The main reason for providing number of reinforcing bars at a support in a simply supported beam is to resist in that zone

In a simply supported beam, the primary purpose of providing reinforcement bars at the support is to resist tensile stress.

At the supports, the top fibers of the beam experience compression, while the bottom fibers experience tension.

To resist this tension, reinforcing bars (tensile reinforcement) are provided at the bottom near the supports.

This reinforcement ensures that the beam can safely handle the tensile forces, preventing cracking or failure in that zone.