aluminium strenght ?

Discussion in 'Boat Design' started by ericrunner, Apr 15, 2011.

  1. ericrunner
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    ericrunner Junior Member

    Im currently in the process of building a hard top for my pontoon . The materiel is rectangle aluminum tube 2x1x120 wall roof is 12 x 8 all the contour and the middle Long cross is 2x1 . Every 24'' cross are 2x1 x 1/8 channel . My question is would this be solid enough for someone to walk on this structure ? And if so how many pounds per scare feet this would take
     
  2. ericrunner
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    ericrunner Junior Member

  3. ericrunner
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    ericrunner Junior Member

    [​IMG]

    the structure is a permanent hard top for my pontoon i just want to make sure if for some reason i can walk or crawl on it ...lights ect ... roofing materiel is uhmw 1/8
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Bit confused...you seem to be mixing units?

    Can you provide in just one..imperial..or metric....(I prefer metric)

    Also, how are you joining the structural members together? and how are supports joined to the frame?..and their distances from the ends etc.

    Finally what grade of aluminium are you using?
     
  5. ericrunner
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    ericrunner Junior Member

    top is 12 feet x 8 feet
    materiel is 2 inch x 1 inch .120 mm and 1/8
    tig welded

    supports are 78 inch long bolted to the pontoon railing and plywood floor
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Im sorry but this is clear as mud to me.

    You must be consistent with your units, you're mixing feet and mm. This is where errors creep in.

    Here, you cite a tube. A tube has only a diamater and a wall thickness, this is 2 dimensions. Yet you cite 3 dimensions, this is where it starts to get confusing??

    This cannot be a tube, so it must be an RHS...if it is a RHS..what is its orientated in your design?

    It is terribly difficult to answer your question correctly when you provide data that is clear to you, but not to anyone else.

    How is it bolted, through what, where how??...what size bolt, grade/material??

    Without ascertaining the load paths and fixings, one cannot provide a proper response.

    You still need to provide the grade/temper of aluminium you're using.

    Whilst performing a structural analysis is "straight forward", it can only be done with all the facts to hand.
     
  7. ericrunner
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    ericrunner Junior Member

    got my answer on another board ! deflection starts at 300 pounds :D
     
  8. Jeremy Harris
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    Jeremy Harris Senior Member

    Actually, deflection of any member will be there with no loading at all, just from the weight of the material in the beam. The way that loads are applied to a beam (distributed or point), the section of the beam (specifically the area M of I) and the structural properties of the beam material (primarily the Young's Modulus) will determine how far it deflects for a given load applied in a particular way with particular types of fasteners and supports.

    The answer you've been given is almost certainly wrong, if only because, as Ad Hoc has already repeatedly pointed out, you haven't given enough information to allow even a very rough approximation to be made.

    For example, if your beams are 2" x 1" box section, with a 10g wall thickness (this is a guess based on my attempt at interpreting your data), made from 6061-T6 aluminium alloy, oriented so that the 2" dimension is vertical, with a length of 120" between supports and if these supports are perfectly attached to the box section, so that the loads are correctly and evenly distributed then a point load of 300lbsf applied to the centre of the 120" span would cause a deflection of about 3 1/4" at the centre with a peak bending stress of around 80%, which is high, but maybe OK for very occasional use.

    If the load is distributed over several beams, then the deflection and stress will be lower, but only if the beams are connected in a way that allows load sharing.

    If the beams are made from a different aluminium alloy, or of a different section, or attached to the supports in a sub-optimal way, then the likelihood is that the deflection under a 300lb point load in the centre of the span may cause failure.

    Jeremy
     
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  9. viking north
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    viking north VINLAND

    When presenting a drawing and asking questions re it's loading characteristics the drawing should be one which contains enough info that it can be built, otherwise there is no references to work from. As pointed out material dimensions alone are not enough, material specs. orientation and fastening have to be specified. Loading? area and locations ? In this case the loading gets complicated because it involves cantilever factors as there are no corner supports. The sad part here is if the poster had approached with a less cocky attitute and presented the info as requested he would have received the best technical info available anywhere. --Geo
     
  10. Scunthorp
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    Scunthorp Hull Tech

    Well said mate.
     

  11. Ad Hoc
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    Ad Hoc Naval Architect

    For amusement i did a quick check, based upon the very little and poor information.
    Those channels, in red, span the frame work. That long.t box section is a red herring, since that member itself, cannot support the transverses under a point load, (a man standing in one place). It is non-contributory. Adds a very small amount of grillage, but not worth considering in this brief analysis.

    So, that channel, with a "normal" 75kg standing at the centre, you get 103MPa...with a corresponding 293mm deflection.

    With all those assumptions from such poor data, and the fact there doesn't appear to be any fixity, a welded aluminium frame, if 6000 series, the as-welded strength is circa 115MPa. This assumes a decent weld etc etc etc...no snatch load from the guy slipping thus the 75kg becomes 150kg as he slips and falls etc etc...and no account for the deleterious effects of sea spray on the welded joint....is the 103MPa, with no factor of safety ok??...and would you be happy walking across a 2.44m span with some 300mm defelction??

    Make up you're on conclusions...
     
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