CRANK BARS IN SLAB

WHY CRANK BARS PROVIDED IN SLAB

Now  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 faces Two types of moments develop which are called.

1.     Positive moment which called (Sagging) develop at mid of the span.

2.     Negative moment which called (Hogging) develop at the supports.

For example:

Place a paper of some thickness on four supports at the four corners. Now with your finger gently press the centre point of the paper. What did you see?

You can see the centre point getting pushed down as well as the four corners getting raised by some extent.

Now this is exactly what happens in slab.

When load acts the middle portion gets sagged down whereas at the edges( portion near beam supports) a hogging moment is developed. You might be knowing that when a member sags, The bottom fibres experiences tension whereas the top fibre experiences compression. This is completely reversed in hogging. The top fibre experiences tension. So in order to take care of this tension we need reinforcements on the top side of slab near the beam supports.

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 provide 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 millimetre 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 having 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 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 is also depends on the span and thickness of slab but normally we go with this.

CONCLUSION:

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 at the sections of supports.

4.     By providing crank 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 reduced. The hogging bending moment will develop at the supports of the slabs and beams.