacity is limited by Lateral-torsional buckling since the compression flange is unrestrained.

Common mistake

Selecting option A by assuming plastic bending capacity controls without considering lateral-torsional buckling in unsupported beams.