Concrete Screw Anchors with Epoxy

[geauxLSU]

I'm having a steal storm shelter anchored to my garage floor with the 3/4" Hilti HUS-EZ screw anchors.

Does any one know if there is a concern about adding epoxy to the holes before we screw them in?

I dont have any other anchor options because that is what the engineering drawings require. I was thinking of adding epoxy to the holes just to add holding power and possibly help with corrosion. I asked Hilti about it and they just replied that they didnt have any data on it. Obviously removing the anchors would require me to zip the heads off.

Thank you

 

[jayrod12]

Why would you do that? seems like an unnecessary additional expense.

If you are going to epoxy just use threaded rod anchors (much cheaper than the HUS-EZ). No point in paying for 2 specialty products.

 

[RobertHale]

I had a friend do this (unbeknownst to me at the time) with the Simpson equivalent. Basically the tighter tolerance of the hole that allows the screw threads to engage, cranks up the friction to such a degree that the epoxy would set before the fastener was driven all the way. He ended up over drilling the holes and turning the anchor into a non-adjustable, untensioned, expensive threaded rod. Either use the threaded rods and the adhesive or use the screw anchors alone.

 

[geauxLSU]

Thanks for the tip, Robert.

Does one else on the forum have any experience or knowledge of this?

 

[cvg]

you can't add "holding power" with epoxy. Just does not work that way. the capacity is based on the concrete breakout strength which will not be increased at all by epoxy on the threads.

either the existing design is acceptable or not. why would you assume it is not?

 

[geauxLSU]

I just knew that epoxy anchor strength was superior to screw anchors according to Hilti datasheets, so I wanted to add epoxy. The other concern I had was that they are using the 7" HUS-EZ which are not intended for exterior applications, yet they will be completely penetrating the slab and into the dirt by a few inches. Thus they will eventually rust out. They only have a thin zinc coating.

I've decided to not do the epoxy, come back and inspect one of them in 10 or 15 years, and put epoxy or wedge stainless steel anchors in if needed at that time. If I do the epoxy now, I wont be able to back them out to inspect.

I don't understand why the epoxy wouldn't add strength though, it should created a bond between the metal and the concrete.

Thank you

 

[jayrod12]

Wait,

Why would they specify a 7" long anchor when there isn't 7" of concrete. You don't gain any capacity past the thickness of the concrete. Don't pay for anchors that are too long for the use. Do not allow them to core completely through your concrete slab. This all seems sketchy. If you need the higher capacity put in epoxy anchors and threaded rod at the correct depth for your application. And if you don't have enough concrete to get the capacities you need then you need to figure out a way to get more anchors.

 

[cvg]

HUS-EZ are rated for indoor, dry conditions. the assumption is that your garage is "indoor and dry".

for a 7 inch anchor you need 8 inch thick concrete. these anchors will not reach the maximum design load if your slab is thinner. They do make a 5 inch anchor which might work if your slab is 6 inches. These are not designed for cored holes, use a hammer drill.

unfortunately, adding epoxy will not help gain additional capacity. regardless of the type of anchor, most anchors rely on concrete breakout strength. they do not rely on bond strength. the assumption is that the bond strength between the anchor and/or epoxy to the concrete is adequate for the load. so failure occurs when an anchor pulls out a chunk of concrete.

 

[geauxLSU]

It did seem sketchy to me too. After talking to several of these shelter companies, they are all doing something weird with the anchors (including the most notable ones). The owner claimed they did't make the short version when he started.

Ground radar showed I have approximately 6" of concrete and some of them will be going into the grade beam which is much deeper. Perhaps I would be better just using the 4.5" or 5.5" anchors?

I believe you use a impact driver into a 3/4" cored hole. If the installers use a drill, is that a problem? I know there is a maximum torque rating which must be observed. I'll try to keep an eye on them, but I don't have any experience with anchoring.

 

[cvg]

no, you do not use a cored hole. the product guide is available, clearly requires a drill. You will have to either a) deepen your concrete slab to handle 7 inch anchors b) use shorter anchors and possibly more of them

https://www.hilti.ca/medias/sys_mas...chnical_information_ASSET_DOC_LOC_3220868.pdf

 

[racookpe1978]

Definitely shorter anchors.

Good research! Your instincts paid off by asking questions. Probably only 1 in 100 homeowners did the checking you did.

How did you do the concrete radar depth checks? It sounds like a neat idea.

 

[geauxLSU]

Thank you. I paid a guy $400 to come out and locate my post-tension cables and rebar with his ground radar. He kind of said off the cuff it looks like you have 5"-7" of concrete. I wish I had pressed him now to be more specific.

Does any one know how to do the calculation for the force the shelter can tolerate?

Wind Speed- 250mph
Concrete psi -3000
Shelter length - 80"
Shelter height - 75"

I'll probably only get 6 HILTI 3/4" HUS-EZ anchors on each side, so 12 total. Hilti says at 4" embediment, each can handle a tension of ~4800 lbs.

Thank you

 

[BadgerPE]

What force are you looking for? Are you specifically looking to see what the shelter is rated for? If so, then you would most likely be looking for a FEMA rating within the manufacturer literature. Perhaps FEMA 320 or FEMA 361. Both are available on FEMAs website for free:

FEMA 320
FEMA 361

My experience has been solely with FEMA 361 designing community safe rooms so I am by no means an expert.

If you are looking for a way to convert the wind speed to wind pressure so you can determine your required anchorage capacity, you can use ASCE 7 and the basic wind speed (v=250 mph) to determine a pressure.

 

[woodman88]

Say you have 4 bolts (one at each corner) good for 4000 lbs each. For a 6'x6'x6' cube, you have 2 x 4000lbs x 6' = 48000 ft-lbs. 48000 ft-lbs / (6'/2) = 16000 lbs. Applied on a 6'x6' area gives 16000/36 = 444 psf.

If the safe room can take this kind of forces, I would say your post-tension slab will fail first.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[cvg]

probably need to check shear also...

 

[geauxLSU]

I figure the wind pressure exerts a force on the side of the shelter and then to the anchors through some distance of steel wall. The wall of the shelter then acts as a lever, now having additional torque. I was wondering how to do a back of the envelope calculation for this to figure out if the anchors can hold.

PS. The shelter weight about 2600lbs.

 

[woodman88]

Hopefully shear is the controlling factor. Than the slab may work.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[canwesteng]

Combined shear and tension, plus more than likely some concrete breakout failure governs, not the 4kip of steel strength. It's a shed, do you care if it stands up to a hurricane?

 

[cvg]

It's a shed, do you care if it stands up to a hurricane?

its not a shed, its a tornado shelter, of course we cares if it stands up in a hurricane. otherwise, see ya Dorothy... the tensile strength is far more than 4 kips per anchor and so is the shear strength

perhaps you should bury this thing

 

[canwesteng]

Seems just skimming it all I picked up on was that it was anchored it to a garage slab and assumed it was a shed... Ooops. Seems like this should have an engineered foundation