Big Dig Boston ceiling collapse 21

[hokie66]

I think all the engineers with substantial field experience who have commented here agree that epoxy anchors are fraught with danger, and that their use in this situation was a poor choice. Back to my answer to MikeHalloran's question, I imagine the use of drilled in anchors was schedule driven. Cast in anchors would have been right on the critical path in the way of the rebar, and the drilled in anchors could be done later. For a cut and cover tunnel, that would allow earlier completion of the work above the tunnel. But the choice of sequence does not explain or excuse the use of epoxy rather than mechanical anchors.

 

[DRC1]

There are a few epoxys that work in overhead applications. Most are too viscous. Most anchors require careful cleaning to develop proper capacities. Proper mixing and distribution of the epoxy in the hole is also critical. Cored holes need to be scored. In many applications the full capacity of the ancor is not needed, so sloppy instalation woreks. Here instalation was obviously critical, as well as proper product selection and preoof testing. I would not use epoxy overhead unless i had to. Here there seemed to be many othe options. One thing that bothers me are the turn bukles. The rods provied a highly redundent support path for the pannels.The loads in the hanger anchors could varry considerably from what may have been assumed for design based on how the rods were designed. This may have a significant contribution to the failure.
My sympathies go to the woman's family. It is also a shame engineering has suffered yet another black eye from this project.

 

[mbullism]

This is looking bad...

http://www.telegram.com/apps/pbcs.dll/article?AID=/20060727/NEWS/607270465/1052/NEWSREWIND

 

[TDAA]

That last link screams criminal negligence. Low factor of saftey on the testing, known issue of bolt pull out, and not enough done to correct / verify the situation. At this point, I think the odds are pretty good we will see some loss of license / jail time.

 

[bigAbdou]

Finally had to add my $.02. As more and more info is released to the press and we get to see what was going on the more this looks like a total screw-up. Sorry this is so long but just had to get this out.

Gov Romney called the adhesive bonded bolt failures a systemic failure meaning many if not all of the bolts were of questionable strength. What he should have said was the entire project was a systemic failure.

A 1998 Inpspector general report on bolts used in the Ted Williams tunnel raised serious doubts about the use of this type of fastener. While this report probably started as a look at cost overruns, the technical gaffs are blatant. No overhead hangers were included in the tunnel section design so all were later added as drilled and bonded bolts. WHY! These tunnels were built very early as prefab sections, I believe at a shipyard in Baltimore, and are steel shell with an inner rebar strengthened concrete liner. Rebar placement is very dense and difficult to drill into.

IG report found that plan was to locate rebar via x-ray etc. and then drill. This proved impossible, blame was placed on density of rebar grid or use of epoxy coated bar. Don't understand why epoxy would mask location of bar. Report indicates some 26,000 fasteners would be needed in tunnel alone. Sure looks like better plan would have been to have embedded rails or cast in anchors etc. Drilling was difficult since they kept hitting bar. Plan was if you hit bar, move drill about 2 inch and drill again, grout in first hole.

This also proved difficult (read $ and Time) so brought in contractor to core drill holes right through rebar. Drilling through bar was questioned but approved by someone higher up. Of course CORE drilling is usually not recommended for adhesive anchors, depends on manufacturer and adhesive system, due to smoother hole.

Pull tests on sample of bolts were poor with failure rates at proof loads of 10-30+% of bolts tested. Nothing indicates that this poor performance was ever adequately resolved. Report indicates that suppliers and independant expert advise was that proper installations should have < 1% failure rates and anything higher should be cause for concern.

These are overhead, CRITICAL applications. Failure means someone will likely be injured or killed. You wouldne't use unrated and uninspected cranes, hoists and shackles for overhead lifting. If these were welded steel joints they would have been done by certified welders and inspected by certified inspectors. Not suggesting the work crews installing the bolts did shoddy work but rather they were never trained how to do job right. If you were doing this on an assembly line and had those kinds of failure rates, you would stop and retrain or retool etc until failure rate was reduced. Ideally installers should have done dozens of test bolts with pulls to failure to demonstrate ability to do consistant installations. I seem to recall testing like this is part of a welders certification process.

