How to Calculate Moment of Inertia of Steel Beams


Multiple quantities in mechanics go by the name "moment of inertia." The rotational, or polar moment of inertia of a solid describes its resistance to rotation. The moment of inertia of a beam, however, generally refers to its second moment of inertia, also known as its area moment of inertia. This value describes the solid's resistance to different forces -- to bending and deflection. A steel beam with a higher moment of inertia can resist higher loads without suffering any damage.

  • Find the cube of the distance between the beam's two flanges. With this example, imagine a distance of 7 inches and calculate: 7^3 = 343.

  • Multiply that answer by the width of the length you just measured. With that area, the beam's webbing measures 1.2 inches in width. Calculate the following: 343 x 1.2 = 411.6.

  • Find the cube of the length of the flanges. If each flange measures, for instance, 5.5 inches in length, calculate as you did in Step 1: 5.5^3 = 16.5.

  • Multiply this cube by the flanges' combined width. If, for instance, each flange is 0.8 inches wide, the calculation would be as follows: 16.5 x (0.8 + 0.8) = 26.4.

  • Add the answers to Steps 2 and 4: 411.6 + 26.4 = 438.

  • Divide this answer by 12: 438 / 12 = 36.5. This is the steel beam's moment of inertia, measured in inches raised to the power of 4.

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