WHY CRANK BARS PROVIDED IN SLAB

Today we are going to discuss the most important topic, why we provide crank bars in slabs which also called bent up bars. Generally in the fixed R.C.C ended Slabs, means column at the ends of the supports. Those slabs face Two types of moments develop which are called.

  1. The positive moment which called (Sagging) develop at mid of the span.
  2. The negative moment which called (Hogging) develop at the supports.

The continuous beam, overhanging beam, etc. all face two types of the bending moments positive and negative so to resist these moments is beams and in slabs, we provide bars and give technical names which we discuss in detail below.

BENT UP BARS:

1. Bent-up bars or known as Crank bars. Which we provide in the ends on the top of the slab or also in the top of the mid supports (if any type of mid-support is present) to counter the negative moment called (Hogging) which produce at that sections of the slabs and beams.

2. Different Angles on the crank bar (Means bends) is provided in the slabs. In some cases we just provide Straight Bars at the end on the top of the slab and in some cases, we provide angles on the crank bars which is 45°.  In some cases, the 30° bent is provided when shallow beams are present at the end of the slabs.

3. Normally the length of the crank bar are minimum 300 millimetres and the slope or bend on the crank bars (Bent up bars)  are 1: 10.

4. By providing crank bars in the R.C.C slabs, The Strength of the slab will be increased very much from those of the slabs in which crank bars are not present.

5. The spacing between the main bars which has a diameter 12 mm or 10 mm should be 6 inches centre to centre c/c and if you are using 8 mm diameter bars it should be 4-inch centre to centre c/c.

6. The spacing of distribution bars having a diameter 10 mm should be 9 inches to 12 inches centre to centre c/c. For 8 mm diameter bar 7 inches to 9 inches centre to centre c/c. The spacing also depends on the span and thickness of slab but normally we go with this.

CONCLUSION:

why crank bars are provided in slabs?

  1. To resist Hogging (Negative Bending Moment) at supports.
  2. To counter the risk of the failure of the slabs.
  3. To encounter the shear force. The shear force is always greater in the sections of supports.
  4. By providing crank with the strength of slab increased.

The crank bars or bent up bar are very important in R.C.C beam or slabs construction because without countering the Hogging (Negative moment) the structure will fail or the strength of the structure will reduce. The hogging bending moment will develop at the supports of the slabs and beams.




 

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