Bracing reasons

Discussion in 'Anything DIY Related' started by AdrianTregoning, 17 Feb 2014.

  1. AdrianTregoning

    AdrianTregoning

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    Good day,

    Calling any engineers here who can shed some light.

    I did a quick finite element analysis on a tank I'm designing. My choice is no bracing. I have a shop who is insistent on adding bracing. I'm a mechanical engineer but definitely no FEA expert so have a look at the design with and without bracing. Did I make a mistake or is the real reason for adding bracing to bring the maximum stress away from the bottom join as the results imply?

    With a max stress of 14 MPa and glass yielding at 70 this gives a FOS of 5. An online calculator gave me 4, and two other Excel sheets I downloaded around the 6 mark. So the stress values seem to be legit which makes me think the results I obtained are accurate.

    It is quite interesting to have run this and definitely something to examine further. I think too many people do things on tanks because it was done before but without seeing why. I like the why. Any thoughts?

    Cheers,
    Adrian

    [​IMG]

    [​IMG]
     
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  3. JJF

    JJF

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    Wow, this is very interesting. I am also an engineer, but of the electronics kind, and have also wondered about this.

    If I may ask, what would be the effect of a point load at the middle top of the glass, say someone pulling or pushing or bumping against it while under full distributed load? I suspect that most people get very nervouse to see a pane of glass bend, at least I do, even knowing that the design is overdesigned by a factor of 5.

    My own tropical 1.2m tank is made from 12mm glass with longitudial bracing as well as two crss bracings. Let's me sleep like a baby at night... :)
     
  4. dallasg

    dallasg Moderator MASA Contributor

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    can you give us the dimensions of the tank
     
  5. Gareth0508

    Gareth0508

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    Awesome stuff! I have braced my tank for peace of mind. Interesting to see where the main stress points are. Is there any bracing on the bottom of you rendering?
     
  6. AdrianTregoning

    AdrianTregoning Thread Starter

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    With a 50kg point load and no bracing, note where it shifts the max stress to, the opposite side:
    [​IMG]

    Outside dimensions are 1320 x 470 wide x 512. 10mm sides, 12mm base.

    No bottom bracing.
     
  7. RiaanP

    RiaanP Moderator

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    Think on that tank you can get away with only bottom bracing in place. That would strengthen the long sides a lot preventing bowing at the bottom.

    10mm glass with actual water depth after taking the bottom pane in mind would be about 470mm
     
  8. JJF

    JJF

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    Ok, so am I correct in deducing that, unless the glass gets a serious knock, it should not break due to tthe water pressure?

    Something else that I note though, is that the unbraced tank tends to conentrate the forces at the top corners and bottom seam. I am not sure what the nature of theseforces would be, shearing or otherwise, but the forces on tthe silicone seals would be unbalanced and may cause it to split open eventually, especially at the top corners? The braing moves tthe forces awayfrom these possible failure points?
     
  9. AdrianTregoning

    AdrianTregoning Thread Starter

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    How serious is serious? :whistling:The 50kg force would be a point load applied evenly for the sake of this calculation. But a screw driver thrust against the glass for a split second at a greater force? Well, these things become more complex and are beyond me. Interesting to note if you change the direction of the force 180 degrees - say you're moving a big rock on the inside and bump the sides with the same force, suddenly you bump it up to 20 MPa. Which is still safe. :biggrin:

    But anyway, the question was the reason of bracing, and the dimensions of this tank are irrelevant unless someone wants to duplicate the experiment on their chomps :p
     
  10. JJF

    JJF

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    Well, as you say, jamming a screwdriver into the glass is bound to give you problems, but that has nothing to do with bracing.

    But any thoughts on my question regarding the fact that the bracing moves the location of the max forces away from the seams? Ive heard of a few tanks splitting on the seams, due to the silicon failing.
     
    Last edited: 17 Feb 2014
  11. AdrianTregoning

    AdrianTregoning Thread Starter

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    Well here is another interesting point to consider:

    displacement: 2.872mm without bracing.

    displacement: 0.678mm with bracing.

    The bracing reduces the deflection which is obviously a maximum at the top. How it reduces the stress so much at the bottom is either an error in my methods, or reality. The bracing only reduces the stress by around 0,5 MPa. Which is minimal. However displacement is much improved and people are afraid when they see the bow - even though it's harmless. :eek:

    I may play around with this some more. However I would really enjoy it if someone could tell me if what I'm doing is correct and if I did go wrong, where and how.

    It's good to challenge beliefs. And I hate rules of thumb where people do things by rote. It annoys me! :lol: Maybe this thread will open our eyes a little :1:
     
  12. AdrianTregoning

    AdrianTregoning Thread Starter

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    Just a single 50 x 12 cross brace reduces the stress much better, and also gives minimal deflection. Very interesting result...

    [​IMG]
     
  13. Dian6430

    Dian6430

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    This is a great thread. My new tank keeps on cracking the front brace. Sides and back brace no problem but front stays for 3 days then it cracks. Any idea? Tank is 110x65x60 out of 10mm glass. Bracing is 6mm thick and 5cm in width. The second brace that cracked was 2x6mm pieces. Also front pane doesnt really bow
     
  14. AdrianTregoning

    AdrianTregoning Thread Starter

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    I would say make the brace same thickness as the tank, ie 10mm. Even make it 12mm. ASSUMING my model is correct there is a lot of stress in the bracing itself so I would go with 12mm. Very interesting comment. Thanks for that, never heard of only bracing cracking but this does indicate that the models I produced might be accurate then. :thumbup:
     
  15. mariusmeyer

    mariusmeyer

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    What tool do you use? Would love to run my current tank through it to see.
     
  16. AdrianTregoning

    AdrianTregoning Thread Starter

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  17. Dian6430

    Dian6430

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    Think im going to use 10mm bracing and make it 8cm or 10cm wide. Just very strange that none of the other bracing is showing any signs of problems
     
  18. AdrianTregoning

    AdrianTregoning Thread Starter

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    That is strange. My only guess is the tank is not entirely level. Increasing the width will help for sure. But definitely 10mm thick.
     
  19. Dian6430

    Dian6430

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    Why would the level play a role? And wont that then crack the back brace as well?
     
  20. RiaanP

    RiaanP Moderator

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    bottom brace, like in euro bracing...
    Right around.

    Will give a lot more surface area for the silicone to adhere to, increasing the tank integrity a lot. And it will prevent tank splitting around the bottom edges.

    Some rimless tanks have another 10mm (or more) glass strip up in each corner, to increase the bonding of the silicone.
     
  21. RiaanP

    RiaanP Moderator

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    That bracing is not strong enough.
    Maybe if its a double layer, up to 12mm then maybe.
     
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