Above Ground Residential Storm Shelter

[phamENG]

I have a client interested in including a site built storm shelter in their new residence. It's going to be on the coast, so we're looking at a hurricane shelter. Flooding is an issue, so finished floor is about 7' above grade. It's an A zone and not a coastal A zone, but only by about 3 feet (edge of house to edge of LiMWA). The intent is to design this per ICC 500 and FEMA P-320/361.

The conventional wisdom with these is to have them in or on the ground. In is certainly not an option here, and on is great - but I'm not excited about what 7' of fill will do to the underlying soils (mostly soft clays in this region) and the short and long term foundation settlement prospects. (I seriously doubt a geotech will be in the picture - just a fact of life for residential work here.)

All of the prescriptive safe rooms set slab on grade foundation as a hard and fast requirement. My thought is to design it per ICC 500 loading requirements, but use a structural slab over a crawl in lieu of fill and slab on grade, and then the superstructure will either be masonry or the double stud with steel sheet setup that FEMA has tested. The foundation walls would have flood vents to allow hydrostatic pressure to equalize, and I'd look at wave impact above and beyond what is strictly required.

Anyone see any significant issues going this route? A non-prescriptive requirement that I'm missing somewhere? Thanks.

 

[BAretired]

The most significant issue that I see is that a geotech may not be in the picture.

BA

 

[IRstuff]

The higher up you are, the stronger the wind; you'll likely need ballistic projectile protection, which might be covered in the FEMA docs.

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[EngineeringEric]

I love the fun projects we run into!

If you aren't in the LiMWA or even if you are, you could make the lower portion of the structure dry flood proofed but inaccessible and then place your elevated slab above the DFE. I'd even make the lower walls grouted solid CMU (CMU because that is all we do in VA or even go with concrete) and then the upper walls the same and a concrete ceiling. This will be like a shaft with an insane roof that can support the building falling on it. If this is a storm shelter, i wouldn't add windows even if they are impact resistant. but have a crazy door with extra locking mechanisms.

You may want to look into the tornado requirements for some elements.

I would urge against the double stud. Stick to solid grouted CMU. The wood connections are always difficult as you know

 

[phamENG]

BA - do you get a geotech on your residential jobs? If so, maybe I should move there...

IRStuff - for building structures we cut off the variation at 15 feet - 0 to 15 is all assumed to be the strength of the wind at 15, and then vary it with height from there. Wind borne debris impact requirements are covered in the FEMA docs and ICC 500.

 

[phamENG]

Eric - thanks. This one's actually south of the border a ways, but construction techniques and trends aren't much different than here at home. I wasn't too sure about the double stud either, but it looks like Simpson has done some testing on it and it's pretty slick: [URL unfurl="true"]https://seblog.strongtie.com/2015/05/which-tornado-saferoom-is-right-for-you/[/url]. I agree that I'd rather have CMU/concrete - but if that's all they can end up affording it's probably better than hiding in your tub.

Side note - were you guys bought out by a big national firm?

 

[EngineeringEric]

HGAM clip at each stud at 12" OC can't be cheaper than 8" CMU! I suppose a double stud wall like that is better than I have at home. I think you are following ICC-500 you should be good honestly, I've used it for EOCs and tornado bunkers. Something to pay attention to, if you are on the first floor of two stories, make sure you can support the weight above falling down on your ceiling/walls. I also am sometimes nervous about being locked into a room with limited exits in the event of other elements failing, I guess make the door open inward not outward? (in the event of debris locking you inside)

I found your post funny as I just did a storm shelter in Hampton but it was all concrete walls and an elevated composite 6" slab all supported by a 12" thick mat slab foundation under the whole house, a unique home.

You sir a great detective! I will keep my offer as always if you ever want a change of scenery, I'm sure you'll be able to find me in the firm.

So far, the transition actually seems pretty good. We get to do the work we like, little corporate feel outside of back-of-house things like billing.

 

[1503-44]

This should work,

 

[phamENG]

Eric - Given current market conditions, you're probably right. I'm going to develop concept sketches of both and let the contractor give him a price on each before getting too far into the detailing. A house like that in Hampton? That is interesting. And I'm not that great a detective. LinkedIn keeps sending me invitations to apply for the open senior engineering position, and I noticed the "a ___ Company" under the logo. I appreciate the offer - I'll certainly keep it in mind, but I've been making my own scenery the last few months and it's been really nice.

1503-44: Maya I ask you where to find the design guide for one of those?

 

[kipfoot]

I like the idea of a mat foundation under the whole thing to give you ample overturning resistance. I'd vote in favor poured in place concrete and some means to allow water to flow underneath, though I'm not familiar with storm shelter requirements. Also, think about the depth of foundations with regard to scour.

My worry would be that the owners would use the shelter rather than evacuate in the face of a hurricane and that the storm surge would come up higher than anticipated. (There's a lot of construction activity in Ocracoke, NC where people are placing their houses on taller piles after being surprised by the 2019 storm surge)

 

[1503-44]

South of the border means Rio Grande to me.

How to Build an Aztec Pyramid School Project

 

[phamENG]

kipfoot - good points all. We did discuss the leaving vs. staying some. At least verbally the owner seems to understand that this is an emergency feature when other options aren't available, and not something to be used routinely. I was on Ocracoke a couple months ago and was checking out some of the work going on down there.

1503-44 - my apologies. Eric and I work in the same market (and I'm pretty sure kipfoot is close by, too). So for us, south of the border means the state line just to the south - at least in this context.

 

[Enable]

Hmmmm...just to add more controversy to the mix I always thought "South of the border" referred to where the pretty colours (errr colors) stopped and the brown began

BTW I have nothing productive to add to this discussion on account of projectiles not being one of our design criteria. Though I suppose icicles can get pretty big on roofs. Seems rather cool. Update us with the design once it's sorted if you could! I'd like to take a gander.

 

[EdStainless]

But now I will work on my daughter to get her to upgrade her front door (PBC FL).
The winds from a storm have bent the hinges on hers once.

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[BAretired]

BA - do you get a geotech on your residential jobs? If so, maybe I should move there...

Usually I do, but there have been exceptions. In your case, the short and long term foundation settlements, and particularly differential settlements, would be an aspect I would not attempt to resolve without geotechnical advice.

If you were to move here, you would find some clients unwilling to retain a geotech on a residential job, giving you the opportunity of considering your continued involvement on the project. Foundation design is best performed at the outset, not after problems occur with the finished structure.

BA