Ribbed (waffle) slab as a T-beam system: The effective flange thickness of a T-beam formed by a rib and a thin topping is generally taken equal to which dimension?

Introduction / Context:
Ribbed (waffle) slabs consist of a thin slab (topping) cast monolithically over regularly spaced ribs. Each rib-and-slab strip behaves like a T-beam, where the slab acts as the flange in compression and the rib acts as the web in bending. Determining the effective flange thickness is essential for flexural calculations.

Given Data / Assumptions:

• Regular waffle/ribbed slab with thin topping over ribs.
• Monolithic behavior assumed; loads produce compression in the slab topping over ribs.
• Elementary textbook rule: the flange thickness is taken as the slab (topping) thickness for ribbed systems.

Concept / Approach:

In ribbed systems, the thin slab topping provides the compression flange. Its actual thickness, not the rib depth or width, defines the flange thickness. While effective flange width depends on spacing and code equations, the flange thickness itself is simply the topping thickness.

Step-by-Step Solution:

Verification / Alternative check:

Design manuals and codes specify formulas for effective flange width based on rib spacing and span. However, they consistently treat the flange thickness as the topping thickness for ribbed slabs.

Why Other Options Are Wrong:

Rib width or depth relates to web properties, not flange thickness. Halving rib thickness is arbitrary. Twice the rib width addresses breadth, not thickness, and has no physical basis here.

Common Pitfalls:

Confusing effective flange width (a plan dimension) with flange thickness (a depth dimension); using large bar diameters in thin toppings leading to cover issues; neglecting shear and punching checks at supports.

Final Answer:

Thickness of the concrete topping (slab)