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

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

  1. fallguy
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    fallguy Senior Member

    Another thing to consider is whether the entire structure needs to be built of the same material or whether it is wiser to beef up the engine area.
     
  2. Michael Hyder
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    Michael Hyder Junior Member

    Yes, good point. I already considered this. Hence, the idea of doubling up beams OR using more beefy beams in the engine area. I'm trying to keep everything uniform in terms of Beam height. For example, use 6061 T6, 4" X 3 X 0.326 thick in engine area, 4" X less beefy in all other cross members. Lots of variables to consider, Main concern is ensuring enough support overall, and specifically in the engine area. Thanks for your help.
     
  3. fallguy
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    fallguy Senior Member

    I apologize I do not know allowable deflection limits for the beams, but my gut hunch is somewhere around 1/8" max for a goal which seems like much to much when written. And this is why I would start with the 600 pound point load. It is only a starting point. Enter the load. Enter the beams. And run the calcs to see deflections. Then you will develop some intuition about how much each beam choice supports the work. It is back of napkin. We have an expert that posts here, but he is on a time difference and doesn't always jump in to all questions.
     
  4. fallguy
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    fallguy Senior Member

    Deflection calc. You can google it, but this is the calc.

    72432AAE-3DFB-4B5F-B882-302AFD46948F.png
     
  5. Michael Hyder
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    Michael Hyder Junior Member

    Fallguy, again thanks for the assistance.... Yes, intuitively I would think 1/8 " deflection is the most you'd want to see on a beam with a 600 lb load. The responses here are helping me to zero in on the engineering involved without understanding the nomenclature. As I understand, I have a load being supported by beams, the main consideration is Maximum Deflection (the main point Maximum ALLOWABLE Deflection). Therefore, the whole exercise here is to determine this value. Given that I will be purchasing 6061 T6 I beams, 3" or 4" height. Max Deflection is considered against loading per beam. For example, engine mounted b/w two beams, then 600lb engine load/ 2 = 300 lbs load per beam, Yes? or if beams are doubled then 600/4 equals 150 lbs per beam. Am I looking at this the correct way???
     
  6. fallguy
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    fallguy Senior Member

  7. fallguy
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    fallguy Senior Member

    More or less, yes.

    apply a force, calculate the ability of each beam option, determine deflection of each scenario; it is back of napkin

    In this case you are more interested in the deflection in the y direction, but in more complex situations; the other direction gets considered.
     
  8. fredrosse
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    fredrosse USACE Steam

    I have been a Registered Professional Engineer for several decades, and with these credentials I can firmly state that the original question is very incorrect on several counts. As Gonzo, and others, have stated, the design of structures requires technical considerations far beyond classic simply supported textbook beam models. Trying to cram all of the technical considerations relevant to this design into a few forum replies is not going to produce results that are reliable and safe.
     
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  9. Michael Hyder
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    Michael Hyder Junior Member

    'fredrosse'. Thank you! Which is why I am seeking out a professional to make the relevant determinations. The good thing with this forum is that it has given me sufficient info to educate myself on the subject matter and more accurately state the Question, and the desired outcome. ) Now I just gotta find a PE that does this sort of thing ).
     
  10. bajansailor
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    bajansailor Marine Surveyor

    Michael, I think that you are trying to make this scenario way too complicated.
    Do you have a shop where you can wander in and say you want to buy some ally I beams for your boat, or do you have to get them by mail order?

    If the former, go to the shop and have a look at some beams. Maybe take a couple of small bricks with you to act as supports. Ask if they will allow you to put a test load (yourself) on a beam, simply supported on two bricks at the required spacing. Don't jump on it - add your weight gradually. If it starts to deflect noticeably, try a deeper beam. If you can jump up and down on it and it doesn't budge, then it should be ok, especially if the load of the diesel engine is across 2 beams.

    If you do have to get your beams by mail order, maybe order one initially to test it?
    Or, if the above calculations are suggesting that 3" or 4" deep will be ok, then just go for 6" deep beams, and then you know you will have a good factor of safety, and the extra weight and cost will not be a lot in the long run?
     
  11. fallguy
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    fallguy Senior Member

    The only trouble is weight. The more he adds; the more the pontoons sink.

    I built a spreadsheet for him with the point loading formula.

    Someone can shoot holes in it and he can modify it if he wishes for other loading schemes.

    Internet went down. I will try it again in a second.
     
  12. bajansailor
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    bajansailor Marine Surveyor

    Here is a useful link to I beams -
    https://www.easternmetal.com/pdfs/aluminum-i-beam-extrusions.pdf

    A 4" deep beam with a 3" flange is 2.79 lbs/ft; a 6" x 4" beam is 4.02 lbs/ft.

    If the engine is spanning two beams, each 11' long, then you need 22' of I beams.
    The two 6" deep beams will weigh 27 lbs more than the two 4" deep beams.
    Ok, they will cost a bit more as well, but I think it would be worthwhile having a reasonable factor of safety re possible dynamic loads (which will be difficult to quantify), and additional static loads (such as more than one extra person standing on the platform when doing maintenance).
    And that 27 lbs is going to have a fairly negligible effect on the loaded displacement of the barge.
     
  13. fallguy
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    fallguy Senior Member

    keep in mind bdeck clearance and center of gravity are both affected by such a proposal; this is why FredRosse gets involved in the fashion he does, because sometimes simple solutions can cause other issues; however, I can tell you the 4" beams are pretty light; still having trouble with my pc internet.
     
  14. gonzo
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    gonzo Senior Member

    He could use the drip pan under the engine as a structural member if it spans the pontoons.
     

  15. missinginaction
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    missinginaction Senior Member

    As I read through all of the posts this one sentence seems to me to be the most sage advice offered here. Can't you find a pontoon that has a similar rig to what you want to do and just copy that? I've gotten some of my best ideas just wandering around boat yards and walking the docks.
     
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