How to Size Steel I-Beams
- 1). 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.
- 2). 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.
- 3). Raise each length to the power of 3 and multiply each by the steel's thickness: 5³ --- 1.5 = 187.5; 7³ --- 2 = 686.
- 4). Add the twice the answer for the beam's flanges to the answer for the steel between them: (187.5 --- 2) + 686 = 1,061.
- 5). 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.
