Plain concrete (unreinforced) columns: Up to what limit of slenderness may plain concrete columns be used, expressed as the ratio of unsupported length to least lateral dimension?

Introduction / Context:
Plain concrete has negligible tension capacity and limited ductility. Therefore, code provisions restrict its use for columns (pedestals) to very stocky members with small slenderness. This question asks for the typical slenderness limit stated as the ratio of unsupported length to the least lateral dimension of the section.

Given Data / Assumptions:

• Plain concrete column or pedestal under axial load.
• Unsupported length measured between restraints.
• Least lateral dimension is the smaller cross-sectional dimension.

Concept / Approach:

To avoid instability and to keep eccentricity effects minimal, the unsupported length should be very short relative to the cross-section size. A commonly cited restriction is that plain concrete columns should not have unsupported length exceeding three times the least lateral dimension; beyond this, reinforcement must be provided to ensure ductility and stability.

Step-by-Step Solution:

Verification / Alternative check:

Euler-type instability and accidental eccentricities become critical as slenderness grows; providing reinforcement and ties addresses these concerns for taller columns.

Why Other Options Are Wrong:

2 is overly restrictive and uneconomical; 4–6 are too slender for plain concrete and risk instability and brittle failures.

Common Pitfalls:

Ignoring minimum eccentricity; misidentifying the unsupported length; assuming plain concrete can be used for tall piers without reinforcement.

Final Answer:

Three times