History Of Bar Bending Schedule:
Long years back, when there was no today’s bar bending schedule, certain recommendations given by Prof. BN Dutta were used for estimating steel for different components of a building.
But these are now backdated (not wrong), and we don’t use them today. Because No accurate values were specified if we use more bars in a single member. Nowadays we are constructing 150+ floors building. It is now possible with our modern techniques, equipment, etc. The estimation of steel becomes easier because of BBS.
Civil engineers who are working on site or freshers who are going to work at construction sites must know how to prepare bar bending schedules. In this article, I will discuss some important things used in preparing bar bending schedule.
These are just basic things. In the future, I will come up with some other articles related to BBS such as BBS for beams, columns, slabs, footings etc.
What Is Bar Bending Schedule?
Bar bending schedule commonly known as BBS is one of the most important terms in Civil Engineering. Because it plays a vital role in building construction.
Like other building materials estimation of steel is also required for constructing a building and here BBS comes with an easy solution. Bar bending schedule provides the reinforcement calculation and some other important details such as bar mark, bar diameter, bar shape, cutting length, number of bars, the weight of bar, total weight of steel, etc, so that we can order the required amount of steel in advance.
The hook is the extra length left at the 4th corner of a stirrup so that the stirrup retains its shape.
Generally, hook length is taken as 9d for one side.
Where d = Diameter of the bar.
The total length of stirrups = Total length of the bar + 2 x hook length (for two hooks)
= L + 2 x 9d
= L + 18d.
Where L = length of the bar for stirrup.
The bar is bent at the column end to tie with the footings. This extra length for bend is called bend length.
Bend length is generally considered as 16 d
Bend Length = 16d
Development length is a vital point in bar bending schedule. Development length is the length of the bar required for transferring the stress into the concrete.
In simple words, the quantity of the rebar length that is actually required to be embedded into the concrete to create the desired bond strength between steel and concrete and furthermore to produce required stress for the steel in that area.
The formula for development is given below:
Development length (Ld) = d x σs/τbd
d = Diameter of the bar.
σs = Stress in the bar at the section considered as the design load
τbd = Design bond stress.
Lap length is an important term in BBS. Lap length is the overlapping length of two bars side by side which gives required design length. In RCC structure if the length of a bar is not sufficiently available to make design length, lapping is done.
Suppose we need to build a 20 m tall building. But is there any 20 m bar available in the market? No, the maximum length of rebar is usually 12 m, so we need to join two bars to get 20 m bar.
Lap length for tension members = 40d
Lap length for compression members = 50d.
d = Diameter of bars.
Look at the image below. You may be seen this on the terrace (top floor) of buildings. This extra rebar is left for future construction purpose. I hope you understand this.
Generally, bars are bent near the support at an angle of 45°. The angle of bend may also be 30° in shallow beams. The purpose of bend near the support is firstly to resist the negative bending moment which occurs in the region of the support and secondly to resist the shear force which is greater at the support.
Crank bars are mostly provided in slabs.
Crank length = D/sin45° – D/tan45° =1.42D – D = 0.42D
So Crank Length = 0.42D
Where D = Clear height of the bar = Thickness of slab – (Top cover + Bottom cover) – Diameter of the bar
Unit Weight Of Steel:
To prepare BBS at site you need to calculate the weight of steel first.
The weight of bars is calculated by the following formula
W = d²L/162
Where W = Weight of bars.
L = Length of bars in meter.
d = Diameter of the bar.
Example: Calculate the weight of 20 meters long 16 mm ø bar
W = 16² x 20/162 = 32 kg.
So basically bar bending schedule is a very important thing in civil engineering. If you are a civil engineer you must need to prepare it at construction site.
I hope you have now enough knowledge about it. There are more topics related to bbs which I will cover in other posts. If you have any questions please feel free to as in the comments. Thank you.
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