Cantilever pool for a house

[MonsieurR]

Hi guys,

An architect is doing remodeling for a house on a hill (high seismicity region on sloped soil) and wants to place a cantilever pool (see attachment). Width is ~3.5 m, depth ~2.0 m, length pending.

Considering the house and terrace are already there, meaning this pool would not be monolithic with the rest of the structure, do you see this project reasonably possible?

I'm thinking the main problem is stability and constructibility. I was thinking maybe anchor bolts to the ground (dywidag or similar) may solve the problem but I don't know the practicality of such solution. Another approach would be a big foundation below the pool but my numbers tell me it would need to be huge.

Since the length is pending the most obvious solution would be to choose one with a cantilever that doesn't lead to a stability problem. This seems the obvious thing to do since at first sight the photo seems something not achievable, yet the problem is it deviates from what the architect wants.

Do you have any thoughts on this?

Kind regards.

 

[WARose]

<shaking head> Oh the joy of working with architects.

Just eyeballing it......it doesn't appear it goes very deep into the terrace. So I'm not sure if you have the room for the option I am thinking of: a micro pile pile group (with some of them at angles) with a reinforced concrete cap on top that the end of your pool could tie into. That's about the only arrangement I can think of that is going to give you vertical and rotational stability at that end. (And not have the typical vibration issues of pile driving.)

By the way, love the title of your attachment.

 

[kipfoot]

My first thought is that the renovation has to include work on the interior at the lower level: cut through the slab and excavate so that you can pour a back span to the cantilevered sides of the pool. That can all be patched back up and put back together.

 

[BAretired]

I vote for an "L" shaped support underground with a footing or pile centered under the c.g. of the pool. The vertical and horizontal leg of the "L" must be designed to resist the required moment, but the footing is centrally loaded.

BA

 

[Prestressed Guy]

That isn't so crazy. When I was a kid back in the 1960's I saw a show about a house up on Mulholland drive in LA that had a pool that rotated up out of the floor and into the wall to convert the space into a tennis court. As you have it drawn you should be able to get the required strength out of the side walls of the pool. You will need to be very careful with the bearing pressure at the top of the slope because you could easily get into a slope stability issue. And as kipfoot said, extend the pool walls back into the basement to pick up back-span leverage and dead loads from the rest of the structure. Would it be easy? NO. Would my fee be high? You bet!

Living on the edge can be interesting.

 

[dik]

Low seismic, maybe. High seismic, maybe not... I can see the channel shape going into a torsional mode in the event of a serious event. You would have to make some sort of tube out of it for it to maybe work... and that' s assuming the back span can be made fairly rigid. You may be able to use part of the tube as a walkway around the pool.

Dik

 

[BAretired]

Here is an elevation of my earlier proposal (see attached).

BA

 

[KootK]

Honestly, I don't see this coming to pass without incurring prohibitive costs and doing a lot of intrusive work to the existing structure. You might try posting in the geotechnical or foundation forums. Most of tough stuff here is the purview of those specialties. If you can catch the attention of member PEinc, that would surely be a boon.i

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Maybe build a tower behind the house and support the outside end of the pool with cable stays. That would satisfy a lot of "crazy" architects.

 

[dik]

The pool portion would still have to be made into some sort of 'tube' for torsional resistance.

Dik

 

[LittleInch]

Can it be done as drawn as a retrofit - Not within anyones reasonable budget. Far too much piling / excavation.

Something like though might be feasible and wouldn't spoil the view from the pool. Still quite a structure to lock in place.

https://dornob.com/one-cool-pool-cantilevered-design-boasts-a-window-in-the-floor/

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[KootK]

I don't suppose the house has a basement?

What do we know about the wall that forms the terrace face?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[IRstuff]

Apparently, someone has built that concept, although it's in Germany:

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[rb1957]

is germany seismically active ?

I get the "sky hook" tower idea ... maybe struts supporting the overhanging end ? maybe extend the side walls as a foundation ??

another day in paradise, or is paradise one day closer ?

 

[DETstru]

IRstuff, that image is very deceiving.
See this one of the same house:

 

[DETstru]

Maybe you can get them to give you a pair of kickers to the far edge of the pool.
What am I thinking... architects...

 

[rb1957]

i wonder why you'd go to the effort of making this pool, and then not making it an infinity pool ?

I like the fence along the slope edge, but not around the pool ? Maybe it's a "thinning the herd" strategy ... if you're stupid enough to walk along the (wide) edge, and clumsy so as not to keep your balance, then you deserve what happens ?

another day in paradise, or is paradise one day closer ?

 

[KootK]

Mount a high capacity, downwards blowing fan to the end of it?

I think that much comes down to weather the terrace is rock or retained soil. If it's rock, a Dywidag anchorage solution may well have legs. If it's soil... not so much.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

The attached file illustrates two concepts which could be considered. The first is a double cantilevered pool which the architect may not like but which makes sense structurally. The second is a singly cantilevered pool which requires a well reinforced "L" shaped structure below grade. I have used this concept on a lighter structure. The pressure on the footing is virtually the same for the two schemes but the pool walls and the "L" shape require plenty of reinforcement.

BA

 

[MonsieurR]

Thank you for sharing your thoughts! It appears I'm not inflexible then.

There is no debate this would be prohibitive in costs, and also being a high seismicity zone with a steep slope I'm afraid local site amplification plus vertical, horizontal and torsional seismic demands would result in a monster structure . I would need a geotech to help me for stability, and engineering would be costly as well (lots of analyses I suppose). I would be even a bit scared to place my signature on this.

So I think I'll just call the architect and tell him it's not reasonably possible, and give him some alternatives. I bet he will ask me how come we can place people in the moon and can't build a simple cantilever pool. I still haven't thought the reply to this.

@Kootk, house doesn't have a basement, there are traditional foundations below the shear walls.
There is soil below but it appears to be very stiff.

@BAretired, I think what you proposed would be the way to go but still such a big substructure would result in stability issues for the steep slope, which would result in an even more costly work

@dik, I understand the torsional problem but what do you mean with the tube shape?

Thanks!