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SESK: Stuff Everyone Should Know · sesk.info/eng-beam-top-compression-bottom-tension

A beam rests on two supports with a load in the middle. Which side is in tension?

The bottom is stretched (tension); the top is squeezed (compression).

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  1. The top
  2. The bottom ✔
  3. Neither; beams carry only shear

Why: As the beam sags, the top fibers get shorter and the bottom fibers get longer. Brittle materials like chalk or glass crack from the bottom, tension side first.

Huh, didn't know that: That is why steel reinforcing bars sit near the bottom of a concrete floor slab between supports.

The dad joke
The beam was feeling down in the middle, but at least the bottom held it together under tension.
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References

Answer: The bottom. As the beam sags, the top fibers get shorter and the bottom fibers get longer. Brittle materials like chalk or glass crack from the bottom, tension side first.

  1. MIT OCW 3.11 Mechanics of Materials (Roylance): Stresses in Beams ocw.mit.edu/courses/3-11-mechanics-of-materials-fall-1999/96d839b02e4a6c63cf8031800e89cccd_MIT3_11F99_bstress.pdf
    the material near the top of the beam is placed in compression along the x direction, with the lower region in tension
    Checked 2026-10-10.
  2. MIT OCW 3.11 Mechanics of Materials (Roylance): Stresses in Beams ocw.mit.edu/courses/3-11-mechanics-of-materials-fall-1999/96d839b02e4a6c63cf8031800e89cccd_MIT3_11F99_bstress.pdf
    like chalk or glass, it will do so by crack initiation and growth from the lower tensile surface
    Checked 2026-10-10.

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