Anchoring at Grade Residential Swimming Pool to Foundation - High Seismic 2

[KootK]

Here's the situation:

- Fancy house design in California.

- 20 miles from the San Andreas.

- 6' deep, outdoor swimming pool with some plexiglass windows on the side.

- Pool rests on a pile supported concrete slab with the bottom of the pool about 18" above grade on the downhill side.

- Pool is a steel framed thing designed by somebody else.

- Anchorage may well be designed by somebody else.

Here's the question: do I/we need to get serious about the anchorage of this thing as though I were designing a tank structure per ASCE7? You know, free-board, sloshing, over-strength, ductile failure modes... all that jazz. On the one hand, I know of no relaxation that would apply to the design of such a thing relative to a conventional tank. On the other hand, I struggle to imagine conventional residential design getting into this level of detail for an at grade, outdoor pool. What are other folks doing in this regard? Is there, perhaps, some IRC/CRC get out jail clause that I don't know about?

In terms of real seismic risk, I'll confess that I don't consider this to be a very high risk situation. The worst I could see happening here is the pool sliding a bit off of it's support and maybe the the slab corner punching up through the pool bottom or something. Maybe a twisted ankle if somebody happens to be standing in the corner of the pool at go time. Or a crushed toddler if one happens to be taking a nap nestled up against the pool foundation when the 9.0 strikes. It's not like we'd be dumping toxic wast into the water supply or anything.

Please advise. Big below of a similar situation but not my situation. Mine's grade beams and piles and on a site with much more slope to it.

 

[JStephen]

On a tank, the freeboard is provided so you don't damage the roof or lose tank contents, and I would think neither would apply to a pool.
On a tank of any size, consideration of sloshing leads to a reduction in the lateral forces on the tank (much longer period for sloshing gives you reduced loading for that component).
Checking the index of IBC 2015 leads me to Section 3109, which references "International Swimming Pool and Spa Code".

 

[KootK]

With regard free-board, sloshing etc, I'm familiar. I'm more trying to ascertain whether or not I need to get into serious seismic design of the thing in the first place.

International Swimming Pool and Spa Code? Jeez, there's a code for every damn thing these days. I'll check it out. Thanks.

 

[KootK]

Was able to source 2012 ISPSC. No mention of seismic I'm afraid.

 

[GC_Hopi]

I guess it would fall to engineering judgement. And your previous statements on the risk seem spot on. Even if the pool does move it would not move much assuming 0.35 coefficient of friction (or you can use ASCE 7 15.7.6.1.5) and the earthquake increases in acceleration over time, its not instantaneous, so a person could conceivably move out of harms way.

It may be worthwhile to put together a small write up of the design conditions and engineering judgement for your records but heavy calcs hardly seem necessary.

 

[KootK]

..assuming 0.35 coefficient of friction...

That I'm curious about too. Either the pool is empty and there's next to no seismic weight or the pool is full and there's probably gobs of friction available. Unless I get railroaded into one of those conditions where I can't rely on friction because of vertical seismic accelerations.

I guess it would fall to engineering judgement.

Baring any evidence to the contrary, I'll probably do just that and lean on friction in a way that will make the seismic design effectively moot. It's a weird world in this regard nowadays however. No sooner will I decide to use my judgement and somebody will chime in with "are you aware of the Intergalactic Building Code for Seismic Design of Residential, at Grade Pools and Spas". Then I'll be stuck through bolting to Madagascar or something.

 

[GC_Hopi]

KootK: Friction is allowable see the ASCE 7 15.7.5 or 15.7.6.1.5

 

[WinelandV]

Is the pool a one-off by a fabricator or a legitimate manufactured product? If the latter, I'd imagine they'd have to have at a minimum some guidance on anchorage. Of course, the guidance may just be "retain a qualified engineer".

That being said, it appears to already be in the ground, and there's no obvious access for an anchor within the wall, and no plates outside the wall. Perhaps it's meant to just sit there?

And I missed the last sentence before the pic. Apologies.

 

[KootK]

Is the pool a one-off by a fabricator or a legitimate manufactured product?

Legitimate manufactured product.

If the latter, I'd imagine they'd have to have at a minimum some guidance on anchorage.

I'm afraid not. The calcs for the pool superstructure weren't terribly inspiring. And the anchorage guidance so far has been:

1) "Here's a pic of the base lug."

2) "It might not need to be attached at all but you should check local requirements."

Perhaps it's meant to just sit there?

Yes, perhaps.

That being said, it appears to already be in the ground

No, the picture is a very similar pool but not my pool. Plenty of time to sort thing out.

 

[WinelandV]

1) "Here's a pic of the base lug."

Depending on the lug's geometry, it might be easy enough to back out its design capacity, then size the anchorage to exceed that capacity. I know this is akin to "the building will fall down before the loads are reached", but, on the other hand, it's a swimming pool.

 

[KootK]

FWIW... the lug thing. Pretty beefy although I was expecting a horizontal base plate of some kind. Thankfully, we wont be supporting he pool one some pieces of... pumice?!?

