Storm Shelter Concrete Anchors 1

[geauxLSU]

Hello,
I'm an EE in the market for a steel storm sheltered to be anchored to my garage floor. I've read the Hilti datasheets/specs (HUS-EX & Kwick Bolt 3) and basically anything with a diameter => 1/2" and a >2" embedment requires a 5"-6" slab, but the standard house is 4". To meet Fema 320 specs you need to be about 3". From all the research I've done, all the storm shelter manufacturers are violating the Hilti datasheets big time, which puts me on edge. I feel like its just a guessing game now if the anchor will hold since they a either completely going through the slab or not leaving any room below the anchor. One prominent company uses 7" anchors which will rust out since they are in the dirt!

Does anyone have any advice? The shelter certification companies have basically told me they don't care, go away.

Thank you

 

[geauxLSU]

....Fema 320 recommends a 3" embedment (not a 3" slab).

 

[jayrod12]

Do you need the 1/2" anchor? Hilti has a 3/8" diameter anchor using the HVA capsule that has published values for 3 1/2" embedment.

 

[KootK]

but the standard house is 4".

Given what I've seen of residential construction, I'd be very surprised if it's not more like 2.5" in some places.

Some place, I thought that there was a limitation on expansion anchors stating that they couldn't be embedded more than 2/3 of the depth of the substrate?

Does anybody on the design side of these shelters even check to see that the anchors engage enough SOG mass tor resist uplift? While a shelter is surely a big improvement over no shelter, there may be some design holes to be legitimately concerned about here.

You've obviously got some options available to you if you're willing to demo parts of your garage slab and pour a little concrete. Hopefully it won't come to that. Maybe you could anchor the shelter to concrete blocks that just sit on top of your slab on grade?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[geauxLSU]

Thank you. I noticed the 3/8" anchor, but since FEMA 320 requires a 1/2", I dont think any shelter company would use a 3/8" anchor. Part of the problem seems to be that no anchor can meet the criteria. (>=1/2" diameter, ~3" embedment, 4" slab, pull out strength ~1800 lbs, made in the USA)

The company I'm considering now uses a 3/4" x 7" Hilti HUS-EZ with only a zinc coating. It will definitely rust in the dirt, I'm just not sure if it will continue to rust once its length has corroded to only the slab thickness of 4".

 

[jayrod12]

Yes it would continue to rust. The rate of corrosion may slow once it reaches the concrete however it will still continue.

I don't suggest going that route (even though you are slightly hand-cuffed) although if you do make it a stainless steel anchor. It will last longer than a strictly galvanized anchor would.

 

[geauxLSU]

Thanks guys,

Considering that the concrete depth could vary a lot & I might not have enough concrete below the anchor to meet Hilit's specs, what would be the best choice: Epoxy, wedge, or screw anchors (HIT-Z, Kwick Bolt3, HUS-EZ) As I already stated, I'm concerned the zinc HUS-EX will rust. I can get the Kiwck bolt and Hit-Z in galvanized or stainless I think.

 

[wannabeSE]

Galvanized or stainless steel anchors won't help. The slab still needs to be thick enough for the anchor. It may be possible to engineer a concrete pad anchored to the top of the garage slab. But, it may be better to demo the existing concrete for the storm shelter anchors.

 

[geauxLSU]

Yikes! The idea of finding a competent person to tear through my slab and post tension cables scares me, it would take a lot of time and resources.

 

[KootK]

PT cables? That changes things. Have you verified the slab thickness and presence of cables? You probably should.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[geauxLSU]

I found this table on steel corrsion rates. When the anchor is in the dirt it would corrode faster. Any thoughts?
At this point, I think I'm more likely ot die trying to solve this than I am to get hit by a tornado!


The (uniform) corrosion rate of steel in different atmospheres:

Atmosphere Corrosion Rate (µm/year)
Rural 4 - 60
Urban 30 - 70
Industrial 40 - 160
Marine 60 - 170
Source: Living Steel

http://www.galvanizeit.org/corrosion/corrosion-process/corrosion-rate

If I assume 70um per year, that would only be 0.05" over 20 years; hard to belive it would be that little

 

[ztengguy]

I am with KootK, not sure what good a shelter is if it can blow away with the slab attached.

 

[jayrod12]

I agree with KootK, are you sure it's PT? seems odd for a residential application.

 

[geauxLSU]

My house was built in 1999, so I've been told everying after '95 or so has them. I see steel poking out of the side of the house every few feet, I assumes that was the post tension calbes?

 

[ztengguy]

If they are requiring a 7" anchor, dont you think they want to get the capacity of a 7" anchor in at least 7" concrete? Having it hang out of the slab does nothing for your capactiy, other than reduce.

 

[cvg]

I'm assuming gumbo clay, it would definitely be post tensioned.

your corrosion rates listed are in atmosphere (air), not in water or soil. you would need to test your soil to determine how corrosive it will be.

 

[KootK]

While residential PT is rare in the US as a whole, it's quite common in CA and TX. And I have heard of it in LA, where I assume OP is located? Visible slab edge anchors is a good tell.

If it's PT, it'll be thicker than 4", right? As long as the cables aren't damaged, PT may be a boon.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[geauxLSU]

I live in Texas, we have major foundation issues here because of the clay soil. One local shelter company told me they won't do INGROUND shelters on newer homes in the garage because they dont want to cut the PT cables. Hmm. Wonder how I can get this slab thickness measured?

 

[hawkaz]

If the PT is for shrinkage and temperature only- 4" is possible. Developers around me like this type of slab because the slab cracking is limited- so they have fewer callbacks.
If it is a structural slab, then it should be thicker.

If the slab is 4" thick, can you use 1/2" diam. Hilti Kwik Bolt TZ anchors with 2" embedment- and tighten up the spacing? Make sure to identify tendon locations before drilling.

 

[geauxLSU]

Yall, made a good point about having it measured/checked. Noted! One website said PT slabs are usually 8" (heck if I know). If that is true than I could use just about any anchor and be in good shape. FEMA requires a 4" minimum slab, so I shouldn't blow away! I just didnt want the anchors to be incorrectly installed on slabs they were not intended for. This is a major hole in shelter design right now.