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

    Well I did some reading and it doesn't talk about actual real world usage strength after it has been turned into a sandwich structure. The core material data sheets are one thing, but I can't find any information on an actual laminated sandwich being tested in different instances. How do you turn the data sheet of the core material, data sheet from the laminate material, plug in core thickness, and panel span and figure out a deflection number for said panel? That is what I need help on and it doesn't look like I am going to find it for free. Screw you Internet!!!!! j/k
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    And they weren't de-mountable ? If not, maybe you should make them in descending sizes and nest them inside one another for easier travel/storage. And those kids pools are available in square and rectangular shapes. Anything that will remain watertight without a liner, and be easily disassembled, will cost a bomb. But you don't like liners ! You have the champagne tastes, which is OK unless you have the beer budget ! :D
     
  3. Rurudyne
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    Rurudyne Senior Member

    ^

    Can I offer another plug for a round tank with a clear liner?

    The engineering is simpler if only because most everything structural actually holding back the water is in constant tension. Compression strength is mainly needed to hold up the walls. Not only that but the water pressure will actually help you maintain a favorable shape rather than work against you.

    But if you have to have a square then you need to abandon the simple structure you have proposed for something more substantial.

    Specifically, by using horizontal laid truss elements, along the top, middle (or thirds), and base of tank. These should anchor into similarly built vertical members to not just hold the whole together but buttress the corners, and probably the midsection of each side. These wouldn't be necessarily difficult or expensive to build from ordinary mild steel tubing. The actual tank walls could be made like the round tank shown earlier, with the wire mesh being part of each panel.

    The working foundation should be likewise, not just a slab, but it should support the slab used. Some provision in the working foundation for leveling will be important because any twist in the tank will concentrate forces in ways that may well be ultimately unhealthy for both humans and koi.

    The tank bottom can then be backed by sections of plywood under the liner. You can also have an external shell on the whole structure to spiff it up a bit.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Then either you have not read enough..or don't understand how to use the tables/data for design.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Making the panels to withstand the stress, easy, making it so it is easily assembled/disassembled, yet stays watertight, not so easy without a liner, imo. Zac is linerphobic.
     
  6. Ooks
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    Ooks Ian

    I am usually asking questions rather than responding, but I think I can add something here.
    I also would recommend you go with a round tank and liner, but if you are adamant about a square tank I would recommend still using a liner and keeping the walls just for structural stability.
    At only 4' of water you really don't have much have any significant load - I work in metric but at only a ~1.2m you only have ~ 12kPa. Any floor should handle that (a guy rocking his chair back probably creates more!).

    You would need to check that whatever is holding the floor up can handle the total additional load from the tank (~7200kg)

    I did a quick bit of googling and this is a good guide to calculate the load on the walls here - url]http://udel.edu/~inamdar/EGTE215/Hydrostatics.pdf[/url]
    I have not done the numbers but I am fairly confident any composite panel will not have an issue with the loads you will encounter from the proposed design. Should be pretty easy to check on the hexcell document.

    The challenge will be in the corners where you connect - the load from the whole panel is now held by a small area, so the forces are much much higher. In your case you are reducing the panels in this area and transitioning to a very small surface area - so I would not at all be surprised to see it fail. You could try using something more like a finger joint with a decent pin, or use pillars??

    If it was me I think I would just avoid trying to manage the loads at that area with joints altogether. Make the panels so they can freestand without load (2 panels hinged together to make an L shape?) Then use a strong collar to hold it together from the outside - simplest would be some tie down straps. (bit like a picture framing strap)
    [​IMG]
    Again no numbers crunched but I am sure a 1 or 2" strap will be orders of magnitude above what you have to worry about. One at the bottom and maybe centre with a nice timber trim around the top - she will look lovely!
    PS this is really just a sneaky way to make you go to a round tank as the strap achieves the same result.
     
