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SESK: Stuff Everyone Should Know · sesk.info/eng-why-i-beams

Why is an I-beam shaped like an I?

It puts most material far from the neutral axis, where bending stress is highest, raising stiffness per kilogram.

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  1. To make welding the ends easier
  2. It is cheapest to roll that shape
  3. It puts most material far from the bending axis ✔

Why: Area far from the neutral axis contributes strongly to the area moment of inertia. The flanges carry much of the bending stress, while the web separates them and also carries shear.

Huh, didn't know that: Steel wide-flange beams are listed in standard tables by depth and weight per foot, like W12x26.

The dad joke
Why do I-beams make great leaders? They put their best material where it counts.
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References

Answer: It puts most material far from the bending axis. Area far from the neutral axis contributes strongly to the area moment of inertia. The flanges carry much of the bending stress, while the web separates them and also carries shear.

  1. Engineering Statics (Baker and Haynes): 10.1 Integral Properties of Shapes engineeringstatics.org/Chapter_10-integral-properties-of-shapes.html
    A shape with most of its area close to the axis has a smaller moment of inertia
    Checked 2026-10-11.
  2. Engineering Statics (Baker and Haynes): 10.1 Integral Properties of Shapes engineeringstatics.org/Chapter_10-integral-properties-of-shapes.html
    make the beam stiffer
    Checked 2026-10-11.

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