> But, as I understand it, epoxy anchors typically have a safety
> factor of at
> least 4:1.
That's based on short term testing of properly installed anchors. As
time goes on, that margin drops off, and it keeps dropping off
forever. If there are installation issues the margins are smaller and
they may drop off faster. Again, epoxy like all plastic is subject to
creep and in many case environmental degradation. I've been through
this with reinforced plastic pipe, some very fancy glass-epoxy
compressed gas tanks and other items. Epoxy is great stuff for
household repairs, but critical items (like airframes, for example)
need permanent ongoing surveillance that's a great deal more
sophisticated than some guy poking a rigging knife into a joint
looking for dry rot. I don't know about concrete residue but the
kinds of epoxy I've run into don't like dampness or ultra-violet or
sea water or acid. Like so many other miracle products, epoxy isn't a
cure-all or some kind of magic bullet.
The failure mode is something like creep-rupture--the accumulation of
permanent strain until the material reaches its fracture limit (not
the per cent elongation, but something like it) and fails. As the
epoxy deforms it simply keeps straining. Moreover most plastics, even
reinforced plastics accumulate plastic strain even at low stress
levels. Strictly speaking, there is no yield point other than some
arbitrary measure like the 0.2% elongation.
Short term overload testing isn't enough to establish service
suitability. Absent installation or environmental issues short term
testing may work for earthquake loading (although for earthquakes,
you never really know) but the roof supports were subject to
continuous statically determinate loading. Statically determinate
loading isn't relieved or redistributed by localized yielding, so the
creep strain accumulation would have been ongoing. In fact the NTSB
report says that other anchors were found which had undergone large
deformations.
If you haven't read the NTSB report summary <http://www.ntsb.gov/
Publictn/2007/HAR-07-02.htm> you should do so. It's a very good
illustration of the Petrosky envelope-pushing principle.
Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw@skypoint.com | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania
1864)
http://www.skypoint.com/~chrisw/
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