Reinforced concrete design — a reinforced concrete (R.C.C.) column is classified as a long column if its slenderness ratio exceeds which limiting value?

Introduction / Context:
Classifying a reinforced concrete (R.C.C.) column as short or long is essential because slender (long) columns experience additional bending due to buckling effects (P–Δ), which lowers their axial load capacity compared with short columns. The classification is made using a non-dimensional slenderness ratio, and selecting the correct limit is a basic design knowledge check in building design and civil engineering exams.

Given Data / Assumptions:

• The term slenderness ratio refers to effective length divided by an appropriate sectional dimension parameter.
• For routine conceptual questions, a common cut-off used for R.C.C. columns is taken as 40.
• The intent is conceptual classification (not a full code check of effective length factors, end conditions, or biaxial bending).

Concept / Approach:

Slenderness ratio is a measure of susceptibility to buckling. In reinforced concrete, when the ratio rises above a customary limit, second-order effects become significant and design must allow for magnified moments or a reduced axial capacity. A widely used educational threshold is 40 for a column to be treated as “long,” whereas “short” columns are below this limit and governed primarily by material strength and first-order bending effects.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

In practice, designers also check code-specific rules (effective length factors, end restraints) and may use more conservative thresholds depending on the situation. The conceptual value of 40 aligns with many standard references and examination conventions.

Why Other Options Are Wrong:

30 and 35 are too low for the usual R.C.C. classification; 50 or 60 are overly high and may under-predict slenderness effects, risking unsafe assumptions.

Common Pitfalls (misconceptions, mistakes):

Confusing concrete column limits with structural steel limits; forgetting to use effective length (not clear length) and the least lateral dimension or appropriate radius of gyration in actual design checks.

Final Answer:

40