Bending backwards for bent up bars ?

Overview :

A particular shape of reinforcement called “Bent up bar” is provided in the flexural members such as beam & slab.This article brings the practical aspect of these bars to you in a very simple language.

Purpose:

If I tell you that bent up bars are provided to take care for the diagonal shear would it suffice ?

Probably no.because then  you would like to first know what is diagonal shear .

So, instead of going further ,let us say that in a beam or slab , there are two types of internal stresses ;

1.Bending moment : Main bars in a beam or slab cater to this stress.

2.Shear stress : this can be further re-grouped into:

2.1 Vertical shear stress: Stirrups are provided to provide resistance against this stress.

2.2 Diagonal shear or Diagonal tension : Bent up bars develop resistance to counter this stress.

In short , if you see a beam you would observe micro cracks near supports .These are due to diagonal tension.

BENT UP BARS IN A BEAM

Bent up bars for beams:

The bent up bars run at bottom in the middle half span of the beam.The bending up normally is done at a distance of 1/4 of the span from the supports.

Difficulties:
Usually the reinforcement cage of the beam is tied at the location of the beam only. There  is restricted space in between the two beam sides and hence inserting the bent up bars in between the top & bottom bars is cumbersome for the fitters.

Can it be avoided ?

The designers have developed alternate method for bent up bars in a beam as follows ;

1. Provide curtail bars for middle half ( L/2 ).

2. Provide EOS bars ( Extra over support ) on the Top side for a distance of L/4 from the supports.

3. Reduce the stirrup spacing to half in the L/4 span near supports on either sides.

 Bent up bars for slabs:

Difficulties:
Bent up bars in the slab are very susceptible to the movements of men & machinery on the slab before & during casting .Comparatively, the bent up bars in the beam are quite sturdy after getting positioned.
1.If the concreting is done with pump then the laying of the pipe across the slab results in getting the bent up bars disturbed  due to continued jerks in the pipeline.
2.If the concrete is lifted to the slab by hoist /lift and carries to specific spots by trolley or carts,then the tire of the carts jump again & again on the elevated bent up bars and eventually bends them down.

 Can it be avoided ?

Yes.It is possible to protect the bent up bars getting disturbed by providing ample number of strong chairs below them
It must be made compulsory to have a regular path made for the empty carts.
A pathway can also be made for the personnel walking on the slab.
For the concrete pipe stands are available on which the pipes are mounted and thus bent up bars are protected.

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