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Old 03-10-2013, 07:08 AM   #7456
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I never even thought about sandbags, but you're right, they do seem to do incredibly well in flood situations.

I'm still trying to wrap my head around just how much outward pressure there might be. And to that end I was thinking about those above ground swimming pools with 4 foot high walls. We had one at a house over in North Vancouver many years ago, and I think the walls were just some thin sheet metal screwed onto some flimsy square sheet metal posts. But somehow they seemed to hold up.

Maybe I will double up on the nails and see if that works.
Are you using ring shank nails or just coated nails. Heck if it has worked for you in the past there is no reason it won't work now. I liked that idea of the saw dust for the floor. I am not trying to pick at you buddy, just curious.
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Old 03-10-2013, 10:10 AM   #7457
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Are you using ring shank nails or just coated nails. Heck if it has worked for you in the past there is no reason it won't work now. I liked that idea of the saw dust for the floor. I am not trying to pick at you buddy, just curious.
These are galvanized spikes Jim. They are a real SOB to pull out. If anything goes I think the plywood will need to break first, and I can't see that happening. It is 1/2" pressure treated plywood.

I hope A Squared gets back to me soon, so we can see how this plays out.
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Old 03-10-2013, 11:58 AM   #7458
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Yesterday afternoon I got 4 layers of 6 mil poly inside the tank for the liner.

If you have never tried to do that with poly, you're missing out on one of life's great experiences. The stuff goes everywhere except where you want it to, it's so slippery.

The hose is on now draining the remaining water out of the original back tank. There is maybe 500 gallons at the most in there. So once I have 1,000 pounds or so of water in the new tank, then I will try pulling the poly straight and fix it to the top of the walls.

Once the back tank is empty, then I can demolish it as well.
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Old 03-10-2013, 12:53 PM   #7459
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Just think, this will be the last time you will have to build water tanks again.
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Old 03-10-2013, 12:58 PM   #7460
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Just think, this will be the last time you will have to build water tanks again.
Oboy! Don't I hope you're right!!
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Old 03-10-2013, 04:03 PM   #7461
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Hi Coco. Its been a long time and I'm happy to continue following your project. I would be fooled that it is indeed one board. Brilliant work....
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Old 03-10-2013, 06:16 PM   #7462
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Keith, sorry for not getting back to you sooner, busy work schedule.
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It looks like you have taken a one foot square area multiplied by 4 feet high to get your water weight, is that right? That would certainly be right about 250 pounds. I think the weight of the water, based on a full tank, would decrease by a hair over 2% for every inch of rise, 48" being very close to 2% per inch. Let's call it 5 lbs per inch of rise.
Yes, this is all correct. The pressure (in all directions) will be a function of the height of the water column above it. A water column with a one ft square cross section and 4 ft high will have approximately 250 lb of water, so the pressure will be 250 lb/sqft. in all directions, not just down. Of course it's independent of the size of the vessel. a 1/4 inch tube, 4 ft high and filled with water will have the same pressure at the bottom, as your tank with 4 feet of water. That's *pressure*. Obviously, the total *force* is much greater in your tank than the tube, because the same pressure is acting on a much greater area.

You probably already know most if not all of this but I'm just trying to make sure we're on the same page.

Now, the bit about the pressure being the greatest 1/3 up from the bottom, I don't want to just say that you're completely mistaken, because I think that you're correct that there is *something* that is *greatest* 1/3 up from the bottom that is a consideration in concrete forms. But it's not pressure, by itself. Pressure is greatest at the very bottom. It can't be any other way. The deeper you dive in water, the higher the pressure. The 1/3 height thing may be a rule of thumb for where the greatest *deflection* occurs, given that the bottom of the forms are securely fixed, or if might be a rule of thumb having to do with the rate at which the concrete sets up, or something else. I'm not sure, and I don't want to say that whatever you're thinking of is incorrect. But I do know that the hydrostatic pressure of a homogenous fluid in a vessel will be the greatest at the lowest point, not 1/3 of the way up.

As far as the above ground pools go, yes they seem very flimsy, but hold a great deal of water, but they do that by taking advantage of the strength of the materials. Sheet metal, although not particularly rigid is pretty strong in tension, and in a vertical cylindrical tank the forces are either :

a) down, which is against the ground, which obviously ain't going anywhere,

or

b) outward in all directions, evenly, which translates completely to tension in the sheet metal sides. The little poles around the outside really don't hold any hydrostatic force at all, all they do is serve to hold up the sheetmetal sides and liner when it's empty. Once it's filled with water, you could take away the little poles and not compromise the pool's ability to contain the water at all. The critical strength in the pool is that it's strong enough in *tension* around the perimeter, because that is where all the force acts.

