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Thread: Calculating PSI at base of pool

  1. #1
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    Calculating PSI at base of pool

    I have a 32' x 16' x 4' pool that we have dismantled and put back together. Problem is the bottom track was stolen during the move and I need to come up with an alternative. To replace this track from Kayak would run me well over $1000...

    Last year I made a makeshift track with aluminum angle and braced it equally in 2 places in between the longest walls. This worked for the most part last year, but the bottom has kicked out a bit in some places... I am throwing around several ideas of strapping it together at the base (wire cable/turnbuckle combo, reinforced concrete, even ratchet straps...) but would like to figure out what the tensile strength/working load/breaking strength/etc) would need to be of whatever medium I choose.

    Basically I know there is approximately 15,000 gallons of water. (the sides are 4' plywood, but obviously it isnt filled it to the brim)

    So approximately 125,100 lbs of water.

    Calculating the pressure of fresh water at .433 per ft of height gives me 1.73 psi at 4'. Adding atmospheric pressure of 14.7 and I get 15.56 psi. But this is the pressure exerted downward.

    Using a formula for Force (using the area of the longest side):
    F= 1/2 x density x length x height^2
    F= 1/2 x .036 x 32'(12in/ft) x 48^2
    Force = 15,925 lbs....

    I've been looking over so many formulas all day that my head is starting to spin....

    Can someone please help?! And by all means, if you can think of a better way to brace the bottom of the pool (aside from shelling out $1000 to Kayak) - I am all ears!

    Marian

  2. #2
    Technical Fellow jboggs's Avatar
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    First off, the pressure of the water at the bottom doesn't matter. And the atmospheric pressure will be acting in an upward direction on the bottom of the pool just as it acts in a downward direction on the surface of the water, so that is also a non-issue. This is like one of those tests in college where the prof would give you lots more information than you need and it was up to you to figure what was relevant and what wasn't.

    If by "bottom track" you mean a support beam under the pool, the main factors to consider are what share of the total weight that beam will support, how that weight will be distributed on the beam, and what type supports the beam will have.

  3. #3
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    Hi Marian,

    As JB states, don't over-think this. It would be a great help if you could post a pic or two of the current situation. That way we can see what you are trying to achieve and maybe suggest a different approach. If not then we can certainly help with forces and pressures.

    Maybe give us the model of the pool and the maker's website too. The better the information that we get then the better solutions we can come up with to help you.

  4. #4
    Lead Engineer RWOLFEJR's Avatar
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    Hi Marian,
    Throwing a strap or cable around the thing isn't going to do what you want it to do. It'd just crush your corners. When you say the long walls kicked out some... how much is some? Is the pool now empty? Reason I ask is this... If the thing is full and you want to try to move things it's gonna be rough to do without damaging things.

    A buddy and myself did a summer of building these things about 33 years ago so my recollection of exactly how they go together is a little cloudy. The big problem if I'm not mistaken... if the base moves out too much you're going to lose your wall panels which will make a mess of everything. My recollection is the walls were screwed into the uprights that dropped into the base channel and they also acted as a support for the deck. So if the base is moving the screws into the walls could be tearing through the wall panels in order to allow it to "grow." Then up top on the top channel there was a heavy strap to keep the top of the long walls straight. The strap acted like a big leaf spring.

    Only thing that seems to make sense to me would be to drain the thing... pull the liner... square it back up... add straps of flat stock at about three or four places from one side wall to the other that attach at the dirt side of your base. This way the straps will be covered with your approx. 3" of sand for the floor. You'll probably need to patch the hole for the drain in the liner and cut a new one when you re-install it. Be a miracle to line it up and not have it pull once loaded. If the drain is offset to one side a bit then flip the liner so the patch won't be in play where you cut the new hole. When you get the liner spread you put an inch or two of water in it and make sure you've smoothed out any wrinkles or bumps then bolt down your drain ring and cut the liner drain hole.

    If you have a piece of fairly stout I-Beam or maybe a piece of channel, and a piece of heavy equipment you might be able to lay the beam up against the areas that shifted out and shove the base over then stake the outer edge to keep it there but I'd guess you'd be flirting with disaster there. If you used the machine as an anchor to push off of and then use bottle jacks to slowly and ... gently push on the base... you might get away with it? Depends on how far you need to move it and if there are gaps in your walls from it moving. You don't want to shove the wall base in and have the wall boards buckle up or pinch your liner in between the boards.

    You might have better control by using a nice piece of oak and a sledge? Scrape awy the dirt etc. along the areas that kicked and lay the board along it and swat the base back in... then stake it.

    Good Luck...!!
    Bob

  5. #5
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    Thanks everyone for the replies! Bob, the pool is empty (almost) there is about 8" or so. We left some water in it to prevent the liner from drying out or shrinking (BTDT).

    When we moved several years ago, we left the bottom track at the house until we could get a u-Haul to move it (each side track was one piece, so the longest track was 32'!). Anyway, someone must have seen it and cashed in on it...

