Concrete Failure Point
Concrete Failure Point
(OP)
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
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





RE: Concrete Failure Point
Please clarify with a sketch if possible.
RE: Concrete Failure Point
-tg
http://xternal.me
RE: Concrete Failure Point
As Ron suggested, a sketch would help.
Polecat
http://www.spiraleng.com
RE: Concrete Failure Point
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
RE: Concrete Failure Point
RE: Concrete Failure Point
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
RE: Concrete Failure Point
http://www.spiraleng.com
RE: Concrete Failure Point
-tg
http://xternal.me
RE: Concrete Failure Point