The compression stress in the top flange converts to tension stress in the bottom flange. Think about how this happens. Compression within the beam is a maximum at the very top of the flange, but decreases within the beam, becoming zero at the neutral axis. From there, tension grows from zero to a maximum at the very bottom of the beam.
Therefore, bending stress is zero at the neutral axis, and maximum at the flanges! This is significant information for deciding where to poke holes in a beam. The “tension face” in a wooden member requires clear wood (i.e. knot-free), especially in the middle third of the span.
We have discovered how moment stress varies within the cross-section of a beam. Now let us look at how it varies along the beam. The beam broke in the middle, where the maximum moment stress occurred.
Therefore, the moment stress varies from a maximum in the middle of a simply supported beam to zero at the ends. This is more significant information, useful for strengthening a beam or avoiding weakening it.
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