"k" values for Bolt Thru concrete anchor?? 1

[smitty9898]

I'm analyzing a steel bolt thru anchor that was installed thru a hardened reinforced concrete slab. The slab was core drilled, and then the anchor was pushed thru the hole. On the backside is a 3"x3"x1/2" back plate. The anchor is loaded in pure tension.

I'm trying to determine the concrete breakout strength of an anchor in tension in accordance with ACI 318 (2002) D.5.2. The capacity of the anchor is highly dependent on the "k" coefficient found in equation D-7; the basic concrete breakout strength equation. I'm having trouble determining an appropriate "k" value for a thru-bolt anchor. I would consider the thru-bolt anchor post-installed, however I believe it will act more like a cast-in anchor. The code refers to post-installed anchors as expansion and undercut, and has a reduced “k” value compared to cast-in. The code also allows for "k" values higher than 17 as permitted for post-installed.

Does anyone know the "k" value for a bolt-thru anchor? Thanks!

 

[Ron]

If you were able to get a plate through the core hole, it isn't doing anything to interface the concrete.

Epoxy bolts in concrete do not fail in the same manner as expansion bolts or embedded bolts. They fail by shear interface failure between the epoxy and concrete, so a pullout cone is not developed.

 

[smitty9898]

Ron -

The plate does not fit throught the hole. The plate is attached at the underside of the concrete deck. The diameter of the core drilled hole is approx. 1/16" greater than the diameter of the bolt.

I'm not talking about adhesive anchors, however adhesive anchors can fail with a pullout cone. It all depends on the material properties and configurations.

Thanks!

 

[KBVT]

You have a punching shear issue here. I don't think this is applicable to Appendix D. You need to confirm the tensile strength of the bolt and perform a punching analysis on the concrete utilizing the plate as your "column."

 

[ishvaaag]

If I understand well the setup you have a steel back-plate to which a bolt passing the concrete depth is either fitted to a nut or welded. In this condition, the bolt is more than anything the means to pass a punching action to be exerted by the backplate. Assuming that the backplate and its rod connection are good enough, this resolves to the punching force for the dimension of the backplate on a slab with the standing reinforcement, if any. Menétrey had a paper on it on ACI Structural Journal.

 

[Lion06]

I don't see why App. D wouldn't be applicable. I would probably check punching shear, too, but I wouldn't leave out App. D. I would use kc=17. This accounts for potential micro-cracking at the anchor location. Cast-in anchors obviously have less cracking than mechanical, post-installed anchors; and mechanical, post-installed anchors likely have more cracking than your situation (because of the expansion/undercut nature of mechanical, post-intalled anchors. That being said, the anchor is not cast-in and I would expect some micro-cracking from drilling. The code doesn't give you an alternative unless you perform testing to determine a more appropriate kc value. Use 17 and call it a day.

 

[smitty9898]

Ishvaag & KBVT - Great points! I will include punching shear in my analysis.

StructuralEIT - Thanks for the response! Do you know of any references that state the "k" values are proportionate to micro-cracking? From my understanding the "k" values are determined from testing.

 

[Lion06]

I didn't say that it is proportional to micro-cracking, just that is the variable that it is trying to capture. This is the reason that for a code-compliant post-installed anchor there is a kcr, and a kuncr.
They are determined for post-installed anchors (were determined for cast-in) anchors by testing, but, again, the variable that this is trying to capture is the micro-cracking around the anchor.

 

[ekhaw]

ACI 318 D.2.2 clearly excluded through bolts from the appendix. Is there any provision for through bolt design besides applying rational analysis such as punching shear? Thanks!

 

[structuresguy]

I might also check bearing of the plate against the concrete. Not likely to be limiting factor, but it could be under certain conditions.

 

[smitty9898]

Thanks ekhaw and strucuturesguy for the insight.

ekhaw, I believe you have answered my question!!! Sometimes it might help to read the entire chapter. Thru bolts are not governed by ACI 318 Appendix D.

I have no knowledge of provisions for the design of thru bolt anchors other than punching shear.