Where do tension cracks begin in an unreinforced beam when loaded?

Where do tension cracks begin in an unreinforced beam when loaded?

• A. Bottom
• B. Top
• C. Mid-span
• D. Outside face

Answer: (A)

When a beam bends under load, the top fibers are in compression and the bottom fibers are in tension. Concrete isn’t strong in tension, so cracks start where the tensile stress first exceeds the material’s ability to carry it—the bottom surface of the beam. This is especially true near mid-span where the bending moment is greatest, so tension cracks typically initiate at the bottom in the tension zone and then propagated upward.

When a beam bends under load, the top fibers are in compression and the bottom fibers are in tension. Concrete isn’t strong in tension, so cracks start where the tensile stress first exceeds the material’s ability to carry it—the bottom surface of the beam. This is especially true near mid-span where the bending moment is greatest, so tension cracks typically initiate at the bottom in the tension zone and then propagated upward.