Securing Safes on Post Tension Foundation


March 24, 2013, 11:22 PM
Experts please help. What are the methods available for securing a safe onto a post tension concrete foundation. My house has them & the plans indicate they are tensioned to 18,000 pounds. I definitely dont want to damage one while trying to drill a hole for a 1/2" red head. So far I have been able to find a specialst who will "Scan" the slab with a ground penetrating RADAR unit for $450. Ther are also X-Ray services available for similar prices. Are there any other methods available. Also does a heavy safe such as an Amsec RF6528 need bolting to prevent being carried off or tipped and having the door attacked. Thanks

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March 24, 2013, 11:35 PM
Yeah, you don't want to cut a tensioned rebar. I'd imagine some kind of imaging equipment will be necessary and that any of it will cost a few hundred.

And yes, bolting down a safe is a pretty important extra safety measure. If it can be lifted or tipped AT ALL, it can be moved. You don't want that. I'd figure the extra money is just good insurance -- especially if you're already paying for a large safe, and presumably have firearms worth enough to justify it.

March 24, 2013, 11:38 PM
The radar is what we use at work. I would spend the money if I were you, as opposed to using some poor man's method (I won't even the very bad ideas I've heard). Snapping one of those pieces of steel is bad times.

March 25, 2013, 09:23 AM
Do it the right way, and consider the cost an investment in security defrayed over the years you'll be living in your home. The cost of repairing a tensioned slab or having your safe stolen is going to seriously outweigh getting the radar done.

March 25, 2013, 10:06 AM
Being that Amsec RF6528 weighs 3400 pounds it would indeed be a bear to steal.
But of course it is possible.
I would bolt it down simply because of the tipping issue a lot of safes have when the door is fully opened and no one wants a 3400 pound safe to fall over on them.
On the other hand the safes body may be so heavy that tipping is not even an issue on this safe.
If I owned a TL rated safe I am very sure it would have been purchased to store more than guns in it so that being my angle I would most certainly go through the minor expense of locating the cables and have it firmly bolted down.

March 26, 2013, 10:51 AM

I don't have to deal with this type of construction in this area. But, how thick is the concrete on the floor? What diameter are the tensioned rods? Here's what I'm thinking, if the floor isn't too thick, use a common stud finder & see if the magnet can't locate the rods. Of course, if you don't get solid indications go with the more sophisticated method of determining where the rods are.


March 26, 2013, 02:17 PM
Hi CB,

The foundation is a 4" slab of 2500 PSI rated concrete. I obtained the test reports from back when the house was built and it tested at 3077 PSI @ 14 days. The tendons under pressure are actually cables strung within the foundation. They were stressed to 4,000 lbs three days after concrete placement and the received a final tension of 18,400.

The issue with a metal detector is that there is also welded steel wire under the slab and some rebar. Its not a disaster to drill through either of those. Cutting a cable on the other hand can result in about 100K of damage to the house as it rips through the foundation - something to be avoided.

Neither my safe dealer (who also installs) nor the specialist mover/installer I am using will have anything to do with drilling a post-tension slab under any circumstances. The specialist recommended that if I was really worried about it that I coule use a 4x5 plate of 3/4" steel plate fastened to the bottom of the safe to provide stability in the event of an earthquake, to add a bit more weight, and to make a base so big it wouldnt fit out the door even if someone tried.

March 26, 2013, 02:19 PM
Hi CB,

Sorry I re-read your post. I am just not sure I trust a stud finder. One miss and I am in serious trouble.

March 26, 2013, 02:25 PM
If you think the scan is expensive, just wait till you price a buggered up tension system.

March 26, 2013, 02:38 PM

OK, with all that other metal in the floor, forget the stud finder, you're going to have to do it the expensive way.


March 26, 2013, 06:53 PM
What about bolting a 1/2" or more thick piece of plate to the safe bottom, then bolt that plate to a wall or bolting the safe directly to a wall instead of the floor? I could have done my safe either way.

Good luck!

March 26, 2013, 10:14 PM
Safes that heavy are not very easy to move. It is rare for us to bolt safes like that in jewelry stores, and they are full of jewelry. I would imagine somebody would want one of those before they wanted your gun safe. If they aren't walking off, I doubt yours would.

As far as your specific question, you already have your answer. If it needs to be bolted, you'll need to scan the concrete. When scanning isn't worth messing with, we glue them.

March 27, 2013, 09:46 AM
I live in Texas and I have a post tension foundation.

Would it be better to glue some 2X6's to the foundation and then bolt the gun safe to the 2X6's?

This way the bottom of the gun safe is not glued to the foundation directly (i.e. if you ever move it sure would make moving the safe alot easier than un-glueing the safe from the foundation.

Outlaw Man
March 27, 2013, 11:12 AM
I would think something like 2x6s would be a good start. Depending on the length and type of anchors you're using, if you keep the in-concrete depth to no more than an inch, you should be alright on a 4" or deeper slab. I'm not guaranteeing anything, though.

March 28, 2013, 08:00 AM

If you glue the bottom of the safe directly to the slab, then how do you un-glue the safe later if you move? And what type of glue??

March 28, 2013, 09:19 AM
If you have predrilled holes in the bottom you can use them to make a template and have some threaded 1/2" rod welded onto a 1/4" steel plate.
Epoxy the plate to a very well prepped floor. Industrial grade epoxy like the stuff they used on those ceiling panels for the big dig in Boston. As far as the back and sides, that depends on whether this is a permanent installation. If so I would surround it with concrete block, well painted with a moisture blocking paint, and complete the attachment process with a couple of cans of expanding foam insulation, sprayed in via hoses. The only problem is that you will never get it out. Also think about electrical and alarm wiring before you build, it is a lot easier than retrofitting. If that thing is secured by two different surfaces, or even better three, it is virtually impossible to rock it and get any leverage under the base to get a prybar on it.

March 28, 2013, 12:00 PM
If you glue the bottom of the safe directly to the slab, then how do you un-glue the safe later if you move? And what type of glue??

The prep work is probably more important than the actual adhesive. Even regular old Liquid Nail does a pretty good job. There are several two part epoxies that we have used as well.

It's more or less a permanent bond. It can be broken with time, effort, and chemicals. You can also glue plywood to the floor, then the safe to the plywood. Not as good of a bond, but does make it somewhat easier to pull apart at a later date.

Steel Horse Rider
March 28, 2013, 12:25 PM
What about bolting it to the wall studs?

March 29, 2013, 01:47 PM
Found out a while back that my 2007 construction house has a conventional rebar slab. Made my week! :D

March 29, 2013, 03:03 PM
Why not just pour an additional footing on top of the current slab? You can drill into that and add as much weight as you want without damaging the tension slab.

March 29, 2013, 05:26 PM
The weight of the additional concrete + the heavy safe would likely exceed the load carrying capacity of the existing slab.

We would need a structural engineer to spec out the details for us, but my understanding of such things this is that just because the concrete has a compressive strength of 3000 PSI. doesnt mean that it can carry a load of 3000 PSI. When the load carrying capacity of the ground under the concrete (and many other things) are figured into the engineers equations you get a carrying capacity of something more like 500 - 800 PSF. Of course your mileage may very. There are many components that go into the equations which is why licensed PE Structural Engineers get paid what they do for consults.

March 29, 2013, 06:38 PM
The normal purpose of a footing is to spread the weight out over a larger area to prevent over stressing the foundation in the first place. If the slab can handle the safe, then a footing that is wider than the safe would improve it's weight distribution. If the slab can't handle the safe an engineer will probably recommend a footing to spread the load out over a larger area.

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