What Is Crank Bar? Reason and Specification of Crank bars.

 

What Is Crank Bar? 

The various shape of the crank bar is provided in the slab and other structural members. Bars are bent near the supports normally at an angle of 45°. The angle bent may also be 30° in shallow beams where the (effective depth < 1.5 breadth). 

In the image, we can see how the bent-up bars are provided in the slab. The slab is supported at two ends.

The maximum tensile stress that is positive moments (sagging) acting in the middle of the slab and the maximum compressive stress that is negative moments (hogging) acting at both ends of the support.

So bottom steel is required at the mid-span and top steel resists negative moments at the supports. A crank bar is provided to make RCC slab safe from compressive stresses.

When these bars are provided, the strength and deformation capacity of slabs with bent up bars compared to slabs without bent up bars is sufficiently increased.

Reasons: 

Crank rods are provided for the following reasons.

·         To save cost

·         For economical usage of steel

·         To avoid the risk of brittle failure

·         To resist the sagging and hogging by single steel reinforcement

·         To withstand the shear stress developed at the end support

Crank Bars Specification

·         Only alternate bars will be cranked.

·         Crank bars are provided at the slope of 1:10.

·         Crank bars will be provided for at least 300 mm length.

·         Crank bars will be provided at different angles such as 45° or 30° based on the beam depth.

 

•  Also Read : Unit Weight of Building Materials. It's Importance and calculation.
• Also Read : Unit weight of steel. Formula For Weight of steel.