WHY CRANK BARS ARE PROVIDED IN SLAB?

Beam generally shows maximum tensile stress at the mid span of beam. This tensile stress is known as sagging bending moment. And at the end supports, It shows maximum negative compressive stress which is known as hogging moment. So the reason behind the providing crank bars is to prevent additional use of bars. Crank bar is substitute for providing extra bars to resist negative compressive stress (Hogging moment) at both end supports. Hence the beam with the crank bars give maximum cross section of bar at top at end supports and at bottom at mid span for resist sagging bending moment and hogging moment. So main function of crank bar is to resist negative compressive stress without addition of extra bars at top at end supports.


In crank bars or bent up bars, the portion of bending the bars are near to the end support with specific distance and this bend is usually at 45 degree. It increases strength and deformation capacity of slab compare to slab without crank bars or bent up bars.

Benefits of provided crank bars in slab:

  • It resist negative compressive stress (Hogging moment) at end supports.
  • It resist shear force which is acting greater at supports.
  • It increases the strength of slab with compare to slab without crank bars.
  • It combats with both, the positive bending moment (sagging) at bottom of mid span and negative compressive stress at top of end supports in use of crank bar. 
  • It reduces the failure of slab from the support.
  • It reduces the usage of steel.

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