Crank (bent-up) bars in slab construction. /Why crank bars are provided in slab work? ~ PARAM VISIONS

Now, let us look into some of the FAQs related to crank bars or bent-up bars in slab construction.

1. Why crank bars are provided in slab construction?

When the dead load of the slab and uniform live load over it act in the downward direction, the forces in the end support or continuous support act in the upward direction to counterbalance the slab load.

This action creates two types of bending moments in the structure. They are +ve or sagging bending moment & -ve or hogging bending moment as shown in the diagram below.

Crank bars are provided to resist these both types of bending moments. For your reference, I have included the crank bar in the bending moment diagram (see below) to clearly understand this concept.

As you can observe in the above drawing, the bottom reinforcement part of the crank bar resists the +ve or sagging bending moment, and the upper bent-up part of the crank bar tackles the -ve or hogging bending moment.

2. What are the uses of crank bars in construction?

The crank bar

1. Helps in resisting bending moments.

2. Helps to withstand shear force i.e. maximum at the support.

3. Reduces the overall weight of reinforcement bars required in the slab.

4. Helps in reducing project costs.

5. Prevents the failure of the slab due to shear force.

3. Why do we provide alternative crank bars in slab reinforcement?

It is recommended that a maximum of 50% of the bars can be cranked to resist the bending moments so that steel is present in every cross-section of a slab. The alternative bent-up bars are sufficient to withstand the resulting shear force near the support.

From an economic point of view, we provide alternate crank bars so that these bars are kept within a specified limit.

4. At what distance bent-up bars are cranked in a slab?

I have downloaded the drawing below that helps you to clearly understand the bent-up distance in a slab.

As you can observe in the above diagram, crank bars are bent up at a 45° angle with the bottom end at 0.15L1 from the center of the end support. At the continuous support, crank bars bent-up end are at 0.25L1 & 0.25L2 distance on either side from the centerline of the support.

Continued 👉 Calculating the cutting length of the crank bar