Braces are not holding properly

[Pablos]

I have a tank:
Length: 170cm = 67"
Depth: 70cm = 27.5"
Width: 60cm = 23.6"
It's been built from 12mm crystal glass.

On top of the tank I have four braces as you can see there is also small triangle in the corner as well as overflow box is built (inside) from 12mm glass.

It should be heavy duty setup ... but my braces don't hold as expected!
As you can see on the pictures below two central braces are going away from front and back side. Front and back is bowing, so top of the tank is wider around 8mm, out of original 60cm.
Braces are still holding, but it doesn't look proper.
Braces has been glue with SMP polymer adhesive (Bison Poly max Crytal Express)

What would you advice to do:
1. leave it as it is?
2. change design and add more? braces?
3. maybe glue is not good, but what I know SMP is much stronger than silicon. ( see Polyurethane and Silicone vs Polymer | Allfasteners Australia )

 

[T-J]

Did this bow before you even put water in, or was this after you did a leak test?
I'm not familiar with that bonding material, but looking at their website, I wouldn't trust that stuff in my aquarium. It's a polymer and I don't know what it may leak into my tank.
If you look around the forums, everyone uses 100% pure silicone on aquarium/sump builds.
Also, looking at your last picture, I can see lots of air bubbles in your seam, on both sides. I wouldn't trust that seam.
Pull it apart, clean everything, re-do with pure silicone.
Good luck!

 

[Joe Rice]

If you look around the forums, everyone uses 100% pure silicone on aquarium/sump builds.

I'm always amazed at the strength of silicone when bonding glass to glass. But you don't have a lot of surface area to work with there. You might consider doing something like the tank shown in the video to increase the bonding area.

He's basically combined a euro-brace with cross bracing presumably to reduce the width of the euro-bracing. Of course, you could just do a traditional euro-brace.

 

[Pablos]

I'm always amazed at the strength of silicone when bonding glass to glass. But you don't have a lot of surface area to work with there. You might consider doing something like the tank shown in the video to increase the bonding area.

He's basically combined a euro-brace with cross bracing presumably to reduce the width of the euro-bracing. Of course, you could just do a traditional euro-brace.

It's a really interesting setup, however the guy hides the top of the tank behind the wall-panel, so without a panel it might not look great.

 

[Pablos]

Did this bow before you even put water in, or was this after you did a leak test?
I'm not familiar with that bonding material, but looking at their website, I wouldn't trust that stuff in my aquarium. It's a polymer and I don't know what it may leak into my tank.
If you look around the forums, everyone uses 100% pure silicone on aquarium/sump builds.
Also, looking at your last picture, I can see lots of air bubbles in your seam, on both sides. I wouldn't trust that seam.
Pull it apart, clean everything, re-do with pure silicone.
Good luck!

Thanks for an advice. I might try with a pure silicone instead.
It was fine with a water test, even a day after. Maybe after 2 days I noticed 2-3mm bowing, than 3rd day 8mm
Alternatively I think about adding stainless steel frame around the top.
Do you think those 4 braces would be sufficient if a pure silicone would be used instead?

 

[T-J]

Thanks for an advice. I might try with a pure silicone instead.
It was fine with a water test, even a day after. Maybe after 2 days I noticed 2-3mm bowing, than 3rd day 8mm
Alternatively I think about adding stainless steel frame around the top.
Do you think those 4 braces would be sufficient if a pure silicone would be used instead?

I'm not a tank building expert, so I'm not going to guess on whether those braces will hold or not. I can tell you my 120 has a single plastic rim with a center brace. My 125 had a single plastic frame with two braces. At 6 feet long, I would THINK you'd need at least 3 braces, but I'm no expert.
You can also look at using pre-made ones. Here's a site I found:

Over The Edge

Industry Leader in Wholesale Plastic Aquarium Tank Frames,Aquarium frames,Aquarium Lighting and Iron Stands

www.aquariumframeswholesale.com

I've never used them, so don't hold me to their quality or cost.

 

[Brett S]

Your braces are sitting on top of the edge of the glass whereas I believe that normally glass braces are installed even with the edge of the glass so the edge of the brace is bonded with the flat side of the side glass.

This will cause the force to try to pull the brace away from the tank glass which I think is a much stronger connection than the way yours are configured where the force is causing the brace to sheer away from the tank glass.

Also I think that typically the braces would be made of much thicker glass to provide for more surface area on the end of the brace where they would be bonded.

 

[Pablos]

This will cause the force to try to pull the brace away from the tank glass which I think is a much stronger connection than the way yours are configured where the force is causing the brace to sheer away from the tank glass.

As the thickness of braces is the same as thickness of glass of the fish tank, the surface area is exactly the same for both cases. Does angle of the forces make a difference there?

 

[Qtfrags]

Just cruising by check out how I braced this 40b I took the trim off of. Also consider when you go to scape/stock the tank how the braces will affect positioning/maneuvering. I used the same thickness glass with tank and brace along with 100% GE silicone

 

[Brett S]

As the thickness of braces is the same as thickness of glass of the fish tank, the surface area is exactly the same for both cases. Does angle of the forces make a difference there?

