Pull Test is insufficient
I have had numerous comments from my engineering colleagues agreeing with my 7/16 letter published in your paper concerning the ceiling panels in the Airport Tunnel of the Big Dig.
I assumed in this earlier letter that the information available to the public was correct and failure mode of the hanger system is that described by me as a ”creep failure” of the epoxy cement holding the hangers into the ceiling.
I now hear that Governor Romney has approved a “pull test” on the remaining hanger system to be definitive in deciding which panels need to be re-hung.
Unless my earlier assumption of a creep failure is shown to be incorrect, a simple pull test is not sufficient to determine the structural integrity of the system. The momentary and present strength of the epoxy cement may be perfectly adequate to support the panels NOW and therefore pass the pull test.
The long time, continued stress resulting from the panels’ weight, WILL eventually cause the polymeric epoxy material to creep and fail. It becomes not a question of WHETHER but WHEN.
Hansjoerg Stern, PE
167 Bog Pond Road, Brewster, MA 02631 508-896-5795