Importance of providing crank bars in slabs and beams

In this video tutorial, brief explanation is given on the importance of setting up crank bars alias bent up bars in slabs and beams. Besides, learn types of bending moments occurred in slabs and beams.

In a slab there are two types of bending moment i.e. positive bending moment alias sagging and negative bending moment alias hogging. Due to negative bending moment, slabs bend and cracking occur. In order to resist the possibility for collapsing of building, crank bars are provided.

Cranks bars are mainly utilized for saving of materials. Usually, for fixed ended Slabs positive (sagging) moments exist at the mid span and negative (hogging) moments at the supports.

Therefore, bottom steel is necessary at the mid span and top steel withstands negative moments at the supports. Crank bars are arranged to get the steel on the top at supports because bottom steel is not generally essential at the supports (only nominal steel is adequate at the supports).

In brief, crank bars are mainly provided for the following reasons :-

To withstand negative bending moment
To withstand the shear force that is larger at supports
To lessen the risk of brittle failure of slab-column connection

The area with 0 shear force, maximum positive bending moments occur whereas the area with maximum shear force, maximum negative bending moments occur.

The angle of the crank bars should vary among 30 degree to 45 degree if shallow beams are used there.

Spacing in crank bars depend on thickness and span of the slabs. Usually, if main bars with thickness 10 or 12 mm are used in slabs, then there should be 6 inch center to center spacing.

If distribution bars with thickness 10 mm are used in slabs, there should be 9 to 12 mm center to center spacing.

If distribution bars with thickness 8 mm are used in slabs, there should be 7 to 9 mm center to center spacing.

When bent up bars are arranged, the strength and deformation strength of slabs with bent up bars compared to slabs exclusive of bent up bars is adequately raised.

To get more information on crank bars, watch the following video tutorial.

Video Source: F&U-FORYOU