Minimum spacing rule for distribution steel in a simply supported slab: The minimum spacing of distribution reinforcement should be four times the effective thickness (effective depth) of the slab or _____, whichever is smaller.

Introduction / Context:
Distribution (secondary) reinforcement in slabs controls temperature and shrinkage cracking and distributes local effects. Codes limit bar spacing to ensure adequate crack control independent of the principal bending direction.

Given Data / Assumptions:

• Simply supported RCC slab.
• We are concerned with the maximum permitted spacing of distribution steel.

Concept / Approach:
Spacing is limited by a multiple of the effective depth and by an absolute cap, selecting the smaller of the two. A common prescriptive rule is 4 * effective depth or 300 mm, whichever is smaller, to provide adequate crack control and stress distribution.

Step-by-Step Solution:
Compute 4 * d for the given slab during design.Compare with 30 cm (300 mm) cap.Adopt the smaller value to satisfy spacing limits.

Verification / Alternative check:
Many design guides and codes specify similar or slightly different numeric caps, but 30 cm is a widely cited maximum for distribution steel spacing in simply supported slabs.

Why Other Options Are Wrong:
20 cm: Overly restrictive as a general cap; may be used in exposure-sensitive cases but not the stated rule.40–60 cm: Too large to ensure reliable crack control for typical slabs.

Common Pitfalls:

• Confusing main and distribution steel spacing rules.
• Forgetting the “whichever is smaller” clause, leading to excessive spacing.

Final Answer:
30 cm