This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
Fob Steel Angle Bars
To Obtain the Actual Stebngth of Flange in Tons
Multiply the safe uniform load (U) by the effective width of flange, that is, the total width of either flange, less the greatest Dumber of riv et holes coming on any line drawn across the top of flange, and at right angles to same (us-ually two rivet boles).
Never use less than one-quarter Inch thick plates.
For STEEL FLANGLES
Add one-quarter to safe load on flange hut length of span (in feet) must not exceed twice the depth (in inches) or deflection will be too great for plastering.