Big Dig ceiling collapse answers forthcoming?

[Prometheus]

So one of the parties blamed in that ceiling collapse is paying up and promising some answers:

http://www.eyewitnessnewstv.com/Global/story.asp?S=7541980

The answer I'm hoping for though, and this has been bugging me for a while, is: Why the heck were they gluing 2-ton panels to the ceiling in the first place? I mean, it can't be a structural reason, can it (engineers, please help me understand!)? Can a tunnel ceiling be strengthened in any way by suspending such weight from it? And if it's not structural, then why are the panels there in the first place? And if ceiling panels must be used, then why do they have to weigh two tons each?

It just seems to me that blaming the company that made the glue is a little like blaming the bullet manufacturer after shooting yourself in the foot.

 

[ben m]

The drop ceiling generally was part of an air-circulation system. You blow fresh air into the space above the panels, which acts as a conduit to carry the air to the middle of the tunnels. The air capacity needed to be quite large to deal with the possibility of a mid-tunnel fire, so it was better to use the entire ceiling space rather than running a handful of smaller ducts.

They said that the panels had to be heavy because lighter panels would be rattled by the wind when, e.g., semi trucks drove by. I find it hard to believe that 2-ton panels are the only solution, but I can imagine circumstances in which they looked like the best solution at the time. (Of course, it's possible that one of those circumstances was "We're over budget again---hey Bob, if you can finish that drop-ceiling design in under an hour, we can move your salary back to the leak-remediation budget.". Or perhaps "My brother's contracting company makes 2-Ton Concrete Panels." But I have zero insider knowledge.)

 

[Prometheus]

The drop ceiling generally was part of an air-circulation system. You blow fresh air into the space above the panels, which acts as a conduit to carry the air to the middle of the tunnels. The air capacity needed to be quite large to deal with the possibility of a mid-tunnel fire, so it was better to use the entire ceiling space rather than running a handful of smaller ducts.

They said that the panels had to be heavy because lighter panels would be rattled by the wind when, e.g., semi trucks drove by. I find it hard to believe that 2-ton panels are the only solution, but I can imagine circumstances in which they looked like the best solution at the time. (Of course, it's possible that one of those circumstances was "We're over budget again---hey Bob, if you can finish that drop-ceiling design in under an hour, we can move your salary back to the leak-remediation budget.". Or perhaps "My brother's contracting company makes 2-Ton Concrete Panels." But I have zero insider knowledge.)

Thanks for the info. I've never seen that explanation before. I'm still curious as to the decision process, but at least the panels now make some kind of sense.

Do you know how other tunnels have dealth with this issue? Why not put the ventilation duct under the roadway, or in a wall? Why not just drill a shaft near the midpoint of the tunnel (wouldn't that also be useful for rescue purposes in the event of a collapse)?