Table of Contents
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 slabs deform into the cylindrical surface and hence bending moment develop only in one direction.
In a 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 quantity of steel let’s take an example for better understanding.
Data Given
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
Extra To Bar = 12 mm, Spacing – 250 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 the numbers of the 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 – Calculation of Cutting Length
Now we will calculate the cutting length of the main bar and distribution bar.
For Main Bar
Cutting Length = Length + 2 x Ld + (1 x 0.42D) – 2 x 1d
Where
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) – 2 x 12
= 2972.96 mm say 2972 mm or 2.972 meter
Distribution Bar
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 – Top Distribution
Numbers of Top distribution bar for longer span = {(L/5)/spacing + 1} × 2 sides
= {(2000/4)/125 + 1} × 2
= 10 Numbers
Cutting Length of Top Distribution Bar = Clear Span + 2 Ld
= 6000 + (2 × 40 x 10)
= 6800 mm or 6.80 meter
Step 4 – Extra Top Bar
Numbers of Extra Top Bars for shorter span= {L -2(L/4)/spacing} × 2 Sides
= {6000 -2 (6000/4)}/250 × 2
= 24 Numbers
Cutting Length of Top Extra Bar = L/4 + Ld
= 2000/4 + 40 × 12
= 980 mm or 0.98 meter
Step 5 – 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 2.972 x 0.888
= 160.98 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
Top Distribution Bar Weight = 10 × 6.8 × 0.617 = 41.95 Kg
Extra Top Bar Weight = 24 × 0.98 × 0.888 = 20.88 Kg
BBS of One Way Slab
The bar bending schedule of one way slab is as follows.
S.No. | Bar Description | Bar Shape | Nos | Length of Bar (m) | Total Length of Bar (M) | Bar Dia (mm) | Unit Weight (kg) | Total Weight (Kg) |
---|---|---|---|---|---|---|---|---|
1 | Main Bar | 61 | 2.972 | 181.292 | 12 | 0.888 | 160.98 | |
2 | Distribution Bar | 17 | 6.8 | 115.6 | 10 | 0.617 | 71.32 | |
3 | Top Distribution Bar | |||||||
Longer span | 10 | 6.8 | 68 | 10 | 0.617 | 41.95 | ||
4. | Extra Top Bar | |||||||
Shorter span | 24 | 0.98 | 23.52 | 12 | 0.888 | 20.88 |
Conclusion
10 mm diameter Steel = 113.27 Kg
12 mm diameter Steel = 181.86 Kg
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