Bending of a beam can occur in two possible ways: concave upwards or concave downwards. The bending moment developed in the beam depends on the direction of the bending. In this case, since the bending is concave upwards, the bending moment developed is called a positive moment.

Here's an explanation of why a positive moment is associated with concave upwards bending:

1. Definition of Bending Moment:
- Bending moment is the algebraic sum of the moments about the neutral axis of a beam section.
- It represents the tendency of a beam to bend or rotate under the applied loads.

2. Concave Upwards Bending:
- When a beam bends concave upwards, it means that the beam is sagging in the middle and the deflection at the midpoint of the beam is downward.
- In other words, the curvature of the beam is such that the center of the beam is lower than the ends.
- This type of bending is typically seen when there is a positive moment at the section.

3. Positive Bending Moment:
- A positive bending moment is associated with tensile stresses at the bottom of the beam and compressive stresses at the top.
- It creates a sagging effect in the beam, causing it to bend concave upwards.
- The positive bending moment causes the beam to resist the applied loads and maintain its structural integrity.

4. Sign Convention:
- In structural analysis and mechanics, a commonly adopted sign convention is that clockwise moments are positive and counterclockwise moments are negative.
- Since the concave upwards bending creates a sagging effect, it can be considered as a clockwise rotation at the section.
- Therefore, the bending moment developed in a beam with concave upwards bending is considered positive.

In summary, when the bending of a beam is concave upwards, it indicates a positive bending moment. This positive moment is associated with tensile stresses at the bottom and compressive stresses at the top of the beam, allowing it to resist the applied loads and maintain its structural integrity.