One Way Slab
One way slab is provided when the L/B ratio is equal to or greater than two. It is supported only on two opposite sides by rigid supports and it carries the load by flexure in the perpendicular direction to the support.
Uniformly loaded one-way slab deform into the cylindrical surface and hence bending moment develop only in one direction.
In one-way slab, Main bars are provided in the shorter span with the crank to counteract positive bending moment and distribution bars are provided in the longer span for distributing the load and to prevent shrinkage cracks.
For calculating the the quantity of steel lets take an example for better understanding.
We have a one way slab drawing with following data
Length = 6 Meter
Width = 2 Meter
Main Bar = 12 diameter, Spacing – 100 mm c/c
Distribution Bar = 10 mm, Spacing – 125 mm c/c
Slab Thickness = 150 mm
Clear Cover = 25 mm
Development Length Ld = 40d
Step #1- Calculation Of Numbers of Bars
Firstly we will calculate numbers of Main bar and Distribution bar required.
Numbers of Bars = Total length/ spacing of bar + 1
Numbers of main bar = 6000/100 +1
= 61 numbers
Numbers of Distribution bar = 2000/125 +1
= 17 Numbers
Step#2 – Calulation of Cutting Length
Now we will calculate cutting length of main bar and distribution bar.
For Main Bar
Cutting Length = Length + 2 x Ld + (1 x 0.42D) – 2 x 1d
L = Clear Span of Slab
Ld = Development Length (Take = 40d where d is diameter of bar)
0.42D = Crank or Inclined length of bar
1d = For 45 bend
Calculation of inclined Length D
D = Slab Thickness – Clear cover (top and bottom) – Diameter of Bar
D = 150 – 2 x 25 – 12
D = 88 mm
Cutting Length of Main Bar = 2000 + (2 x 40 x 12) + (0.42 x 88) – 1 x 12
= 2984.96 mm say 2985 mm or 2.985 meter
Cutting length of distribution Bar = Clear Span (Longer Span) + 2 Ld
= 6000 + ( 2 x 40 x 10 )
= 6800 mm or 6.8 meter
Step#3 – Calculation of Steel Weight
Now we have to calculate the weight of steel.
Weight of Main Bar = Numbers of bar x Cutting Length x Unit Weight of Bar (12 mm = 0.888)
= 61 x 2.985 x 0.888
= 161.69 kg
Weight of Distribution Bar = Numbers of bar x Cutting Length x Unit Weight of Bar (10 mm = 0.0.617)
= 17 x 6.8 x 0.62
= 71.672 kg
BBS of One Way Slab
Main Bars = 161.69 Kg
Distribution Bar = 71.627 Kg
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