Wooden stand. Critique and advice needed please

[Jwhite]

First post, so go easy on me! I am looking for recommendations on the following project. It is a stand for a salt water aquarium. It needs to hold approx 2,100 lbs, as shown in the attached picture. As mentioned, the front needs to remain open to access equipment below the tank. Normally I stick to the aerospace side of things, so thanks for the help with this!

 

[berkeman]

First post, so go easy on me! I am looking for recommendations on the following project. It is a stand for a salt water aquarium. It needs to hold approx 2,100 lbs, as shown in the attached picture. As mentioned, the front needs to remain open to access equipment below the tank. Normally I stick to the aerospace side of things, so thanks for the help with this!

Welcome to the PF.

I'm no ME, but it looks too weak as drawn. You would need to put sheeting on at least the back and sides to keep it from racking -- there are no triangles in the design yet.

It seems safest to look at how production stands are designed and constructed, to get an idea of what is necessary to bear that weight. Also, do you have earthquakes where you live? If so, you should probably figure out a way to make anchoring it to wall studs an integral part of the design...

 

[AlephZero]

If looks like there is plenty of material to take the load, but joints don't look very well designed. I would be more inclined to put your L-shaped "legs" INSIDE the top and bottom frames, extending down to the floor and up to the level of the tank. Then you have 4x6 overlapping areas to fix together, not 2x4 where one of the pieces is end-grain.

If the back and side panels in your thumbnail photo are load-carrying (i.e. they provide the equivalent of diagonal bracing), that will make a huge improvement to the stability of the frame structure. Otherwise as Berkeman said you need some diagonal braces.

 

[Q_Goest]

Hi jwhite, welcome to the board. Sorry Berkeman, AlephZero but I have to largely disagree with you both here. Although it's probably not intuitively obvious, the glass sides of the tank can handle all the bending stresses from end to end (the long way). In fact, I have to believe that would be a criteria for the design of the tank in order to ensure it is safe, otherwise the manufacturer of the tank would be liable for breakage of improperly supported tanks. Consider that the glass sides are in compression along the top and tension along the bottom which allows the entire tank to be supported only on the far ends. That might sound a bit odd, but the glass sides are relatively tall, meaning they have a very high moment of inertia to resist bending. (I = bh³/12) and the modulus of elasticity of glass is also very high, around the same as aluminum (10,000,000 psi). Compare that to pine which has a modulus of only 1,300,000 psi, so the 2x6 yellow and red pine boards will flex much, MUCH more than the glass over the length of the tank and will be unable to provide any support. Those 2x6 beams are just way too flimsy to support anything. Put into numbers, assuming a linearly distributed load due to water (2100 pounds over 72.5 inches) the stress on the glass sides supporting the entire weight of the water and assuming the glass is only 1/4" thick, is only around 1400 psi which is well below what I'd expect glass to handle safely. And I have to believe the glass is more than 1/4" thick.

Note also, the deflection in the center of the tank, assuming it is supported ONLY on the two ends is only about 0.0015", so the wooden rails running from end to end do NOT provide any support for the tank. In fact, they don't have to. You only need to support the far ends of the tank.

To prevent the box from tipping over, you need some triangulation. That comes in when you secure the side & back panels. Side panels on a rectangular box are terrific for turning 4 vertical poles into a very rigid structure. The design is fine as is, though I do like AlephZero's suggestion here:

I would be more inclined to put your L-shaped "legs" INSIDE the top and bottom frames, extending down to the floor and up to the level of the tank. Then you have 4x6 overlapping areas to fix together, not 2x4 where one of the pieces is end-grain.

I like the idea of extending the 4 legs from the ground all the way up to the tank. That's where all your load is, not on the long, relatively thin 2x6 boards colored red and yellow in the picture. Remember, for those boards to support the load, they have to deflect less than the glass, which isn't reasonable to expect.

 

[Jwhite]

Thanks for the good info! Gives me something to think about. I am sure I will have some more questions tonight and I will post back.

To answer a couple quick questions... The glass is either 1/2" or possibly 3/8". No earthquakes here in Kansas. Only tornadoes!

The sides and back will be wrapped in plywood and I can add triangle braces on most sides. Attached is version 2. But after reading the info above it sounds like some re-design may be needed.

 

[AlephZero]

Sorry Berkeman, AlephZero but I have to largely disagree with you both here.

I don't think we are really disagreeing about anything here. I took it as "obvious" that the tank would be stiffer than any sensible-looking wooden stand, therefore if the tank is not strong enough to support taking all the load out at the corners you have got a major problem. I took the The OP's comment on the attachment, "a flat base to support the tank is not an option" as consistent with that idea.

Even if you make the frame as stiff as the tank, you still have the problem of geometrical tolerances which will mess up the load distribution to an unknown degree.

Hence my basic idea that the four legs should support the weight, without any wood joints between the tank and the floor, and main function of the rest of the frame is to keep the legs upright.

 

[Q_Goest]

I don't think we are really disagreeing about anything here.

I guess I was responding without reading into your responses carefully enough. Anyway, I think we might all agree now the design is sufficient and having panels on the sides should provide sufficient bracing. I don't think the diagonals are doing anything, the panels are much stiffer. Also, the idea of having the vertical columns from floor to bottom of the tank is a good one. And I just noticed Berkeman suggested anchoring to the wall... hmmm... I kinda like that idea too! Maybe I need glasses.

 

[berkeman]

Yeah, hurricanes shake houses too... :shy:

I think the tolerances issue is a good one. Should we suggest some tolerance absorbing mechanism like a rubberised mat or something? What's the best way to equalize the loading in the 4 corners with some tolerance issues in the 4 corner posts?

 

[Jwhite]

KI6EGL, thanks, but no mat under the glass. If it was an acrylic tank that would be the way to go. I am thinking the upper and lower frames will distribute the weights ok to the floor. Then wrapping the 3 sides in plywood will take care of any racking. My main concern is the lack of a center vertical column on the front. - KF9TJ

 

[Q_Goest]

My main concern is the lack of a center vertical column on the front. - KF9TJ

As mentioned, the center column doesn't do you any good. It'll be fine without it.

 

[larrybud]

Are you guys aware of how the tank is constructed? There is a frame around the top and bottom of the tank, so the load will be transferred to that frame all the way around. The bottom glass probably sits about 1/2" above the bottom of the lower frame. Seems to me like you'd want to spread the load not only to the tank, but to the floor as well.

EVERY single commercial stand made out of wood has center supports in the center on the front and back. Besides add support, it also gives you a center column for a door to over lap. I think it would be a huge mistake to make an open stand since all the junk to run the aquarium sits below, not to mention the supplies that you can hide away.

I don't see why someone would want to reinvent the wheel here.

Might want to check out this:

http://www.aquariumlife.net/projects/diy-aquarium-stand/56.asp

 

[Q_Goest]

Hi larrybud,

Are you guys aware of how the tank is constructed?

Actually, I've had a couple salt water aquariums, the larger being around 50 gallons. They were made with glass sides and a solid bottom connected by what appeared to be a silicone adhesive.

EVERY single commercial stand made out of wood has center supports in the center on the front and back. Besides add support, ...

If I told you the center support had to be less than 0.001 inches away from a line traced between the corner supports would you think that anyone could manufacture a stand to that accuracy? The problem is that the glass sides of an aquarium are so rigid they can't bend or sag at all without there being so much stress in the glass that the glass would break. I realize that sounds counter-intuitive, but the analysis for that is straightforward and should be something any mechanical engineering student could do before their senior year.

 

[AlephZero]

If I told you the center support had to be less than 0.001 inches away from a line traced between the corner supports would you think that anyone could manufacture a stand to that accuracy? The problem is that the glass sides of an aquarium are so rigid they can't bend or sag at all without there being so much stress in the glass that the glass would break. I realize that sounds counter-intuitive, but the analysis for that is straightforward and should be something any mechanical engineering student could do before their senior year.

In reality, the tank isn't made to that degree of tolerance either, and the floor of the room certainly isn't. However many legs there are, most of the load will be going through three of them (though you don't know which three) and the rest don't do much structurally except provide some reassurance that the stand "looks safe".

 

[Q_Goest]

I would absolutely agree. As you say, the point of course isn't that the stand and the tank need to be built with precision, the point is that the load needs to be distributed in some way that can accommodate imperfections. The imperfections in manufacture of all the various parts and the implied rigidity of the stand when compared to the aquarium result in the requirement to limit any bending in the glass to less than 0.001" which is clearly not reasonable. Further, it isn't difficult to then make an assumption about the supports and determine the stresses in the aquarium which turn out to be acceptable when supported only from the ends. One could try and support the aquarium across the entire bottom and there are some clever ways to do that. Putting a rubber mat underneath as suggested by Berkeman might help distribute the load, but that really isn't necessary once the stresses in the aquarium are determined. I guess all this is well understood by experienced engineers but it takes a lot of explanation to go into all the details of why a center support isn't needed.

 

[ColinW]

If you haven't already got one then buy or borrow a chop saw. Getting perfectly square cuts is half the battle. It would be a good idea to reinforce the joints where you have screws going into end grain. The method described here is a simple way to do this, especially if you have a pillar drill.
http://woodgears.ca/shop-tricks/endgrain_screw.html
As AlephZero hints, joints made with side grains glued are much, much stronger than those with end grain, so putting the legs inside to increase the side grained, glued area will make things much stronger, or at least glue extra blocks inside.