Glass thickness calculators

[hobbzz]

I'm looking for glass thickness calculators that take in to effect bracing and tempering. The ones I've found using the usual google search turned up none that take these things into consideration. I've also seen the base formula and that too does not take this into acount. And NO, I'm not making the tank myself, just looking to make sure I get the right thickness. Thanks.

 

[nibor75]

All of the glass thickness calculators you find online are based on the calculation/values from this page:

http://www.fnzas.org.nz/?p=1732

They assume a theoretically perfectly braced tank, with no deflection of any edge allowed:
"The calculations that follow expect the glass to be supported around its perimeter on all four sides".

Tempered glass has a tensile strength over double that of normal glass. The Elastic modulus is pretty much the same. You can use the formulas in the link above and just double the value of tensile strength.

None of these calculators give any consideration to construction technique, strength of adhesive...... Experience trumps theory IMO.

 

[hobbzz]

Thanks for the link. Is there any way to calculate different kind of bracing to get a more real-world rating rather than just relying on an assumed "prefect" brace?

 

[nibor75]

Not without a much more in depth study.

Assumptions in engineering calculations are not so bad though. Imagine the edges of the tank are euro braced with a two inch strip of glass. The force from the water pressure on the sides of the tank are going to bend the glass bracing strip in the direction it is most rigid as the side bows out. It is going to bend very little and as long as the adhesive is strong enough the edge of the side of the tank will not move much either. Cross bracing will bring us closer still to that "perfect brace". The safety factors in the calculation give us a bit of room to play here. The real world is never an exact match for the theory

The plastic trim you find on mass market tanks typically has more give in it than a glass euro brace. Longer tanks have cross braces to offset this.

The calculations for rimless tanks are just as simple but you won't find a calculator for them online. You sometimes see posts where people say "make the safety factor 5 or 6 for rimless" but this is meaningless as the mathematics is not the same.

What dimensions are you looking at and what construction/thickness is being suggested? Why tempered, weight/cost?

 

[hobbzz]

10' x 4' x 30". They recommend 5/8 for that size and they do aluminum rims on all their tanks. Not sure how many cross braces with this length, but they do use them. They also offer tempering and they can drill before hand if you want tempered, which is why I asked. What's the standard safety factor before bracing if you plan on bracing? Thanks for the help.

 

[nibor75]

I would not use a different safety factor for different methods of construction or bracing. The safety factor is simply how many times the glass is "too strong" in a perfect world. Much better to use a method of bracing that is sufficiently rigid that we can consider it does not bend at all.

I am not sure where this 3.8 figure comes from originally but it sounds sensible - to allow for the tank being knocked, minor blemishes or weak spots in the glass and is reasonably conservative. The consequences of a tank blowing out could be considerable. The bigger the safety factor the better.

Glass is manufactured to metric dimensions. Glass sold as 5/8 is 16mm thick. 16mm glass has manufacturing tolerances of 15.09 to 16.66mm. Better quality glass production will be more consistent and closer to 16mm.

The first thing I would do is run one of the calculators you have found at the low end of this tolerance for the longest side (10'x30") : 3048mm x 762mm with a thickness of 15.09mm.

Personally, if I was having a tank of this size built, I would calculate the theoretical thicknesses for rimless and for perfectly braced at a safety factor I felt comfortable with. I would then compare this to the recommendations of a professional and experienced builder to see if they tally. If the budget allowed, I would always go thicker and not tempered.

I can run the numbers for rimless for you tomorrow if you like. I just need to dig out my copy of Roark's formulas for Stress and Strain. The constants for a plate supported on three sides are not the same as for four. It's in my office somewhere but where it is, your guess is as good as mine.....

 

[hobbzz]

No thanks, I can use the link you found. So basically 6-7 for rimless and 3.8+ if properly braced. Why would you say go thicker instead of tempered?

 

[nibor75]

That doesn't work for rimless. The calculation is not the same as the glass is only supported on three sides not four.

Tempered glass may be stronger but if it fails it shatters into tiny pieces immediately emptying all the tank contents on the floor.

 

[hobbzz]

That's a good point.