Maximum spacing (pitch) of main tension reinforcement in slabs: The pitch of main bars in a reinforced concrete slab should not exceed which multiple of the slab’s effective depth (subject to absolute spacing limits)?

Introduction / Context:Bar spacing rules in slabs ensure crack control, load distribution, and proper bond. For the main tension reinforcement, both a relative limit (as a multiple of effective depth) and an absolute limit (a maximum in millimeters) are applied in design practice.

Given Data / Assumptions:

• Reinforced concrete slab in flexure with main tension steel.
• Effective depth d is known.
• Absolute spacing limit (such as 300 mm) also applies, but question asks the multiple of d.

Concept / Approach:To control flexural cracking and ensure adequate distribution, the maximum spacing of main bars is commonly limited to 3d or an absolute cap (whichever is smaller). This balances crack width restraint with constructability and bar economy.

Step-by-Step Solution:Determine effective depth d of the slab.Limit main bar spacing to ≤ 3d and also ≤ the absolute cap (often 300 mm).Provide distribution bars orthogonal to the main reinforcement per code minimums.

Verification / Alternative check:Crack control checks (bar diameter, spacing, stress) confirm that limiting pitch to 3d is compatible with acceptable service crack widths and load distribution.

Why Other Options Are Wrong:

• 4d and 5d exceed typical limits and can lead to wide cracks.
• 2d or equal to d are overly conservative and uneconomical for most slabs.

Common Pitfalls:Forgetting the absolute cap (e.g., 300 mm), using too-large bars at wide spacing, or not providing adequate distribution reinforcement.

Final Answer:Three times the effective depth.