What size square mild steel pipe will support load

  1. Can anyone please help me with a question re the minimum size square steel pipe needed to bear a load. I loved physics but never took mechanical engineering. I am rigging a winch to one end of a pipe to lift a static load (holding up a gangway temporarily to repair). The gangway weighs around 500 lbs., and will be supported in the middle (essentially all of the weight). The proposed pipe is mild steel, 2.5 inch square, 12 feet long, 1/8 inch wall thickness, and connected at the other end and in the middle to supports. In essence, would the pipe bend if holding that load at 6 feet out from the support? What size pipe would bear the load? Thanks!!!
     
  2. jcsd
  3. nvn

    nvn 2,124
    Science Advisor
    Homework Helper

    So far, it sounds like you have a simply-supported beam or second class lever. If the yield factor of safety is 2.0, and if the allowable bending stress of your square tube material is Sb = 157.5 MPa, and if your load is statically applied, then your beam stress level would be R = sigma/Sb = (151.9 MPa)/(157.5 MPa) = 96.4 %. For a given value of Sb, you want to ensure R does not exceed 100 %.
     
  4. Sorry for my ignorance... but does that mean that the square pipe in question will support the load? Thanks
     
  5. nvn

    nvn 2,124
    Science Advisor
    Homework Helper

    R > 100 % indicates a component is overstressed. Notice the value of R in post 2, which is for the proposed beam size described in post 1.

    After lifting, support (prop) the load with additional, stable supports, before getting human body parts underneath the load.

    How do you propose to attach the midspan load to the beam such that the load is not concentrated on a fine line across the beam, and also does not slip longitudinally along the beam if (when) the lever angle is steep?
     
  6. Thanks for the follow-up.
     
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