Access For Building The Manhattan Bridge

The construction of a large bridge presents a problem that the construction of a tall building does not: the bridge has to, in some way, be bootstrapped. What does that mean? In order to build a structure over a river, you need a way to get over the river. You don’t need to start 1000 feet in the air to build a 1000-foot building, but if your bridge is going to be crossing a river channel about 2000 wide, you need access to the full 2000 feet at an elevation at or near that of the final bridge. There are many ways of doing this, such as cantilevering out from the land on each end, but the pattern for suspension bridges in New York (and, I assume, elsewhere) is to haul a small cable across the river by boat after the towers have been completed and then lift it up into position. Once you have a few of those cables lifted, you can use them to carry other temporary works.

The picture above shows the Manhattan bridge in the earliest stages of main-cable construction. The towers are carrying four temporary cables, which in turn carry two wood walkways that would be used for the cable-spinning process. The bridge has four main cables, whose future locations can be easily seen at the tower top; the solid portions of the deck are where workers will be walking. The use of two wide wood structures instead of four narrow ones is to reduce the amount of sway on the temporary walkways, a type of structure famous for flapping around in the breeze; keeping the middle portion undecked reduces the sail area that catches the wind. The lack of handrails of any kind is typical of the pre-OSHA era. Here are some carpenters taking a break:

I’m not sure which is more disturbing, the fellow on the left nonchalantly dangling over a killing-height drop, or the implication that one of the main tools in building the walkway was a hatchet. Here’s a view very shortly after, as work is beginning preparatory for cable-spinning:

Here’s the work some months later, with the main cables in place, completed and wrapped, and the hangers being installed:

And another view:

The hangers are pairs of small cables slung over saddles sitting on top of the main cables, so each hanger appears to be four cables. They’re tied together at the bottom and weighted to keep them from flying around too much.

And the final product, with the steel open-ball finials on top of the towers:

It’s quite graceful, considering that it’s carrying a double-deck roadway and four subway tracks. When I lived in Brooklyn, the moment when the Q train came out from underground and started the long climb up this bridge was always fun.