Structural steel – definition of a “long column” for design checks In steel design, a column is termed a long (slender) column when the ratio of its effective length to its least lateral dimension exceeds which limiting value?

Introduction / Context:
Columns are classified as short or long (slender) to decide whether crushing or buckling governs the strength. The classification hinges on a geometric slenderness parameter, enabling engineers to select the correct design formulae and safety checks.

Given Data / Assumptions:

• Effective length refers to end-restraint–dependent buckling length used in column design.
• Least lateral dimension is the smaller of the cross-section's lateral dimensions.
• Conceptual/terminology question; no numeric calculation is required.

Concept / Approach:
A column with a large effective length relative to its thickness (least dimension) is prone to elastic instability. Traditional teaching problems often state that a column may be treated as “long” when the effective length to least lateral dimension ratio exceeds a threshold (commonly taken around 12–15 in many practice problems), beyond which Euler-type buckling effects dominate.

Step-by-Step Solution:

Verification / Alternative check:
Design handbooks and exam-oriented summaries classify columns as long when geometric slenderness is high enough that buckling, not material crushing, becomes critical; the chosen limit reflects that intent in such problems.

Why Other Options Are Wrong:
More than 3 m long is not a reliable criterion because end conditions and cross-section size also matter. Least dimension < 25 cm alone ignores length and restraint. Free at top refers to boundary conditions, not a definition. The distractor with a much smaller ratio does not align with the common threshold used.

Common Pitfalls:
Confusing overall length with effective length; ignoring that the least lateral dimension, not the larger one, controls buckling.

Final Answer:
Effective length / least lateral dimension > 15