My father recently asked how to figure the strength of an I-beam. I could not awnser the question. So I ask for more info as to why he wanted to know. He wants to rebuild his bridge and make hold 40000 lbs. Use 4 I-beams at 80 feet long clear spaning 70 of the 80 feet. Meaning that 5 feet is supported on each end. Assuming that each beam holds 10000 lbs how big of a beam is needed.