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

    I would say 95% of the time it will be a concrete slab, but I do know if two shows that will have either concrete pavers or grass as a substrate which could get a little tricky.

    Yes the filtration will be remote located outside the tank, and for most of the testing/shows the suction and return plumbing will be just up and over the side of the tank, so no penetrations in the liner. However I do want to have the option of installing bulkheads in the liner and pass the plumbing through the panel. I will keep all penetrations at least 20" off the bottom however to have lower pressures on the panel there, and make there is plenty of space between penetrations on a single panel.

    As for the gussets I am waiting on a more detailed explanation as to the localized loading at the joints. I had thought of them before but like I explained in the previous post I thought the long extrusion would distribute the forces better and make for an easier assembly.

    Thanks for the helpful post though.
  2. gonzo
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    gonzo Senior Member

    The force on the panels and the seam can be arrived by with integration. If you are not good with calculus, it would be hard to put three semesters of college on a post. That is why hiring an engineer is a good option. Using gussets or channels in the corners is going to produce huge stress concentrations that will require a lot of reinforcements. There are better options, like straps. You can always build it and test it too. If it doesn't work, try something different. It is a tried and true system.
  3. TANSL
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    TANSL Senior Member

    I think not but perhaps I do not understand what you mean.
  4. philSweet
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    philSweet Senior Member

    The strength is less of an issue weight-wise than the 8' unsupported span, and as Fred alluded to, you need to fix the panels in shear. Edge extrusions like you show are not as handy as you might think. One way I have seen them done is to stretch a bicycle inner tube around the flanges, which holds stuff together, then push an extrusion over, and inflate the inner tube. But those extrusions and flanges have to be beefy, and the flanges should interlock somehow. The good news is these joints are tolerant of some out-of-squareness.

    In order to lighten the panels, you want to unload the span either by sitting the edges in channels which are connected to the opposite side, or by wrapping some kind of hoop around the thing and wedging the panel in against the hoop.

    If supported entirely by hoop tension, about 2000 pounds tension is needed for a 4' deep container 8 X 8. Similarly, the tie rods holding the channels will have a total of 4000 pounds of tension on them. The bottom edge will have 2/3 of this.
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    And to think you can pick up rectangular, and various other round and non-round above-ground swim pools 4' deep, dirt cheap, short assembly times, and no need to talk to the professor who designed the space shuttle ! Crazy stuff.
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Ah, dear, the day will dawn when you agree with gonzo, one day, just that once.
  7. Ooks
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    Ooks Ian

    Thought I better chime in again and say that you should heed Petros's advise re loads from the 4' water. Didn't quite ring true for me in terms of loads that floors would see from other examples (guy rocking on chair again) but I had a quick look and he is right. That is actually pretty high compared to what they are speced to take. May limit where you can safely take this thing.

    I had another thought for you on construction - how about making your panels out of fairly thin veneer and then attaching a perpendicular beam. The beam will be much stronger than the panel to limit bending. you could also extend it beyond the panel and use a fairly simple pin or bolt to join them together - essentially creating a collar. Have some attractive fabric to cover it all up and you can even hide your pipes for the filtration system.

    I did a quick internet search trying to find a picture to illustrate - not exact but hopefully you see my point.

    if it looks like a floor it is probably because it is. but if you offset the beams on adjacent walls it will be light, stackable (2 panels can slot into each other back to back) and very easy to put together. (also loads on beams are way easier to calculate)
  8. Barry
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    Barry Senior Member

    Gonzo is quite correct. Many of the "easy" formulas that we use to solve engineering problems are derived from solving integration equations which then for simple applications become just a x b x c = something. But if say you are involved with varying forces, cross sections, pressure differentials, or dynamic loading of complicated, but mathematically defined by equation shapes, then you would have to set up a differential equation and integrate it to calculate a value.

    An example would be the simple 3 directional integration to develop the simple formula for the volume of a sphere. A triple integration of the x y and z axis. But say you had a parabolic 3 dimensional shape intersecting through the sphere at a given spot, and you need to figure out the volume. The only alternative would be to integrate the volume using both definable equations.

    To the OP
    I would suggest you consider aluminum bulkheads for several reasons.

    1) relatively light weight
    2) a 4 x 8 bulkhead with 6each 2 x 4 x .250 rectangular tubing longitudinal stringers and perhaps a .188 inch sheeting will meet the stress parameters, plus two vertical on each end.
    3) Joining the corners can be done with a bolt schedule through the corner uprights and with a cordless impact wrench take maybe an hour to assemble
    4) They can take a beating much more than a composite bulkhead can withstand. If you put a hole in it, a quick trip to a welder can fix it for minimal cost
  9. waikikin
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    waikikin Senior Member

    Hi Zac Penn,
    I'd be making multiple square tanks of 1160mm so they can be palletised with a draw angle so the suckers nest like a set of tupperware cups or like skip bins but light, break the span & you save material.... concentrate the extra effort on some integrated plumbing manifolds, maybe in the reinforced top edges and to the bases so they can be linked...., maybe in the principle of a tea cup & saucer have a "saucer" molding(also tapered and stackable and the right size to form a lid) also to contain spillages, surely if you're at a show you need risk assessment & mitigation strategy for spill.... come to think of it you would definitely need saucers with your modular walls..
    ... & definitely put some windows in a couple... great show drawcard, folks love looking at fish...
    When stacked the footprint will be smaller than your demountable tank and it's framing, you can stack the tanks back into the tooling as well and maybe even hang up out of the way.
    There you go.... I just imagineered a product range for you, but you can prolly already buy from Alibaba.. who knows?

    All the best from Jeff.
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Next time I look at a big glass fish-tank, I'll ponder all the engineering that went into it. Not !
  11. waikikin
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    waikikin Senior Member

    You never know what may happen, the structure must be engineered for all contingencies......

  12. Mr Efficiency
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    Mr Efficiency Senior Member

  13. cor
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    cor Senior Member

    There is no need to over engineer this. I would build it with plywood and 2x4s.

    I once built a 8'x8'x4' hot tub with 1/2" plywood and 2x4s. There were four 2x4s screwed on edge to the plywood, one on top, one on the bottom and two spaced out in the middle. The 2x4s stuck out past the end of the sheets and overlapped the 2x4s on the next sheet. The overlaps were then bolted together at the corners. I did not use a liner, but simply fiberglassed the seams on the inside. If using a liner you could simply unbolt the corners and take it apart.

    1/2" ply is about 45 lbs per sheet, with the four 2x4s screwed to it the whole panel would probably end up at about 60lbs and cost $60. Should only take a few hours to build.

  14. SamSam
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    SamSam Senior Member

    If you were able to put cross pieces across the top, like this [​IMG], it would colossaly strengthen the weakest point, the top edge. It could be solid bars, metal cables or straps or whatever was least intrusive, easiest, etc.

  15. Barry
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    Barry Senior Member

    To Cor from Alaska

    Only in Alaska, chuckle, ( the land of improvisation, baling wire and duct tape)

    If you did not have a liner, then you had a bottom, which means that the part of the panel that had the highest load and hence stresses ( the bottom) , were attached through to the other wall which would reduce the bending stresses in the lower 2 x 4.

    If it should have failed you might have been washed out into the tundra/muskeg/ocean much to the delight of brown bears, moose and seals, not having a bunch of water hitting a bunch of lawsuit leaning show attendees.

    In a situation such as this a reasonable factor of safety, due to liability if the structure fails, might be in a factor of 3 or 4. Which requires a stronger design.
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