Before we see how to prepare a bar bending schedule, let us first try to understand what it is. A BBS is a table that lists all the information about different steel reinforcement bars being used.
By the end of this article I will give you a free tool for preparing bar bending schedule
How to calculate the weight of a rebar
Imaging a pencil weighs 1kg( I know it is insane but stay with me). Then 10 pencil will weigh 10kg, 100 pencils will weigh 100kg. Similarly, we can find the weight of rebar, we first calculate wt of 1m of a bar and then we simply multiply that weight with the no of meters of a bar we have.
Then the question is, how do we find weight of 1m of any rebar. The answer is simpler than you might think, it is simply d*d/162 where d is the diameter of the bar
So 8mm bar will weigh 8*8/162=0.395Kg/m
12mm bar will weigh 12*12/162=0.89Kg/m
And so on.
We call this the weight of 1 Unit of bar or simply the Unit Weight of Bar.
So a 10m of 12mm rebar will weigh 10 * 0.89=8.9kg
Or simply Length*UnitWeight
Bent Reinforcements
Now we are rarely going to use a straight bar. Most of the time we will have to bend the bar as per the designs received. This doesn’t change a thing, just add the length of all the bent sections of the bar and multiply it with the Unit Weight of the bar
Calculation of Number of bars
And we are rarely going to use a single bar
Normally you will be given a drawing with length width height of the structure and spacing to be used for a bar, you will have to calculate the no of bars
Number of bars = (Length -2*cover)/spacing +1
Where
Length is the length/width of concrete structure
The cover is the concrete cover
Spacing is the space between individual bars
Why +1?
To answer this let see a simple case where length is 5m and spacing is 1m( forget about cover for now )
In this case, No of bars is 5m/1m=5
Now let us provide five bars
As you can see there is still space left for one more and this is exactly why we add 1 extra bar
But wait we are not done, we still have one last thing to discuss.
Deduction in BBS
Depending on who you ask, this will either be the best thing or this is going to be the greatest sin you’ve ever committed.
Let’s first see what this is and how to apply it( Only If you have to )
When you bend rebar it stretches.
When you stretch a 100g loaf of bread, it still weighs 100g, stretching it will increase its length but not weight.
So instead of directly multiplying the Unit weight with the length, we must first decrease the length by the amount it was stretched.
People who support this say that a bar stretches “d units” for every 45-degree bend where d is the diameter of the rebar
This means
D for 45
2D for 90
3D for 135
And
4D for 180
So we count the number and angle of bends and subtract the length of the stretched bar with these values to get the actual length before multiplying it with the Unit Weight.
In this case Total Length is 2+1+1 = 3m
We have 2 90-degree angles so deduction is = 2D+2D=4D
=48mm ( D is 12mm )
So the actual length is TotalLength- deduction=3-0.012=2.988m
Weight is ActualLength*UW*No=2.988*0.89*6=15.95Kg
Q: So, What should you follow?
A: Whatever the convention is on your site. Don’t get fired for listening to a guy on the internet.
If you want software for Bar Bending we have an excellent app for you. BBS- rebar weight calculator
Just draw the shape of the bar on the app and enter some basic details and that’s it.
The app will prepare the bbs for you.
BBS App Android Google Play Link: Click here to download
iPhone AppStore Line: Click here to download
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