Column ties: The diameter of transverse reinforcement (lateral ties) in reinforced concrete columns should be at least one-fourth of the diameter of the largest longitudinal bar, but not less than what absolute minimum diameter?

Introduction / Context:
Transverse reinforcement (ties) confines concrete and prevents buckling of the longitudinal bars in columns. Codes prescribe minimum diameters to ensure adequate stiffness and robustness of ties during construction and service.

Given Data / Assumptions:

• Tie diameter must be ≥ one-fourth of the largest longitudinal bar diameter.
• An absolute minimum tie diameter also applies to avoid flimsy ties.
• Ordinary reinforced concrete columns in buildings.

Concept / Approach:
Even when one-fourth of the largest bar would permit a very small tie, practice imposes a floor on tie size. Commonly adopted absolute minimum is 6 mm for mild steel ties (and often 8 mm for high-strength deformed, depending on code/project). Thus, when one-fourth rule yields less than 6 mm, the minimum 6 mm governs.

Step-by-Step Solution:
Check one-fourth of largest longitudinal bar.Compare with absolute minimum permitted for ties.Adopt the greater value; for small bars, this is typically 6 mm.

Verification / Alternative check:
Design examples and detailing guides routinely specify 6 mm or 8 mm ties; 6 mm is a widespread minimum for conventional building columns with smaller longitudinal bars.

Why Other Options Are Wrong:

• 4 mm, 5 mm: Too slender and difficult to handle; below common minimum.
• 7 mm, 8 mm: Acceptable but not the minimum in the general case posed.

Common Pitfalls:
Using only the fractional rule and forgetting the absolute minimum leads to under-sized, flexible ties that are hard to place and insufficiently confining.

Final Answer:
6 mm