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, Scott
Answer: Hi Scott, The way P is expressed above is incorrect. The formula should look like this: P = 4*S*Fy/L
To simplify it more, let's Substitute in Fy = .66*46 ksi
The 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.