When any concrete structure gets deflected under load, there are two types of zones that develop due to load in the member. These are tensile & compressive zone.
Concrete material is good to resist compressive load in the compression zone but, concrete is very weak in nature to bear the tensile load. A reinforcement bar is provided at the tensile zone (tension fiber) and the same reinforcement is bent at 450 to the next tensile zone to take the tensile load.
In the case of the RCC slab, the concrete member’s deflection takes place under axial vertical load. Due to this, the tension zone is developed at the top part of the support and the bottom part of the center.
Hence, the reinforcement bar which is provided at the bottom part is bent up alternatively & taken to the top tensile zone of each support & those alternative bars are known as bent-up bars.
Why bent bars are provided in slab construction?
When the uniform lives load and dead load of the slab act in the downward direction, the forces act in the upward direction in the end support or continuous support to counterbalance the slab load.
Two types of bending moments are created in the structure. They are positive (+ve) or sagging bending moment and negative (-ve) or hogging bending moment. Bent-up bars are provided to resist these two types of moments.
The bottom reinforcement of the bent-up bar resists the positive or sagging bending moment and the upper bent-up bar resists the negative or hogging bending moment.
Why bent-up bars are provided in the slab?
The uses of bent up bars are as follows:
- It helps to resist bending moments.
- It helps to withstand shear force (i.e. shear force maximum at support)
- It reduces the whole weight of the reinforcement bars required in the slab.
- It helps in reducing the cost of the project.
- It prevents failure of the slab due to SF.
Why do we provide an alternate bent-up bar in the slab?
The drawing which is given below helps you to understand the bend- distance in the slab.
In the above diagram, crank bars are bent up at 45-degree angle with the bottom end at 0.15L1 from the center of the end support. Cranks bars bent up end are at 0.25L1 and 0.25L2 distance on either side from the centerline of the support at the continuous support.
Calculation of cutting length of bent up bar in slab
As a site engineer, you need to calculate the cutting length of the bar according to the dimensions of the slab and give instructions to the bar benders.
For a small construction area, you can hand over the bar detailing to the bar bender. They will take care of the length of cutting but that must not be accurate because they will not give importance to the bends & cranks.
They may give extra inches to the bar for the bend which is totally wrong. So, as a site engineer, it is always recommended to calculate the cutting length yourself.
Diameter of bar = 12 mm
Clear cover = 25 mm
Clear span(L) = 8000 mm
The thickness of slab = 200 mm
Development length (Ld) = 40d
Cutting length = clear span of slab + (2 x development length) + (2 x inclined length) – (450 bend x 4) – (900 bend x 2)
(Note:- there are four bends at 450 in the inner side (1, 2, 3, & 4) and two bends at 900 (a & b).
Cutting length = clear span of the slab + (2 x Ld) + ( 2 x 0.42D) – (1d x 4) – (2d x 2 (From BBS shape codes)
d = diameter of the bar
Ld = Development length of the bar
D = Height of the bending bar
All values are taken except ‘D’ in the above formula.
So, we need to calculate the value of ‘D’
D =thickness of slab – (2 * clear cover) – (diameter of the bar)
= 200 – 2 x 25 – 12
= 138 mm
Now, substituting all values in the formula,
Cutting length = Clear span of slab + (2 x Ld) + (2 x 0.42D) – (1d x 4) – (2d x 2)
= 8000 + (2 x 40 x 12) +(2 x 0.42 x 138) – (1 x 12 x 4) – (2 x 12 x 2)
Cutting length = 8980 mm or 8.98 m in length.