Bar Bending Schedule Basics | BBS OF STEEL REINFORCEMENT | Quantity surveying | Civil tutor

In this lecture, I have discussed the bar bending schedule basics in which I have discussed the bbs of steel reinforcement, the various sizes of reinforcement bars used.

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I have discussed the reasons of providing hooks in structural members and how to calculate the hook length and cutting length of bend.
I have discussed briefly the provision of overlapping reinforcement bars in structural members. This lecture will also include the provision of crank bars.

I have also discussed concrete cover, spacing of reinforcement bars, calculating the number of bars, and the number of stirrups.

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Diameters:
Reinforcement bars are available in various sizes in which we have 6mm,8mm,10mm,12mm,16mm,20mm,25mm,28mm, 32mm & 40mm dia bars.
In addition to that we can have customized diameters which can be ordered directly from the industry as per our need.

The weight of a steel bar is calculated by using the formula D^2/162 Kg/m in which D is the dia of bar.

Hooks:
Hooks are provided in structural members as per the requirement. The function of hooks in stirrups is to prevent the seismic movements and to prevent the concrete from splitting outward. These also prevent the slippage of steel from the concrete. They keep the longitudinal bars in position and hold the steel tightly. The main function of a hook is to develop anchorage.
Now the question arises why is there a requirement for anchorage?
The tension reinforcement should not be slipped from it's position, otherwise the purpose of providing the reinforcement will not be served.
So to develop the sufficient bond strength between steel and concrete development length requirements must be satisfied. Now there may be cases where there is unavailability of required length to meet the requirements of development length. In those cases, anchorages may be provided in the form of hooks.
The length of hook = 9d, while d is the diameter of the bar

Bends:
These shapes are usually used in footings.
The length of this bend is taken from 10d to 16d in which d is the diameter of bar.

Overlapping:
As we know, the standard length of reinforcement bars is 12m. In case we need to increase the span of a structural member like beam or column beyond the standard length of reinforcement bar, which means we also need to increase the length of steel bars by adding more reinforcement bars.
If we add the straight bars directly from end to end, then there may be chances of slippage of steel which is prevented by overlapping the bars.
For compression members (columns) the lap length should be taken as 40d and for tension members (beams) overlapping should be taken as 50d
But from the practical point of view, we can take 50d for both columns and beams.

Crank Bars:
Cranked bars are those bars that can resist negative bending moments or simply hogging moments.
Bent up bars are provided in slabs as well as in beams since these members are subjected to tensile stresses.

Concrete Cover:
Cover is the space between reinforcement and shuttering or it is the minimum distance between the surface of embedded reinforcement and the outer surface of concrete.
The purpose of providing the concrete cover is to prevent the reinforcement bars from getting corroded by environmental exposure.
The minimum cover for footings is taken as 50mm. For Columns & Beams the minimum cover is taken from 25mm to 50mm and for slabs the minimum cover is taken as 25mm.

For providing the clear cover, covering blocks are used to create a space between the reinforcement and the shuttering surface.
These blocks are available in different sizes as per the need.

Spacing:
The spacing is provided to have a properly anchored reinforcement.
The minimum spacing between two bars should be at least equal to the maximum size of aggregate with a margin of 5mm.

Number of bars = (opposite length/spacing) + 1

Number of stirrups = (Actual length of column or beam/spacing) +1

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