peteroznewman
Subscriber
I suggest you use simply supported beam equations to calculate the value.
Look up the equation for the maximum bending moment for this configuration.
It has a term in it for moment of inertia, I, so look up the formula for that for a square cross-section.
Look up the equation for the maximum stress given the bending moment.
Look up the strength of structural steel. You need to use the safety factor on the Ultimate or the Yield Strength.