QUOTE: Massachusetts State Troopers and another worker (wearing hard hat) inspect hardware removed Tuesday, July 11, 2006, from a Big Dig tunnel where cement slabs fell late Monday night in Boston. The falling slabs crushed a passing car killing a woman and injuring her husband.
Damn press and "cement vs concrete"!
From the photo of steel tie back (turnbuckle with clevise and pin arrangement), seems that two (2) epoxied anchors secured a fabricated T-plate to the concrete panel, and the pullout failure looks like possibly that the drilled holes were not cleaned correctly resulting in premature failure. Speculation on my behalf, but I was expecting to see a cast-in insert to the panel and a pullout cone.
With all due respect to those with the hard job of investigating this tragedy....why is a state trooper inspecting anything?! Structural Engineers should be dispatched to the site to assist any transportation official with the appropriate actions such as closing roads etc.
Regards,
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
I was reading the following in the Standard times which the main newspaper for the southcost of Mass.
"John Christian, a geotechnical engineer hired to conduct an investigation on behalf of the Turnpike Authority, said the attachment bolt design was fairly standard. He said holes were drilled in the tunnel's concrete ceiling and then bolts were inserted along with a pressure-injected epoxy packing."
Now why Hire a Geotechnical engineer to study a structural failure?
All those politicians demanding accountability are probably the same ones that look down their nose at structural engineers and sniff, "You guys are so conservative."
mbullism (Structural) Jul 13, 2006
I'm still a bit puzzled by the overhead use of epoxy anchors in the tensile zone of the concrete tunnel ceiling above... I know they have caps and seals to keep the epoxy in the hole while it sets, but I always thought these type of anchors were a no-no in "cracked concrete". Admittedly, I've yet to hear how thick the base material is or how deep the anchors were. Several outlets have reported that the bolts and epoxy came out of the holes cleanly...
Is it possible that the anchors were meant to be set deep enough so as to "ignore" the tensile region and be actually secured in the top/compression zone?
I suspect the geotechnical because it's a tunnel, though I definitely agree it's a structural concern.
Also, does it trouble anyone but me that vertical epoxy inserts were used such that zero redundancy exists in failure mode. I would have less troubled to see a studded plate cast in the slab than what was done.
Comments are welcome.
Regards,
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
It was likely a Geotech, because we are also the ones that tend to do a lot of the material testing and inspections work. We do tend to know more than just soil.
I do agree however, that a structural should be leading this, and working closely with someone that is quite familiar with the materials and construction aspects of this work.
I believe one article said that the threaded insert pulled out, leaving the epoxy in place. That would indicate that the bond between the anchor and the epoxy failed. I didn't see a report that the epoxy came out of the concrete.
qshake-
One official (forget now, which one) said that the system was designed so that , if one bolt gave, the others could hold the panel. Though looking at the graphic, it's not clear how that would work. It would seem that if one corner gave way, the bolts on the other diagonal would see double the load.
Tension anchors such as this are required to have a factor of safety (by code) of at least 4. Considering that, it appears that the assembly had been holding for a while and gave way. That points moreso toward an installation anomaly, likely exacerbated by vibration or other repetitive loading, or potentially a material failure (excessive creep under load, embrittlement, corrosion, etc.), or as usual, some combination of those.
Such a forensic venture should involve a team to include a structural, a geotechnical, and a materials engineer.
What is the "tunnel ceiling"? What were the anchors anchored to? Rock? Concrete? It seems the assumption that the epoxy would bond to the "tunnel ceiling" with little redundancy is questionable. I could definitely see the entire epoxy/bolt assembly pulling out but not the epoxy away from the bolt... seems unlikely.
For hanging loads - a form of redundancy can be provided by these three methods:
1. Use a much higher safety factor on the anchors you use.
I remember after the Hyatt Regency collapse that a walkway system in Des Moines, Iowa as built with a SF of 10.
2. Use more anchors than needed (per comments above) such that a portion of the anchors can do the full support.
I used this on a stay cable structure where four separate cables were used - 2 of 4 could support the dead load and 3 of 4 could support the dead + live + wind.
3. Provide a supplemental support system that would allow full release by the primary support, and may allow large, damaging deflections, but would not allow collapse. Sort of like a seismic bend-like-crazy-but-don't-fall-down system.
There may be others here but these would be what I would look at.
I also seem to recall someone in another forum discussing the fact that adhesive anchors are very very susceptible to the in-field quality. In fact, I think this is one of the selling points for the new Simpson Titan anchors...as they are not as susceptible to installation QC.
Here's a link to an article that discusses the reason for having such heavy panels in the first place: When the large vent fans kicked in to evacuate smoke in the case of a fire, explosion, etc., the plenum above the ceiling would have enormous air velocity and the ceiling needed to remain static and not flutter with such winds.
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