Cracked Versus Uncracked 1

[XR250]

I am still unclear on this issue. I am working on a metal stud job where I am using epoxy anchors in various situations. Many are for a cantilevered sill, so I have one bolt at each stud (16" O.C.) and it acts in tension only (about 1400 lbs ASD). This is an existing building with 9" PT slabs. They are going to x-ray it to make sure we don't hit anything.
If I assume cracked concrete, my anchor fails unless we go with a deeper embedment. If I assume un-cracked, it passes.
I have asked a few former collegues and they say they would assume un-cracked.
What are the rules?

 

[TLHS]

The AEFAC technical note had a list of references that I started digging through. I found this.

http://www.bbacerts.co.uk/download/...r Bolts for use in Concrete - UK Guidance.pdf

It is amazingly specific and clear. It has areas that can be considered uncracked by inspection, and also clarifies that minor cracking is acceptable at time of installation as long as it is not expected that further cracks will form or that existing cracks will expand. That would take shrinkage cracks out of the equation, for most applications.

Note that this is a british opinion, so their recommendations for areas that remain uncracked don't appear to account for seismic load reversal. I'd never consider a potentially life safety seismic application as uncracked, personally. Even if analysis level loads show that a member stays in compression, the true deformations are going to be higher and part of the assumed ductility mechanism involves concrete cracking.

 

[KootK]

Damn. Way to run with it TLHS. If nothing else, I bring that to court with me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TLHS]

And a bit more...

https://www.hilti.com.hk/anchors

Hilti's non-north american technical information is significantly more in-depth. I have a european manual hanging around somewhere and it goes significantly further into how they've come up with their capacities.

That page goes a bit into cracking and makes it clear that the major concern is that when cracks propagate after an anchor is installed they have the propensity to intersect with the anchor hole itself due to the stress concentration at that point. If a wide enough crack forms, it basically bisects the pullout cone into two pieces.

Presumably, small width random cracking that would existing in almost any concrete beforehand has a significantly smaller effect on the breakout characteristics.

 

[IDS]

It is amazingly specific and clear. It has areas that can be considered uncracked by inspection, and also clarifies that minor cracking is acceptable at time of installation as long as it is not expected that further cracks will form or that existing cracks will expand. That would take shrinkage cracks out of the equation, for most applications.

Shrinkage cracks don't expand? That's news to me.

I'm all in favour of following engineering judgement and keeping it simple, but that means adopting the more conservative option if there is any reasonable doubt.

What are the potential consequences of designing for cracked when uncracked would have been ok?

What are the potential consequences of designing for uncracked if at any stage of the life of the structure it may become cracked, and subsequently be subject to maximum possible loads?

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/