The allowable load for an I-beam is determined by the beam’s designation, nominal size and its span. Calculate the designation by multiplying the beam’s width by linear mass density (pound per foot), and the nominal size by multiplying the beam’s depth by its width, all in inches. The third determining feature of the allowable load for an I-beam is its span, the length between the two supported ends of the beam, measured in feet.
The Engineering Toolbox makes clear that the allowable load calculation is determined for a uniform load measured in pounds. If the load is not uniform, it can put uneven stress on the I-beam and cause it to bend under the uneven pressure.
It is important to ensure that all three measurements are taken into consideration. Leaving just one of the three out gives a false indication of the safe load that can be taken by the I-beam. For example, a beam with a designation of W8 X 10 and Nominal size of 8 X 4 can take an allowable load of anywhere between 6,900 and 15,600 pounds, depending on the span of the beam. As the span increases, the allowable uniform load drops. At these measurements, there is no allowable load for a beam with a span greater than 18 feet.