Science Puzzle
Cantilever Tension
A cantilever beam is fixed into a wall on the left and hangs in the air with a heavy load at its free right end. The top face of the beam is being squeezed (compression) and the bottom face is being stretched (tension).
If the beam were made of plain concrete, which face would crack first, and why?
The Answer
The bottom face, because that is where the tension is, and concrete is very weak in tension.
Concrete is excellent at resisting compression: you can squeeze it hard without cracking it. But it handles tension poorly: stretching pulls the material apart along its weakest points, and concrete has many tiny internal voids that become crack initiation sites under tension.
This is exactly why real concrete beams and bridges use reinforced concrete: steel bars or tendons are embedded in the tension zone (the bottom of a horizontal beam) to carry the tensile load that the concrete cannot handle on its own. Understanding which part of a structure is in tension and which is in compression is one of the most fundamental questions in structural engineering.
The principle: Stress distribution in beams. A loaded cantilever puts the top face in compression and the bottom in tension. Concrete resists compression well but fails in tension, so reinforcement is needed on the tension side.