Big Dig Boston ceiling collapse 21

[Strider17]

Gentlemen:

This is not my field but what IS a tunnel ceiling?
Why are "decorative" concrete panels suspended over
a roadway, mounted by anchors/rods/grout all of which are
loaded in tension? Why go to the effort of putting
large concrete elements in a tunnel with no load bearing
responsibility? That seems like a waste of mat'l.
Hanging a concrete block over a roadway can have two
possible outcomes...and one is bad. Is this normal or
traditional construction?

TL, PE

The more you know, the more you
know you don't know....

 

[JAE]

Strider,
Read the links. They were using the space between tunnel roof and celing panel for a mechanical plenum for venting purposes.

 

[TDAA]

As the article says, the panels are part of the ventilation system. It is similar use as the drop ceiling in many buildings serving to form the air return. In this case: "Michael P. Lewis, the state's Big Dig project manager, said during a press conference yesterday that the panels in the I-90 connector needed to be heavy for stability in the face of 'hurricane force winds' generated by fans near the roof of the tunnel." So, basically they were made with sufficient dead load to counteract vibrations caused by the 75+ MPH winds they expected.

Now for the sad part: "Now that the concrete panels are all being removed from the connector tunnel as part of the safety review, Turnpike Authority chairman Matthew J. Amorello has said he will consider reopening the tunnel without any drop ceiling at all. Some industry observers have said all along that the drop ceiling was mainly cosmetic, concealing the fans from the drivers below." They likely served no real purpose, but it was obviously a dangerous setting.

"Big Dig officials were not interested in using a lighter material, because doing so would have required many more supports to prevent the ceiling from vibrating." <= A good lesson for redundancy.

 

[mbullism]

snippets from an article on thebostonchannel.com:

"Reilly described one of the bolts that failed as a "clean break" and said there wasn't any concrete attached to it."

"Avi Mor, of Dr. Mor & Associates, a California-based consulting firm specializing in analysis of construction defects, said if concrete failure was to blame for the collapse of the panel, investigators would likely find pieces of concrete still epoxied to the tie rods. Reilly said there was no concrete attached to the rod that failed Monday night."

Admittedly, I haven't heard if the epoxy pulled out of the hole as a plug, sheared through leaving some on the bolt and some in the whole, or failed at the bolt which may have pulled out clean (though I'm not sure how as they are generally threaded), but it appears the concrete did not fail around the anchor epoxy and that the anchor rod itself did not fail in tension...

My question remains the same regarding epoxy anchors in the concrete tenisile zone... it seems to me that if you epoxy these in (threaded inserts or not), and then apply by all accounts what amounts to tons of static loading, that the concrete tunnel ceiling structure (not the hung panel system but the actual top of the tunnel) that the tunnel ceiling will deflect under load. While the reinforcing in the tensile zone takes up the tensile loading it will stretch, the concrete in that zone subject to "cracking", even if minute. The whole system is then additionally subject to creep over time... none of the Hilti or Simpson literature we have in the office list an epoxy anchor suitable for overhead application in the tensile zone (with the exception of the "new Hilti "TZ" spiraled anchor which I'm pretty sure was not around as early as 1999...

 

[tw]

I would not want to have post-installed epoxy anchors holding those loads in tension over moving (many times heavy) traffic for many year and with mechanical wind...over that many miles. Come on, that can't be the way they wanted it? That just seems crazy, you're expecting too much of the contractor relative to the liability. Seems like these elements were ok'd by someone and never inspected and probably never intended to be.

Set it and forget it didn't work this time and it looks like they have a BIG problem.

tw

 

[Strider17]

JAE:

Thanks for the links...It does smell like a political
decision...

TL

The more you know, the more you
know you don't know....

 

[haynewp]

Another link

http://www.tollroadsnews.com/cgi-bin/a.cgi/XWoB!g!REduFEIJ61nsxIA

 

[haynewp]

Note: I do not necessarily agree with the opinions stated in the above link.

 

[Ingy]

mbullism, in answer to your earlier question, the typcial "roof" section of the cut-and-cover tunnel sections, as opposed to the immersed tube sections, is about 6-8 feet. I could stand on the travelling form and the top of the pour was above my head...

tw, the anchors were installed as designed and I have no doubt they were inspected. There was never a shortage of inspectors on the project.

What probably should have been used are the Richmond inserts that are embedded in the concrete.

I'm afraid the issue will become a political witchhunt. It already has...unfortunately, much of the same political energy and pressure on display now is what drove many decisions during the project. I hope we get a good forensic analysis out of this that we can all learn from because the careers of many honest, ethical, and honorable engineers are about to get ruined as political fodder. Maybe something good can come of the mess.

 

[Blackhorse007]

wermmm in the first place, how could it have hapened??!?!?!

 

[samdamon]

Gents,

A few preliminary thoughts. I would expect that pullout tests were performed during conctruction on at least a portion of the anchors. Believe it is widely undertood that adhesive anchors are sensitive to poor installation. Poor bond due to debris in the hole or oil in the compressed air used to clean the holes are two examples. SInce failuire occurred years after installation perhaps not just initial installation may have played a role.