Lesson from this early use of bonded fasteners was partially learned as most of the rest of the Big Dig system uses embedded rails or cast in anchors. This is not true everywhere, especially on systems near Williams tunnel as these were fairly early in project and also did not have anchor points designed in from start.

But here they used fewer, smaller diameter bonded bolts to support much heavier panels compared to in the Ted Williams tunnel portion. WHY? Contractors and others raised questions about design being too heavily loaded but these concerns were generally swept aside by the engineering and management contractors like Bechtel. Clearly someone still had resevations about use of the bolts as some areas apperently were to be 100% pull tested although proof load may not have been much over working load.

Again problems were found with high failure rates but it doesn't appear anything was done to rectify. This is not to say fault lies with Adhesive anchors. Properly installed, they have strengths comparable to the undercut anchors being put in now. Any anchor system is only as good as the design and installation make it. Screw either one up and you will have problems

This project was a problem from the start when cast in anchor points or rails were not used. Continued when overhead hangers were designed, clearly with inadequate safety magin and redundancy. Compounded by an apparent lack of design rewiew and checking to catch these issues. Add in work by the construction contractor that may have been deficient plus a lack of proper inspection/oversight by the managing contractor. Also a healthy dose of ignoring or explaining away all of the warning signs/reports about problems. Both state and fed overseers of project seemed to have same blindspot.

Of course by late 90's everyone was up in arms about the huge cost overruns of project, so can't imagine anyone (this includes project managers, state and federal elected reps and appointed Department/Agency officials) wanted to hear about something which might cost a few million to fix. Now it will cost many times that and is having a much greater impact on commerce, never mind the likely 10s of millions from a wrongful death suit.

Also sick of hearing about the supposed cost overrun of project. Just a quick look at salaries + OH/Benefits (let's say approx. $100K/person/year) for the 1000s of people who worked on project for 10-20 years, you quickly get 5-10 billion, never mind millions of tons of materials, heavy equipment, etc. Anyone who looked at this project at state or federal level can't seriously expect us to believe they only thought this was going to cost $2 billion.

 

[kenvlach]

Sounds like a chain of weak links, with different parties liable for each link.

Blame-shifting:
US agency eyes blasting as reason for loose bolts [at a nearby office construction project]

http://www.boston.com/news/traffic/...ency_eyes_blasting_as_reason_for_loose_bolts/

Also, the [core] drilling method and cold weather during construction.

 

[bostonjohn]

The attachment detail is a physical resultant of poor planning and lack of judgment. My first observations of the detail fall into two categories: schedule and judgment.

First, is related to schedule. Mechanical attachments are usually for smaller elements. Substantial elements need to be factored in early on in the design phases and embedded anchors, unistrut, or steel is usually placed for anchorage. Because not even a flexible/adaptable system was put in place it makes one wonder if the nature of the ceiling system and exhaust fans (same mechanical anchorage) were even on the designers’ radar during the early planning stages. This was a design/build project. After investigating this, I found that research done in tunnel fires in West Virginia created the basis for the use of jet fans to exhaust/supply the tunnel. I'm wondering what the original (at the time when the section was cast) schematic plan was for the ceiling and ventilation system; specifically, what the ceiling was.

http://onlinepubs.trb.org/onlinepubs/trnews/rpo/rpo.trn190.pdf

I wonder if these tests influenced the current NFPA 502 (interesting that someone from P.B. was the chair of this proposal)

http://www.nfpa.org/assets/files/PDF/ROP/502-04-ROP.pdf

Second relates to judgment. It appears that the whole assembly seems out of balance. When you look at the size of all the members in the assembly, the little bolts meant to be attached to the concrete seem way out of scale. If the same set of forces and reactions were used as the basis of design, why would the little bolts be so out of scale with the rest of the members? I wonder if someone improperly adapted the assembly meant for a steel attachment for a concrete one.

