## Comments for capacity of square tube steel

 Apr 18, 2012 tube steel formula question by: Anonymous Question:Why doesn't your formula of P=S*M*/4 work with your 4x4 sq tube comparison between rectangular tube and square tube. Ref steel tube tab.Thank you,ScottAnswer:Hi Scott,The way P is expressed above is incorrect. The formula should look like this: P = 4*S*Fy/LTo simplify it more, let's Substitute in Fy = .66*46 ksiThe more simplified P = 121.44S/(L*12) for 46 ksi steel tube, the L*12 converts the length to inches. Please note this formula will not work for metric units.Please note for rectangular tubes there will be two possible answers due to the fact that one side of the rectangle is shorter than the other. Depending on how the tubes are going to be loaded will determine whether Sx or Sy is to be used.To see rectangular tube dimension click hereIf the long side of the tube is to be vertical and the load is to be placed parallel to the long side, use the Sx in the calculations.If the short side will be vertical, the Sy should be used.