To begin with it was to be the largest suspension bridge in the world. It was to be half again the size of his bridge over the Ohio at Cincinnati, for example, and nearly twice the length of Telford’s famous bridge over the Menai Strait, in Wales, the first suspension bridge of any real importance. It was to cross the East River with one uninterrupted central span, held aloft by huge cables slung from the tops of two colossal stone towers and secured on either shore to massive masonry piles called anchorages. These last structures alone, he said, would be a good seven stories tall, or taller than most buildings in New York at the time. They would each take up the better part of a city block and would be heavy enough to offset the immense pull of the cables, but hollow inside, to provide, Roebling suggested, room for cavernous treasury vaults, which he claimed would be the safest in America and ample enough to house three-quarters of all the investments and securities in the country.
The towers, the “most conspicuous features,” would be identical and 268 feet high. They would stand on either side of the river, in the water but close to shore, their foundations out of sight beneath the riverbed. Their most distinguishing features would be twin Gothic arches—two in each tower—through which the roadways were to pass. These arches would rise more than a hundred feet, like majestic cathedral windows, or the portals of triumphal gateways. “In a work of such magnitude, and located as it is between two great cities, good architectural proportions should be observed,” wrote the engineer. “…The impression of the whole will be that of massiveness and strength.”
His towers would dwarf everything else in view. They would reign over the landscape like St. Peters in Rome or the Capitol dome in Washington, as one newspaper said. In fact, the towers would be higher than the Capitol dome if the dome’s crowning statue of Freedom was not taken into account. So this in the year 1869—when the Washington Monument was still an ugly stone stump—meant they would be about the largest, most massive things ever built on the entire North American continent. 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 stonework 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.
Below the water the towers were to be of limestone and each was to be set on a tremendous wooden foundation, but from the water-line up they were to be of granite. In plan each tower was essentially three shafts of solid masonry, connected below the roadway, or bridge floor, by hollow masonry walls, but left unconnected above the bridge floor until they joined high overhead to form the great Gothic arches, which, in turn, were to be topped by a heavy cornice and three huge capstones. The total weight of each tower, Roebling estimated, would be 67,850 tons, but with the weight of the roadway and its iron superstructure added on they would each weigh 72,603 tons.
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 all along 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, or inclined, 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 works. It was to be made in the same way as ordinary hemp 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.
But most important of all, Roebling was talking about making the cables of steel, “the metal of the future,” instead of using iron wire, as had always been done before. There was not a bridge in the country then, not a building in New York or in any city as yet, built of steel, but Roebling was seriously considering its use and the idea was regarded by many engineers as among the most revolutionary and therefore questionable features of his entire plan.
~~The Great Bridge: The Epic Story of the Building of the Brooklyn Bridge -by- David McCullough
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