Sensors Mag

Engineering vs. the Bottom Line

October 4, 2006 By: Stephanie vL Henkel, Sensors


E-mail Stephanie vL Henkel

On your way from Montreal (assuming you eluded that unspeakable overpass collapse in Laval) to Rhode Island you might want to go west of Boston. Tunnel Terror's on the rise again, and with good reason. The bolt story is something out of Dilbert's nightmares. This would be the perfect time to install some much-needed sensor technology.

Bolts—and Nuts

The designer of the suspended ceiling in the Big Dig tunnel, Gannett Fleming, specified 4-bolt anchor plates. In his wisdom, the project manager argued the designer down to a 2-bolt version. Structural engineer vs. Bechtel/Parsons Brinkerhoff. Now, who's going to win this one? And did anyone test this (no doubt cheaper) suspension system? Well, yes and no. Uh, kinda.

Let's back up a moment and remember that the bolts were to be held in place by epoxy, in holes drilled into already suspicious concrete. As it was later discovered—and only after some of those concrete segments of the suspended ceiling fell on an automobile traveling through the tunnel and killed a woman passenger— the work crew had experienced considerable trouble with the bolt holes and the epoxy. They used the wrong drill bit for the job, did not mix the epoxy correctly, and didn't put enough of the adhesive into each bolt hole. Twenty of the bolts that popped out were those that let go of those fatal panels.

Testing, Testing . . .

When it came time to do a pull test, Bechtel brought in a company from Portugal. That firm's test, designed for bolts embedded in rock, exerted a weight only 25%–50% greater than the weight the bolts would be carrying. Odd, said a spokesman for CEL Consulting, noting that the usual practice was to test at 200% their normal carrying weight. Bechtel retorted that the standard used by the Portuguese company came from the American Society for Testing and Materials. But hold on. The ASTM standard does not include safety testing guidelines for bolts.

So of the bolts tested with 3250 lb. pull (instead of 5200 lb.), seven came loose within weeks and five did the same right after the ceiling was hung from them. Subsequent to the initial tests, the pull was increased somewhat, but more than 80% of the bolts were already installed—so weren't even yanked a little bit. Among them were the 20 that led to that hapless woman's death.

It gets better. The CEL Consulting spokesman remarked that because epoxy bolts are notorious for failure without warning they are rarely used to hold heavy objects over areas where people might go. Gannett Fleming didn't favor them either, but, in an initial compromise with Bechtel, came up with the original 4-bolt design that was later reduced to a 2-bolt version. The firm's only other option was to prove that two bolts couldn't do the job—not an easy task, proving that something won't work.

Meanwhile, in the Sumner Tunnel

This 72-year-old tunnel, a major route from Boston's Logan Airport into the city, is showing signs of decay. Specifically, chunks of concrete are falling from the roof. The problem was noticed during the tunnel ceiling's most recent inspection, three years ago. Water from Boston Harbor sneaking through the tunnel shell and damaging the steel rebar has been fingered as the likely cause. (The outbound Callahan Tunnel has loose ceiling bolts; during a 1996 renovation, workers shoved new ones up into damaged concrete without taking time to first repair it.) The idea now is to put a suspended ceiling in the Sumner to catch whatever falls. Does this sound like a good idea?

So the inspection teams are going to make more frequent visits while the Big Dig mess gets straightened out. Well, goody. Inspectors viewing the overpass at Laval thought it looked just fine. Until it didn't. What better way can a concrete structure communicate its condition than by shedding parts of itself? Remember that even before the bolt problem was discovered, the Big Dig tunnel had begun to drop chunks onto the roadway and the vehicles traveling over it.

Talking Concrete

Actually, there's at least one other way concrete can talk. Smart Structures Inc. assembles Endevco Picochip accelerometers and other sensors into a sophisticated, embedded, wireless monitoring system for roads, bridges, and other structures. After construction (or reconstruction) has been completed, the sensors provide state and federal agencies with continuous information on the operational health of their infrastructure in terms of corrosion, wind, weather, normal usage, natural disasters, and man-made threats.

Sure makes more sense than the equivalent of a new paint job.


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