As far as your previous tank goes, as I recall, and without going back in the thread to find pictures of it, I think it was built log cabin style out of interlocking pieces of 2X2 lumber, making something akin to finger joints at the corner, and presumably, as you assembled it, you nailed down thru each of the overlaps at the corners on each course of "logs". In this case, the hydrostatic forces which are trying to spread the walls apart are being transmitted to those nails as shear forces between the "fingers" and of course nails are pretty strong in shear. Looking at your new tank, if I understand correctly what you're doing (and I may not) you are assembling the panels by face nailing between the sides (the "studs" ) of adjacent panels. If this is the case, then the force of the water, because your many sided polygon is approximately a circle, will be primarily transmitted to your sides in tension around the perimeter, which means that the predominant force on the nails is withdrawal force, which as you know isn't the nail's strongest direction. There won't be much in the way of shear forces as there isn't much force that will tend to cause the panels to move relative to each other. It may be that you can put enough nails in it that you would get a withdrawal failure. I wouldn't like depending on the nails withdrawal strength though. A few years down the road, when the members get a little soft......

If it were me, building the tank, I think I would do this:

Get two lengths of cable long enough to go around the tank circumference and then some and some cable clamps. I'd wrap each cable around, say 1/3 up and 1/3 down for symmetry, and then I'd figure out a way to take up a little tension on the cable, maybe with a come-along, or block and tackle. Wouldn't have to be much tension, just enough to make it snug, then put the cable clamps on the overlap area of the cables. Wouldn't be too costly, or require changing your basic design much and would add a lot of safety factor. I bet that you would even be able to find a way make it look good, like a decorative feature. Or you could just keep that in your back pocket, and go ahead with just nails, and use the cables as a fix if you start seeing nails pulling apart after filling it. A retrofit wouldn't be a big deal either, just need to drain the tank.
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Old 03-10-2013, 08:33 PM   #7463
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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.
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Old 03-15-2013, 03:19 PM   #7464
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OK, for what it's worth, I've done some testing to see how much force is required to extract a 3" galvanized nail right from the 2" x 4"'s I have used in the water tank.

The results tell me it takes about 264 pounds of force on a direct pull to extract one nail.

Given that there are at least 8 such nails in the bottom 12" of every section of the tank framing, I'm no longer the least bit concerned as to whether or not the tank will hold. I think a safety factor of 8+ will do the job.
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Old 03-18-2013, 02:27 AM   #7465
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Well maybe that will work might work for you then, and I certainly hope that it does. I may tend to discount tthe nail's strength more than necessary. And worst case scenario, if they do start to pull apart, you could retrofit with a cable/band/strap reinforcement without too much trouble. It wouldn't be like you'd have to go back and redo everything.
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Old 03-18-2013, 01:15 PM   #7466
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Well maybe that will work for you then, and I certainly hope that it does. I may tend to discount the nail's strength more than necessary. And worst case scenario, if they do start to pull apart, you could retrofit with a cable/band/strap reinforcement without too much trouble. It wouldn't be like you'd have to go back and redo everything.
Something else I think I will do, purely as a safety precaution, is to add some steel strapping right at the very bottom of the framing on the outside, which would prevent the possibility of anything pulling apart.

I was able to speak with a retired physics professor from the local university as well about this. He pointed out that the force of the water is not actually pushing directly sideways on the nails as was the case when I did the nail pull test. Unfortunately, he wasn't able to give me any sort of formula to determine what the actual force really is. He pointed out that he is a scientist, and I would need an engineer to try and determine the final answer.

I think I'm well covered, but the strapping will do it for sure.
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Old 03-20-2013, 08:16 AM   #7467
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Keith sounds like you got all your bases covered on this project. Should be filing up quite fast with all this rain we have been having.
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Old 03-20-2013, 11:11 AM   #7468
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Keith sounds like you got all your bases covered on this project. Should be filing up quite fast with all this rain we have been having.
Good grief Batman, you're not kidding about the rain. It hit here yesterday around 4 PM and it is only just now stopping. There's 26" in the tank already! Or just over 14,000 pounds.

This guy is outside over Vancouver Island as we speak.
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Old 03-20-2013, 12:59 PM   #7469
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right over me head, but I can't find the dang pot of gold.........
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Old 03-20-2013, 04:36 PM   #7470
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Man, I think you are living in the gold, it is beautiful up there in your area.
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