Which term describes a bending moment that produces compression at the top fiber and tension at the bottom fiber in a beam?

• A. Positive Moment
• B. Negative Moment
• C. Sagging Moment
• D. Torsional Moment

Answer: (A)

In bending of a beam, the stress distribution across the depth is tied to the curvature: when the beam sags (concave up), the top fibers are compressed and the bottom fibers are in tension. In the common sign convention used in structural analysis, this sagging, or concave-up, bending is called a positive bending moment. So, a positive moment describes a bending moment that produces compression at the top fiber and tension at the bottom. A hogging moment would reverse the stresses (top in tension, bottom in compression), and a torsional moment involves twisting rather than the pure bending stress pattern described here.