For example, a 25â span would be 25x12 / 20 = 15â. The width of this beam would be between 1/3 and Â½ the depth. The dimensions of a girder would be the same, but the flange would be thicker.A beam overhang can be a maximum of 3/8 of the supported span.Floors (Decking and Joists) Metal decking with shear studs, 2â deep spans 10â, 3â spans 15â in the direction of the decking ribs.he dimensions of a girder would be the same, but the flange would be thicker. A beam overhang can be a maximum of 3/8 of the supported span. Floors (Decking and Joists) Metal decking with shear studs, 2â deep spans 10â, 3â spans 15â in the direction of the decking ribs.