Bar spacing rule (different diameters): For horizontal parallel reinforcement bars of different diameters placed side-by-side, the minimum clear spacing should not be less than _____.

Introduction / Context:
Minimum clear spacing between parallel bars ensures adequate concrete flow, aggregate accommodation, and bond. When the adjacent bars are of different sizes, the code-based limit is governed by the larger diameter bar.

Given Data / Assumptions:

• Two or more horizontal bars placed in the same layer.
• Bars of unequal diameters are adjacent.

Concept / Approach:
Codes specify that the clear spacing shall not be less than the bar diameter (for equal bars). For unequal bars, the controlling dimension is the larger diameter bar. Additional aggregate-based limits may also apply (e.g., not less than 5 mm more than nominal maximum aggregate size), but the bar-diameter criterion here is decisive.

Step-by-Step Reasoning:
Identify the larger of the adjacent bar diameters.Set minimum clear spacing ≥ diameter of the thicker bar (and also ≥ aggregate-based limit as applicable).

Verification / Alternative check:
This rule promotes proper compaction and bond; it is consistent with widely used RC design standards for bar spacing.

Why Other Options Are Wrong:
One diameter of thinner bar / half diameter: May be too small, risking honeycombing and poor bond.Sum or twice thinner bar: Not a general minimum requirement and may be overly conservative or arbitrary.

Common Pitfalls:

• Ignoring aggregate size limits which may govern in some mixes.
• Confusing clear spacing with centre-to-centre spacing.

Final Answer:
one diameter of thicker bar