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  • Aluminium box section compression load cal

    Discussion in 'Calculations' started by enzyme, Jul 21, 2011.

    1. enzyme

      enzyme New Member

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      Hi

      Please can anyone help with the following?

      Can anyone help me determine the strength in compression (weight loading capacity) of a aluminium box section (30 x 40 x3mm wall thickness)that is 1400mm long in the vertical plane?
       
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    3. AndrewNew

      AndrewNew Well-Known Member

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      Assuming you are loading it along the long axis (i.e. using it like a prop), you need to know the strength of your material (check the data sheet for the yield strength or the proof strength, you will proabably only have the tensile strength and not the compressive strength but they are usually very close so you can use the tensile strength) and to calculate the cross sectional area of the material in your cross section (i.e. (40 x 30) - (34 x 24) mm²). Multiply the strength (N/mm²) by the area (mm²) and that will give you the failure load in compressive yielding.

      Since your part is long and thin, you will also need to calculate the buckling load (this is likely to be less than the load at failure in compressive yielding because of the shape of the part). The buckling load depends a lot on how the loads are applied to the ends of the part. Pin jointed at the ends is a worst case. The Euler buckling load (which unfortunately tends to over-estimate the buckling load, so make sure you include a suitably large factor of safety) for pin jointed ends is given by (pi² x E x I) / l² where E is the Youngs modulus of your material (around 70000 N/mm² for aluminium alloys), I is the minimum second moment of area and l is the length. Minimum second moment of area for your part is (40 x 30³)/12 - (34 x 24³)/12.
       
    4. maniacal_engineer

      maniacal_engineer Well-Known Member

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      there is also local crippling to be accounted for. I am too rusty on it, but the method is found in NACA stress memos and involves the same analysis as an euler column, but applied to a panel.
       
    5. enzyme

      enzyme New Member

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      Thanks for your replies but they are very much way over my head.

      Can anyone just tell me how much weight, (even if it was not theoretical possible to sit that much weight) on the end of the box section stud upright?

      Thanks
       
    6. AndrewNew

      AndrewNew Well-Known Member

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      How is your upright supported? Is it fixed into the ground at one end and supporting a weight at the top? How is it connected to other stuff? Or is it completely free standing? These questions do considerably affect the answer.
       
    7. enzyme

      enzyme New Member

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      It is more of theoretical question!

      Assume "perfect" fixing top and bottom.
       
    8. Radhoine

      Radhoine New Member

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      I think a such problem is related to buckling analysis, many finite elemnent code can do this type of simulation (Abaqus, Ansys..) this is a non linear analysis problem and it need an appropriate algorithm to be resolved.
      Just an advice check the providers of aluminum profile they usally give a full caracterisation of their product (inculding compression strenght)
       
    9. enzyme

      enzyme New Member

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      Thanks for the reply. I have been to the manufactures, and they have sent me a full spec.
       
    10. hmck57

      hmck57 Member

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      To be able to calculate the buckling, crippling, and yield allowables I need a little more information about the material: The Modulus of Elasticity (E) and the yield strength (preferably in compression) are required. In addition, the end-fixity of both ends of the column is required. Above you said "assume perfect fixing top and bottom". Do you mean by this that the ends are "fully" fixed, allowing no rotation at the ends of the column? This is a very unconservative assumption if you are unsure of the fixity.

      If you are unsure of degree of fixity, if you could provide a picture or diagram or some type of description that would probably help.
       
    11. AdamW

      AdamW Member

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      The indispensable reference: 'Roark's Formulas for Stress and Strain'...

      ... and a bit of patience :)
       

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