For ribbed (joist) slabs in building construction, the in-situ topping (wearing/finishing slab cast over the ribs) generally has what thickness range?

Difficulty: Easy

Correct Answer: 5 cm to 8 cm

Explanation:


Introduction / Context:
Ribbed slabs consist of closely spaced ribs with a thin topping slab forming the flange. The topping provides distribution of loads, diaphragm action, and a surface for finishes and services.


Given Data / Assumptions:

  • Conventional building floors with ribbed slab systems.
  • Normal live loads and finishing requirements.


Concept / Approach:
A topping that is too thin may crack and fail to distribute loads; one that is too thick adds dead load without significant benefit. Practice typically adopts about 50–80 mm (5–8 cm) thickness for a good balance of performance and weight.


Step-by-Step Solution:
Identify typical topping thickness used in ribbed slabs.Select the 5–8 cm range as most common in building practice.Confirm that this thickness allows cover to reinforcement, conduits, and finishing layers.


Verification / Alternative check:
Structural detailing manuals and manufacturer guidance for joist systems commonly specify a minimum topping thickness in the 50–80 mm range.


Why Other Options Are Wrong:

  • 3–5 cm: Often inadequate for crack control and embedment of services.
  • 8–10 cm or 12–18 cm: Heavier than necessary for typical ribbed slabs, increasing dead load.


Common Pitfalls:
Forgetting to include screed/finish thickness in dead load; using too thin a topping over deep rib spacing causing localized punching or cracking.


Final Answer:
5 cm to 8 cm

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