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Re: 'Epoxy creep' factor in Big Dig death

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On Jul 11, 2007, at 8:59 AM, David Fisher wrote:

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(--nospam--at)skypoint.com   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw/



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