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Old 02-09-2009, 11:03 AM   #1
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Exclamation Advice for Repairing Octagonal Tank

I have an older ~50 gallon factory-made glass Octagonal Tank with dimensions of approximately 29" across and about 21"tall that needs repair -- the seals blew on the tank with lots of water everywhere.

The problems: the original octagonal plastic frames for the top and bottom of the tank have fallen apart, the silicone seals leak and need to be redone, and there was never a brace going across the top of the tank for additional support.

The good news: the glass is in perfect shape and was originally cut to spec for the octagonal shape.

Questions: Does this tank need an external octagonal frame around the circumference of the top and bottom for support? Alternately, would it be okay without any frame or does it need an additional support like a brace across the top? Does anyone have any advice or pointers on how to do this? The last thing I want to happen is for the seals to blow out again and all that water all over the floor.

Many Thanks,
Kathy
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Old 02-10-2009, 12:27 PM   #2
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Welcome to AA!

First of all a disclaimer - I have never worked on octagonal tanks. I have resealed a couple of rectangular tanks .... so some of this is extrapolation. Experienced tank builder please feel free to disagree & point out mistakes!

In a rectangular tank, you can do without the rim except in tanks longer than 2-3'. An octagonal at 29" prob doesn't need a center brace. But I rather suspect that the rim is helping the tank keep its shape. Personally, I would err on the side of caution & reinforce the joints.

The best solution is to find a replacement rim, but that might be difficult.

A euro-brace may be a DIY option. You basically silicone a stripe of glass 2-3" wide "on edge" to the side panes <at the top> to prevent the glass from bowing & to preserve the angle of the joint. <You don't need bracing at the bottom as the bottom pane acts as the brace.> This is easily done on rectangular tanks, but for an octagon, you will need some precision cut pieces, perhaps something like the picture <top view, red are glass braces>:
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Old 02-10-2009, 05:27 PM   #3
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Thank you for the advice! I've been looking extensively for a rim online that will fit to no avail. I might do your suggested Euro-brace option. What do you think about making a rim for the tank -- what material would be best? I've seen tanks built with plywood and plexiglass or glass -- I have a table saw and jigsaw and was thinking about making a rim out of wood, but am not sure if that is an appropriate solution. Any thoughts?

- Kathy
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Old 02-11-2009, 03:04 PM   #4
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I see a couple problems with a wood rim:

1. It will have to water-proofed very well or it will fall apart. Marine epoxy or a spar finish might be an option, but I don't think that any finish will last too long when in contact with water (as a rim would likely be). So you are looking at redoing the finish every year or 2.

2. It is going to be difficult to do joints <at those 6 corners> that is strong & still be reasonable thin & esthetically pleasing. I am thinking of something like a deeply channeled stripe of wood for the rim. But because of the glass that is sandwiched in there, you would need to have fairly thick pieces (maybe 1-1.5") to get enough surface area for gluing. Even at that thickness, you would prob only be able to do butt joints. I can't see anything like a 60 degree box joint in something that thin, and splines are out due to the glass .... And butt joints are weak.... I built my canopy & did some trim using butt joints. The joints <done with Type II waterproof Titebond> open whenever some moisture gets on it. <And I am messy enough to get water on my canopy with each pwc!> In my case, it is no big deal, as those are only trims & the canopy itself is properly jointed. <The gaps closes when the wood dries.> I would be wary of doing butt joints in a structural setting.

In summary, A wood trim would require some pretty fancy joinery to be both strong & airy looking.

I know someone had posted a DIY rim made of metal. He used channeled stainless steel & welded all corners. This might be another option if you have a welder.
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Old 02-11-2009, 03:15 PM   #5
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I like the idea of a channeled piece of wood. What do you think if I got one large and thick piece of wood (larger than the size of the octagon) and then use my RotoZip to make one confluent octagonal channel? I could then cut an opening in the top and trim out the sides with molding with my laser-guided miter saw. How much wider than the glass do you think I would need to make the channel to account for any normal expansion/contraction of the glass?

- Kathy
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Old 02-12-2009, 08:07 PM   #6
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Glass don't expand much, so all you need is maybe 1/16 to 1/8" wider in the channel to accommodate the silicone when you inset the glass. However, wood will contract & expand with moisture, so you will need some room to allow for wood movement ... maybe another 1/8" if you are doing all 6 sides with the grain. <Wood expands a lot more across the grain.>

I am not sure you can get away without doing any joints. You almost never see any solid wood blanks 30" wide. <Plus they cost an arm & a leg!> Plywood & other composites are not suitable for wet application. In addition, using a single piece like you suggested will put part of your rim across the grain. This is structurally weak & the large cross grain movement will likely cause cracks. <And if your channel is not wide enough, this will pull apart your tank seam!>

If I were to make a wooden rim, I would use 6 pieces, all running with the grain. <Ideally, you want to use Quarter-sawn clear hardwood to minimize wood movement.> You would need to have a fairly thick piece to properly join the boards. I might use a 1.5" thick piece & place the channel so there is 1/4" of wood outside the glass & 3/4" inside the glass. <That would leave a 1/2" channel ... good for ~3/8" glass.> I would do a mitered butt joint on the outside 1/4" for looks & a box or finger joint in the inside 3/4" for strength. That is a pretty complicated joint to cut, esp. at 60 degree. It would prob. need a table mounted router & some custom made jig .....
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Old 02-12-2009, 09:11 PM   #7
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Just a thought here, but you might be able to make a top rim out of hardie board. I'm not sure, but I think they make it in thicknesses that would work. Harder product to work with though and the joints would be tricky.
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Old 02-12-2009, 09:45 PM   #8
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Wood could rot, twist, swell, or shrink. Metal would be fine if using stainless steel so rust wouldn't be an issue. I would likely go with something fairly inert like acrylic to fashon the rims. I would do both the top and bottom of the tank with an band that takes the pressure off the joints. The bands would prevent deformation of the tanks shape and relieve some of the strain on the joints.
Just my opinion. The tanks I fix are made of wood(cedar) or steel and approximately 35' tall. Usually 100,000 to 500,000 gallons. Quite a different beast really. Good Luck.
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Old 02-17-2009, 06:13 PM   #9
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You might be able to buy an aquarium frame online or possibly a framing kit? A metal frame is the best in my view as it avoids all the shrinking /swelling problems of timber. You might need to track down someone who can weld though. Good luck... and make sure you use aquarium silicone for your sealant!
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