How do you convert steal yield and tensile strength to pounds?

If you load a beam in tension or compression only ( pull or push) then you multiply the tensile yield or ultimate strength by the area of the beam cross section. Thus applies to a simple beam (bar) only.

If you load the beam any other way, such as bending or twisting, or perpendicular to its axis you need to use formulas from an engineering strength of materials course or handbook.