What shape of bending creates a positive moment?

A positive moment in bending is characterized by the interaction of forces that create tension in the bottom fibers of a structural member and compression in the top fibers. When a beam is subjected to bending moments, it can take on various shapes, the most relevant of which are concave up and concave down.

When a beam deflects in a concave up shape, the midsection of the beam is pushed downward while the ends are held up, resulting in tension at the bottom side and compression at the top side. This configuration aligns with the definition of a positive bending moment, as the stressed regions (tension and compression) develop as a result of the downward force causing this upward curvature.

In contrast, a concave down shape would indicate the opposite scenario, where the top fibers are in tension and the bottom fibers are in compression, which corresponds to a negative moment. Other shapes like flat or concave sideways do not effectively convey the interaction between tension and compression in the manner that correctly defines positive moments in a bending scenario. Therefore, the choice indicating concave up is associated with a positive moment due to the tension-compression relationship that this shape establishes within the material during bending.