Easy To Reach

My previous speculation about how the old Tappan Zee Bridge would be demolished was wrong. I had thought that the backarms of the cantilevers would be lifted off their piers and dissembled elsewhere. That was too subtle. The picture above, taken from a train, shows the actual process: one backarm has been dropped from its piers with explosives and is lying in the river, sitting on the river bed, where it’s being disassembled. The other is still in place, waiting for the same process to begin. It’s probably easier to see if I enlarge the center of the picture (click on it to enlarge it more.)

“A” is the backarm of the west cantilever of the old main span, still in position. “B” is what’s left of that cantilever, greatly shortened by piecemeal demolition, worked from the bridge itself. “C” is the new bridge, just north of the old bridge. And “D” is the backarm of the east cantilever, somewhat crumpled, lying on the river bed.

Few permanent structures are designed to make their demolition as easy as possible, but cantilever bridges are especially difficult. You can get rid of the cantilever spans readily enough and the suspended spans between them (if there are any), but the backarm spans are tough. They either can carry their own weight between their two piers or they can’t, so partial disassembly really doesn’t work. In the case of the Tappan Zee, the cantilever spans were over the navigable river channel, so dropping the backarms into the water doesn’t block the channel.