Minimum diameter of main longitudinal bars in RCC columns: What is the minimum nominal bar diameter permitted for the main (longitudinal) reinforcement in a reinforced concrete column?

Introduction / Context:
Main longitudinal reinforcement in columns must satisfy minimum size requirements to ensure ductility, robustness during handling, and adequate bond with surrounding concrete. Very small bars are prone to buckling between ties and offer poor confinement compatibility.

Given Data / Assumptions:

• Reinforced concrete column (short or long).
• Main bars refer to the longitudinal steel, distinct from lateral ties.
• Standard building-code practice is assumed.

Concept / Approach:
Common RC codes specify a minimum nominal diameter for main column bars. A widely adopted limit in building practice is 12 mm minimum for longitudinal bars in columns, which balances constructability, stiffness, and bond while avoiding excessive bar count.

Step-by-Step Reasoning:
Check code minimum diameter for longitudinal column bars.Select the smallest diameter satisfying the requirement: 12 mm.Ensure tie spacing and bar arrangement are then designed for confinement and buckling control.

Verification / Alternative check:
Practical detailing handbooks and code commentaries reinforce the 12 mm minimum as a robust lower bound, especially under construction tolerances and the need for adequate development length.

Why Other Options Are Wrong:
6, 8, 10 mm: Too small for main column bars; risk of instability and poor bond.

Common Pitfalls:

• Confusing main bars with confinement ties (which can be smaller).
• Ignoring clear cover and spacing constraints when increasing bar count.

Final Answer:
12 mm