Patrick Marini

luglio 17, 2006

Brooklyn Bridge – 1883

Filed under: New York — π@3κ @ 2:24 pm

John Augustus Roebling (Muhlhausen, Thuringia 1806 – New York 1869)
Brooklyn Bridge
2 torri gotiche in granito sorreggono il ponte tramite 4 cavi d’acciao così descritto da McCullough:

The towers would dwarf everything in view… On the New York skyline only the slim spire of Trinity Church at the head of Wall Street reached higher.

The towers were to serve two very fundamental purposes. They would bear the weight of four enormous cables, and they would hold both the cables and the roadway of the bridge high enough so they would not interfere with traffic on the river. Were the two cities at higher elevations, were they set on cliffs, or palisades such as those along the New Jersey side of the Hudson, for example, such lofty steelwork would not be necessary. As it was, however, only very tall towers could make up for what nature had failed to provide, if there was to be the desired clearance for sailing ships. And as the mass of the anchorages had to be sufficient to offset the pull of the cables, where they were secured on land, so the mass of the towers, whatever their height, had to be sufficient to withstand the colossal downward pressure of the cables as they passed over the tops of the towers.

The suspended roadway’s great “river span” was to be held between the towers by the four immense cables, two outer ones and two near the middle of the bridge floor. These cables would be as much as fifteen inches in diameter, and each would hang over the river in what is known as a catenary curve, that perfect natural form taken by any rope or cable suspended from two points, which in this case were the summits of the two stone towers. At the bottom of the curve each cable would join with the river span, at the center of the span. But along all the cables, vertical “suspenders,” wire ropes about as thick as a pick handle, would be strung like harp strings down to the bridge floor. And across those would run a pattern of diagonal stays, hundreds of heavy wire ropes that would radiate down from the towers and secure at various points along the bridge floor, both in the direction of the land and toward the center of the river span.

The wire rope for the suspenders and stays was to be of the kind manufactured by Roebling at his Trenton (wire) works. It was to be made in the same way as ordinary hemp wire rope, that is, with hundreds of fine wires twisted to form a rope. The cables, however, would be made of wire about as thick as a lead pencil, with thousands of wires to a cable, all “laid up” straight, parallel to one another, and then wrapped with an outer skin of soft wire, the way the base strings of a piano are wrapped.

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