When estimating rebar for a rcc member you have to understand few terms clearly regarding reinforcement.
4 Terms to understand to truly estimate rebar quantity
Below are 4 terms you should understand clearly to truly estimate the rebar quantity –
- True length of bar
- Cutting length of bar
- Bending length of bar
- Bend deduction length
1. True Length of Bar
For estimating rebar quantity we just calculate the length of some leaner shapes of bar.
For example, take a top longitudinal bar of a beam. The shape of the bar will look like the image below.
For estimating purpose we just get the length of the bar and convert it to the weight of bar.
Let’s calculate the bar in the image above. The length of that bar is,
Suppose, A is = 20′, B is = 1′ and C is = 1′
So the length of the bar is,
= 20′ + 1′ + 1′
This is the true length of bar.
2. Cutting Length of Bar
As we know steel is ductile and subjected to elongation. When it bends it is increased in length.
So if you we cut the bar as its true length and bend it we won’t get the desired shape after bend. To get the desired shape, the bar should be cut lesser than the true length.
The required length to get the desired shape of the bar is calledcutting length of bar.
3. Bending Length of Bar
Now the question is how much a reinforcing bar is increased in length when it is bent?
It depend on the size of bar and angle of the bend.
The increased length for specific diameter of bar for specific angle is called bending length of the bar.
4. Bend Deduction Length
It is confusing. Because bend deduction length conflicts with the bending length.
Actually both the terms are same. Then why the engineering world using both terms to confuse the fresher? This question is mine.
The purpose for writing this post is to eliminate the confusion between these two terms.
Don’t be confused. Both terms are same. When we deduct the bending length from the true length of a bar then it is calledbend deduction length.
Mostly used bend angle in civil construction world are 45° and 90° bend.
For 45° bend, Bend deduction length is equal to dia of bar.
For 90° bend, bend deduction length is equal to two times of bar dia.
Bend deduction length is actually used for calculating the cutting length of bar in construction.
Let’s calculate the cutting length of the bar shown in the image above.
Cutting length = true length of bar – (2 x dia of bar) x number of bend.
= 22′ – 2 x ¾” x 2 (20 mm = ¾”)
For estimating purpose only, you don’t have to deduct the bend deduction length form the true length of bar. It is required when you are cutting bar or making bar bending schedule for placing reinforcement in the rcc member.
How do you calculate bend deduction length for bar? Please share in comment below.