A Squared...WOW! Fantastic post, many thanks.
I'll go through each section to make sure I have things straight here.
Firstly, I think I have pressure and force mixed up somehow. I'm going to claim my recent brush with CO poisoning for that. It really does serious damage to the memory, hopefully I will get that back.
It seems to me that we usually use pounds per square inch (psi) when we refer to water pressure. So it is easy enough to figure out what pressure a column of water would have if it was exactly one square inch in area and 4 feet high. I believe that would be 1.7341 psi + a very small amount. So an area of one square foot should be the weight of four cubic feet of water, or 249.72 pounds.
Now, as to the pressure - or force if you wish - being greatest 1/3 up with concrete, I believe that is true. Concrete is not a homogenous material as is water. The instant that concrete hits the bottom of a form it starts to become a solid. Then there is the friction on the sides of the form itself. Water has no such friction, or at least it is very nearly immeasurable. In any event, we are discussing water here.
So I do agree with you that the pressure of water will be greatest at the very bottom of a given vessel.
You are quite right in your description of the old water tanks, I used some mill cut 1 1/2" by about 3" lumber of whatever stripe I managed to pull out of the ocean. And yes, it was obviously nailed as you suggest.
One thing against these tanks was the fact that they were square. This produced a greater outforce in the middle of the 10' long sides which eventually caused the bowing. However, they did stay intact until the wood itself started rotting. I am hoping that the more round design of this new tank will more evenly distribute the outward pressure, actually, I'm certain that it will.
There are 14 sections each of 32" in width.
As far as the nailing itself goes, I have driven the nails in from both sides of the adjoining pieces of 2 x 4, and they are driven at an angle, rather than dead straight. I honestly doubt that they will pull out.
The cable idea is a good one, and to that end there is a very large water tank on the island here which utilizes several such cables. I have heard rumours that this tank is 53,000 gallons, which means it could contain over 1/2 a million pounds of water when full. I think it is about 8 feet high but I can't remember for sure. Which means, I guess, that there would be something like 500 pounds per square foot pushing outwards on the bottom of that tank!
I haven't looked at this tank in several years - it's at the far end of the island. But if I get a chance I'll take a trip down there and take a pic or two.
Somewhere around here I have some numbers for cable (or steel rod) heights above the bottom of a 4' tank. You actually wouldn't put them at 1/3 and 2/3, but rather much closer to the bottom of the tank where the pressure is greatest. But then, I have a hunch you probably already know that.
Alternatively, I could simply use some steel strapping right at the bottom since that is where the maximum force will be exerted. I'll have a look around here to see what's on hand.
And thank you very much again for such an informative post. :thumbsup: