Main reinforcement is provided above the neutral axis in a cantilever beam carrying gravity load. This is done to ensure that the beam is effectively reinforced against the tensile forces that are induced in the bottom portion of the beam due to the applied load.

Below are the reasons why main reinforcement is provided above the neutral axis:

1. Tensile forces: When a cantilever beam is subjected to gravity load, it experiences tensile forces in the bottom portion of the beam. These tensile forces can cause the beam to fail if not adequately reinforced. By providing main reinforcement above the neutral axis, the tensile forces can be effectively resisted and the beam can withstand the applied load.

2. Neutral axis: The neutral axis of a beam is the line along which the beam experiences neither compression nor tension. In a cantilever beam carrying gravity load, the neutral axis is located above the centroid of the beam section. By providing main reinforcement above the neutral axis, the tensile forces acting on the bottom portion of the beam can be effectively resisted.

3. Bending moment: In a cantilever beam, the main reinforcement is primarily designed to resist the bending moment induced by the applied load. By placing the main reinforcement above the neutral axis, it can effectively counteract the tensile stresses that are generated in the bottom portion of the beam due to bending.

4. Compression zone: The top portion of the beam, which is located above the neutral axis, experiences compressive forces. These compressive forces are resisted by the concrete itself, which is a good material for bearing compressive loads. Therefore, there is no need to provide additional reinforcement in this region.

In conclusion, main reinforcement is provided above the neutral axis in a cantilever beam carrying gravity load to effectively resist the tensile forces induced in the bottom portion of the beam. This ensures the structural integrity and stability of the beam under the applied load.