Steel Girders – Lateral Restraint at Supports For a simply supported girder whose compression flange is fully restrained against lateral movement at both ends, the effective laterally unsupported length is taken as:

Introduction / Context:
Lateral–torsional buckling (LTB) limits the bending strength of beams when the compression flange is not adequately restrained. Codes provide effective length factors that convert the clear distance between restraints into an equivalent laterally unsupported length for design. This item checks the factor used when the compression flange of a simply supported girder is fully restrained laterally at both ends.

Given Data / Assumptions:

• Simply supported girder.
• Compression flange laterally restrained at both supports (no lateral movement).
• Standard code-based effective length factors apply.
• Span is the distance between supports/points of zero moment for a typical UDL case.

Concept / Approach:

End restraint to the compression flange reduces the effective laterally unsupported length because the flange cannot sway at the bearings. Empirical/analytical studies reflected in code tables yield a reduction factor from the full span to about 0.85 × span for such end conditions in simply supported girders.

Step-by-Step Solution:

Verification / Alternative check:

In the limiting case of no restraint, effective length would be close to the full span. With full lateral restraint and additional warping/rotation restraint, even lower factors may apply in special cases; 0.85 × span is the commonly adopted value for end lateral restraint only.

Why Other Options Are Wrong:

span assumes no beneficial restraint; 0.7 × span and 0.5 × span are too low for the stated boundary condition; they would overestimate capacity.

Common Pitfalls:

Confusing lateral restraint (preventing sideways movement of compression flange) with full rotational fixity, and misapplying factors intended for continuous beams or braced points along the span.

Final Answer:

0.85 × span