How does one work out the glass thickness on a brace less / rimless tank?

Discussion in 'Anything DIY Related' started by Express Reef, 23 Oct 2013.

  1. Express Reef

    Express Reef

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    Is there a formula for this?
     
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  3. Visser

    Visser MASA Contributor

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  4. Express Reef

    Express Reef Thread Starter

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    Thank @Visser, but the calculator is nog good if you go rimless, for 1200 x 1000 x 600 with water level on 550 it gives side glass ass 10.3 and base as 12.2, and I know for a fact its going to pop... was thinking off 15mm glass but once again the glass bend slightly... and I`m not a huge fan off lamitaned glass...
     
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  5. brentv

    brentv

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    Following with interest.
    I'm also deciding what glass thickness to do for my build.
    A few factors to consider for me:
    Thicker = Safer
    But Thicker = less clear (Unless Low Iron)
    Thicker = heavier duty magnets
    Thicker = price double

    My thoughts are to do a thin euro brace trim around the top, so still get the rimless style as such, without the snails etc escaping to easy.
    This also helps with a bit of that bend.
    But will follow this, coz the calculator doesn't factor in rimless (I think?:blush:)
     
  6. Express Reef

    Express Reef Thread Starter

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    :iagree:

    since @Visser is the engineer think he must design a new calculator for MASA:biggrin:
     
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  7. Visser

    Visser MASA Contributor

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    I have gone through the calcs on the spreadsheet, & it is factored for rimless tanks. (meaning - only fixed on one of the horizontal edges & 2 vertical.)
    The calculator calculates the bending of a glass pane with the forces that would be exerted by water on the pane if it was freestanding (supported on 4 corners, but not fixed).

    The maximum bending of a "fluid" retaining container is considered to be at one third from the bottom, & thus, the forces on the top edge would be substantially less.

    If you add enough bracing to a tank, you can make any tank from 3mm glass...

    If you want to play it safe, you can simply increase the "If side glass thickness is changed to..." value until you get a safety factor (in green block) of higher than 6

    That way you know it will be safe...

    Maybe some day I will create a new spreadsheet to make a selection between braced & rimless. (Just need to go through some of my study notes again, lol.) But not now...
     
    Last edited: 23 Oct 2013
  8. Visser

    Visser MASA Contributor

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    @Express Reef... There is no way on earth you can build a tank with "economical thickness glass" without the glass bending... It will always bend... Only have to keep the bending limited to less than max bending capabilities.

    Just as an example... When I still kept freshwater tanks... I had a 1800 x 1000 x 1000mm deep tank (without bracing) & the side glass thickness was 16mm & bottom of 20mm, & I kept the tank for 2 years without any problems... (slightly under factored, but it held up)
     
    Last edited by a moderator: 26 Nov 2015
  9. HOT SAUCE

    HOT SAUCE

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    i think most calculators will calculate the glass thickness as its safe and wont crack but the bending factor is difficult to calculate because of the different quality types of glass with different strengths
    probably best way is calculate and the result you get add 1-2 mm or increase the safety factor
     
  10. Visser

    Visser MASA Contributor

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    Glass is a very difficult material to do calculations for in structural elements as there are literally hundreds of factors that could affect the glass properties.
    All calculations are done according to specifications of commercially available glass & the best you can do is to calculate enough of a safety factor to overcome the weaknesses in glass.
     
  11. JackSplat

    JackSplat

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    If I can throw in my 2cents worth…

    The thickness of glass for a given load is a very complex problem because glass will fail due to crack propagation and not, in contrast with ductile metals, due to plastic yielding.

    If a pane of any solid material is bend the outside surface is in tension while the inside surface is in compression. In a metal the stresses are mostly homogeneously distributed because of the crystal structure and the ductility of the material. This has the benefit that the whole pane (in this case a sheet) carries the load equally and it is not likely that the material will fail under static loads.

