In reinforced concrete design per IS practice, a column is classified as a long column based on its slenderness. Specifically, if the ratio of its effective length to its least lateral (transverse) dimension exceeds what threshold value, the member is treated as a long (slender) column for design and detailing?

Introduction / Context:
Columns are categorized as short or long (slender) depending on their propensity to buckle. This classification strongly influences design checks, permissible stresses, additional moments due to slenderness, and detailing rules in reinforced concrete design practice.

Given Data / Assumptions:

• Classification is asked in terms of effective length divided by the least lateral dimension (breadth or thickness) of the column section.
• Standard building design conditions and conventional IS-based practice are assumed.
• No eccentricity or load specifics are required; only the threshold ratio is needed.

Concept / Approach:
The slenderness ratio for R.C.C. columns is commonly judged using two measures: effective length to least lateral dimension and effective length to radius of gyration. A widely used rule of thumb in basic exam questions is that a column becomes “long” when the ratio effective length / least lateral dimension exceeds 12. Below this limit, the member is usually treated as “short.”

Step-by-Step Solution:
Identify the parameter: slenderness ratio = effective length / least lateral dimension.Recall the threshold used in many design guides and objective questions: 12.Compare with options and select 12.

Verification / Alternative check:
Introductory reinforced concrete design texts summarize this as a quick classification rule. Detailed code checks may additionally compare effective length to radius of gyration (with other limits), but for one-mark theory, 12 is the expected answer.

Why Other Options Are Wrong:

• 10: Too conservative relative to common threshold.
• 15, 20, 25: Exceed the typical threshold; these would under-flag slenderness.

Common Pitfalls:
Confusing radius-of-gyration criteria with least-dimension criteria; forgetting that actual code design may add secondary moment checks even if the simple ratio is near the limit.

Final Answer:
12