Hard As A Rock

That’s the base of the Manhattan tower of the Manhattan Bridge. (Click to expand.) There’s a curious detail: the plinth that the steel legs of the tower sit on appears to be made of ashlar masonry. Construction on the bridge began in 1901, and reinforced concrete was certainly in use by then. That backward-seeming detail is especially odd given that the structural design of the bridge was quite advanced, for example using flexible towers that bend with changes in load on the deck.

If we look a little to the south, the stone towers of the Brooklyn Bridge continue into the water as stone.

(These incredibly glamorous photos, by the way, were shot from under the FDR Drive.)

The Brooklyn Bridge towers give a hint as to what’s going on with the Manhattan Bridge. The towers are granite from top to bottom because of the stone’s resistance to weathering. They sit on top of wood caissons filled with brick and mass concrete. The part of that description that matters in terms of looking at the next bridge over is “resistance to weathering.” The Manhattan Bridge also rests on wood caissons, but with an upper foundation of, at least in part, reinforced concrete. The problem with concrete in a marine environment, as was already known by 1901, is that the material is not waterproof. Concrete will absorb water and that greatly speeds deterioration, wave action and impact from floating debris speed it more. I suspect that the stone wrapped around the pier foundation below the Manhattan Bridge’s towers was meant as a weathering protection for the less resilient, more  modern materials inside.