A solid slab has overall depth 20 cm and effective depth 15 cm. What is the maximum permissible horizontal spacing between parallel main reinforcement bars in the slab?

Introduction / Context:
Bar spacing limits in slabs control crack widths and ensure adequate distribution of reinforcement. Codes cap spacing as a function of effective depth to keep flexural crack spacing within acceptable bounds and to promote load sharing between bars.

Given Data / Assumptions:

• Overall depth = 20 cm; effective depth d = 15 cm (i.e., 150 mm).
• Main reinforcement under consideration (tension zone).
• Use standard spacing rule for slabs: maximum of 3d or 300 mm, whichever is smaller.

Concept / Approach:

For slabs, the maximum spacing S_max is min(3d, 300 mm). This ensures crack control independent of absolute slab thickness beyond a threshold.

Step-by-Step Solution:

Verification / Alternative check:

Even if the slab were thicker, the 300 mm cap would still govern once 3d exceeds 300 mm, maintaining consistent crack control.

Why Other Options Are Wrong:

• 40, 45, 60 cm: Exceed the 300 mm cap; would risk excessive crack spacing.
• 35 cm: Still greater than 30 cm limit in this case.

Common Pitfalls:

• Using overall depth instead of effective depth in 3*d calculation.
• Confusing slab rules with beam spacing provisions, which differ.

Final Answer:

30 cm