Country: United States
Date: November 2007
Why are beams shaped like the letter I?
The cross section of structural beams is shaped like the letter "I"
to conserve material and reduce weight. When such a beam is laid
across two column to support the roof of a building, as an example,
it has the same weight carrying strength as a beam that has the same
height as the "I" but a width equal to the widest width of the "I".
So, by using an I-shaped cross-section instead of a rectangular one,
one can save over 50% in material and weight.
Ali Khounsary, Ph.D.
Argonne National Laboratory
I-beams are shaped like to letter I to maximize the moment of inertia.
This maximizes its resistance to bending and deflection and is why
I-beams are used in building construction as opposed to other beams with
the same area. If the beam is only subject to bending about one axis
the stress in the beam is inversely proportional to the moment of
inertia (or more strictly speaking the "second moment of inertia").
Imagine an ordinary bar of metal that has a rectangular cross
section, being bent. The surface of the bar that is on the
"inside" of the bend is subject to compression, and the
surface of the bar that is on the "outside" of the bend is
being stretched and therefore is in tension. Midway between
these surfaces, is an imaginary plane inside the middle of
the bar called the Neutral Plane. Here, the tension on the
outside surface of the bend, and the compression on the
inside surface of the bend, cancel out and there is no stress
at all when the bar is bent.
Since the center area of the rectangular bar has little or no
stress, and stress caused by bending mainly appears near the
outer surfaces, there is no point in having all that material
doing nothing near the center of a rectangular beam.
So as a result, an "I" shaped beam is used because it is much
lighter than a rectangular beam, and just as strong. Another
way to look at it is, that an "I" beam puts its material
where it best resists bending, and does not waste material by
placing it in the central area where it does little or no
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Update: June 2012