In many cases, reinforcing bars must be bent into different shapes. This is done for several reasons. Let us return to the reason for reinforcing concrete with steel-it is to increase the tensile and compressive strength.
As an example, you might compare the hidden action within abeam from live and dead loads to breaking a stick over your knees.
When you apply force, the splinters next to your knee push toward the middle of the stick, while the splinters from the middle to the opposite side pull away from it. A similar phenomenon occurs inside the beam.
Consider a simple beam (one that rests freely on two supports near its ends). Whenever a beam bends or sags, it is due to the dead load (weight). As the forces increase from the center to the bottom, the bottom portion of the beam tends to stretch or lengthen.
The steel reinforcing bars are required for this part because it is in tension. The tensile strength in the beam is enhanced by the combination of concrete and steel. This prevents the beam from breaking because of the force of the load.
There is no stress in the exact center of the beam, between the compressive and tensile stresses.