Ribbed (joist) slab: For a rib containing two longitudinal bars, the practical breadth (web width) should be within which range for good detailing and placement?

Introduction / Context:Ribbed slabs have narrow ribs supporting a thin topping slab. The rib breadth must be sufficient for bar placement, concrete flow, cover, and shear capacity. With two longitudinal bars, typical practice keeps the rib width around 100–120 mm to avoid congestion and ensure proper compaction.

Given Data / Assumptions:

• Two longitudinal bars in the rib.
• Standard bar spacing, concrete cover, and link (stirrup) detailing constraints.
• Ordinary building-grade concrete and construction methods.

Concept / Approach:

Minimum width must accommodate 2 bars plus clear spacing and cover, while meeting shear web requirements. Hence, 10–12 cm is a widely used practical range.

Step-by-Step Solution:

Verification / Alternative check:

Project detailing manuals and site experience confirm that ribs narrower than ~100 mm cause congestion and poor compaction.

Why Other Options Are Wrong:

6–10 cm too narrow for two bars and links; 12–15 cm is workable but often uneconomical for typical rib spacing; the question targets the commonly recommended band.

Common Pitfalls:

Ignoring link width and nominal cover; choosing too-narrow ribs leading to honeycombing.

Final Answer:

10 cm to 12 cm