When a beam is loaded uniformly, what kind of moment acts around its mid-point?

When a beam is loaded uniformly, the nature of the moment about its mid-point is referred to as a positive moment. This positive moment occurs due to the bending action caused by the uniform load, which tends to create a sagging effect in the beam. As the load is distributed evenly across the length of the beam, the mid-point experiences the greatest amount of deflection, resulting in a moment that causes the beam to bend downwards.

The positive moment is characterized by the curvature that forms at the mid-span of the beam, causing the top fibers of the beam to be in compression while the bottom fibers are in tension. This behavior is typical for simply supported beams under uniform loading, where the maximum moment occurs at the midpoint.

In contrast, a zero moment would imply that there is no bending effect at that point, which is not the case for a uniformly loaded beam. The terms negative moment and constant moment refer to different loading conditions or structural scenarios, where negative moments usually occur at supports and constant moments are indicative of beams or sections under uniform or symmetrical loading. However, in the case of a beam subjected to a uniform load, the identifiable effect is indeed a positive moment at the midpoint.