Engineers describe a steel beam's strength in terms of its ability to remain rigid in spite of the bending forces that act on it. This value is the beam's moment of inertia, sometimes called the second moment of inertia, or the area moment of inertia to distinguish it from the other similarlynamed value related to solids' rotation. Any increase in a beam's dimensions increases its moment of inertia, but the dimensions that contribute most are the lengths of its flanges and the length of the steel between them.

Raise the distance between the flanges to the power of three. If this distance measures, for instance, 8 inches  8³ = 512.

Multiply this answer by the thickness of that stretch of the beam. If the steel there is, for instance, 1.4 inches thick  512  1.4 = 716.8.

Raise the flanges' length to the power of 3. If each has a length of, for instance, 4.5 inches  4.5³ = 91.125.

Multiply this answer by each flange's thickness. With a thickness, for instance, of 0.85 inches  91.125  0.85 = 77.46.

Multiply the result by 2, to account for the two flanges  77.46  2 = 154.92.

Add to this answer the answer from Step 2  154.92 + 716.8 = 871.72.

Divide the result by 12  871.72 ÷ 12 = 72.64, or approximately 72.5. This is the beam's area moment of inertia, measured in inches, and raised to the power of 4.
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