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Introduction to Bar Bending Schedule [BBS]:-
“BBS” The word BBS Plays a significant role in any construction of High rise buildings. BBS refers to Bar Bending Schedule. Well, Whats the use of BBS? Why we use BBS?
BBS is termed as Calculation of Steel required for the construction of a building. We use steel to make concrete to be reinforced and for tension requirements. But how much steel required for constructing 15 floors building?? How much Steel I have to order? All these questions are answered in BBS.
Well, from 1950 to 2017 lots of modifications and enhancements happening in our world. At that time three storeyed buildings are high rise buildings now we are constructing a building with 200 floors+. There is a massive growth in construction. More population, more need of facilities, more need of space, more construction.
How BBS Changed from 1950-2000:-
Father of Estimation B.N. Datta has given certain recommendations for the usage of steel (bar bending schedule) in different components of buildings. But, he didn’t mention any values if we use more bars in a single element.
| Bar Bending Member | Percentage |
|---|---|
| Slab | 1% of total volume of concrete |
| Beam | 2% of of total volume of concrete |
| Column | 5% of total volume of concrete |
| Footings | 0.8% of total volume of concrete |
At that time we’ve used only four bars in columns; now we are using 12+ bars in columns based on load analysis. So, the percentage of steel is increased in a column which reveals that the above-cited values are outdated. ( I am saying outdated “not wrong”) He wrote that book in 1950. Now we are in 2017. He gave recommendations according to the potentiality of construction at that time. Now we are constructing 200 + floors in the small area.
Hope now you are clear that BN Datta didn’t give any recommendations of Bar Bending schedule for high rise buildings.
Well, one article isn’t enough to calculate the amount of steel required for the whole building. In future, I will come up with different posts for calculation of various components of the building. Here I am showing how to calculate the steel required for various components.
Bar Bending Schedule [BBS] :-
1. Hook Length:-
The hook length is commonly provided for stirrups.
Hook Length = 9D [D is Diameter of the Bar]
Total Length of stirrup = Total length of Bar + 2 x Hook Length (Two hooks)
Total Length = L+2(9D)
Therefore Total length = L+18D (D is Diameter of bar)
2. Bend Length:-
Bend Length is usually provided in columns at ends to tie the columns to the footings.
Bend Length = 16D [D is Diameter of the Bar]
3. Overlap Length / Lap Length in Reinforcement (Bar Bending Schedule) :-
Suppose the length of building is 20 m. The standard length of Rebar is 12m. We have to join two Re-bars for the total length of building. In order to Join the two bars (overlap). Overlap length is adopted.
Overlap Length for compression members = 40D
Overlap Length for tension members = 50D
[D is the Diameter of the bar]
Have You seen the below picture on your top floor of the building? We generally leave some length of Bar in last floor i.e., 50D. It is used for further construction purpose. (Constructing a new floor)
4. Crank Length :-
Crank length is provided in Slab Shear stress is maximum at supports in Slab. So, to resist this stress we usually crank the bars at the ends of supports in slab.
Crank bar is bent up at an angle of 45degree with the length of 0.42D
D = Depth of Slab-Top Cover-Bottom cover
5. Extra Bar:-
Now a days Extra bar is also provided under every crank bar. To keep crank bar in position. (Refer above image for clear idea)
The length of Extra bar is L/4.
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