Concrete Failure Point

[telecomguy]

I have an electronic enclosure being bolted to a concrete platform with 4" concrete thickness. The installation consists of a 3/8" thick stainless base plate sitting on top of the platform and a 3/8" thick stainless backer plate (fishplate) underneath the platform. 5/8" stainless threaded rod is used to secure everything so the 4" thick concrete is sandwiched.

I'm asked to provide a wind load analysis. I've already determined the expected moment and reaction forces at all anchor points, and qualified the design for standard concrete anchors in a slab. The anchor manufacturer provided significant pull-out data so I could easily show a factor of safety of 4x on all anchors in the previous design. But since this is a new installation method, I was asked to re-analyze.

My dilemma is not knowing how I should treat the concrete. I can show that the tensile strength of the 5/8" threaded rod will not be exceeded (with safety factor) but I'm not sure how to show the concrete won't be the failure mode. I assume it will depend on re-bar spacing and other factors but it's just not something I deal with in my 9-to-5 so I'd appreciate any guidance you might provide. It's probably a simple table look-up. Just not sure where to go for it.

-tg

http://xternal.me

 

[Ron]

If I understand your system correctly, there really is no fastening to the concrete....you are sandwiching the concrete between two plates. With that in mind, the concrete pullout is irrelevant. If the enclosure is subject to wind loads, then your enclosure "system" will induce a moment in the concrete slab.

Please clarify with a sketch if possible.

 

[telecomguy]

You nailed it. I really don't see the fasteners being the weak link with this approach so I think I need to consider the strength of the concrete. But I reserve the right to be way off in the weeds...

-tg

http://xternal.me

 

[Polecat]

If the 4" slab is a suspended floor, then you will have some considerable amount of steel in it already. We just don't know how much of it is required by the other loads applied besides the cabinet you are installing. Is the wind load reaction to the cabinet really considerable, or relatively small?
As Ron suggested, a sketch would help.

Polecat

http://www.spiraleng.com

 

[telecomguy]

No, the wind load is really not a concern in my mind but the customer is always right and they want the analysis. I attached a crude sketch on the previous post. The red is my enclosure and the plate underneath the platform. The threaded rod runs thru all and will have washers/nuts on top and bottom. (not to scale)

I've already submitted the following and haven't heard back so they may be content at this point. The easy analysis was to consider failure of the stainless rods. I already had reaction forces at the bolt positions so I just took a ratio of that to the tensile strength of the threaded rod. I showed about a 32:1 safety factor which is obviously silly to be worried about. So I think they must be concerned with the platform itself. But I'm in the dark just as much as you are about the construction of that platform. Just thought there may be a standard table or something that indicated the strength of various types of concrete so I could look at a reasonable worst case.

On the previous installation method using Hilti expansion anchors, Hilti specified pull-out strength in cracked concrete so that's what I used to show a minimum 4:1 safety factor which is their requirement. If I gave you the moment on the base, you'd laugh. I just needed to provide the #'s for their files.

-tg

http://xternal.me

 

[Saboral]

In this case given the steel plates on each side I don't see a pullout failure as possible. I think the appropriate analysis for the concrete would be to treat it as a beam and analyze for the bending moment induced in the concrete and reinforcing as a result of the wind load. You can compute the maximum allowable bending moment of the "sandwich composite beam" i.e. (steel plate/concrete w/ reinforcing/steel plate). Once you have that you than compute the moment induced by the wind load. Bob's your uncle you now have the factor of safety to concrete failure. Additionally you would also do a shear failure analysis of the sandwiched concrete to the surrounding slab. Unless a jet engine or a space shuttle is thrusting opposite this enclosure I have a hard time seeing a failure in either of these two modes due to wind.

 

[Polecat]

OK, if the moment from the cabinet is negligible, then you can probably estimate the strength of the slab by assuming that it has NO steel in it, even tho we know it does.
Let's just take a section of slab the same width as the cabinet and compute the moment needed to crack the section. Let's call the width (b)inches. You said that it is known that the slab is 4" thick (d), hence the section modulus of the concrete section we are looking at will be bd^2/6, or b*4^2/6 = 2.67*b in3. The floor will have at least 3000 psi concrete strength, so the modulus of rupture will be 7.5*sqrt(3000), or 410 psi.
Hence the cracking moment of the slab will be f*S or 410*2.67*b inch-lbs, where b is whatever the width of cabinet is normal to the wind direction. As long as the wind moment on the cabinet is less than that number, you will have no problem.

http://www.spiraleng.com

 

[Polecat]

One other item we have not addressed is the weight of the cabinet. If if is considerable then you would need to consider punching shear on the slab. The weight would be added to the compressive load on the slab coming from the toe of the base plate.

http://www.spiraleng.com

 

[telecomguy]

Thank you all. As I mentioned, I already submitted the bolt analysis to the customer. If they come back wanting more, I believe I now have a better understanding of the potential strength of the concrete platform to once again show them that they are worried about nothing.

-tg

http://xternal.me

 

[jayrod12]

You could check it as a sort of reverse punching shear check, however you will likely enact a bunch of the rebar which can't really be accounted for in strictly two way shear checks. It is also quite likely that the 4" slab would fail a two-way shear check so you might not want to go that route. But if they want calcs and are paying for them, you could give them calcs.