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