Stand deflection

[Gp!]

I am working on a new stand without a center brace in the front and trying to decide what on an allowable amount of deflection is acceptable for safety over time. I'm not so much worried about my stand failing, but I don't have first hand experience with long term effects on acrylic or glass seams and the effects of center sagging stands.

Is 1/16" of sag going to cause problems over time? Do I need to design for less than 0.01"? Or is something like 0.001" or even less required?

 

[Matt Carden]

I built my own without front center support using 2-2"x6" sandwiching 1/2" plywood. Only been wet 4 months so I can't speak about long term.

 

[malacoda]

More info on the tank type would help as would more info about the stand top.

Is it a rimless tank or a braced/rimmed tank?
What size tank?
If rimless, will you be putting 1/2" foam or 1" foam between it and the stand top?
What type of material is the stand top and what kind of material is it --- MDF, plywood, wood plank (i.e. dimensional lumber), metal?
How thick is the material?
When you say deflection, are you actually seeing the top deflect when you put pressure on it before the tank is even resting on it?

As for long term effects...

Within a few weeks to a couple of years, significant deflection will allow the bottom and/or front and back panels of a glass tank to sag under the weight of the water and rock. That will put excessive and uneven pressure on the silicon joints causing one of them to eventually fail.

Deflection in the top can cause this, as can a top that is uneven (e.g. not perfectly flat).

Putting foam beneath a the tank compensates for slight deflection and unevenness. It essentially conforms to the 'voids' and helps to more even distribute the weight of the tank across them.

I'm no pro, but the maker (solid reputation) of the new 65g I am setting up informed me 1/2" foam should be adequate to compensate for any unevenness or slight dips of 1/16" or less. 1" foam might let you get away with a bit more. But in all honesty, even with 1" foam, anything more than ~1/8" would probably keep me up at night.

The other consideration of not having that deflection braced is that, although it might only be 1/16 now, over time the material may stretch and sag even more.

On the other hand, with a smaller rimmed tank, the rim/bracing may well support the tank just fine regardless of not making full contact with the top all the way around its perimeter.

Hence the need to know much more about the stand and the tank before any can even try to provide a reasonable answer...

 

[Ebslinger]

Any deflection will cause stress. It is just a matter of where that stress is applied.

That said, aquariums are not designed to be under stress at any point.

 

[Gonebad395]

On your front of the stand just beef the top board over the door

 

[ccombs]

@evolved

https://www.reef2reef.com/threads/wrasse-retreat-2-0-evolveds-270g-peninsula.256483/#post-3030030

He did a bunch of calculations with 80/20 and this is right up his alley, so maybe he can give his input.

 

[Gp!]

Thanks for the replies. I'm on my cellphone now so sorry for the brevity.

Stand isn't actually built. I'm trying to determine a setpoint for design. Didn't know if anyone had any numerical data for a decent safety factor.

Most people can't see 1/16th of an inch. 0.005" is typical tolerance of a laser. Just trying to get a feel for how important flat is.

Stand will be made out of steel. Space is about 11' wide

 

[ccombs]

Also, how big of a tank/how much weight are we talking here?

 

[Gonebad395]

Here’s the one I did for my 90 3 years no sagging

 

[Gp!]

Here’s the one I did for my 90 3 years no sagging

You are very skilled!

 

[Gonebad395]

You are very skilled!

Ty very much

 

[RobW]

I am working on a new stand without a center brace in the front and trying to decide what on an allowable amount of deflection is acceptable for safety over time. I'm not so much worried about my stand failing, but I don't have first hand experience with long term effects on acrylic or glass seams and the effects of center sagging stands.

Is 1/16" of sag going to cause problems over time? Do I need to design for less than 0.01"? Or is something like 0.001" or even less required?

How big of a tank are we talking? Also does the tank have a flat bottom or does the bottom sit up higher than where the side panels come down? If it sits on the edges the load is concentrated to the surrounding edge of the tank. If it is flat bottom than the load is distributed evenly amongst the entire bottom area of the tank which is much less of a load.

 

[cracker]

First How big is the tank?
How long is the run between the front corners.
Is there a brace. middle back ?

 

[RobW]

Here’s the one I did for my 90 3 years no sagging

Nice job on that stand

 

[Adalius]

In structural I see L/360 and L/400 as 'allowable deflection' for floor loads. So in a 6' span, for instance, you'd be allowed 0.18" - 0.20" of deflection in the middle, or slightly under 3/16" to a bit under 1/4" on a 6' span.

Obviously that is crazy for fish tanks. So the rule I was always told is no more than 3mm of deflection in the middle of any span, of any length, for a tank. That's just a hair under 1/8th inch, around 0.118".

Should be plenty of reasonable options that will hold that deflection on a reasonable span if you're going steel. AISI book would come in handy to verify whatever you choose.

 

[KStatefan]

Thanks for the replies. I'm on my cellphone now so sorry for the brevity.

Stand isn't actually built. I'm trying to determine a setpoint for design. Didn't know if anyone had any numerical data for a decent safety factor.

Most people can't see 1/16th of an inch. 0.005" is typical tolerance of a laser. Just trying to get a feel for how important flat is.

Stand will be made out of steel. Space is about 11' wide

I looked for this when I designed my stand but could not find a good answer. If I remember correct I used 0.050” in my design that I am working on now. I am only spanning 4 feet so it is much easier to over design.

 

[Gp!]

I looked for this when I designed my stand but could not find a good answer. If I remember correct I used 0.050” in my design that I am working on now. I am only spanning 4 feet so it is much easier to over design.

Thanks. Kinda where I was leaning. Good to know others haven't found it before I give up lol