The need of bent up bars in construction purposes
Bent up bars are provided to counter the Shear force.
We are just taking a general example of the simply supported beam with uniformly distributed loading. Now see the shear force and bending moment variation along with the length of the bar.
As you seeing that towards the ends of the beam shear is increasing and bending moment is decreasing.
We also know that longitudinal reinforcement is provided to counter the bending moment stress or flexural stress. As towards the ends, BM is decreasing, so there is no need of the same quantity of longitudinal R/f as of middle portion of the beam. But there is somehow required more quantity of shear R/f as shear is increasing towards the ends.
Now if we bent some bars at some specified location we can economically provide an extra safeguard to shear force without compromise the flexural strength of the beam.
Bent up bars are always provided along with shear R/f.
Note1: in case of continuous beam negative BM or hogging BM is generated at the ends of the beam if we bent the bars at the ends, those bent up bars will counter the negative BM at the ends.
Note2: Capacity of Bent up bars to counter Shear is not taken more than the half of the shear force taken by the shear R/f. That is, the maximum capacity of bent up bars is = 1/2*( shear force taken by shear R/f).
This is the need or use of providing bent up bars in beams or construction purpose.