How strong do bands on water tank need to be?

How do I know how strong of a strap to use when I make something like this?



I did the math on this example and the hydrostatic pressure is about 12,700 pascal. It's been so long since I failed physics that I barely even know what last sentence means anymore. Looking for an answer in units common to ratchet straps (ie: 12,000lbs working load, etc...) I'm more of an ad hoc kinda guy but I'd rather not lose this much water due to catastrophic failure if possible.

Answers from professional engineers and experienced hillbillies equally considered.

Thanks!

I would start by converting pascals to PSI. That would be 1.8PSI. A foot of water produces about 0.43PSI at the bottom, so that would imply a tank that is a little over 4 feet deep. So far, this all seems reasonable.

Obviously the pressure on the top of the tank will be less than at the bottom, and if you look at old wooden tanks, you will note that they have more bands at the bottom than at the top. Lets say you want to try and use 2 bands. Then the bottom one will need to hold in the bottom 2 feet. The pressure at the bottom we said was 1.8psi, and at the top it will be half that, 0.9 psi This averages to 1.35psi. Then youd take the circumference of the tank, (lets say it is a 12 ft diameter tank). Circumference would be 12*3.14 = 37.68 ft or call it 450 inches. Since we are talking about a 2' depth, multiply by 24 and we get a value in square inches; So you have 10,800 sq inches, and on average 1.35 pounds per sq inch of pressure. That means that there would be 14,600 lbs of pressure on the band when the tank was full. That would make me think it would be smart to put 2x 12000lb straps on the bottom 2 feet, and one would be enough for the top 2 feet.

That is how I would go about doing the math, although I am not an engineer, so it is only worth what you paid for it :)

In eyeballing that picture, I'm going out on a limb here and guessing the structure could be designed to transfer the water pressure to the horizontal pieces of plywood (if constructed properly). So knowing the shear strength of the screws/bolts/wood joining everything together would need to be taken into consideration. I personally wouldn't want to rely soley on the integrity of nylon straps over the long haul due to weathering/UV degridation concerns, not to mention possible abrasion due to expansion & contraction (and possible stretching?).  In any case, it's never a bad idea to incorporate a good factor of safety on top of your calcs.

Thanks so much Carl.  I was close but I only got to 1.8 PSI and it seemed to me it would be a little bit higher, hahaha. I think your recommendation as how many and what strength strapping to use is spot on.

And thanks Pete, I hear you about the straps not lasting. I do plan on surrounding this tank with something. Both as a sun-shield and for looks. I'm thinking a junkpole kind of look. Another thought is also that this tank will be emptied every year so I can inspect the nylon easily,

I am hoping to build this this year in time for our semi-annual rainstorm here in California. If I do I will post it up here. And owe you one! Thanks again.

Clever idea.
You could replace the orange straps with steel cables, usually used on clothes lines in Australia. Its stranded and galvanised 3/16 inch dia.
With turnbuckles it would work well.
You could fit a few on the side and they do not cost much.
wood stave pipe wound with wire to boost pressure capacity

The bands I have from a 10ft x 3 ft wooden trough are made from 1/2 inch round stock and there were three of them. My old redwood hot tub that was oval and 6ft x 4ft x 5ft used the same 1/2 inch round bar and there were three on that as well.

That steel cable is a clever idea. Those 1/2 steel bars would likely be rated for 15,000 lbs in tension. Another thought I just had was high tensile fence wire. It is rated for 1500lbs, so you would need a bunch of wraps, but it might be pretty easy to find it basically for free. We sent miles of it to the scrap yard when took out some of our vineyard.

If the radius was big enough you could maybe even make lap joints in plywood that would hold the required tension. A simple framing nail takes like 600lbs to shear, if memory serves, so a line of nails or screws a ways back from the panel edge might do it. I am not sure what the tensile strength of plywood is, but I have a hard time imagining that it couldnt stand up to 4 feet of water pressure.

Anyway, keep up posted, because this is a clever way to store a lot of water on a sort of temporary basis.

Wow, all the excellent responses above have stolen my thunder! Nicely done people.

I would only add: factor in the hydrostatic shock of you and your guests cannonballing into this pond whilst in your cups.  