That’s good. It looked much thinner in the pictures, but I guess it was just the angle.

However, I believe that the glue does have less strength against sheering.

 

[Soren]

As the thickness of braces is the same as thickness of glass of the fish tank, the surface area is exactly the same for both cases. Does angle of the forces make a difference there?

I have no experience yet in custom tank building, but I am pretty sure from research and general principles learned in my engineering occupation that the angle of the forces makes a significant difference here. Silicone should adhere glass much better when in tension (brace ends being pulled by the side walls) than when in shear (current setup where water pressure outward on sides is trying to slide along the surface of the brace).
It will also matter how much silicone is in the gap between the two pieces of glass. As I have studied custom tank building, I have read that the 1/16" gap between pieces is critical, since the silicone in this gap is what provides the most strength (not the external bead in the corners).

 

[GainesvilleReef]

Both the tensile and shear strengths of the joints are proportional to the connected surface areas. A normal euro braced configuration would have a strip across the front with a much larger connected surface area. The rigidity provided by the perpendicular brace across the front prevents the bowing and the length distributes the shear force. In this configuration, you have concentrated shear forces and very little rigidity along the length on top of smaller connected surface area.

 

[Pablos]

this configuration, you have concentrated shear forces and very little rigidity along the length on top of smaller connected surface area.

actually surface area it’s not so small, there are 4 braces in total. Each is 20cm (8”) wide, so braces covers almost half of the tank length.
If you’re saying it’s not enough, what would be the better setup? Euro braces only?

 

[GainesvilleReef]

But it is 1/2 of the surface area and thus 1/2 the shear strength. Also, has half the stiffness of having a full-length brace.

Additional just reading a few things about that kind of adhesive. Apparently, they can continue to cure after the initial curing in the presence of moisture. It is permeable to moisture and the additional moisture will cause additional curing inside the material. And this can increase the volume of the applied adhesive. Where pure silicone will actually shrink.

So it could be all of these things combined.

 

[Pablos]

I made some research and those my observation.
First of all it make a HUGE different if braces are sitting on top of sides of a tank or within a tank. It's tensile strength vs shear strength. One of the recommended sealant RTV108 is two times stronger against tensile than a shearing.

Then I wanted an answer, is it enough with the current braces and the setup which I have, but just with a better sealant. I did some calculations.

assumptions:
- I omitted silicon strength on the corner and bottom of the tank, since actual pressure on top would be < 35%
- I calculated on full water volume ( 100% filled tank, no sand, no stones, fish ;-) which reduce pressure). Actual pressure would be <80%
- I didn't count small triangle, only four braces. Even if it's two time smaller, tensile strength applies there, so it would have same strength as one of braces. Actual pressure would be reduced, so total would be 80%
- surface area is count on glass/glass not a real sealant surface
- calculation are on one side, pressure on one side of a tank and surface from one edge of braces

maths
Total surface = 4 braces have a total 14.88 in2 surface
Total shear strength = 2,976 lbs (based on the RTV108 specs, Shear strength =200lbs/in2)
Pressure on side of a tank = 0.5 * 0.036 * 27"^2 * 67" = 879 lbs
Estimated actual pressure on top (without those estimates from assumptions) would be 0.35 * .8 * . 8 = 197 lbs only

conclusion:
4 braces have a strength of 2,976 lbs which is ~3.4 times more than pressure on front/back side of a tank.

Taking into account 'actual' pressure braces would be 15 times stronger than pressure ... quite a good safety factor

Does it make sense to you?

 

[Pablos]

I haven’t found GE SCS1201s nor Movement RTV108 in my local hardware stores.
I found only
- GE 1200GC
- Red Devil 100% silicone
- DAP 100% silicone window, door & siding

which one would be the best?

 

[GainesvilleReef]

Don't forget with the cross brace you have to include the forces on both sides of the tank. Each of the 2 joints has to handle the tension. Also, the center braces will be under more tension than the sides.

 

[Pablos]

Don't forget with the cross brace you have to include the forces on both sides of the tank. Each of the 2 joints has to handle the tension. Also, the center braces will be under more tension than the sides.

you are absolutely right. Calculation has been done for one side, but of course tension would be equal on front joint as on back one.

regarding central brace, for sure it would take the biggest load, but it's just estimation. in the same way I haven't added glass resistance / flexibility for bowing.

Do you have an experience with mentioned sealants ?

 

[Grill]

I've built a few tanks and have always gone with Momentive RTV series for the seams (usually RTV 103 black) ASI is another good brand. I like to go with aluminum bracing because its cheap, easy to acquire and easy to cut. Plus it gives it a clean industrial look. Everyone has their favs. Euro, rimless etc. Here are a few picks of bracing and seams. Good Luck.

 

[Pablos]

That’s an amazing tank! As alternative solution I’m thinking about having similar to yours aluminium frame.