What Does Crank Bar Mean In Construction?

What Does Crank Bar Mean In Construction?

A Crank Bar also referred to as a Bent Up bar is a reinforcement element placed on top of concrete structures to resist negative bending moments or hogging at the supports.

This type of bar is bent at an angle, typically 45° near the supports, but in shallow beams, the angle may be reduced to 30°.

The bent-up bars are seen in the slab which is supported at two ends, with the maximum tensile stress acting in the middle of the slab and the maximum compressive stress acting at both ends of the support. To address these compressive stresses a crank bar is added to increase the safety of the reinforced concrete slab.

This addition leads to an improvement in the strength and deformation capacity of the slab compared to those without bent-up bars.

Why Crank Bars Are Provided In Slab Construction?

Crank bars are an important aspect of slab construction as they provide the necessary support to counteract the downward forces acting on the slab. These forces are created by the dead load of the slab and uniform live load, causing upward forces in the end support or continuous support.

This creates two types of bending moments in the structure: +ve or sagging bending moment and -ve or hogging bending moment.

To resist these bending moments, crank bars are used. The bottom reinforcement part of the crank bar is designed to counter the +ve or sagging bending moment, while the upper bent-up part of the crank bar is designed to counteract the -ve or hogging bending moment.

Crank bars play a crucial role in ensuring the stability and longevity of slab structures.

Uses Of Crank Bars In Construction

The crank bars are used in construction for several reasons. Firstly, they help to resist bending moments. Secondly, they help to withstand shear force, which is maximum at the support. Thirdly, they reduce the overall weight of reinforcement bars required in the slab.

Fourthly, they help to reduce project costs. Finally, they prevent the failure of the slab due to shear force.