Lets work with square feet and I am going to mostly take the decimals out and just do fractions.  Approximate the increase by 1/2 a psi per vertical foot.  The first vertical foot goes from 0 psi to 1/2 psi.  Average is 1/4 psi.   Now lets get that to square feet.  144 square inches per square foot.  1/4 of 144 is 36 lbs per square foot pushing straight out.  Now multiple that times your perimeter length in feet.  So lets pretend it is 100 feet around for 3600 lbs pushing out at the middle of  the top foot of water 6 inches down

Now the second ring a foot deep will add 1/2 a psi or 7200 lbs.  7200+3600 = 10800 lbs for a circle with a 100 foot perimeter length  So a second band 18 inches down from the top should pull in with that much.

Now a third ring a foot deep will add another 1/2 psi or another 7200 lbs = 10800+ 7200 = 18000 lbs for the 100 foot circle.  So a third set of bands at 30 inches down should pull in with that much

Now a forth ring adds another 7200 lbs bringing it to 25200 lbs so a 4th  set of bands at 42 inches down from the top of the water should pull with 25,200 lbs.  So if those are 10000 lb straps you would need 3

So the top 2 feet will need 2 bands,  The next foot will add another 2 and the 4th foot will add 3.  So 7 bands, 100 feet long would handle it with margin for error.  If you play with the math a bit you will find you have 4800 lbs margin on band set 4, 2000 lbs margin on band set 3 and 5400 lbs margin on bands the top 2.  for a total margin for error 12200 lbs.  That means that properly distributed you could likely get away with just 6 bands for water 4 feet deep and a 100 foot circumference .  But you only have 2200 lbs margin for error depending on just the bands.  Now if you add wood work for increase margin you should be safe to start with.  Remember those straps rot in the sun and get weaker so likely you want to keep the 7th band as well as shade them from the sun.  Now if you do diagonal bracing to the top and set posts into the ground taking base load you can reduce the needed number of bands to 5 or 6 and still have good margin for error.  The other answer is to play with your diameter.

In my case I'm talking about coopered wood staves made from 2 in stock. So not as pictured. Hog panels with a course of tarpaper and then pond liner seem to work well.

Now multiple that times your perimeter length in feet.  So lets pretend it is 100 feet around  

One things that none of these analyses have taken into account yet is what the actual design diameter is... 100 feet of circumference is nearly 32 feet across, and at 4 feet deep would hold almost 25,000 gallons.

I suspect that if the liner is going to be a tarp, then there will be a practical limit to how big the tank can get. I forget how big of a tarp one can buy at home depot, or the like. If you made it narrow enough for a 20' liner, you could also use black polyethylene sheeting. Not sure which would last longer. Material costs per stored gallon go up as you get farther from the height being equal to the radius, but you also get increasing pressure with depth.

How much water are you planning on trying to capture?

I did some more number crunching, because I am starting to think I want to build one of these too. I had been planning to build a concrete cistern this summer, but the materials I was going to use for the "ferrocement" do not make a stiff enough armature to apply cement with formwork. Now I am envisioning a plywood ring that I put a tarp inside and line on the outside with multiple wraps of high tensile wire.

So; 20x20 seems like a common upper limit for cheap tarps. You can get one for 30 bucks. You can also get a roll of slightly thicker 6 mil black poly, that works out to 28 bucks per 20x20 square. I suspect that the poly might be more water tight. With 20 feet to work with, you should be able to line a 12 foot radius with 4 feet to spare on each side. In practice, it might come up a little short? Still, that size would hold about 3400 gallons. You would need 5 sheets of plywood. 3/8ths sheating should easily bend to a 6' radius if you cut it in half and bent across the grain:
https://schmeling.com/file_library/plywood_bending_radius.pdf

My thought is to cut 2x4s and stand them on end at each panel seam and center, which would let you connect the panels together and give the plywood extra rigidity. Honestly, with enough screws it might not even need strapping at that point, but I will likely strap it anyway since I have free wire. If you had to buy straps, it would add maybe 100 to 150 bucks. The 10 2x4s are going to set me back another 80 bucks, so if nothing can be scrounged, it would cost:

Plywood 5x sheets of 3/8ths - $200
2x4 8ft 10x                               - $82
tarp 20x20                               - $30
strapping 3x 10k by 40 ft       - $150
box of nails or screws            - $20

total = $482 which is about $0.14/gallon. I think I will be able to cut that in half just with stuff I have laying around. It will need a roof to get any sort of service life out the liner, but that could also function as catchment area.

