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re: big dig structural failure - epoxy anchors overhead supporting gravity

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i sent this message yesterday morning, and again last night. but it didn't show up. perhaps it was too long, so i deleted the copied stuff. i hope it goes thru this time. . . . ------------------------------------------------------------------------ -------------------------------------------
Folks,

Now that I've seen the picture, I have a few more points for discussion.

1. Based on the relative capacities of the four anchor rods (if installed correctly) and the cable, it appears there was no consideration of ultimate strength design. the basic design of this should have considered that the only ductile failure would be the cable yielding in tension, so it should be the weakest link by a reliable factor. Just based upon experience it is clear that those four anchor rods would fail in tension (if installed correctly) well before the cable yields. Result is a brittle and sudden failure with no warning. If the connection system had been designed such that the cable yielded first, there would likely have been warning (sagging ceiling) and possible redistribution of the gravity loads to other members. Less likely to kill someone.

2. The anchor rods in each group should have been drilled at an angle, so that they are not parallel to each other, and are not parallel to the primary tension force. Failure would then require combined shear and tension, or combined bending of the anchor and fracture of concrete. Drilling far enough into the concrete such that the rebar cage is engaged would help prevent the brittle fracture mode. I realize this is more expensive to install, but engineers are supposed to enforce public safety.

3. Note one of the photos in the link provided below shows some of the rods are bent. Apparently some anchor rods failed before others and the T section rotated before the remaining rods failed. Of course the investigators will have to verify this.

4. I still maintain that using non-galvanized threaded rod is a problem. I agree that it appear the epoxy engaged the threads somewhat, so it would never be a truly "clean" break. However, the non-galvanized rods have a coating of oil and rust which is likely not compatible with the epoxy. It would get mixed in with the epoxy, and probably reduce the bond strength. Time will tell on that one, but I would like to hear your opinions.

5. Clearly mechanical, rather than chemical, anchors should have been considered. [of course i am assuming cast-in-place anchors were somehow out of the question.] Expansion anchors and undercut anchors require less inspection effort and are more difficult to install INCORRECTLY. As long as the hole is the proper length and diameter, it is fairly hard to mis-install these. And at least in an overhead application, you don't have a pile of dust sitting at the BOTTOM of the hole!

6. I predict the epoxy manufacturer will be held second-most liable (after the engineer of record). This amount of epoxy clearly was not bought off the shelf at Home Depot or Lowe's. The supplier would have provided free technical advice and their engineer would have been involved in reviewing the application (if not the actual design) and in reviewing the installation & testing.

My points above are some pretty basic engineering principles that have been taught in engineering school, studied, researched, and practiced for decades. I still maintain that the culture in Boston (see my email from Friday 14 July 2006) contributed to the lack of care for proper engineering, proper construction practices, and proper inspection practices.

I do want to make it clear there are glaring examples TO THE CONTRARY in the Boston area. There are many firms and individuals who practice state-of-the-art engineering and demand excellence and accountability. Sadly, however, I believe they are in the minority.

Mark Swingle



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