My husband is a swimmer. While looking at the pool designs in this thread, he raised the question of how thick the glass, or plexiglass would have to be on these three pools:
That lead to the question of the pressures exerted, especially on the walls in example #11.
Thanks.
[sub]I know he won’t build it for me, anyway.[/sub]
Hydrostatic pressure is linearly related to depth. P=Dwz Where, Dw=the density of water and z is the depth. So for an 8ft deep pool it’d be 62.4 pcf8ft~=500psf.
I would be very grateful for simple answers. My mathese is pretty minimal.
Pressure increases at the rate of about .433 psi per foot of depth. At the bottom of a pool 8’ deep, that would mean a pressure of 3.464 psi, which is pretty close to 500 lbs per square foot.
The picture of #11 shows some substantial steel beams, no doubt intended to deal with the weight of and pressure in this pool.
But, hubby’s question was how thick does the glass have to be.
Xema, the steel definately is there to support the weight, but how would steel beams under the structure deal with the pressure? The pressure is exerted laterally, right? I guess what I’m asking is there a difference between pressure and weight, relating to downward force?
I’m not sure I understand what you are asking. The structure under the pool will have to deal with the overall weight of the water, and the glass will have to be strong enough to support the point pressure of the water. Glass is very strong in thick panels, and the glass used in the pools you link to will just be thick enough to withstand 500 lb per square foot (or whatever).
The water will also exert pressure sideways, and the same considerations apply.
I’m afraid it’s a little complicated. The pressure is how hard the water is pushing on each square inch of the glass, and that varies over the water depth. The force is what happens when you add up all that pressure over all the square inches. That’s how hard the water is trying to push the whole slab of glass out the side of the pool. But even that won’t tell you how thick the glass has to be, because the water is trying to bow the glass outwards into a curved shape. The thickness required to resist this depends upon the length and width of the glass slab and how it is supported. And there should be a great big safety factor built in, ~300% or so…
This aquarium site indicates a glass thickness of 20mm is required for an 8ft x 3ft glass panel, with a safety factor of 2.67. it’s a start. Saltwater Aquariums Glass Thickness and More
This site allows to calculate thicknesses for larger dimensions, if you can come up with some numbers: http://www.fnzas.org.nz/articles/technical/glass_complete/
The steel beneath the pool is mostly concerned with supporting the weight. But those vertical beams along the sides of the pool seriously reduce the tendency of the glass sides to bulge under the pressure exerted by the water. This allows the use of glass panels substantially thinner than would otherwise be necessary.
Both downward and laterally. If there were significant gaps in the base on which the floor sits, then distortions due to pressure could be a problem there, too. One suspects the base is fairly solid.
How about if we were to frame it like this:
I have a 20 foot thick sheet of unobtanium hovering, parallel to the ground, in mid-air, extending off into infinity in all directions. In the sheet of unobtanium, there is an 8’ diameter circle cut out. I want to construct a cylindrical pool of glass on top of this hole. The bottom panel of the glass must be fitted within the hole in the unobtanium, secured with unobtanium-glue at its edges, its surface exactly even with the top of the unobtanium sheet. Around the perimeter, but not entering into the space above the hole in the sheet, I will also build an 8’ tall wall of glass, fixed to the unobtanium with uno-glue at its base, and fill it with water.
How thick do the two glass items (the floor, and the cylinder wall) each need to be based on average glass strength?
(Assuming this helps picunurse)
You also need to choose a glass type.I found tensiles of 10KSI for “regular” glass and 24KSI for tempered.I suspect there are other types,and I didn’t examine plastics.
Transparent aluminimum would be the best material.
Well it certainly won’t help her husband, who has to cut out a circle of thick glass (fun!) and then source a cylindrical glass wall and have it delivered.
My second link allows you to calculate the glass thickness for flat walls and floor of an arbitary cuboid tank. Without dimensions, the question can’t be answered in any more detail. Cylindrical pools are a different ball game again.
has Scotty stopped by yet? 
Is this the same, no matter how big the pool is?
For example, if one pool is 100 feet long, 20 feet wide, and 8 feet deep, and another “pool” is 10 feet long, 1 foot wide and 8 feet deep, would the pressure (in PSI) on the walls at the bottom of the pool be the same?
That seems unlikely to me, but i’m not a physics expert.
Yup. The pressure 8’ below the surface of the Pacific Ocean is the same as at that depth in a small tank (provided it contains salt water of the same density).
Keep in mind that you must account for both pressure and total force being exerted by the water. They each present differing design requirements. For instance, a person wearing high heels exerts a tremendous pressure on a floor but not much total force. A water bed exerts a tremendous total force but not much pressure.
A thought experiment that suggests pressure doesn’t depend on the size of the container:
Consider an 8’ long pipe open at both ends. Stick one end down into the ocean. If pressure in the small container (i.e. the pipe) were lower, water would spray up out the top end.
Another view of Image #11. And another view