 

[WinelandV]

Looks like they took a shipping container and waterproofed it.

Well, I'm out of easy ideas. I'll just say that being on a slope is a bit worrisome, as you'll have soil on the buried side trying to push the pool off of its foundation. Weld the lug to an embed plate that's tied directly into one of the piles?

 

[KootK]

I'll just say that being on a slope is a bit worrisome, as you'll have soil on the buried side trying to push the pool off of its foundation.

No problem there. The dirt loads all go right into my pile and grade beam system which has been designed for it.

Weld the lug to an embed plate that's tied directly into one of the piles?

Not bad. If aesthetics/trip hazard allows, maybe I could post install some angle clips around the lug just to act as "keepers" after a fashion

Thank you very much for your interest/support so far.

 

[Celt83]

if it is just a modified shipping container could embed one of these, [URL unfurl="true"]https://en.wikipedia.org/wiki/Twistlock[/url], into the slab and throw some additional anchor reinf. steel around it for piece of mind.

or surface mount option: Link

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[BridgeSmith]

Don't know if it's necessary for something containing water, but we have always tried to isolate our retaining wall structures in high seismic regions from fully the embedded piles under them. Rigid connections between rigid structures and stiff piles result in very high shear loads. We've accomplished the isolation using a 1 ft layer of fabric-reinforced crushed rock.

 

[bones206]

If aesthetics/trip hazard allows, maybe I could post install some angle clips around the lug just to act as "keepers" after a fashion

That was my first thought too when I first read this yesterday. I see it more as a property damage prevention issue than a life safety issue, and retaining clips would be cheap insurance for the owners to hopefully prevent major damage. I would leave a little space to still allow the pool to slide around a little bit to dissipate some energy through friction.

As far as code compliance goes, I'm really not sure what governs in this case. Might be best to ask the local AHJ. My gut feeling on this is that anchorage is overkill and probably not code-mandated, but maybe retaining clips could be offered to the owner as a sort of "optional seismic protection upgrade".

 

[Agent666]

I've done a few concrete swimming pools, including these infinity type pools you see on the edge of a cliff waiting to fall off the cliff under a seismic event. Never seen a steel pool, so advice below is based on my experiences with concrete pools for what its worth.

Initially when I was first getting into designing them I asked others more experienced in the design how do anchor this thing to stop it sliding? Only answer I got was we don't, it can slide if it must, you can create some small thrust blocks (your keepers) at the corners to prevent it moving too far, or a continuous nib around the perimeter to retain it. It just hits these if you like and prevents it moving to the point where it might fail by falling off the foundation or something.

For an above ground concrete pool it was always poured onto another concrete slab on grade or suspended slab (usually required when hanging over a cliff). So there was appreciable friction when you considered up to 1.8-2m depth of water+ the self weight of the pool and a somewhat rough surface + some inevitable bond between the pool and the underslab. Generally as well restraining the pool is a bad thing, my initial thoughts were we would have to drill in some bars all round to tie it to the foundation. But because it can be affected by temperature effects (shrinking/contracting) this isn't recommended as it tends to make a concrete pool crack, and cracks leak in a water retaining structure. The crowd I used to work for used to go to court all the time to report on others pool failures, guy I worked with had become an expert in it. When you are dealing with $500k pools including all the support structure to get it to the cliff edge there is money to be made when it goes wrong

Unlike a real tank design there didn't seem to be too much consideration of what went on at the corners. Usually it was design a cantilevered wall, ensure you have sufficient moment coverage in the walls and into the slab. Check for cases where you had pool full/empty plus any areas that might have been retaining soils in the empty state. That was it. Reinforcement was always the same for a typical 1.8m deep pool, 12mm bars (500MPa) at 150 ctrs, higher in the walls and middle of base this reduced to 300 ctrs.

Unlike a tank holding some dangerous liquid, if a bit of water sloshes over the top it not that big a deal. Generally with a concrete pool you are designing for limiting crack width criteria so they had plenty of strength for small increases in water pressure from convective and impulsive loads due to the water sloshing. Keeping in mind for gravity case you might have a load factor on the water pressures, but for the seismic case it was accepted you could use 1.0. That ratio alone is usually enough to cover the increase in pressure.

I guess you need to consider consequences of failure. If the whole thing goes and its going to flood out the guys entire house and his bedroom is in the basement or something then you might pay a bit more attention and provide an overland flow path away from other structures, than say if the pool was located away from other structures.

Edit - We also have a swimming pool design standard (NZS4441), all it says on the matter of design is the following for in ground pools:-

 

[Tomfh]

Wow. A shipping container pool.

Agree with adding some twist locks. if they're using a shipping container then no point reinventing the wheel.

 

[WinelandV]

KootK,

Out of curiosity, what are your Sd1 and Sds for the site?

It's sounding more and more like the angle to keep it from sliding is the way to go, assuming it can get anchored appropriately.

 

[Huevo]

Some good information and details for connecting and anchoring containers here and here.