Silicone and bracing

[805HD]

Back story: I acquired a 72 x 18 x 30 170 gallon rimless tank that had some pretty crappy top bracing. Clearly a mix of the manufacturer and previous owner/ owners attempting to add strength to the tank. I decided to remove said bracing and start from scratch in hopes to clean up the look of things in addition to achieving some structural gains.

Considering location of the of the overflow weir and with access to plumbing in mind a traditional euro brace was not going to be an option so I proceed with a design that sits on top of the tank utilizing shear strength instead of tensile. A gamble, but I rolled the dice. See pictures.

I used eight 4” strips of 3/8” glass adhered to the top of the tank using kraken brand 100% aquarium silicone which had more than ample time to cure.

Confident in my resulted bracing I filled the tank carefully. I’m aware that 1/2” glass at 30” tall is not the most ideal when it comes to bowing/ glass deflection. I knew undoubtedly, the two center braces running front to back would be under the most stress being closest to center of the tank. Nonetheless, the tank was filled and everything seemed to be just fine.

Until it wasn’t.

24 hours in to holding water I heard a huge pop/ thud sound from the fish room. My fears were confirmed. The two center braces running front to back had let go at the silicone seam under shear force. See pictures.

After changing my shorts to an unsoiled pair i immediately started to drain water from the tank. Noting almost 5/8” of deflection in the glass. Insane. So much movement. Undeniably barely dodging a bullet that would have put 170 gallons of water on the floor. Pure luck if nothing else.

So if you have made it this far, the question is obviously where to go from here?

My new plan is to replace the two center braces that let go from 4 inch to 10 inch wide glass. Which is the widest I can go while still having access to the weir plumbing. This will add 6 additional inches of surface area. That plus the use of a stronger silicone like RTV108 which has a shear strength of 200psi should offer a substantial gain in strength from the original design and material. I can’t find the shear strength spec on the silicone I used but the tensile is only 174psi so I think it’s safe to say the RTV108 is superior in comparison.

Does this make sense or am I setting myself up for another soiled pair of shorts and potential soaked carpet?

Any and all input is more than welcomed and appreciated!

 

[Timfish]

That had to be an unnerving experience! I would stick with the 4" wide glass strips. It looks like the problem was with the black silicone not sticking to the glass cross support. If it was my tank I would get an industrial strength with 400 PSI or more (157 is 1000 I think) making sure all the joint surfaces are cleaned real well with. I would also get 1/4" glass rod and reinforce the inside of the joint.

 

[805HD]

@Timfish it was! I was literally standing maybe 10 feet away. Heard the thud and the light clamped on top the tank was shaking like someone SMACKED it. It’s amazing that glass didn’t give.

How exactly did you do that math? I was trying to figure out how to do that. Take the psi value and multiply it by the length of the seam?

From my research that RTV108 silicone is the way to go. That’s the consensus anyway.. but I’ll look into it. Technically speaking this is above the water line so I do have options.

Also glass rod.. I’m assuming you’re suggesting to silicone something in to the bottom of the brace and the inside of the of the front and back glass? Kinda like a inside seam so to speak?

 

[805HD]

Also, if my math is "mathing" correctly square inches is measured by multiplying length X width.

So, 4" x 1/2" gives us 2 square inches.

The kraken has unknown shear strength. I'm going to call it 87 PSI which is half its tensile strength giving a value of 174 PSI @2 sq in.

The RTV108 has a shear strength of 200 PSI giving a value of 400 PSI @2 sq in.

Do those values multiply since there are four points of contact between the two braces or does it stay the same regardless?

My guess is it DOES NOT multiply by points of contact.

If my guess is true, I had somewhere in the neighborhood of 174 pounds of shear strength up against 1,411 lbs of water force against a 30'' tall piece of glass? It's amazing it held as long as it did.

Even the RTV108 wouldn't stand a chance in my opinion.

Increasing surface area contact (wider glass) would increase those number dramatically, as well as adding in tensile strength by @Timfish suggestion of a glass rod creating an inside seam.

Is this theory jiving with anyone else or am I way overthinking this here?

It's starting to make sense to me but again, I'm not a numbers guy or an engineer... Clearly.

I'm ultimately trying to determine if this was a adhesion issue or a strength issue.

 

[Timfish]

@Timfish it was! I was literally standing maybe 10 feet away. Heard the thud and the light clamped on top the tank was shaking like someone SMACKED it. It’s amazing that glass didn’t give.

How exactly did you do that math? I was trying to figure out how to do that. Take the psi value and multiply it by the length of the seam?

From my research that RTV108 silicone is the way to go. That’s the consensus anyway.. but I’ll look into it. Technically speaking this is above the water line so I do have options.

Also glass rod.. I’m assuming you’re suggesting to silicone something in to the bottom of the brace and the inside of the of the front and back glass? Kinda like a inside seam so to speak?

I was using the given PSI rating for Momentive RTV, 108 is 400 PSI and 157 is 975 (108 is much cheaper), wasn't really doing any math.

I'm not an engineer but I think you need to figure in all teh joints when calculating the load on a specific area of a joint.

Using a glass rod on the inside of the joint give you both tensile and shear strength. But from the pics you posted it looks like for some reason the silicone just didn't stick to the glass used for the cross bracing, it looks like it came off very clean.

 

[805HD]

@Timfish I hear ya. I’ve done a boatload of more research over the past few days. I ordered some glass and went with rtv 103 same as 108 but it’s black the rtv157 only comes in grey so I passed.

I think between widening the top cross braces, adding some glass to the inside seam and using the rtv103 I will have significantly beefed things up in both shear and tensile strength. Should be more than adequate this time around. From my calculations we are talking about more holding power than water weight.

That kraken brand stuff is more of a sealant than anything else. Doesn’t really offer much as far as structural stuff goes.


Gonna make sure glass is surgical clean and give ample cure time.


Something along these lines.