2mm aluminium treadplate on 200mm/8 inch centres?

Discussion in 'Materials' started by RSD1, Oct 9, 2019.

  1. RSD1
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    RSD1 Junior Member

    I've got an aluminium workboat with 30 x 30 x 3mm aluminium box tube longitudinal purlins laid with 200mm centres. How much weight / force would be needed to bend/sag 2mm aluminium treadplate? Looking to replace an unsuitable deck material with the 2mm aluminium.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Get an offcut and test it, but it will weigh about the same as 8mm ply, you would think.
     
  3. RSD1
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    RSD1 Junior Member

    It will - but the ply it is replacing is 18mm so a big weight saving - about 300 lbs saved on the boat. Good thinking about trying a piece! Must be past my bedtime!
     
  4. bajansailor
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    bajansailor Marine Surveyor

    Re carrying out a practical experiment, be aware that if you have a sheet of 2 mm plate simply resting on the longitudinal purlins, the force required to bend it (when you stand on it) between the purlins will be much less than if the plate is securely welded to the purlins.
    When the plate is just resting on the purlins, then this could be regarded as 'simply supported', but when it is firmly welded then this could be regarded as being 'built in' (in beam theory parlance).
    Hence it would be prudent to weld your test sample to the purlins.
     
  5. fallguy
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    fallguy Senior Member

    It will sag under any weight, ftmp.

    The question is how much.

    And you are wondering how much it will sag underfoot.

    You can calculate the deflection, but you need the grade. 6061-T6 is probably the plan?

    Also, the decking is not a simple beam, but loaded on 4 sides.

    I believe the actual dimensions are 200mm x 25" no ?

    We assume 2 mm tread deck is same as 2mm flat; probably.

    Also important is that number where it will not return.
     
  6. RSD1
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    RSD1 Junior Member

    I believe that it is 6061-T6

    200mm spacings on 30 x 30 x 3mm box tube purlins in one direction, but effectively unsupported in the other direction as the purlins have been laid on top of the beams.
     
  7. fallguy
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    fallguy Senior Member

    I don't have time now, but this is an easy calculation. You basically want to know deflection of a 200 pound weight on an 8" spacing. A formula on xcel would be best, so you could adjust the plate and purlin spaces.
     
  8. RSD1
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    RSD1 Junior Member

    Can't adjust the purlin spaces as they are already welded in (except for cutting every second one out and I don't think 400mm spacing would be adequate). Best to make it 250lbs - I'm not the smallest guy around...
     
  9. bajansailor
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    bajansailor Marine Surveyor

    The purlins are spaced on 200 mm centres, and they are 30 mm wide, so the distance between them is 170 mm - a bit under 7".
    If you do go for this 2 mm thick plate, and have it 'built in' to each purlin (rather than just resting on the surface - simply supported) - perhaps using a grid of plug welds from the top - then this should create a pretty rigid and stiff structure.
    I would be surprised if you get much deflection from somebody walking randomly on it, as the average foot / shoe is going to be longer than 7", and will hence be longer than the span between the purlins. You might see something if you jump up and down with your feet specifically aligned longitudinally to fit between the purlins......
     
  10. RSD1
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    RSD1 Junior Member

    Last email I got from the boat builder was that he was planning to lay the 2mm treadplate across the purlins with a bead of silicon (daboff) and then rivet it down...

    If they put their feet that way it should be OK, but if their feet are parallel to the purlins and in between them?
     
  11. RSD1
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    RSD1 Junior Member

    Can't weld it because the built in fuel tank already has fuel in it apparently.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    What is the span of the plate/RHS...as this determines the load you can carry.
    Also is the plate/RHS just laid onto of a frame or is it fully welded to the frame...this also determines the max load.
     
  13. bajansailor
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    bajansailor Marine Surveyor

    Is he really going to use silicon, rather than (eg) 5200 or Sikaflex?
    If he glues and rivets the plates to the purlins, then that should count reasonably well as being 'built in'.
    I just measured my shoe width - it is about 4". Yours is probably bigger, maybe 5" (?).
    If you do align your feet fore and aft, between the purlins, you then have a uniformly (well, reasonably) distributed load from your foot in the middle over approx 5" (rather than eg a single point load at the centre of the span). I would still be surprised if I saw much in the way of deflection as a result.
     
  14. RSD1
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    RSD1 Junior Member

    Apparently - this sounds like the stuff he is using http://media.wuerth.com/stmedia/shop/catalogpages/LANG_en/2109126.pdf

    Hopefully - am rapidly becoming tired of all of his nonsense

    Pretty much

    Fingers crossed!
     

  15. bajansailor
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    bajansailor Marine Surveyor

    I am thinking that if he is going to use sticky stuff between plate and purlin, then at least use something that is a proper adhesive and not just a sealant. Although I can see why he is reluctant to do this, because 5200 and Sikaflex probably cost about 10 times as much as cheap discount crap.
    And he will need a LOT to bed down the plate on to the purlins.
    With an adhesive you will get a proper bonded joint, and the rivets are the backup. Make sure that he uses 'proper' marine grade rivets.
     
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