Ribbed (joist) slab detailing: which set of limits is appropriate for safe and code-aligned design of a ribbed slab system?

Introduction / Context:
Ribbed (joist) slabs achieve economy by concentrating concrete in ribs while using a thin topping slab for distribution. Proper geometric limits on rib spacing, rib width, and overall depth relative to rib breadth are required to ensure stiffness, shear capacity, fire cover, and good load distribution.

Given Data / Assumptions:

• Monolithic ribbed slab with regular ribs and a thin topping slab.
• Need typical detailing limits widely used in building codes and practice.
• Focus is on spacing, minimum rib width, and overall depth cap.

Concept / Approach:
Minimum rib width ensures adequate space for reinforcement and cover, reducing congestion and improving fire resistance. Limiting clear spacing controls two-way action in the topping and prevents excessive transverse bending. Limiting overall depth relative to rib breadth maintains a realistic rib slenderness and avoids brittle behavior.

Step-by-Step Solution:
Adopt rib width ≥ 75 mm to allow main bars plus cover and to provide concrete confinement.Keep clear spacing ≤ about 450 mm so the topping acts as an effective flange between ribs.Restrict overall depth to ≤ 4 × rib breadth so ribs remain stocky, controlling shear and deflection.

Verification / Alternative check:
Check shear at the rib–slab junction and distribution steel in the topping slab; deflection calculations corroborate that these geometric limits lead to serviceable deformations.

Why Other Options Are Wrong:

• Options b and d: rib widths too small and overall depth too restrictive for practical reinforcement and capacity.
• Option c: spacing excessively large and depth overly slender, harming distribution and shear.
• Option e: unlimited spacing is unsafe; topping alone cannot maintain action.

Common Pitfalls:
Ignoring topping reinforcement, using too-narrow ribs causing congestion, and exceeding spacing that the topping can bridge.

Final Answer:
Clear spacing between ribs ≤ 450 mm; rib width ≥ 75 mm; overall depth of slab ≤ 4 × rib breadth.