    In glass, because of its brittle nature, any imperfections can cause stress concentrations which may give rise to a cracks, leading to failure. (This also happens in ductile metals, but mostly only under cyclic fatigue)

    Now, for a rimless pane on the side of a tank, the maximum deflection will be on the top edge. It is very likely that there are imperfections on this edge due to the cutting process, especially if the edge wasn’t finished properly. These imperfections in conjunction with high deflection will cause stress concentrations and may lead to cracks and failure if the stress concentration is large enough.

    If you put a rim on a tank the likelihood of these stress concentrations are reduced dramatically.

    If you are going with a rimless design, try to get glass with as smooth as possible top edges and go for as large as possible safety factor without breaking the bank (using the calculator).

    The alternative would be to do a FEA on the tank but that would be expensive, unless you are building a monster...
     
    Last edited: 23 Oct 2013
  12. dallasg

    dallasg Moderator MASA Contributor

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    the calc is made for rimless, it doesnt factor in bracing
     
  13. dallasg

    dallasg Moderator MASA Contributor

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    bracing stops flexing/torsion, it doesnt add to the vertical strength of the pane, thats where thickness comes into it.
    so factor your height first, then work on length of pane and adjust to your needs and safety of 3.8
     
  14. Visser

    Visser MASA Contributor

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    Ive done FEA's on all my own tanks + some for friend's tanks on solidworks, & the weakest point always turns out to be at the bottom 1/3rd of the pane.
    Unfortunately, solidworks doesnt take imperfections on cut edges into consideration, but a tanks is a lot more likely to crack from imperfections at the bottom than at the top...

    If anyone needs an FEA analysis for their tanks, I would be willing to help out, but just take into consideration that it takes me about a week to complete one depending on time I have available...

    & please... Dont even ask for an FEA for a normal tank under 300L... Only odd shaped tanks or tanks with unusual dimensions...;)
     
    Last edited: 23 Oct 2013
  15. leslie hempel

    leslie hempel Moderator MASA Contributor

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    hey dude, my apologies i remembered you send me a design sometime ago but i havent had sketckup installed on my pc to view it..


    i definatley believe a tank of that size is best built out of 15mm with bracing around the bottom, allowing an extra bond area for the glass at the base...

    as for 15mm it comes beautifully cut (laser i think) and bevelled on the edges.. its s superb finish in contrast to what we get with even a "poilished" edge in 10 or 12mm

    depending on your overflow it will be likley that only 3 of the sides are at risk of bowing as the 4th will have an overflow attached to it (have not seen the design yet)

    an important thing i urge you to consider is to go approx 7cm higher with your total glass height from your proposed water level, this will help to a degree with jumping fish and eliminate glass hopping over a lower edge when wavemakers are used and when cleaning the glass with a magnet cleaner..
     
  16. Express Reef

    Express Reef Thread Starter

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    Thank you les,
    So I must rather do 1200 x 1000 x 620 with water level on 550? The glass will be bright polished or 1200 x 1000 x 600 with water level on 530?
     
  17. dallasg

    dallasg Moderator MASA Contributor

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    my new tank is 1250x1000x500 in 12mm starphire, and got a good safety rating
     
  18. Express Reef

    Express Reef Thread Starter

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    Where`s the thread? And pics... or it did not happened
     
  19. leslie hempel

    leslie hempel Moderator MASA Contributor

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    i cant speak theoretical.. i speak in practice :tt2: ! my system of 1.500 x .800 x .650 has a small bow of 1-2mm and it stressed me at first but if i had hone 50mm lower in depth it would change the bow situation..

    the bow is on the 1,5m mane (back is braced with internal center overflow.. so on a 1.2 i cant see a bow occurring...
     
  20. leslie hempel

    leslie hempel Moderator MASA Contributor

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    +1 :whistling:
     
  21. JackSplat

    JackSplat

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    This does not sound right, I would agree if the tank is properly rimmed, but on a rimless tank I would expect the maximum deflection at the top, and hence the greatest likelyhood of it cracking there, or am I missing something...
     
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