Assumptions:
1.
Slab is supported on all edges e.g. with the help of beams.
2.
Slab can carry any kind of loading like point load , U.D.L (uniformly
distributed load) etc.
3.
We know the drawing details of slab like no. Of steel bars and type of
concrete.
4.
We just want to know theoretical capacity of the slab not actual otherwise we
may need to do plate load test which is destructive in nature.
Lets
find out the capacity of slab.
Step 1 – Find out the no. Of bars and their
dimensions in one meter span of slab in shorter direction.
Step 2 – Find out the grade of concrete.
Step 3 – Using the IS 456 page 90 formula,
calculate the area of steel present in tension and the thickness of slab and
thereafter find the moment of resistance of slab.
fck
= Grade of concrete.
fy
= Grade of steel.
B =
Width of beam.
d =
Effective depth of beam.
xu
= Depth of neutral axis (NA) from the top of beam section.
xu,lim
=limiting depth of neutral axis (NA) from top of beam section for balanced
section.
Ast
= Area of steel.
Compressive
force, C = 0.36fckBxu
Tensile
force, T = 0.87fyAst
Lever
arm, LA = d−0.42xuLA
Moment
of Resistance, MOR = C×LA = T×LAMOR
MOR
= 0.36fckBxu(d−0.42xu)
MOR
= 0.87fyAst(d−0.42xu)
The
above are general formulas for MOR.
For
under-reinforced section, xu
xu,lim
depends on fy & d only.
Step 4 – After knowing the Moment of
resistance you can find out the load on beam as you know the span of beam
because,
Moment
= force × perpendicular distance.
From
this you can calculate the strength of slab without breaking the slab.