Personally I probably would not use adhesive anchors in an overhead application, but that doesnt mean it cant be done. Does anyone recognize the red cap on the anchor in the photos and graphics? What adhesive system was used? A cast-in-place anchor may not have been feasible depending on how the tunnel wall concrete was placed. Generally also it may have been simpler to lay out these anchors precisely after the concrete was done. That means using either a mechanical or adhesive based anchor. Many types of post-installed mechanical anchors are susceptible to load reverals, so they may not have been the best choice.

Wonder what the real loads these anchors resisted were. No data out on that yet.

Hope that full data on what happened is made available to public.

Very sorry for the loss of life.

 

[miecz]

Been discussing this with some non-structural engineers. They all have the same question: how can you guys hold tons of concrete over the public with glue? Worse, epoxy, that everyone knows requires precise preparation, mixing, and curing, or else it doesn't work.

I tell them about lunch seminars I've dutifully attended for the last 25 years. Where salesmen tell me of the latest products and research and so on. My wife, who's a nurse practitioner, tells of the drug representatives who do the same in her field.

 

[haynewp]

It seems like common sense to avoid overhead epoxy installations but I am wondering if there is mention in any of the chemical anchor manufacturer's literature regarding not installing in overhead applications?

 

[WillisV]

Most of the manufacturers specifically allow overhead installations - e.g. section 4.10 of the following Simpson product:

http://www.simpsonanchors.com/pdf/codes/er5791.pdf

provided certain considerations such as elevated installation temperature are adresses. As someone mentioned previously these products typically have a safety factor of 4.0 on them, and I for one would not have a problem specifying them in the same type situation as long as I felt I was following the manufacturer's recommendations and that proper inspections and testing were taking place.

 

[haynewp]

I don't think I would have even if this had not happened, just by the quality I usually see in the simple applications. I specified metal stud trusses to be anchored with 3/4" dia epoxy bolts into cmu recently, I got to the site and they had put in 3/8" expansion bolts at every location and none of them were even straight and half not tightened down all the way. But maybe the work is generally better in other areas.

 

[Qshake]

I agree with Haynewp and having noted this earlier in the thread am completely at a loss to explain why such an application was used to hang concrete panels of such mass.

Over traffic such as it is, no matter if the number of anchors is twice necessary there is always the one per panel that may fail and induce not only more axial load on the remaining anchors but possible torsion on the system as well.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Ingy]

There seem to be two camps here, overhead epoxy okay, overhead epoxy not okay. I'm wondering if those camps separate along heavy structural/general building designers? In California Caltrans specifically allows the use of epoxy anchors in overhead applications on their bridges. Their use requires heavy inspection which is much more prevalent in heavy structural public works projects.

I think it is important to keep in mind that the tunnel sections were not designed by one person or even one company. The entire project was broken up into a large number of design segments, and further broken down into construction contracts. Many structural engineers from different firms were involved in the decision to allow the epoxy anchors. I don't know, however, if different manufactured systems were used in different segments of the tunnels. Were they all supplied by one manufacturer? Seems unlikely given that the first panels were installed in 1995 and the last ones in 2004.

 

[haynewp]

Ingy, it sounds like from your earlier response that you were actually involved with the construction of this part of the project?

Unless someone was there monitoring every hole installation, I don't see the inspection as being completely effective. It is not like inspecting a weld after it has been placed.

 

[dik]

I've used epoxy anchors often and if properly installed, they can have a greater capacity than mechanical anchors. I still have confidence in them but now with heightened concerns. There are a couple of caveats... epoxy anchors are very susceptable to the condition of the material that they are bonding to as noted in earlier posts, and they are very temperature sensitive. I don't recall ever using them for a similar situation. For life safety issues, I would have specified a regimen of testing to ensure that a large number were tested... realizing that thousands of anchors may be installed, the number tested could easily be in the hundreds with additional testing if proof loads failed. Some engineers may have been lulled into a false sense of security because of the large safety factor without, perhaps, fully appreciating the problems of installation.

I've been aware of safety issues, and spec that the installer be experienced and that the anchors be installed strictly in accord with the manufacturer's printed specs, but my greatest concerns until now have been temperature related.

From the links, it appears that the cause of the problem goes far beyond the engineering aspects of it... a matter of time, cost over-runs and political interference... but, the responsibility for the tragedy rests with the engineering. We are the tekkies that are responsible to make things work... but, the lawyers will have a 'field day' with this. Based on my own past experience, it could be a bit dicey... the engineer of record may have advised others verbally or in writing of concerns or may have been frightened or insecure for real reasons such as loss of employment... Also, in some large firms, contract management may be handled by others completely separate from the design group...

Hopefully, we'll see the issues unfold as time progresses...

Dik

 

[Qshake]

Ingy -

I'm in the heavy infrastructure market which includes everything from bridges to ports and harbors. That said I've never designed a building for occupancy by the general public.



Regards,
Qshake

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