Physics of an unlevel tank

[Harry Muscle]

I was wondering if there are any physics buffs out there that might help me figure something out ... does an unlevel tank (that is perfectly supported from below) actually encounter any extra stresses than a level tank. I know it's generally assumed that a tank that is not level has a greater chance of cracking, but is that really the case. It's obvious that a tank that's not supported well from below has a greater chance of cracking since the weight is not being distributed properly and that put's pressure points on the glass and/or seals. However, what if the tank is supported perfectly (ie: the surface it sits on is perfectly flat), but it's slightly unlevel. Does that in reality place any extra pressure on it? If it does, how much? Physics should be able to answer this for us.

I'm good with math and was good with physics years ago in school, but the problem I'm having is trying to figure out exactly what the logic is behind an "unlevel tank is more likely to crack". Obviously if I can't figure out the logic behind this idea then there's not much I can calculate.

Thanks,
Harry

P.S. The best logic that I can come up with is that since the tank is unlevel that means that the glass panes are not vertical, so instead of just holding water in the tank, there will also be a certain amount of water sitting on top of the glass (on the side of the tank that is lower). You might have to draw a picture to follow the previous explanation. But the amount of water is very small, for example, if a 4 foot tank (48" x 12" x 24" for example) is 1/4" unlevel from left to right, the glass panes on sides will be off from vertical by about 1/8". That means that that there will be about 18 cubic inches of water resting on the glass pane. That's 0.65 pounds of weight. I highly doubt that 0.65 pounds of water is able to increase the risk of a tank failing, or can it? So if anyone has any better ideas of why an unlevel tank is at a greater risk of failure let us know ... or maybe that's just a myth. Thanks again.

 

[unhpian]

Without going into the in-depth math of the fluid mechanics of this situation - you are basically correct. When a tank is level, the only forces working on the "vertical" glass (i.e., the part we look through) are in the x or y direction - and there should be zero forces in the z direction. When the tank is not level - and the panes of glass are now at an angle, there are forces from the x/y AND Z direction acting on it. Now, granted, the sum of all forces in the x, y and z direction should not change for the tank as a structure as a whole, they're just distributed differently. While i've never studied this closely, if I had to guess, I would say that most tanks that break when on an non-level surface don't break due to the extra forces on the glass - but probably break due to seam failure as a result of twisting into a non-rectangular shape, or from the frame putting pressure on on a point on the glass due to shape distortion.

 

[Harry Muscle]

Thanks. That's what I thought. Also just to add more fuel to the idea that this (unlevel tank more likely to break) might be just a myth, I wanted to see how much pressure is already on the glass vs. how much is added to one side due to the unlevelness of a tank.

The 18 square inches of extra water mentioned in my first post adds 0.65 pounds of weight or pressure in the z direction. However, in the x direction, that same piece of glass (12" by 24" side) is already experiencing 114.37 pounds of pressure. So it's obvious that a slight unlevelness of the tank adds only a very small amount of pressure to one side. About 0.56%.

Harry

P.S. So far, what I'm seeing is that if the stand that the tank is on has any chance of warping due to being shimmed it might actually be better to leave a tank unlevel instead of risking an non flat surface developing for the tank to sit on. However, I am looking for more feedback and comments from others if possible.

 

[Bri Guy]

I know nothing of physics, but I always looked at the tank like an egg, as long as the pressure is exerted evenly, then it won't break.

When the tank is not level, all the water that moves to the lower side also exerts less energy on the high side.

 

[Elliott]

<a href=showthread.php?s=&postid=14160576#post14160576 target=_blank>Originally posted</a> by unhpian
Without going into the in-depth math of the fluid mechanics of this situation - you are basically correct. When a tank is level, the only forces working on the "vertical" glass (i.e., the part we look through) are in the x or y direction - and there should be zero forces in the z direction. When the tank is not level - and the panes of glass are now at an angle, there are forces from the x/y AND Z direction acting on it. Now, granted, the sum of all forces in the x, y and z direction should not change for the tank as a structure as a whole, they're just distributed differently. While i've never studied this closely, if I had to guess, I would say that most tanks that break when on an non-level surface don't break due to the extra forces on the glass - but probably break due to seam failure as a result of twisting into a non-rectangular shape, or from the frame putting pressure on on a point on the glass due to shape distortion.

just curious, how do wavemakers, closed loops and powerheads change this? would they not apply forces to the z plane regardless of the tank being level or not?