    I've attached a picture that I found online of a newer style construction (ours is older-style). The original track was a 4-3/4" aluminum channel that wrapped around the inside of the wall and the outside of the braces that are located every 4' (the triangular braces are positioned where the marine-plywood butts up to one another. The straight braces are spaced between those. In the old style Kayak, the braces are merely aluminum with 2x4 pieces placed inside where screws attach the walls to the brace.

    When I put the pool back together I bought 1x1x1/4 angle (with a tensile strength of 45,000 psi) and I strapped them together in 2 places about 11' away from each end wall. The angle wrapped around the walls only. I think that is my problem. I didnt think about the braces separating from the walls as the water pressure caused the walls to bow. So the walls are still in the bottom make-shift track, but the braces started to kick out from the walls. Still attached and tucked under the top track (which like huge steel bow-shape - exactly like the leaf-spring Bob explained).

    So this is why I was thinking I could wrap something around the braces to stop them from separating from the walls. My husband's brother works at a fiberglass company, so we have a ton of fiberglass sheet rejects. Anyway, to address one of Bob's concerns, I doubt that the screws ripped through the water wall as we reinforced the old vinyl coated marine plywood with the fiberglass overlapping the seams (like a running bond) on the inside. However, I would wager that the screws ripped away from the chintzy pieces of 2x4's inside the aluminum braces. I thought about dropping the liner from the walls (but not moving it on the bottom leaving it intact with the drain and protecting the sand) and BOLTING the plywood walls and fiberglass to the braces, in addition to wrapping the braces on the outside with tow strap (which I already bought and put on it last year just to hold it from busting apart all the way) or cable and turnbuckle or whatever...

    Anyway, I hope I wasnt too confusing. Thanks for all the help! Marian

    Above-Ground-Kayak-Pool-1.jpg

  6. #6
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    Here I am again. We've been playing around with some ideas... third time is a charm and we want to make sure this is the last year we are redoing this thing! I think some of our ideas are overkill... but better safe than sorry... anyway, that's why I am here.

    I've attached a few pictures to show show you where and how it failed. The tow strap was added after the fact to get us through the season without it blowing out further (which did seem to work - Work Load 2830 lbs, Break Strength 8500 lbs). The straight braces were not bracketed to the pool at all, just screwed on through the pool wall (and fiberglass sheeting that we used on the outside of the marine board). The triangular braces were bolted on with angle braces in 2 places. You can see where I didnt brace it to the pool is where it pulled apart. In hindsight, I should've braced it directly to the aluminum angle that we used in place of the missing track.

    Idea #1 - Concrete 4x4 posts at every 8' intersection (where marine boards butt together), below frost line - as if building a deck.

    Idea #2 - use coil strapping from brace to brace (running under the sand, and connecting it to the outside of the braces). Would have to decide what gauge strapping would be needed and if it would be needed at only the 8' intersections, or every 4 ft (at all the triangular braces) or every 2 ft (at all the braces).

    As I said before I did use flat bar equally spaced in two places to span the 16', bolted to the aluminum track (under the sand). The problem really wasnt the track moving, but keeping the upright braces secured to the bottom of the marine boards. Also, the short sides barely moved at all. We mainly had a problem with the far end and I'm thinking it was worse here because we have a rolling lot and we didnt have to dig down into the ground on this end, the other end has a concrete driveway less than 1' away that it goes below. Anyway, I'm leaning toward idea #2, but would like to know what gauge strapping I should use and how far apart you recommend for that gauge. Hubby wants idea #1, but I know he is not looking forward to digging the 4' post holes! He also suggested using the strapping idea and still concreting around it in conjunction with the driveway but leveling out the slope beside the pool and use that space and deck as a bar with stools on the concrete pad at a later date.

    What do ya's think???

    20120605_1204.jpg20120605_1205.jpg

  7. #7
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    I know you guys told me not to over-think this, but in my inpatient mind, I'm thinking if I use the force that I calculated before (15,925 - as noted in first post) and divide that by the 32' it spans, I get 500 lbs/foot. CS16 (16 gauge coil strap) is rated at 1850 lbs with a clear span + 24" going into SPF (which I will be going into pine with the aluminum jacket).

    Of course I know the installation purposes are totally different and my force calculations may not even be relevant (or right)... but I'm thinking off the top of my head - trying to get ready for the repairs this weekend.

  8. #8
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    Mariane, why not fabricate the channel using flat bar and angle with the edge up? 1/4" x 2" x 2" plus 1/4" x 1-1/2" plus 1/4" x 2" x 2"

    If you make it in steel, stitch weld the angle to emulate the existing one.

    Don't worry about pressure.

  9. #9
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    Quote Originally Posted by PinkertonD View Post
    Mariane, why not fabricate the channel using flat bar and angle with the edge up? 1/4" x 2" x 2" plus 1/4" x 1-1/2" plus 1/4" x 2" x 2"

    If you make it in steel, stitch weld the angle to emulate the existing one.

    Don't worry about pressure.
    Is it good to use steel or some other material can be used?

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