A steel Ibeam's size describes how well it can resist compression and tension. The value that specifies this resistance to loads is the beam's area moment of inertia. This value is also known as the second moment of inertia or the bending moment of inertia, and it is unrelated to the other measure called moment of inertia, which describes objects' rotational inertia. The beam's size, or second moment of inertia, is proportional to its length and width.
Things You'll Need
 Tape measure

Measure the length and thickness of each of the beam's flanges. For example, a beam's flanges may each be 5 inches long and 1.5 inches thick.

Measure the length and thickness of the steel between the flanges. For example, this steel stretch may measure 7 inches long and 2 inches wide.

Raise each length to the power of 3 and multiply each by the steel's thickness: 5³  1.5 = 187.5; 7³  2 = 686.

Add the twice the answer for the beam's flanges to the answer for the steel between them: (187.5  2) + 686 = 1,061.

Divide this answer by 12, a conversion constant: 1,061 ÷ 12 = 88.4. This is the beam's second moment of inertia, measured in inches raised to the power of 4.
References
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