 

[Harry Muscle]

<a href=showthread.php?s=&postid=14161389#post14161389 target=_blank>Originally posted</a> by Elliott
just curious, how do wavemakers, closed loops and powerheads change this? would they not apply forces to the z plane regardless of the tank being level or not?

Actually if the glass is perfectly vertical then no they would not apply any pressure in the z axis (the z axis in our discussion here is the up down axis). The light sitting on top of the fish tank though for example is applying pressure in the z axis since it's pushing down on the tank due to gravity. The wave maker simply increases temporarily the pressure in the x or y axis (left right or front back axis) assuming the glass is perfectly vertical. If the glass is not perfectly vertical, then it would temporarily increase pressure in all three axis.

Harry

 

[syrinx]

If the tank is level (evenly supported) on the stand, and then the stand is not level due to the floor there is little risk- other than toppling. However if the stand is not evenly supporting the tank, due to poor craftsmanship, there can be problems.

 

[therealfatman]

A wave box greatly increases stresses. The possibility of problems caused by induced waves in a tank are much greater that any problems created by unlevel tank pressures. Problems of streses created by constantly running closed loops and powerheads are minimal. Surge systems are always suspect when they are large capacity surge sytems with very quick outflows.

 

[dots]

It depends on the design of the system, how far it is off, how tall it is. There is no clear answer without the specifics of it.

Best case scenario, no effect except aesthetic.

Most of the time, it isn't the static load that going to cause failure, its that dynamic load that exceeds the factor of safety.

Most realize the loads are transfered through the system to the ground, through the floor. Getting there directly and balanced evenly is the key. When we do things like cut braces, remove center pieces, cut holes.....we make the forces once flowing down in thier own place, the force doesn't go away, like water it finds its way to the ground the easiest way. Sometimes all through the same member, only designed to hold it own forces originally.

By inclining it, your making the system work harder to redistibute the forces. So now, instead of going in nice up and down patterns, it has added a little movement in another direction. This force translated in another direction can add new stresses on areas such as joints at the connectors now experiencing more shear than originally designed. It complicates things.

I would be most concerned with the center of gravity being shifted, being more prone to be toppled.......like a 55g, 8ft off the ground, leaning 1/2" front to back for instance.

All of this is highly unlikely unless the stand was already way inadequate, in which the additional unlevelness was just a "drop in the bucket".

Small tanks, don't worry.......500g,+ or really tall, is when you start considering such things....but like I said.....depends and is speculation until then.

 

[Elliott]

given that a wavebox greatly increases stresses, what would be the most important supports in tank construction to accommodate a wavebox?

 

[CIGDAZE]

Your system would have to be seriously out of level, for it to have any significant impact on its structure - provided of course that it's evenly supported.

The 1/4" cited in the first post is nothing to worry about.

Even if it were a whole degree out of level, which would equate to nearly a 1-inch drop over a typical 4 foot long tank (which I would consider to be pretty extreme simply from an aesthetics perspective), the horizontal component of the total load would be about 1.7%.

I'll use my 75 gallon tank as an example which probably weighs in around 700 pounds, the horizontal load would be a whopping 12 pounds - slightly more than a little nudge against the tank or roughly the forces experienced while simply mag-float scraping your tank.

I'd be much more worried about these "waveboxes" where you have the possibly of hundreds of pounds of water sloshing around - depending on tank size.

If your floor/stand are out of level by even a degree, there's more issues needing to be addressed than simply the tank's level.

 

[TandN]

I have a question somewhat on topic I think............

We set up a new 180g Acyrlic 4x3x2 3 months ago.

My husband had surgery when it was going on so wasnt around for it. He noticed the otherday when under the tank that the tank is about 1/2" off the steel stand in front. The tank sits completely level its on a 1/2" steel tube stand w/ 3/4" plywood and 1/2" foam. Theres nothing going on w/ it but is it s huge problem ?

