Ribbed (waffle) slab reinforcement: What is the maximum diameter generally permitted for bars used in the slab component of a ribbed slab system, considering serviceability and detailing practice?

Introduction / Context:
Ribbed (waffle) slabs consist of thin topping slabs cast monolithically over closely spaced ribs. While ribs may contain larger longitudinal bars, the slab (topping) reinforcement is governed by cover, spacing, crack control, and practical placement. Excessively large diameters are unsuitable for thin slabs due to spacing and serviceability constraints.

Given Data / Assumptions:

• Typical ribbed slab with a relatively thin topping slab.
• Standard cover and bar spacing limits for crack control apply.
• Question refers to bars in the slab portion, not the main rib longitudinal bars.

Concept / Approach:

Codes limit bar diameters as a function of slab thickness and spacing. For thin toppings, practical bar diameters are small (commonly 8–12 mm). Larger diameters complicate placement, risk inadequate cover, and impair crack dispersion, which relies on multiple smaller bars at closer spacing.

Step-by-Step Solution:

Verification / Alternative check:

Design manuals and detailing guides commonly show 8, 10, or 12 mm top meshes for waffle slabs; ribs carry larger bars as required by flexure.

Why Other Options Are Wrong:

6 mm may be too light for structural slab reinforcement beyond distribution; 20–24 mm diameters are generally reserved for ribs or beams and are impractical in thin slabs.

Common Pitfalls:

Confusing the slab mesh with rib reinforcement; violating minimum spacing/cover; using large bars that create honeycombing or poor consolidation around reinforcement.

Final Answer:

12 mm