# Design of RC Structures Questions and Answers – Thickness of Slab

This set of Design of RC Structures Multiple Choice Questions & Answers (MCQs) focuses on “Thickness of Slab”.

1. For a simply supported slab of dimension 3m*4m, the minimum thickness of the slab should be less than which of the following (mild steel reinforcement)?
a) 86mm
b) 100mm
c) 120mm
d) 300mm

Explanation: According to clause 24.1 of IS 456:2000 the minimum thickness D<lx/35.
D should be less than 3000/35 = 85.71mm~86mm.

2. The thickness of the slab depend upon which of the following?
a) Span
b) Depth
c) Span/depth ratio

Explanation: The guidelines about the thickness of the slab is given in clause 24.1 in the IS 456:2000 and ACI. The thickness of the slab primarily depends on the span/depth ratio as it determines the nature of the slab.

3. What is the highest dia reinforcing may be used for an 80mm thick slab?
a) 20mm
b) 25mm
c) 16mm
d) 10mm

Explanation: The highest dia of the reinforcing bar is = thickness of the slab/8
= 80/8mm = 10mm.

4. The thickness of a slab of dimension 5m*6m subjected to moment of 9kN-m/m width of M20 concrete and Fe415 steel is given by which of the following?
a) 100mm
b) 150mm
c) 200mm
d) 300mm

Explanation: For the M20 grade concrete and Fe415 steel,
R=.914 ; M=9*106 N-mm/1000 mm of width
The thickness is given by, d=(9*106/1000*.914)1/2
=99.24mm~100mm < 4000/35=114mm
Hence 100mm satisfies every criteria of slab thickness.

5. What is the effective depth of a 120mm thick slab having clear cover 25mm and rebars of 20mm?
a) 75mm
b) 85mm
c) 65mm
d) 55mm

Explanation: The effective depth of the slab is = Thickness of the slab-clear cover- dia of the rebar/2
= 120-25-(20/2) = 85mm.

6. For a continuous slab (Fe 415) of 3000*6000 what is the maximum thickness of the slab?
a) 94mm
b) 84mm
c) 74mm
d) 44mm

Explanation: According to the codal provision for a continuous slab the thickness D<lx/32=3000/32 = 94mm.

Sanfoundry Global Education & Learning Series – Design of RC Structures