What is the formula to find crank length in steel bars for RCC slab?

Hi Welcome to "thecivilengineer18.com",

 So, In the Bar Bending schedule, providing the enough required amount of crank length is very essential to make the slab stable. First let’s dive in some basics before finding the crank length of the bars in slabs.

What is Crank Length?

The Crank length is the length of the bar in the from the face of the Vertical structural support member which is the column up to the point where the rods starts to bend upwards in the slab.

Why and What is the need for Crank Length in Slab?

To tell you short in technical terms, due to bending of the slab, tension will be more in the top of the slab near the supports to overcome it we provide crack length and reinforcement on top layer of slab.

If you are from other background, let me explain you with example, take a rectangular book, try bend it by placing the book over water bottle on 4 side below. Now you can see the middle part goes down( Tension in bottom and compression at the top) and the longer ends of the book moves( which is the opposite of the middle, so tension on top and compression on bottom), Hence the Crank length to provided to over come the tension on top of the slab near supports.

How to find the Crank Length?

Engineering goes with math which we can’t avoid, so lets have some math formula L=2d+2t, Where d is the effective depth(Distance between Compassion i.e. top end of the slab to the center of the tension reinforcement in the bottom) of the slab and t is the thickness of the slab.

And since you have read this much let me give you a tip on how much the bars additional length of bars that need to be provided while calculating the crank length. Since the bars were cranked from bottom to top, that needs to be done without deformation so while calculating the cutting length of the rods for the slab reinforcement add 0.42D for the bending of bars.

Lets say you slab’s main reinforcement is 10mm diameter , the length for the bend of the bar will be 4.2mm or 2 inches.

Real time calculation,

Lets say your slab is 10 feet(L) X 10 feet(B) X 0.5 feet (D) , and the reinforcement details will be 10mm diameter and 6 inch (Centre to Centre) . Then, the cutting length of the one bar will be, 10feet 2 inches without deducting cover. In this 10 feet 2 inches, 10 feet for slab and 2 inches for crank bend. Among the 10 feet , lets find the crank length using L= 2d+2t = 2*122mm + 2*150mm = 544mm which is 1 feet and 9.75 inches is the crank length of the 10 feet 2 inches main reinforcement cutting length.

Hope this answer helps you, Like and Share to those who need it. Happy Learning!