A black plastic cistern is up to 2000$ for a 2500 gallon tank, which is like 80 cents a gallon. You can almost buy bottled water for that (although refilling all those jugs is going to be a hassle :)

Carl Nystrom wrote:

Now multiple that times your perimeter length in feet.  So lets pretend it is 100 feet around  

One things that none of these analyses have taken into account yet is what the actual design diameter is... 100 feet of circumference is nearly 32 feet across, and at 4 feet deep would hold almost 25,000 gallons.

I suspect that if the liner is going to be a tarp, then there will be a practical limit to how big the tank can get. I forget how big of a tarp one can buy at home depot, or the like. If you made it narrow enough for a 20' liner, you could also use black polyethylene sheeting. Not sure which would last longer. Material costs per stored gallon go up as you get farther from the height being equal to the radius, but you also get increasing pressure with depth.

How much water are you planning on trying to capture?

In Dan's example photo, that tank is made from what look to be metric pallets. 9 pallets that I can sort of count, and so, treating the nonagon as a circle...
1.0m high x 1.2m wide. x 9 = 10.8m circumference  and 1.0m tall. 3.438m diameter/1.719m radius. 9.282 cubic meters volume.
40" high x 48" wide.                425.25" circ.                           40" tall.         135.375" dia./67.688" rad.     2452 U.S. gallons

Sorry, I've been away fro the weekend so I missed this post really getting good.

Anyways, I had the wire epiphany over the weekend as well. Nice to hear John say the same thing since he's knowledgeable on the subject. I am still thinking about using pallets and strapping it (although the plywood and lumber sounds pretty dang good) just to save money. Thinking about a pretty durable liner and then a cheapo tarp for a sunshade/lid. I am pretty fired up about this, the drought was just about to run me outta here but this should at worst buy me some time. I was looking at ferrocement too but it just seems to be too big of a project. This I know I can slap together. If it goes well I am already planning to make a "real one" following that, using techniques more like what Carl said.

The funny thing is I just happened to see that picture at the top while looking up how to place my 2500 gallon tank. It was a year old, a guy bought it and didn't ever use it. Still $1100....

Boys I think we are on to something here. I won't be making mine until late fall but I am stoked!

Yeah, I think this is a topic that might be of interest to others as well. I am thinking of using a wooden frame as a stop-gap to an eventual concrete tank, and I figure the plywood ring will make nice formwork to apply cement to.

Also, if I lay down a compacted gravel bed, filling a tank with 27,000lbs of water a couple of times should help the ground do any settling it is going to do before I move to a less flexible concrete liner. I have access to a bunch of old 12 foot pieces of sheet metal, so I think I will put some sort of shed roof up over mine to hopefully give the liner a fighting chance at lasting more than a year. Also, I am going to build it in the woods, so It will keep leaves out.

Carl Nystrom wrote:

Now multiple that times your perimeter length in feet.  So lets pretend it is 100 feet around  

One things that none of these analyses have taken into account yet is what the actual design diameter is... 100 feet of circumference is nearly 32 feet across, and at 4 feet deep would hold almost 25,000 gallons.

I suspect that if the liner is going to be a tarp, then there will be a practical limit to how big the tank can get. I forget how big of a tarp one can buy at home depot, or the like. If you made it narrow enough for a 20' liner, you could also use black polyethylene sheeting. Not sure which would last longer. Material costs per stored gallon go up as you get farther from the height being equal to the radius, but you also get increasing pressure with depth.

How much water are you planning on trying to capture?

In greenhouse plastic 50' x 100' is readily available as a single piece in 6 mil thick.  Folded double that would still give you a 50x50 liner to drop in.  If you went 5 feet up the sides  to allow for waves that would still let you line a 40 foot diameter circle.