Ribbed (waffle) slab applications in building design: Why is a ribbed slab system typically selected instead of a conventional solid slab?

Introduction / Context:
Ribbed (waffle) slabs consist of a thin top flange (slab) and a grid of ribs beneath. They are commonly used in halls, parking decks, and buildings where larger column spacings are desired with efficient use of concrete.

Given Data / Assumptions:

• Comparison is with a conventional solid slab of similar span and loading.
• Normal building use; no special acoustic linings assumed.

Concept / Approach:
By removing concrete from low-stress regions (between ribs), a ribbed slab significantly lowers self-weight while maintaining stiffness through deep ribs. This enables longer spans or fewer supports with similar deflection control. The resulting coffered ceiling is often architecturally desirable and can aid in building services routing.

Step-by-Step Solution:
Identify the prime structural advantage: reduced dead load per square metre.Recognize the stiffness benefit from deeper ribbed sections.Note secondary benefits: architectural coffers; potential for improved acoustics/thermal behavior when combined with finishes.Select the option that captures these core reasons: option (a).

Verification / Alternative check:
Design guides show ribbed slabs achieving longer spans at similar reinforcement levels compared to solid slabs, confirming the efficiency motivation.

Why Other Options Are Wrong:

• (b) Ribbed slabs do not produce a plain soffit; they produce coffers.
• (c) Acoustics alone is not the core reason and often needs added treatments.
• (d) Thermal benefit can be incidental; the primary driver is structural efficiency.
• (e) Not always cheapest; economy depends on formwork and repetition.

Common Pitfalls:
Ignoring formwork cost; inadequate shear punching checks near columns; insufficient cover at thin flanges.

Final Answer:
To reduce self-weight while achieving longer economical spans and providing architectural coffers