I have followed the news reports about the problems that have caused the catastrophic failures in the ceiling suspension of the Big Dig. Solely based on the information available in the press and on TV, I draw the conclusion that the basic design of the ceiling is flawed and requires a complete overhaul.
First, the use of free hanging 3 ton slabs of concrete acting as an air duct for the ventilating system is not good engineering practice. Why all this weight suspended in midair over our heads to form an air duct? Air ducts are typically constructed with sheet metal, sound insulated with foam and stiffened by corrugation.
Secondly, the practice of supporting the heavy concrete slabs from hangers that are glued to the tunnel walls or ceilings by epoxy is patently unsound and unsafe. Polymers, such as epoxy, exhibit a characteristic engineers call “creep” at normal ambient temperatures. This means that they flow like very viscous fluids when subjected to a persistent pull such as the weight of the slab. They inevitably will fracture under these conditions of loading some time after solidifying. It is not a question of whether, simply when. In our present case the “when” appears to be 5 to 10 years. To use such a time dependent suspension technique offends the very basic precepts of sound engineering and safety.
All the political wrangling that surrounds this issue notwithstanding, the basic engineering of the tunnel ceiling must be right before the public should be persuaded to use it ever again.
Hansjoerg Stern, PE
167 Bog Pond Road, Brewster, MA 02631