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Thread: Bearing capacity of hollow tube

  1. #1
    Associate Engineer
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    Bearing capacity of hollow tube

    Hi,

    I'm trying to find the capacity of a hollow tube / pin which is subjected to transverse loading. The pin is inserted in to a trestle and two trestles are used to support a beam. My knowledge is limited to basic calculations with simple beams and when I've tried applying what I know, the figures don't seem to be correct. So, as an example, if we take a hollow tubular section of steel that is simply supported at each end and apply a transverse UDL to it, we get a maximum shear force, maximum bending moment and deflection. How do I use these figures to determine if the section will support the force applied to it?

    Let's say we have a 1 m length hollow tubular section of 100 mm diameter and wall thickness of 10 mm. The UDL is applied across 0.8 m of the length of the section from the centre (i.e. a gap of 100 mm each side). Let's say the steel grade is 350 N/mm2. What is the maximum load the section can elastically support?

    Many thanks in adavance

  2. #2
    Administrator Kelly Bramble's Avatar
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    Check out the following --> Beam stress and deflections
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
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    Thanks for the reply. It's not so much the equations I can't find, it's the numbers and what to do with them that are confusing me. Using the 100/90 tube above and applying a load of 5 MN to it, I get a maximum bending stress of around 18.5 GPa. Am I then comparing this to the steel's shear modulus? Won't the shear be higher farthest from the centre (i.e. not where the maximum bending stress is).

  4. #4
    Administrator Kelly Bramble's Avatar
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    The maximum calculated stress needs to be much lower than the Proportional limit stress, Yield Stress as well as Fracture Stress. You will need to apply a reasonable factor of safety based on the end item application (Static loading or fatigue).

    Below is Stress vs. strain curve typical of aluminum



    1. Ultimate tensile strength
    2. Yield strength
    3. Proportional limit stress
    4. Fracture
    5. Offset strain (typically 0.2%)

    FYI, other design considerations might include not exceeding a specified maximum deflection due to applied loading.
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    Last edited by Kelly Bramble; 09-14-2015 at 08:21 AM.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  5. #5
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    Thanks Kelly. This all makes sense. I think what had me unsure was my colleague spoke directly of failure by shear and my training only goes as far as bending of a simple beam. Would it be right to say that metals would generally fail through bending stress before shearing?

  6. #6
    Lead Engineer
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    I would like to add one additional note about the loading configuration on the beam. With tubing, in addition to the standard beam bending stress concerns, you need to be careful about how you are applying the loading. A point loading on a thin wall beam can result in defromation buckling of the beam regardless of the acceptable bending stresses.

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