Physics question (structural)

[MotF Bane]

yeah, but you would be easier to tip over, no?


idk anything about this shit, but it seems like you guys are overcomplicating it

Overbuild. Solves all questions.

 

[MotF Bane]

IMO, that's way too far to span across the front without some kind of center support.

Absolutely agreed. Actually, I can prove it...

 

[bonkers325]

her209:

the front needs an intermediate support, or you need to use a 2x6 instead of a 2x4. an intermediate support or a chevron brace ( looks like this /\ ) to act as a center support will work as well.

edit: or just add a knee brace at the corners of the front frame

 

[MotF Bane]

What I have here is a 9'6" wide bench, 30" deep. It is sitting on 2x4 (vertical) all around the perimeter; those are screwed into the walls. There are two angled supports spaced under the bench. The bench is made of two sheets of 3/4" particle board, with a front 1x3 hardwood trim.

The black object in the foreground is my cellphone, for a sense of scale. The gap is around 3/32". The metal object is a 6' level. The bench is bowed across the front, very slightly, under its own weight. The front of it supports almost nothing except itself.

 

[Fenixgoon]

i could throw this in abaqus, though it really seems overkill

anyway, what i'd be concerned about most is the pressure on the bottom of the tank. im sure it has plenty of strength, but a little support to avoid and bending moments could help, IMO.


<--- not a structural engineer (i have some mechanics background though)

 

[yh125d]

I think you should just get some W8X31

 

[KayGee]

With that &#9488;cross-section (right side view) and some quick calculations, the front could bow as much as 0.15". Add another 2x4 to get a rectangular cross-section measuring 4" wide and 6" high and that should take care of the bowing.

 

[her209]

her209:

the front needs an intermediate support, or you need to use a 2x6 instead of a 2x4. an intermediate support or a chevron brace ( looks like this /\ ) to act as a center support will work as well.

edit: or just add a knee brace at the corners of the front frame

Knee brace like this?

 

[her209]

With that &#9488;cross-section (right side view) and some quick calculations, the front could bow as much as 0.15". Add another 2x4 to get a rectangular cross-section measuring 4" wide and 6" high and that should take care of the bowing.

Add it where?

 

[bonkers325]

Knee brace like this?

yes, like that. i suppose you want it this way so that the front is accessible? if you're sticking with 2x4's, get decent quality species like doug fir or southern pine, and not some low grade furring studs.

do you intend to support your aquarium along the edges? is your aquarium capable of handling those stresses? does it need a flat surface to bear on?

 

[her209]

yes, like that. i suppose you want it this way so that the front is accessible? if you're sticking with 2x4's, get decent quality species like doug fir or southern pine, and not some low grade furring studs.

do you intend to support your aquarium along the edges? is your aquarium capable of handling those stresses? does it need a flat surface to bear on?

Yeah, the tank is supported along 3 of the 4 edges. Front and both sides. The back edge isn't supported because the holes drilled on the bottom are too close to the edge and adding some support would hit the bulkhead.

Edge that isn't supported would be the back:

 

[KayGee]

The knee braces that bonkers325 suggested will reduce the bowing. If you're looking for a clean rectangular look in the front, here's what I meant in my previous post.

 

[bonkers325]

Yeah, the tank is supported along 3 of the 4 edges. Front and both sides. The back edge isn't supported because the holes drilled on the bottom are too close to the edge and adding some support would hit the bulkhead.

Edge that isn't supported would be the back:

have you used this tank before in this kind of setup (i.e. - back edge unsupported)? the tank needs to be well supported so that the seams on the corners of your aquarium remain intact. if the bottom of the aquarium starts deflecting too much under the weight of the water, you may see issues with leaking or cracking of the bottom. this is assuming that the tank was always designed to be laid on a flat surface with supports on all 4 sides.

why do you need the top to be open? a sheet of plywood would be great as a flat surface for your aquarium. it would also prevent the frame from racking.

 

[Imp]

You have bad lateral support.

Imagine 4 popsicle sticks in the shape of a square and put a nail through each corner. Hold the bottom down and push on the top left or right corner. Now imagine that instead of your hand, you have a 1200 lb tank holding it down and someone's ass bumps into the tank.

Also, the front 2x4 at the top appears to be sitting on its weak axis. Looks like you have one under there oriented "properly", but it may still be laterally unsupported depending on how well you attach it along its length.

One last one... with the way your new 'bracing' is set up, you're laterally loading the columns at the center, where you'll produce the greatest bending moment and deflection.

 

[BoomerD]

An acrylic tank needs to be supported all across the bottom. You can't just put it on an open framework stand. You'll have problems quickly...as the bottom bows and breaks.
Cover the top of that stand with 3/4" plywood...with braces across front to back. for a 72 inch long tank, I'd put in two supports at 24" spacing.
Every acrylic tank manufacturer will tell you this.

I think you should just get some W8X31

I dunno...that seems kinda light for this...

I know many folks with big tanks who have custom steel stands built. Power-coated for corrosion resistance.

 

[CycloWizard]

I don't think it's statistically indeterminate. I'm pretty certain, just did resource checking and this can theoretically be solved.

The load distribution is from stats, and the forces can be derived with newton's 2nd law.

I'm thinking it is like as Skorpio said, 25/25/12.5*4. Some quick calcs assuming his stand weights 25kg, fish tank 600kg.
F= ma = 0
F = 5886N + 245.3N = 6131.3N

front 2 legs @25%: 1533N downward force
back 4 legs @12.5%: 766N downward force

You're right - it's definitely not statistically indeterminate. However, it is statically indeterminate. You can decide any load distribution you want by assuming a solution, but that doesn't make it physically realistic, much less mathematically correct. How many reaction forces are we looking at? How many equilibrium equations can you write? The former is greater than the latter, so the problem is statically indeterminate. It's one thing to be ignorant, but it's another to tell someone else that they are wrong when you don't have the slightest notion what they're talking about. While the OP might get lucky and get away with the nonsense you suggest by sheer luck, are you going to foot the bill when your design inevitably fails? If not, quit pretending to know that which you do not.

 

[yh125d]

Do you really want to put that much weight that high off the ground? (looks like the bottom of the tank would be around 4' el., seems like a very bad idea with a very lightweight stand like you have.


I'd do like 6x 4x4's 3 front 3 back, evenly spaced, with a 2x6 plank top and two 2x6's lengthwise alongthe bottom of the 4x4's, and short 4x4's connecting the 4x4's in fron to their corresponding rear 4x4


Wood is cheap bro, and if you're putting this much money into your fishies, you're not gonna regret building a stand you never have to question yourself about

 

[CycloWizard]

Really? I'm the only civil engineer here?

It's technically indeterminate on one side, but like any engineering problem, there are so many assumptions/tips/tricks to solve it and get it close enough.

Please go talk to a carpenter. You're playing with 1200lb. Depending on how long your legs are, how well you reinforced everything, x6 2x4 pieces of wood may be pushing it.

Really, you're a civil engineer and your only contribution is less helpful than that of a chemical engineer? I already said it's statically indeterminate. He could treat the bottom of the tank as a simply supported plate and go from there, but that's a lot more analysis than anyone here is going to do on a weekend.

 

[bonkers325]

her209

make the corner legs 4x4 and make the front 72" piece a 2x8. this is an overdesign, and you won't have to wonder if the thing will snap in half when fully loaded. any grade of lumber will do from home depot, as long as its a hardwood.

add 2 2x4 joists (in the short direction) and nail a 1/2" thick plywood to the top to give yourself a level surface to place your tank on. the plywood will act as a flexible diaphragm and prevent your stand from twisting under load.