  7. waikikin
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    waikikin Senior Member

    Every summer we buy an intex 8' round inflatable ring pool & set up in a fenced courtyard as a plunge pool just to cool off... about $70.... end of summer chuck it out, they do 10' in similar style, I reckon that using them and a few isocubes you would be set, other than that build a big strong scow & use for fishing on the inside or outside, it could fit on a stout trailer & block it really really well when you fill it.... the dimensions would have to change, maybe 2' deep & x 2-2x5 longer to accomadate your h2o.

    Jeff
     
  8. Ooks
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    Ooks Ian

  9. Petros
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    Petros Senior Member

    hydraulic forces on a 4 ft high tank are almost double than a 3 ft high tank. If it is only used occasionally, why worry about how much "real estate" it takes up. If the forces are much lower, than the frame and liner can be much lighter and compact to transport, faster to set up. It will take up much less room when packed and stored, which will be most of the time according to your description.

    I would also advocate for a round tank, you can use an accordion like fence (designed line those round portable pet pen) with a several Kevlar webbing straps at top, center and bottom edge to hold in the water pressure. The liner you can be made from heavy duty polyester reinforced truck tarp, glue the seams with solvent type cement. It will be durable, inexpensive and easy to repair if necessary the kevlar webbing belts take all the pressure.

    No need to worry about the liner lifting up the side walls, there is no lifting force on the assembly, all hydraulic pressure is perpendicular to the surface. Particularly if the liner is no taller than your fence, it can not lift up the fence, it would have to lift the weight of the water in the liner.

    The floor loads will not be insignificant 250 lbs per square foot for a 4 ft deep tank. Floors rated for that much loading are only commercial warehouses, and public roadway decks. A reinforced slab on grade should be okay, if it is not steel reinforced you will almost certainly develop surface cracks. 3 ft deep would only be about 190 lbs/sf, still very high, but not nearly as likely to give you trouble on a commercial building (they are typicality rated from 100 to 150 lb/sf or higher, for permanent loads, temporary transient loading should not be a problem).

    Something like this would be very fast to set up, stretch out the fence and attach it to itself, unfold and throw in the liner, affix the straps around the outside, and you are ready to fill it. It would only require six parts to be unpacked and assembled.

    You can use the same concept on a square tank, but the frame would have to be a lot heavier. Not need for full panels, just a fence like frame that assembles from four panels. It could even be made from varnished wood, would look much nicer for trade shows.
     
  10. Zac Penn
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    Zac Penn Junior Member

    The later is correct. I do not know how to interpret those data sheets into finished laminated sandwich cores. The testing is on the core material alone. It doesn't say what the finished sandwich strengths would be.

    I am not linerphobic, but I am completely against vinyl liners due to their tendency to crack and leak quite quickly. I will be using an EPDM fish safe rubber liner in this tank which does not have the weaknesses of the cheap vinyl liners. "well just throw the EPDM liner in the round swimming pool tanks" you say. Flat liners do not lay nicely into round cylinders. There will be a hundred little wrinkles and folds in the liner which not only looks bad, but it traps debris from moving on to a drain in the tank. With a square or rectangular tank you can create very nice blind folds in the corners so there are no wrinkles or visible folds in the tank.

    Thank you for that. Those are the type of calculations I was trying to find but hadn't been able to yet. I still do not know how to use the Average Pressure, Force on Wall, and Center of Pressure calculations to determine the proper core material and laminating schedule to achieve enough strength for a flat wall.

    I understand you concern about the corners being the weak spot. I agree and will build them up to a thickness that is necessary to handle the load. I just need to know what that thickness would be. I like the simplicity of the straps as a retaining structure and if the extruded channel idea doesn't work I go there as a last resort.

