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Re: ACI 318 App D, and wedge anchors

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Jared,
 
What brittle failure can we/they talk about with the safety factors per Appendix D? 
 
Some normal structure (say, a house) is designed with an overall safety factor of, say, about 3 - against its actual physical failure.  This means that it will take an unheard of wind or earthquake to take the structure down (cause structural failure).  This further means that long before any (brittle or not) kind of failure in the anchors, the structure itself will be ruined, and the performance of the anchor will not matter.  
 
The logic (or lack thereof) of the situation leads to the conclusion that the structure itself represents a "weak link."  The failure of this "link" will inevitably come earlier than that of the anchor, practically assuring that the anchor will never fail.
 
Besides, there are no reported epoxy anchor failures. None. 
 
V. Steve Gordin, SE
Irvine CA
 
----- Original Message -----
Sent: Thursday, July 31, 2008 15:55
Subject: RE: ACI 318 App D, and wedge anchors

Bill,

You are right that the ductility issue applies in both tension and shear.  But if you add enough anchors far enough down on the wall, you can get the concrete connection strength of the multiple anchors to be greater than the single threaded rod in tension to the hold down at top of the bracket.  This was an admittedly over-kill design to meet the requirement for a single location, so it was feasible.  I wouldn't recommend it to be used at multiple locations in a retrofit, since it would get real expensive quick.

As far as the series of calcs goes, I ran ACI appendix D calcs in accordance with the ICC ES report for a number of anchor sizes and different "seismically approved" anchors and quickly realized that is was not possible to get the anchors to fail in a ductile manner in tension.  I emailed Hilti tech support since these were the "seismically approved" anchors to see what I was missing.  The tech support provided the following response in May 2007, in the middle of the post-installed anchor crisis:

"You are absolutely right and you are not missing anything. In most of the design cases concrete failure modes are governing and resulting in a brittle failure."

Hilti went on to provide information on the changes approved for ACI 2008 and the 2.5 factor for non-ductile designs.  Combined with the lower values developed under the new testing criteria, this is unfortunately quite a burden on the post-installed anchors and in many cases makes them very difficult to be utilized and meet the direct wording of the code, especially for single anchor bolts or hold downs.  I suspect that in a couple years, Hilti, Simpson and others will find a way to address this issue through new product developments that will meet the intent of the current codes.   In the mean time, I cringe every time I get the call from the contractor with a mis-aligned anchor bolt and the request to pop in a post-installed anchor.

Jared


The ductility issue (1/2.5) not only applies to tension anchors but shear
anchors as well.

Just curious, what "series of calcs" did you go through? Section D.3.3.5
isn't clear on this.

T. William (Bill) Allen, S.E.
ALLEN DESIGNS
Consulting Structural Engineers

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