Forces On A Tank's Walls

[Veri]

Hey all,

I was thinking about the forces exerted on my new-ish purchased tank - regular five flat glass panels with a fixed plastic base. I recall two important pieces of structural advice in setting up the tank: (1) make sure your base is flat (i.e. not bumpy); and (2) make sure your base is level (i.e. not sloping). I understand the danger of the first, but I don't quite understand the significance of the danger of the second. The most basic first approximation seems to be, ignoring atmospheric pressure:

Water pressure p(height) = rho * g * height, so:
Force on side wall due to water = integrate ( p(height) * width ) d_height as height varies from 0 the depth of the tank.

But this reveals a very small percentage change in force if your tank is slightly non-level, and as long as you're not tipping enough such that one side of the tank exceeds the manufacturer's high water limit, I'm not sure what limit is being exceeded in mechanical terms. Yet the Internet is full of people with glass and acrylic tanks reporting leaking / bulging / cracks / etc which they suspect related to tanks with perhaps under a centimetre depth difference on either side, so I'm obviously getting it wrong.

Is there perhaps a site which analyses the forces on a fish tank in good detail (panels, seal, etc.)?

Thank you!

 

[awilson360]

Hey all,

I was thinking about the forces exerted on my new-ish purchased tank - regular five flat glass panels with a fixed plastic base. I recall two important pieces of structural advice in setting up the tank: (1) make sure your base is flat (i.e. not bumpy); and (2) make sure your base is level (i.e. not sloping). I understand the danger of the first, but I don't quite understand the significance of the danger of the second. The most basic first approximation seems to be, ignoring atmospheric pressure:

Water pressure p(height) = rho * g * height, so:
Force on side wall due to water = integrate ( p(height) * width ) d_height as height varies from 0 the depth of the tank.

But this reveals a very small percentage change in force if your tank is slightly non-level, and as long as you're not tipping enough such that one side of the tank exceeds the manufacturer's high water limit, I'm not sure what limit is being exceeded in mechanical terms. Yet the Internet is full of people with glass and acrylic tanks reporting leaking / bulging / cracks / etc which they suspect related to tanks with perhaps under a centimetre depth difference on either side, so I'm obviously getting it wrong.

Is there perhaps a site which analyses the forces on a fish tank in good detail (panels, seal, etc.)?

Thank you!

If the tank is not level the centroid where the force is applied on the glass will move vertically.
I think the reason they advise placing it on level ground is more due to the shear force on the glue holding the panels together.

AW

 

[OldMan47]

Welcome to the forum Veri.
You are indeed correct that the forces on those glass panels are no different than they would be on a level tank. The forces that tear apart tanks are simple to understand. If you have local high spots on a stand, it creates a place where the force is not evenly distributed across the tank's bottom. That means that concentrations of force in small areas will overstress the glass and break it. Another destroying set of forces is created when the surface of the stand is warped or twisted in some fashion rather than being completely flat. Again, the forces start trying to change the tank's bottom surface to match the stand's top surface. The end result is a twisting force that tears apart the seams of the tank. A simple small tilt off level has only an aesthetic effect. It just doesn't look right when there is an air space at one end of the tank and the tank is full at the other end.