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.

 

[bones206]

If the pool itself hasn't been specifically designed as an anchored vessel under seismic loading, then anchoring it could actually make failure or damage more likely to occur. If they haven't designed a seismic-resistant load path from the anchor points all through the vessel superstructure, then forcing seismic reaction forces into those lugs by anchoring could unintentionally overstress some component.

So even if it's possible to anchor using shipping container connections, you might be going out on a limb from a liability standpoint. But on the other hand, if the AHJ classified the pool as a Chapter 13 non-structural component, then you would be obligated to provide anchorage per 13.4 and the manufacturer would be obligated to certify the vessel design for seismic loading with anchored boundary conditions. If it's classified as a flat bottom tank under Chapter 15, then I guess you can use friction and take advantage of what you've referred to in the past as "poor man's base isolation" (my vote). Section 13.4 specifically prohibits consideration of friction resistance for non-structural components.

 

[FoxSE14]

Some similarities, but not a pool. Apologies for anything out of bounds/redundant/inapplicable as I have not read every post above:

In a past life we designed the anchorage and foundation for a single-story shipping container office/control room subject to blast loads. For the anchorage, we provided a horizontal steel embed plate cast into the concrete with A706 rebar welded to the back and developed into our foundation walls. We relied on the mass of soil confined by the rectangle of foundation walls to resist the blast/impulse load. The office container supplier welded the base plates onto the bottom corners of the container, when container arrived to the site, the contractor fillet welded the base plates to our embed, and everyone was off to the races (i.e. the missing piece in your pumice photo)...can whomever is providing the pool also provide weldable base plates for you to do something similar?

Then there's the labor...we had the fortune that the GC did a bang-up job making sure the base plates were cast dead-level, and believe he only needed one small shim out of 8 base plates.

Edit: Personally, based on doing some seismic retrofit work for a couple years in the past...I would focus on getting the anchorage to the foundation correct for seismic forces, including sloshing, assuring ductile failure at anchorage, to make sure the whole shebang doesn't turn into Santa's death sleigh heading down the mountain. I think you've got a lot more 'cushion' and room for eng judgment once the forces are down to the fdn/subgrade interface (friction, etc.)

 

[BridgeSmith]

I agree with bones206; creating a rigid connection to the foundation will result in very large forces on the pool walls and the connection to the foundation, possibly as high as what it would take to move the foundation slab. If the foundation slab is in turn rigidly attached to piles, the forces could get even higher. I like the idea of letting the pool just rest on the slab, but limiting its movement so it can't move off the slab.

 

[FoxSE14]

HotRod10, bones206 - agree on your points re: potential for overstress at the walls. Not sure if this creates aesthetic concerns or issues with pool mechanicals, but, could a continuous (or semi-continuous if so needed) "beefy curb" be cast tight to the base of the pool wall to prevent movement off the foundation system? I'm thinking something similar in cross-section to a grade beam (maybe 12", 16", 18", 24" square as calcs require), but cast as a second pour atop the slabs/grade beams, appropriately reinforced with longitudinal steel, stirrups, and hairpins/dowels down into the initial pours?

Or maybe that's overkill and you can get away with a few concrete haunches/reverse corbels in strategic locations?