I-Beam Question of Maximum Shear Stress, Please Help

[COLOR=#000000][FONT=Tahoma]I am working on a calculation for a beam and can not figure out how to show exactly how much my beam can support. I have all the info for a point load but how do I know how much PSI it would take to break the beam in half??? I keep finding formulas for shear stress of a load but how do I know my beam can support that load? Attached is the info I have came up with for point load but no idea on if my beam can handle it. Can anyone help me out? Thanks.
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You need to look at your beams section properties. If this is a standard beam section, these will be listed by the manufacturer. If not... then it's back to basics I'm afraid; section depth, grade of material, web thickness etc.