 

[therealfatman]

With a wave box your dealing with harmonics or resonance. Your pretyy n much created with the wave boz what an earth quake would cause in your tank. They efforts you would need tp therefore but into your tank stand are efforts to provide diagonal bracing. Sheathing over a wood frame is much safer when using a wave box or if you live in a earth quake prone area. Picture putting your hand on the top of the tank at one end and pushing it sideways thereby creating a ripple in the waters surface now time your additional pushes in such a way as to increase the size (height) of the waves with each push. That is what a wavebox is designed to accomplish and what also what tends to happen during earth quakes to aquariums and water towers. It can collapse poorly braced supports under the tanks. It can cause sky scrapers to sway during earth quakes and even in the wind and frequenly causes broken windows in skyscrapers.

 

[wmilas]

The problem isn't with it being out of level in one direction, its more that its out of level in 2 direction, causing a torque on the bottom plane. If there is rotational torque that's when in the X-Y or Y-X plane you can have stresses build abnormally along one of the seams, actually along two of the seams that are directly opposite of each other. When this happens you can get premature failure if the stresses are abnormally high.

 

[Jeremy1988]

It depends on the design of the system, how far it is off, how tall it is. There is no clear answer without the specifics of it.

Best case scenario, no effect except aesthetic.

Most of the time, it isn't the static load that going to cause failure, its that dynamic load that exceeds the factor of safety.

Most realize the loads are transfered through the system to the ground, through the floor. Getting there directly and balanced evenly is the key. When we do things like cut braces, remove center pieces, cut holes.....we make the forces once flowing down in thier own place, the force doesn't go away, like water it finds its way to the ground the easiest way. Sometimes all through the same member, only designed to hold it own forces originally.

By inclining it, your making the system work harder to redistibute the forces. So now, instead of going in nice up and down patterns, it has added a little movement in another direction. This force translated in another direction can add new stresses on areas such as joints at the connectors now experiencing more shear than originally designed. It complicates things.

I would be most concerned with the center of gravity being shifted, being more prone to be toppled.......like a 55g, 8ft off the ground, leaning 1/2" front to back for instance.

All of this is highly unlikely unless the stand was already way inadequate, in which the additional unlevelness was just a "drop in the bucket".

Small tanks, don't worry.......500g,+ or really tall, is when you start considering such things....but like I said.....depends and is speculation until then.

I am starting a 90 gallon...I put it in the garage and its 1/4 inch uneven from one side to another. Its only going to have water in it for a day or two because it needs a vinegar bath from the previous owner left a bunch of algae in it. How long though does it take a tank to develop a stress crack over time? I assume the more uneven the sooner...but in my case will I be okay leaving water in the tank for a couple of days.

 

[outy]

does an unlevel tank (that is perfectly supported from below) actually encounter any extra stresses than a level tank.

Yes factually there is a fraction of an amount of extra stress.

But factually so small it makes no difference . what so ever.



CIGDAZE said it well

Your system would have to be seriously out of level, for it to have any significant impact on its structure - provided of course that it's evenly supported.

I would like to add to this that even at extreme angles the tank loses its water negating the angle by reducing pressure.




If a tank fails because it is a hair unlevel then the tank was going to fail from time alone anyway.

 

[Jeremy1988]

question

question

Okay I see...one more question. I am going to be putting the tank in the room soon (from the garage floor where it is now). The area in the room where its going into is slightly unlevel. maybe 1/4 if that much total. Its unlevel from the wall to the center of the room. So basically from the wall to the center it drops downward. I went to Lowes...my tank is 36inches in length in the front. I found some 36 inch steel strips. There about 1 1/2 in width but only about 1/8 thick or a bit more in height. I was thinking of using these going across the front to raise it level with the back of the wall if that makes sense. I was going to put the strips on the floor and put the stand above it. O the tank is 24inch wide and tall btw. I just didn't know if that would somehow be bad...or not a good idea. It seems like a good idea haha.

 

[davocean]

I'd just use shims, shimming a 1/4" is no big deal, and most likely those steel strips will just rust out anyway.

 

[Jeremy1988]

Well...I never thought of rusting as a problem...I was thinking that the shims would go under the stand...away from water...I guess if water runs down the stand it could effect it. Maybe I can find away to protect the steel by coating it with something...then again the CPC may wear off over time. Maybe I will get some EZshims...seen them online, look like a composite material.

 

[outy]

I shim under my stand.


steel will rust just being close to the tank. I store things under the tank and never had a leak and metal get rusty after a few years.


I have a flat bottom stand so I threw a few magazines under one end of the stand and it made it perfect.


Roofing shakes work pretty good too.