Calculating Stress on a Panel, and Material Selection

Discussion in 'Boat Design' started by Zac Penn, Dec 10, 2015.

  1. Zac Penn
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    Zac Penn Junior Member

    Feel free to yell at me for this not being a "Boat" design thread but I figured this would be as good of a place as any to post this problem. I am sure some of you boat designers/engineers are bored of the same problems with boat design and will enjoy a different kind of project.

    I am new to composites and boating in general, but I have a work project that I need some assistance in designing for. I build filtration systems for Koi Ponds and certain times of the year I have a big system that needs to be constructed and water tested so I need a large water holding tank that I can setup temporarily that will hold around 1,800 gallons of water. I also setup at some local trade shows to show off my filtration systems so setting up a holding tank is needed then as well. Generally speaking I would need this tank maybe 10-20 days per year and the other time it will be collecting dust, and taking up valuable storage/floor space at my warehouse. So I want to build a modular system that goes together without the use of fasteners, is extremely light, and can be broken down and stored with very little space wasted.

    I have decided that I want the tank to have an inside volume of 8' x 8' x 4' tall (or slightly less/more based on the materials used), be square in shape, and break down into as few pieces as possible. As of now I have decided on using fiberglass laminate over a lightweight core to create a sandwich structure panel.

    Here are some of my goals for this project….
    Material costs should be as low as possible.
    Deflection in the center of the 8' span less than 3/4"
    Extremely easy setup and take down
    Very light weight panels (Ideally less than 60 lbs per panel but I doubt that is possible)
    Panels should be nestable for storage and shipping purposes
    Panel thickness should be as thin as possible but no thinner than 1" (wishful thinking) and no thicker than 4”

    Here are a few drawings I have made that show some details of the tank design so far...
    Drawings.png

    The tank structure will just be floating over a concrete slab as the base. There will be a square rubber liner that will be dropped inside the fiberglass structure to form the watertight membrane.

    Quick Connect Corner.png

    For rapid assembly and equal load distribution in the corners I am wanting to use a metal channel extrusion to hold the fiberglass corners in place.

    New Corner Design.png

    To make the fiberglass layup easier I have eliminated all hard corners and went with 45 degree angles wherever the fiberglass changes direction. Each layer of laminate in the above drawing is scaled at 1/16" thick so it is very similar to a layer of 1808 open molded thickness. This would give me a total flange thickness of 1/4" with two inside two outside laminates, but I have no problem building those flanges up thicker if necessary. All of those dimensions are flexible so if you have a better idea then please share.

    Flanged Bottom.png

    Seeing as how the tank will be a floating structure over the concrete slab I have a fear of the rubber liner being able to wiggle it's way under the vertical wall (in a dip in the concrete slab) and the water pressure trying to push the wall structure vertically as the rubber liner bulges outward. I don't know if this will be a valid fear or if the friction between the liner and fiberglass wall panels will be so great to overcome any vertical lifting forces from the liner, but I just included the last picture as an option to try an mitigate that fear. I would LOVE to exclude that flange if it is not necessary and with todays extremely strong adhesives I am sure I could adhere a flange onto the bottom after the fact if it is needed.


    So that is where I am at so far, and I am really happy with the design. I am not good with complex mathematics, and calculating all of these stresses is way out of my wheel house. I have been in contact with FGCI.com and they are helping as best they can but they can't provide any real structural recommendations based on liability. However my outside salesman seems to think that a 1" Carbon-Core honeycomb core with two layers of 1808 inside and two layers of 1808 (maybe three) on the outside should gain me enough strength to keep the panel from deflecting outward too much. We also talked about building a slight inward curve into the panel so that as it did deflect outward with water pressure it would just straighten out. However a flat panel will be easier to duplicate on a flat layup table.
    I have also been reading up on a "Club Sandwich" structure and the extra stiffness gained by having multiple cores laminated together, but that does add additional work into the mix, and if this project works well for me I may have some customers interested in the same kind of tank structure, so the less work the better.

    So hopefully you can dust off the old textbooks and slide rulers and see what you can figure out.

    Thanks,
    Zac
     
  2. gonzo
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    gonzo Senior Member

    You really need this to be signed by a certified engineer. Since you are in Florida, it will be better to get one registered in the State.
     
  3. Zac Penn
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    Zac Penn Junior Member

    Boooo Where is your backyard engineering spirit?
     
  4. jehardiman
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    jehardiman Senior Member

  5. Zac Penn
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    Zac Penn Junior Member

    HAHAHA Obviously you are missing the point of making this a modular tank that can be disassembled when not in use. For $22K I could rent a storage unit for 18 years and store a cheap plastic $200 water tank there.
     
  6. SamSam
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    SamSam Senior Member

  7. NavalSArtichoke
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    NavalSArtichoke Senior Member

    Well, $22K could be used to set up a Victim's Compensation Fund.

    The concrete slab at the base better be well-engineered as well. Each foot of water in this tank will weigh approximately 2 tons. This tank doesn't see to be too portable if you have to drag an 8'x8' concrete slab around.
     
  8. fredrosse
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    fredrosse USACE Steam

    Tank on Floor

    Every year at the Cabin Fever Exposition there is a model boat tank setup on the expo concrete floor, about 30 feet x 80 feet, with a water depth of about 3 feet. This must not be too complicated an arrangement, although I am not privy to the details.

    As mentioned earlier, a round portable swimming pool that costs a few hundred dollars might do just fine, no need to re-invent anything.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Seems obvious that a small circular kids swim pool would work, plastic liner sitting on some bedding sand, easily assembled and disassembled, cheap as chips. It isn't hip to be square, though, that would be difficult.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

  11. Mr Efficiency
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    Mr Efficiency Senior Member

    A quick look reveals there are plenty of cheap demountable square kid's pools available, for cheap. Go to a pool centre or Walmart ! No need for an engineering committee on this one.
     
  12. Zac Penn
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    Zac Penn Junior Member

    Seriously guys...I know there is a bunch of cheap junk out there that this could be done with, and there is outrageously expensive tanks as well, but that isn't what I want. I appreciate your opinions.
     
  13. Zac Penn
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    Zac Penn Junior Member

  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Cheap junk ? Why won't kids pools do the job ? If they stand up to the rough-house antics of children, I doubt they'll fail in service for your application. Does it have to withstand a hurricane or high-scale earthquake ?
     

  15. Zac Penn
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    Zac Penn Junior Member

    They are designed for a specific purpose and rapid setup and disassembly is certainly not one of them. They are round, so they do not optimize volume per sq ft of real-estate. The vinyl liners are not meant to be rolled back up, stored for a year and then used again. The vinyl dries out and each sharp crease eventually creates a pin hole and leaks. After three times you are chasing down 30 leaks. I do have lots of experience with with this, as I used to manufacture similar tanks used for koi shows and quarantining fish.

    Another aspect to a fiberglass cored tank is that it will have a better insulating value than a swimming pool. If it were a balsa or foam cored tank it would be a higher R-value but even a honeycomb core will be better than a bare liner. Now you are thinking about thermal losses from the concrete slab. Well there can be a 2" thick sheet of polystyrene laid on the ground before the liner is installed to keep that to a minimum.

    I realize all the factors that would make things easier, and I am choosing to do it a more difficult way to optimize volume, and make it more suitable for my needs.
     
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