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Old 03-21-2011, 02:11 PM   #1
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Pool On Patio?


I plan on building a 12' patio out in to my back yard. It won't be very high off the ground; maybe 2'.

What I am wondering, and have so far not been able to calculate (I'm not a builder, just a homeowner who likes to do what work I can) is if I can construct a patio that would be able to support one of those "softpools" during the summer time.

We have one of the 10' wide soft-side pools that stand up as you fill them and are about 3' deep. We love having it for the kids to cool off in, but it requires a large footprint of the yard where we won't ever have grass growing after this beast sits on it.

Can I build my patio with the proper footers & supports, etc. to be able to hold this thing, or is it crazy to even think about it. I honestly have no idea.

Thank you for any input / suggestions / hope you can give me. My yard is not very large. It will be a wide patio/deck and there won't be much yard space left anyway.

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Old 03-21-2011, 07:45 PM   #2
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Pool On Patio?


That is not a patio, what you are building is a deck. Doubtful that your city would allow it, since the pool being full depending on the size, would probably stress the joists and decking too much. Only way to truly find out, is to find a engineer in your area to do the calculations and tell you if your deck design would stand up the stress from said full pool sitting on it for over 3-4 months.

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Old 03-21-2011, 07:59 PM   #3
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Every foot of water exerts approximately 62 pounds per square foot of load on the deck. A three foot deep pool exerts almost 190 pounds per square foot. Depending on local codes, a deck is typically designed for anywhere from 40 to 60 pounds per square foot live load, plus 10 psf dead load, for a total of 50 - 70 psf. Your pool alone would be approximately triple the design load for the deck.

Could you design for such a load? Absolutely, but you are not going to find the required framing sizes in your deck code. You are either going to have to engineer it yourself, or more likely hire an engineer to size the structural elements, design the connections, and stamp your plan.
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Old 03-23-2011, 01:35 AM   #4
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Pool On Patio?


Thank you very much, this information has been helpful.

I assume this is why when you see a hot tub on a deck, it is usually recessed down below floor level with its own supports underneath?

I think I have an idea to solve my problem that won't involve unusual amounts of framing.
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Old 03-23-2011, 09:34 AM   #5
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Pool On Patio?


Bear in mind that I have an engineering background, but I am not experienced or licensed as a structural engineer. What I am about to offer is based upon general static physics used to extrapolate what you would need to do to adequately design your deck to support the load.

As Daniel was saying you're looking to support 190 pounds per square foot where standard deck design is intended to support 50 to 70 pounds per square foot.

In general, two things are considered in your calculations, stress and deflection. Both numbers are calculated based upon formulas with variables that depend on how the ends of a structural member are constrained, the dimensions and shape of the cross-section of the structural member and the length of the structural member.

The limits of my experience are that I mostly deal with metal, and being that metals generally have the same strength properties in all direction simplifies things in these calculations. Wood, on the other hand, is stronger in the direction of its grain than it is perpindicular to its grain, therefore wood is more complicated to calculate strength than the calculations I'm familiar with, but for this limitted situation I would simply suggest that we resort to overdesigning the deck to compensate for any loss of strength that might come from assuming that the material is uniformly strong in all directions.

Your best strength increase will be reducing beam and joist spans by increasing the number of posts and beams supporting your deck. But you can also gain strength and rigidity from increasing the size of your joists and beams, so let's look at those calculations first.

Stress and deflection of a structural member is calculated using, among other things, the area moment of inertia. Those other things, for now, are not being changed for the purpose of this example - and those things are length, material and end constraints. So we look at the dimensions of the cross section to find moment of inertia, which for a rectangular cross section is calculated using:

When calculated with respect to centroid: I = (BD^3)/12
When calculated with respect to the top or bottom: I=(BD^3)/3

Where B is breadth and D is depth... For example, on a 2x6 joist, B=2" and D=6"

We're going to ignore whether we're calculating with respect to centroid vs top or bottom because we're only changing D.

Here's the bottom line. 6^3=216. 8^3=512. 10^3=1000. 12^3=1728. That means a 2x10 joist with the same span as a 2x6 joist can support nearly 5 times the weight. So if you had a deck that was designed to use 2x6 joists and beams, you could simply up-size to 2x10 to achieve 5 times the strength. And that would achieve the 4x the strength with some extra margin. You could go up to 2x12 if you really wanted some extra safety factor.

It's worth pointing out that I'm referring to simple beams, but I don't think there's a need to get into the ramifications of stress and deflection in other situations so long as I simply point out that there are different calculations for different situations.

As far as the span of your joists and beams, stress is a function (all other things being equal) of length squared. So suppose you have a beam with a 10 foot span between posts. 10^2=100. If you cut the distance in half to 5 feet 5^2=25. What this means is that at the same load, the beam with half the span sees stresses that are 1/4 of that in the longer beam. And the bottom line is that it means that the beam with 1/2 of the span can support 4 times as much load before it achieves the same amount of stress.

So to achieve 4 times as much strength, you could reduce the span between posts and beams by half or more. This might involve more cost and material, but it also achieves something that upsizing your joists doesn't achieve: You're increasing the amount of area that you are bearing on your soil.

And obviously 4x the strength doesn't give as much safety margin, so you might want to use more posts to reduce your span by less than half.

Sorry if I didn't quite get everything pieced together in a manner that is as cohesive as it could be, but hopefully I've presented enough pieces to understand how all of this works and what you might be able to do and what it does for your deck strength.
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Old 03-23-2011, 09:51 AM   #6
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Pool On Patio?


Don't forget about the live load (people going in the tub) You need to figure each person as they step in the tub.
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Old 03-31-2011, 09:49 PM   #7
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Thank you all for your help, but at this point I think I might just have the area where I plan to set up the pool be on ground level - recessed down in to the deck - maybe put down some concrete, then when the pool is not set up; in the fall & winter, the recessed area will have seating around it by sitting on the actual deck and I could put my small fire pit/box in the middle.
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Old 03-31-2011, 11:11 PM   #8
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That would be the best way to do it.

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