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

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

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

Answer: (B)

When a beam bends under a load, the material on opposite sides of the beam’s cross-section experiences opposite stresses. The bottom fibers are pulled outward as the beam sags, so they undergo tensile stress (they’re being stretched). The top fibers are squeezed together, undergoing compressive stress. The line through the cross-section where the stress is zero is the neutral axis. Shear is present as a separate effect, but the primary stress you feel at the bottom in typical bending is tension. That’s why the bottom of a simply supported beam under downward load is in tension.

When a beam bends under a load, the material on opposite sides of the beam’s cross-section experiences opposite stresses. The bottom fibers are pulled outward as the beam sags, so they undergo tensile stress (they’re being stretched). The top fibers are squeezed together, undergoing compressive stress. The line through the cross-section where the stress is zero is the neutral axis. Shear is present as a separate effect, but the primary stress you feel at the bottom in typical bending is tension. That’s why the bottom of a simply supported beam under downward load is in tension.