What size I Beams to carry a 600lb load across 5ft span

Discussion in 'Boat Design' started by Michael Hyder, Jan 4, 2021.

  1. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Happy New Year to you Michael.

    This is a very simple straight fwd exercise.
    But first you need to set your assumptions and goal/objective!

    In my language, your engine weighs 272kg and the span is 1.623m.

    Being aluminium the objective is to satisfy the deflection. Since aluminium is a low modulus material, it is driven by deflection and not stress.
    So, what deflection is appropriate?

    In general one could say 1/100th of the span...this equates to 16.2mm. But considering the item that it is supporting i would suggest this is too high.
    A value of around 10mm is "reasonable". However, again, considering the item that is being supported half this value would be more appropriate i.e 5mm, deflection.

    Now the load we can consider this to be a point load, not a UDL. It makes no sense to analyis this as a UDL or even a partial UDL. Again - because of the item that is being supported.

    So, again, we can add another "factor of safety", by assuming the beam is supply supported - rather than built-in. In reality this is not the case, as the ends will have support to prevent the beam ends moving and also to transfer the load from the beam into the hull....this is also a very very important part of the structural design that you must address!!!

    So we shall assume a S/S analysis with a max deflection of 5mm.

    Finally the load, the 272kg. We shall assume a nominal 1g loading added to this....therefore, a load of 544kg.
    And since, looking at your dwg it appears 2 beams are supporting this engine..which makes sense, not 1, this must be supported by at least 2 beams...then each beam will experience 272kg or 2668N of load.

    Thus the max deflection = W.L^3/(48.E.I)
    Where I = the 2nd moment of inertia of the beam.....we can rearrange this to have I = W.L^3/(48.E.deflc)

    With a deflection of 5mm, the I = 688,863mm^4, or 6.9 x 10^5 mm^4

    The stress is simply = M/Z

    So, now, what is the design allowable stress?

    Noting Regs rules..i.e. Class rules...and the fact this is 6061, and considering you will attach this to the hull - in some form - it will most likely be welded.
    Therefore your design allowable stress = 76.8MPa.

    This gives a modulus of = M/Stress = 14.2 cm^3

    So, we now need a beam that satisfies the I and the Z.

    Looking at the list the AA-5 beam has a Z = 23.4cm^4...so this is ok, and the I = 9.13 x 10^5 mm^4

    These values are over the calculated minimums. But considering many issues can go wrong, more FoS the better.

    Finally the shear stress. Given the web thickness of 0.13inches = 3.302mm and the web height of 66.04mm, this gives a shear stress of 6.1MPa, which is below the 43.2MPa.

    Thus it is as simple as that.

    But you must be very careful how you attached the beams and transfer this load into the hull.

    Good luck.

    EDIT... you must realise the above calc's, this is the bear minimum requirement based upon a simple set of data.
    Last edited: Jan 4, 2021
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  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Ad Hoc has provided an excellent step by step calculation above - he explains it very well (I am going to save a copy for future reference).

    And (perhaps surprisingly), his calculations show that for the purpose of supporting the engine, two of your AA-5 beams (that are 'only' 3" deep, according to the chart in your first post) will be plenty strong enough for the job.

    However it would be prudent to also consider all the other possible things that could influence / affect the loadings on these beams - for example your catamaran 'racking' (twisting) while under way (especially so if there is any wave action), loads from people standing on the bridgedeck when they are working on the engine (there might be more than one person), and even the effect of running aground in a worst case scenario (perhaps fully loaded, at maximum speed?).

    I think it would be difficult to try and quantify what these possible additional loadings might be - so you need to decide how much of a factor of safety you now want to apply to your 3" deep I beams.
    Remember also that if it 'looks' right, then you are half way there already - and I am not too sure that 11' long x 3" deep 'I' beams will 'look' right on your boat, never mind that in theory they are strong enough (aluminium is amazing stuff).
    You will get a better factor of safety if you increase the depth to 4", and even more so to 5" or even 6" - and the extra weight incurred is going to be a small percentage of the all up loaded displacement of the boat.
    Also, for a work boat, which needs to look indestructible, you want it to look the part - so fit some nice chunky beams that fit in and 'look' right.
    So long as you are comfortably in excess of the theoretically calculated beams using Ad Hoc's beam theory, you should be fine.
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  3. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Quite right!
    The calc above is just for a simple beam analysis, nothing else.....no other additional loads etc are taken into account, which from the initial enquiry would probably contuse the OP at this stage.

    Thus on the data provided the beams shall suffice. But, as Bajansailor notes, there is still more to consider.
    But that can only be improved upon - with more data.

    One step at a time...

    So why one step at a time? Because design is a step by step process...you tick off each load case scenario one by one..to ensure you cover all the bases. You can't do it with a one liner or a 1 line hand calc...there is always more to add and consider...thus step by step...every analysis is broken down into smaller and smaller parts... and then you just add them together..at the end. Then you know, you have covered all possible scenarios. Thus identify the load cases, and address each one, on their own merits. You can';t do it all at once in one simple calc, in that sense. Unless you wish to over engineer it, in the absence of each step wise process.....
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  4. Eric ruttan
    Joined: Jul 2018
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    Eric ruttan Senior Member

    Dear Wiser beings; I did not see, although I may have missed, when It was made clear to our OP that one does not, like marching into Mordor, put bolt holes in I beams.
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  5. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I think that fatigue is probably one of the main failure modes for an aluminum structure. A diesel engine with a pump will generate vibrations with large amplitude. That was the reason for my suggestion of soft mounts.
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