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Re: ACI 355.2

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In a message dated 3/20/01 9:13:31 AM, dennis.wish(--nospam--at) writes:


Question: What restriction if any will be placed on the use of epoxy anchors

(or other chemical anchors which do not promote concrete cracking from

expansion pressure?

>Remember, the cause of cracking is not soley due to the expansion forces of 
mechanical anchors.  Shrinkage, live + dead load, and wind/seismic loads on 
the concrete member need to be considered.  For example, consider a large 
shearwall hold down load near the middle of a long foundation.  The uplift 
load into the anchor at that point puts the foundation into bending  and the 
concrete in that area will probably be in a tensile zone.  Now, a threaded 
rod epoxied 16 " deep for that hold down is no longer approved because it did 
not pass ACI (and now proposed ICBO ES) criteria.

2. When the provisions for retrofit of Tilt-up concrete to alleviate

cross-grain tension failures in ledger connections reached its height of

design in the early 90's the questions of capability of expansion anchors

was also raised. It was my understanding that expansion anchors were not

recommended for tension installations in retrofit projects. However,

Undercut anchors were determined to provide appropriate strength with less

risk of cracking concrete and reducing strength. Again, I have opted to use

chemical anchors over expansion or undercut anchors as common sense dictated

that the bond from chemical anchors would be superior to compression type

anchors and would do less damage.

Question: Is this the perception of other engineers and if so, what major

consequence will there be in retrofit work for the restrictions placed on

compression type anchors that can not be resolved with possible changes in

installation requirements of chemical anchors that would insure appropriate

strength, mixture and installation without the need of deputy inspection?

>My impression is that the performance  of expansion anchors under seismic 
tension loading was largely unknown at that time, and there was a low 
confidence level among engineers to use friction type expansion anchors for 
those applications.  The City of Los Angeles still prohibits the use of these 
anchors for this application.

Note that most undercut anchors are really designed to perform like a headed 
bolt.  In comparison to wedge anchors, they put significantly less expansion 
force into the concrete, and for the most part get all of their load from the 
head bearing on the undercut concrete area.

Howard Silverman