    Oh man that looks nice but I think it requires a $99 subscription to the website. Might be worth it in the end if it can calculate sandwich structure though.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I think you need aluminium extrusions your 4 side panels can slot into at the ends, plus a bottom and top rectangular slotted cap. You dont need any base, your liner can sit direct on the concrete. or if it is sitting on the dirt, just rake some loam to give an even base. 4 panels, 6 aluminium extrusion pieces, plus your liner. Your panels really won't need to be built like a brick outhouse to handle 3 feet of water. Those kiddies pools tells you that, the square ones I mean.
     
  12. fredrosse
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    fredrosse USACE Steam

    Composite Panels

    It looks like you can get exactly what you are originally asking for (8ft x 8ft x 4 ft deep, water filled) with a cored fiberglass/epoxy skin, on the order of 2 inches thick. Stresses and bending deflections meeting your requirements can be met.

    While I am an engineer capable of calculating all of this, there should be a more lengthy examination of the trade-offs to arrive at an optimized design. For example, the thicker the panel, the greater strength per unit panel weight. But you don't want the outer epoxy/fiberglass skin to get too thin, so a lower thickness panel (with thicker fiberglass skin) might be preferred. Then there are the cost considerations, thinner skin with higher strength fabrics, what is the weight/cost tradeoff? Plus several other technical trade-offs to examine here.

    The corner joints (4 ft vertical seams) will have a separating force of about 5600 pounds, this is doable, but does require proper design development. You need to engage an engineer to design this puppy, with adequate knowledge of stress analysis. I would think about 10-20 hours work with an engineer who is familiar with the materials to be used, but of course you could spend far more just for the design if you don't get the right worker. Make sure the calculator knows all about shear flow!
     
  13. philSweet
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    philSweet Senior Member

    Okay Zac, we are starting to get the picture. For trade shows, will you be adding additional landscaping around the tank? also, when not at trade shows, will you really have a concrete floor all the time?

    I've done some work in instant containers, and I've also worked with resort landscape fountain filters and pond filters. The filtration system is completely separate from the tank, yes? So you have some fittings in the liner near the bottom for hose connections that need to be accommodated as well.

    I'd use a gusset at the corners top and bottom. The four foot vertical span is nothing compared to the 8' horizontal span. If you used 12" gussets, you cut the span down to six feet, and this will cut the weight of the panels a fair bit. 16" gussets would be even better.

    So to set up, you place four corner gussets on the ground and connect them with flat struts on the ground that also stop your liner from creeping out under. Then erect the panels, run a ratchet strap around tempoarily, add the liner, connect liner fittings to panel fittings with hoses, fold the corners and drape drape over the panel tops. Then add the top corner gussets and panel caps, which fasten to the gussets.

    As far as the panels go, try the prefab Al/foam 2" mobile home or trailer panels. If you can find four refer trailer doors, even better, but maybe a bit pricey.
     
  14. waikikin
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    waikikin Senior Member


  15. Zac Penn
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    Zac Penn Junior Member

    Thank you very much for your detailed remarks. I completely understand your thoughts on strength per panel weight by going with a thicker core, but I would rather have a thinner core and thicker skins for durability during transport/storage/assembly/disassembly. We all know I am going to try an assemble this thing by myself and there will be times that a panel will fall to the floor. I would rather the skin be extra tough to handle these stresses as well as the constant pressures from the water pressure.

    As for the vertical seam separating force of 5600 lbs. That force will be the same no matter how the corners are joined together, so in your opinion which joining option would be more evenly distributed? Gussets at the top and bottom or the vertical extrusion over the entire 4' vertical length? I may be complexly wrong here but the reason I chose the extrusion is to distribute the force more evenly across the entire panel to reduce the chance of overloading a single small portion of the panel.

    I am sure it is a complex equation but would you be so kind as to explain a little on how you came to that number? Where that force is actually calculated from (position wise on the tank)?

    I will be the fabricator of the tanks along with my full time head fabricator at my shop. We are going to have FGCI.com's technical support team help us build the first one and go over all the details about proper layup, wetout, vacuum bagging (if we go that route) etc...

    Thanks again for the help,
    Zac
     
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