The type, span and size of an I-beam determine its load capacity. "I-beam" is a general term used to describe the wide flange, the American standard beam and the bearing pile, which all have an I-shaped cross section.
Engineers consult a chart to find out the load capacity for a specific beam depending on the span and the type of load the beam must withstand. The most commonly used charts are in the Steel Construction Manual from the American Institute of Steel Construction.
Physical constraints determine the load capacities of beams. For example, the allowable uniform load for a W8 x 18 wide flange with a 8-foot span is 30,400 pounds, but it drops to 24,300 pounds for a 10-foot span and to 20,300 pounds for a 12-foot span.