Lastly, after what we now know about this anchorage system in this application, you will still find people that think we should still employ this in future projects. The engineering community needs to determine first, what conditions this approach should be used and second check projects currently online that have employed this system so we can catch any future tragedies.

 

[Tomfh]

http://www.designnews.com/article/CA6357443.html

 

[dik]

In going over some of the articles, it appears that some of the epoxy 'plugs' had worked free from the hole. Could be that with coring, there was little bond of the epoxy to the concrete. Possibly thermal conditions or residue from the coring operation. There doesn't seem to be a bond between the epoxy and the concrete. Still no news on the type of fastener used?

Dik

 

[MikeHalloran]

In the not so fine print of their installation instructions for epoxy bolts, Hilti specifically disallows use of diamond drills, presumably because of the resulting smooth finish.

If you were faced with drilling through rebar, what else would you use?



Mike Halloran
Pembroke Pines, FL, USA

 

[steve75002]

As I am not a structural engineer, perhaps I have missed something, but it appears to me that the choice of epoxy bolts was flawed for another more fundamental reason from the start:

The purpose of the concrete slabs was to facilitate ventilation in the event of a fire. However, in the event of a fire, any heat from the fire would have been rapidly transmitted to the steel structure supporting the slabs.

This heat would then be transmitted to the bolts and the epoxy holding them.

Epoxy strength diminishes rapidly with heat.

Therefore, the risk of roof failure in the event of fire would have been dramatically increased, creating a hazard, the opposite of the design intention.

Or have I missed something obvious.

 

[Ron]

Steve75002....I'm not sure if that was the purpose of the concrete slabs that failed; however, that is precisely the reason epoxy anchors are not used for hanging sprinkler systems or for fire suppression systems.

 

[hokie66]

It is now reported that the epoxy used was supplied by Powers Fasteners, formerly Rawl. Old name in that business. Will be interesting to hear how much input Powers had in the selection of fastener, or if they just supplied material as ordered.

 

[Ron]

I only have experience with some of their mechanical anchors. Good products from my experience. Will be interesting to see the results as the investigation unfolds.

 

[steve75002]

Ron ... according to many different references in this thread and elsewhere, the heavy concrete slabs were necessary to withstand "the hurricane force winds" that would have passed over them in the event of a fire - anything less would have not have had sufficient rigidity and moved/vibrated/resonated under the pressure.

In consequence, it appears that epoxy bolts are not suited for this application, because of lower heat resistance, as fire has to be a major design consideration in tunnel design.

How is it therefore that they were specified, and their use was permitted, in preference to mechanical fixings that, suitably specified, would have been inherently safer, but probably more expensive to impliment?

 

[boffintech]

Steve75002, I think some argued that no system for hiding the overhead ventilation was actually necessary but if a system were used it would have to be concrete slabs to withstand "the hurricane force winds" and not move/vibrate/resonate under the pressure.

 

[steve75002]

boffintech ... either way, hanging masses of concrete from epoxy bolts significantly increases the danger in the event of fire.

How is it that such risks were permitted, not withstanding the actual consequences?

 

[Ron]

Steve...that is a significant question, the answer to which is probably balled up in politics and finger pointing.

 

[KDengineer]

Again, not that the product is necessarily to blame (only the investigation report will tell this) but for those interested in what product was used I found the attached link
[URL unfurl="true"]http://news.bostonherald.com/galleries/?title=behindthescenes07262006&record=3#photo [/url]

 

[hokie66]

KDengineer's reference shows a photo of adhesive being used which is a product of ITW Ramset Redhead. Other reports say Powers Fasteners. Did the contractor just buy from whomever gave him the best deal? Or maybe that was a different part of the project. Bet Hilti is glad their name is only being mentioned as suppliers for the rectification anchors.