The top of a structural beam is subject to what type of force?

The top of a structural beam is subject to what type of force?

• A. Compression
• B. Tension
• C. Shear
• D. Torsion

Answer: (A)

When a beam bends under a downward load, the cross section experiences opposite strains on opposite faces. The curvature shortens the fibers at the top, so the top surface is squished together, i.e., compressed. The bottom fibers are lengthened and go into tension. The line through the cross section where the fibers experience no longitudinal strain is the neutral axis. Torsion and shear describe different stress states (twisting and parallel shear within the section, respectively) and are not the primary effect at the very top under simple bending. So, the top of the beam is under compression.

When a beam bends under a downward load, the cross section experiences opposite strains on opposite faces. The curvature shortens the fibers at the top, so the top surface is squished together, i.e., compressed. The bottom fibers are lengthened and go into tension. The line through the cross section where the fibers experience no longitudinal strain is the neutral axis. Torsion and shear describe different stress states (twisting and parallel shear within the section, respectively) and are not the primary effect at the very top under simple bending. So, the top of the beam is under compression.