If the beam is a W8x18, it comes in two stress levels now 36 ksi and 50 ksi. Most beams now are 50 ksi. Could you make a mistake here? Maybe not as described above with the selection of the section because if you choose A36 you will certainly be OK if it is a 50 yield beam, but what about the connections: number of bolts, length and size of weld, copes, size and thickness of endplates or clip angles...if the beam is the right size, are you OK? Maybe.