Structural Design of Pools

[SCss]

Hi,

I am in the early process of designing a pool. Are reinforced concrete pools designed based on retaining walls or cantilever slabs fundamental principles? And what is the general process to pool design? I was told by one of my colleagues that it should be designed as a retaining wall with two load conditions: 1.) Full Pool and 2.) Empty pool (the latter of which usually governs). I was just hoping for some advice on the general design process. Thank you

 

[HeavyCivil]

I have never done one but would design it as a tank. Obviously the empty pool condition will control for buoyancy and lateral earth pressure.

You could use FEA or pca tables to get moment coefficients if you're going to rely on your horizontal bar for 2way bending. If you design it as a cantilever retaining wall you'll have a conservative amount of vertical bar, but as long as you detail your corners well I can't see this being 'wrong' if a bit wasteful

One thing to note with this kind of design is the amount of liability you potentially open yourself up to. Not for failure, but when someone wants to sue you because their kids cracked their heads off your pool, their horse drowned in it - etc. I'd just make sure your insurance covers that stuff.

 

[JoshPlumSE]

Actually, I would design it mostly like a retaining wall...Using only half the footing width as the length of footing. But, VTEIT is right, the empty case should just about always control the design.

 

[MiketheEngineer]

From what I have seen, I think most residential pools are just built with "That's the way we always done it - or that's what my daddy says"

Scary??

 

[a2mfk]

Mike- I think you are right. I have always felt they are prescriptively designed, and part art and part engineering...

Living in Florida and doing forensic investigations, the problems I have seen almost always relate to emptying a pool with a high water table, or some serious soil problems. Few pools residential pools are rectangular, which helps a lot when you have curved walls (arches). Once they are full of water and backfilled, the hyrdostatic pressure is greater than the soil pressure, and the rebar goes into tension (like a tank, like the others said). Water weighs about half what soil weighs, so if it is an in-ground pool, the soil is pre-compacted by the natural surcharge, so settlement should not be an issue...

With all that said, I wouldn't touch them, there are so many variables and the biggest is the construction. And I am not sure how much number crunching I could do to come up with a reasonable design. Like others have said, if you design it as a cantilevered retaining wall with no water, I think you will get some BIG numbers and it would be your last pool design

 

[Bobber1]

Make sure your drainage is good. Seeing an entire swimming pool floating in an undrained pit is an awsome sight!

Use lots of rebar to keep your cracks tight.

 

[SCss]

I agree with you both. Mike, one way I managed to counter the problem of having two distinct load cases is to use double reinforced. The pool wall is basically a 9" outer leaf of masonry (in-filled concrete blocks), 9" doubly-reinforced concrete, 6" inner leaf masonry (in-filled). That's the way I've found examples of pool construction. I'm sure its over reinforced, but finding literature on pool design is difficult.

 

[csd72]

It is not just a simple matter of designing it as a astandard retaining wall, the reinforcing stresses need to be kept to within limits to ensure that they comply with requirements for liquid retaining structures.

 

[HeavyCivil]

I don't see any reason to follow ACI 350 crack control provisions if the pool will have a liner.

I also don't think you're required to, regardless of the liner. But obviously keeping steel stress low will result in a better finished product, albeit an expensive one.

 

[BAretired]

There are quite a few variables to consider. Outdoor pools in my locale have to be designed to resist frost pressure as well as normal earth pressure. One technique is to leave the pool filled throughout the winter to resist the frost pressure of the ground, but then cedar logs or other frost cushions are required within the pool area to take up the expansion of the pool water when it freezes.

If an outdoor pool is to be emptied in the winter, there must be an underdrainage system.

BA

 

[MiketheEngineer]

I've seen a number of free form shotcrete pools built. They seemingly use layers of WWF - welded wire fabric. It's often so dense I wonder how they get the shotscrete in there.

They seem to work well.

I too have seen pools "pop" out of the ground - esp fiberglass ones. Kind of funny.

 

[SCss]

I am considering excavating the surrounding earth by about a foot and filling it with a low density fill using a geosynthetic material to keep it from compressing. That way I will have a material with a density not much more than that of water. This will imply a minimal amount of strength since in the critical load cases (empty pool) the force imposed by the soil will be low, and in the other load case (full pool) the opposing water pressure will result in minimal resulting forces. I will design it as a retaining structure and make sure it resits overturning and sliding, bearing pressure and bending of both wall and base.

 

[csd72]

I woulnt think it was worth it. the wall still needs to have enough rebar to prevent cracking and whats anothe 2" on the wall thickness.

As for the comments regarding the soil load being less than the water, I would be very surprised if the water exceeds the at rest pressure of the soil.

 

[SCss]

I was thinking of using something like this:

http://www.acfenvironmental.com/geofoam.html

 

[hokie66]

I fail to see the advantage of the low density fill. I have seen a lot of residential free form pools built. Is that the type pool you are asking about? If the sides are stable and water in the excavation during construction is not a problem, the norm is just to trim the excavation to the desired shape, place the reinforcement, and place the concrete. Where I am, the concrete is usually not placed by shotcreting, but rather by pumping a low slump mix, with a lot of shovel and trowel work. Bars are used rather than fabric. Use small bars of the lowest strength available, as that aids in site bending, which is always required in some areas of a free form pool.

 

[SCss]

In that case all you're deigning is the amount of steel to resist cracking and bending due to water pressure when full? As well as bearing pressure of the entire structure. That seems slightly risky considering the unpredictable effects of water underground. No?

 

[a2mfk]

In Florida, where there are probably as many pools per house as anywhere, for concrete in-ground pools they simply dig a hole, put in #4s or #5s at X" o.c. each way (I'd say 6-8" min), and shotcreted. I would say thickness varies from 5"-8". I don't know any engineers who "design" them, thus my original comments on this often puzzling subject. Sounds like from hokie and others this varies depending on the geographic area.

It also seems to be a prescriptive thing, and I think any engineering is really just stamping a design that a pool contractor has used for many years with "no problems". And the reason I do not take umbrage with this way of doing it is the pools are immediately tested to their design loads, and the last thing a contractor wants is a call back on a crack in a pool. So whatever design they are using works. And I cannot envision any life safety issues if a pool shell cracks...

I'd try my best to get a hold of some local pool design drawings before I'd dive into this pool design (bad pun). Better yet would be to talk to an engineer in your area who has designed pools. If you are not comfortable with the designs you are seeing I would not do the job. You will just get push-back from your client when you give them your design if it is not what they are used to seeing (too thick or too much rebar), regardless of how correct you may be from an analytical engineering standpoint. Its just a fact of life in pool design...