Reinforced Concrete – Minimum Diameter of Longitudinal Bars in a Column According to standard RCC practice (IS guidelines), what is the minimum bar diameter permitted for longitudinal reinforcement in a column?

Introduction / Context:
Longitudinal reinforcement in columns must satisfy minimum detailing rules to ensure ductility, durability, and constructability. A key rule is the minimum diameter for longitudinal bars, which helps avoid handling damage and excessive corrosion risk.

Given Data / Assumptions:

• Member type: RCC column subjected to axial load with bending.
• Code context: Indian practice per IS 456 detailing provisions.
• Objective: Identify the minimum permissible diameter for column longitudinal bars.

Concept / Approach:

Minimum diameters are set to ensure adequate stiffness of individual bars, proper bond, and practical placement with ties, cover, and lap splices. Too small a diameter compromises robustness and corrosion allowance.

Step-by-Step Solution:

Verification / Alternative check:

Reviewing typical schedules confirms that column main bars are rarely smaller than 12 mm due to lapping and buckling considerations during construction and service.

Why Other Options Are Wrong:

• 6 mm and 8 mm: Suitable for ties or links, not for main column bars.
• 10 mm: Common for secondary uses; still below the minimum for column longitudinal bars.
• 20 mm: Allowed but not the minimum; using it increases steel without being mandatory.

Common Pitfalls:

Confusing minimum longitudinal bar size with minimum tie diameter. Ties may be 6 mm or 8 mm depending on bar sizes and spacing, but main bars must be larger.

Final Answer:

12 mm