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Re: ACI 318 App. D

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Scott,
 
I also figured this much regarding the inspection, but was not sure as is did not make much sense.  Oh well. 
 
The difference in the anchor bolt pullout values is staggering.  According to Ronald Cook's  "Strength Design of Anchorage to Concrete"  (PCA), this difference results from the "high tensile stresses that exist at the embedded head of the anchor while other approaches... assume uniform distribution of stresses over the assumed failure surface."  
 
As a result of such "new" approach, the "old" breakout strength of concrete is supposed to be divided by a square root of the embedment depth.  Say, if I have a 25" embedment depth, the "new" concrete breakout strength is 5 times lower than the "old" one.  This also changes the governing case (it becomes concrete, not steel) and results in further reduction of capacity due to lower strength reduction factor (of course, the final result may be less different due to the steel-concrete strength "play").
 
We are used to the "new and improved" methods leading to the same decades-old results through more difficult calculations.  In spite of natural frustration, this gives us confidence in the ultimate adequacy of the decades-proven design techniques.  Here, one of the most basic structural engineering concepts appears to have been proven wrong.  Did I miss somebody in California notifying the engineers not to use CBC/UBC Section 1923 that produces non-conservative - at best - results?
 
Thanks again,
 
Steve Gordin SE
Irvine CA
 
----- Original Message -----
To: Seaint
Sent: Sunday, October 08, 2006 7:48 PM
Subject: Re: ACI 318 App. D

Steve:

Table 1704.4 lists the requirement of continuous inspection of anchors in
cast-in-place concrete.  It references section 1912.5.  Now, it appears
that this is ONLY for allowable stress design of anchors in concrete
(section 1912 of the 2003 IBC).  It appears that if you do continous
inspection of anchors in concrete which were designed per ASD (section
1912), then you may increase allowable tension (but not shear) loads by
100%.  HOWEVER, section 1912 specifically says that is CANNOT be used for
seismic loading and that you must design anchors in concrete for seismic
loads using section 1913, which is the strength method (which in the 2003
IBC bascially references ACI 318-02 appendix D).  I don't see any
requirements in the IBC for inspection if the anchors are designed by
strength method (i.e. ACI 318-02 Appendix D)...although I did not look
at it too terribly closely.

Regards,

Scott
Adrian, MI


On Sun, 8 Oct 2006, S. Gordin wrote:

> Good Sunday afternoon,
>
> While working with IBC and ACI 318-02, Appendix D, I found out that the cast-in anchor pullout strength was reduced 2.5 or more times as compared to the previous (say, 1997 UBC 1923) approaches.  Such change seems quite dramatic, but may be I am not reading correctly?  Or does this mean that the most recent research renders all our pervious anchor bolt design (and research, and testing) non-conservative at best?
>
> The other question is that UBC was pretty clear on the anchor capacities with or without special inspection.  Where (if at all) can I find similar requirements in the IBC?
>
> TIA
>
> Steve Gordin SE
> Irvine CA

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