Difficult Disassembly

Another photo taken from a train, this time of the Tappen Zee bridges. If you click on the picture to expand it, you can see the new, gleaming-white, cable-stayed Tappan Zee bridge behind what’s left of the old bridge. Because this crossing of the Hudson River was placed for political reasons at the widest spot north of the mouth at New York Harbor, the central bridge over the navigation channel has a causeway on each end; the old causeways have been removed, leaving only two parts of the main span.

The old main span was a cantilever truss without a suspended span. In other words each half, east and west, of the main span was a cantilever anchored by two piers, and the two cantilevers met in the middle. (Some longer cantilever trusses have the ends of the cantilevers supporting a smaller simple-span bridge rather than meeting.) The portion of each half-truss that is located between the two piers is the backarm, and the portion between the center-most pier and the meeting point at the center of the span is the cantilever. If you look at the picture above, you can see that the cantilevers have been almost entirely dissembled, leaving only the backarms in place. This sequence is necessary, as doing any work on the backarms while the cantilevers were still in place would be dangerous. In order to disassemble the backarms you can either provide temporary shoring similar to what was used 65 years ago to put them in place, or you can lift them off the piers in one piece. Given that a lot of the work of assembling the new bridge and disassembling the old causeways was done using very large floating cranes, I suspect that the backarms will be removed in one piece each and disassembled on shore.

Big stuff and nothing like our work, but fascinating to see. Various trips up the Hudson have got me glimpses of the work every few months, so it’s been like stop-motion animation.