3x3 or 4x4 square tubing bending strength

  1. I need to erect a hoist beam with a 10' span using square tubing. Which is stronger, 3"x3" 1/4" wall or 4"x4" x 3/16"? Also, how much weight could I lift at the center?
     
  2. jcsd
  3. Mech_Engineer

    Mech_Engineer 2,299
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    Gold Member

    To determine which is stronger, all you have to do is calculate the bending moment of inertia for each cross section. The higher the bending moment of inertia, the stronger the beam is in bending. To calculate the moment of inertia for a hollow tube, calculate the MOI for a solid tube, and subtract the MOI for the hole in the middle.

    Wikipedia second moment of area (a.k.a. moment of inertia)

    To figure out how much weight it can hold, you need to read up a bit on beam bending theory, and you will need a bit more information about what kind of material the tube is made out of:

    Wikipedia article on beam bending
     
  4. FredGarvin

    FredGarvin 5,087
    Science Advisor

    Most box tubing is A36. Other than that, you should be able to plug and chug with what M_E gave you
     
  5. Way over my head. Another option is to nail three 2x6x10 yellow pine boards together. I'm hoping this will support lifting 1500 lbs.
     
  6. Mech_Engineer

    Mech_Engineer 2,299
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    Well it sounds to me like you will want to hire an engineer to design this "crane" for you then... especially since it sounds like a safety-critical item.

    It does look like a 4"x4" hollow tube with 3/16" walls should be able to hold 1500lb without too much trouble, but it will depend and what kind of holes are drilled in it and how the beam is suppoorted at the ends.

    ... and a 4"x4" tube with 3/16" walls is about twice as strong in bending as a 3"x3" tube with .25" walls.
     
    Last edited: Nov 4, 2009
  7. FredGarvin

    FredGarvin 5,087
    Science Advisor

    In terms of just what will bend less,
    3x3x.25 Ix = 3.16 in^4
    4x4x.188 Ix = 6.59 in^4

    So the 4x4 beam will be deflecting less. I attached some quick calculations. The first calculation is calculating the load required at the center of the beam to get to yield (You don't want to go that high). Then using that load, a deflection at the center of the beam is calculated. These are pretty rough considering the end conditions, but they can be a guide for you.
     

    Attached Files:

  8. Thank you to everyone. This project is only temporary. I'm building a new floating dock on top of my old dock. I just need to raise and slide it ever so gently into the water. You know what they say. It only costs a little bit more to do it yourself.
     
    Last edited: Nov 4, 2009
  9. Mech_Engineer

    Mech_Engineer 2,299
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    Fred- It looks like you're off by a factor of 2 in your max force calculation, the beams should be able to put up with about twice as much force as you're quoting...
     

    Attached Files:

  10. FredGarvin

    FredGarvin 5,087
    Science Advisor

    Oh yeah. I forgot the factor of 2 in my calculation for M. Whoops.
     
  11. Mech_Engineer

    Mech_Engineer 2,299
    Science Advisor
    Gold Member

    "That factor of two will get you every time," a wise engineering professor once told me... :wink:
     
  12. Hi Folks, Is there a corresponding equation (or equations) for a cantilevered beam, and tables that someone can point to that have the values needed to run the computations for different sizes and wall thickness of square tubing (mild steel I think in my case)
     
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