The Great Bridge Read Online Free
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 way he had designed it, the enormous structure was to be a grand harmony of opposite forces—the steel of the cables in tension, the granite of the towers in compression. “A force at rest is at rest because it is balanced by some other force or by its own reaction,” he had once written in the pages of Scientific American. He considered mathematics a spiritual perception, as well as the highest science, and since all engineering questions were governed by “simple mathematical considerations,” the suspension bridge was “a spiritual or ideal conception.”
His new bridge was to be “a great avenue” between the cities, he said. Its over-all width was to be eighty feet, making it as spacious as Broadway itself, as he liked to tell people, and the river span would measure sixteen hundred feet, from tower to tower, making it the longest single span in the world. But of even greater import than length was the unprecedented load the bridge was designed to bear—18,700 tons.
The long river span was
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 way he had designed it, the enormous structure was to be a grand harmony of opposite forces—the steel of the cables in tension, the granite of the towers in compression. “A force at rest is at rest because it is balanced by some other force or by its own reaction,” he had once written in the pages of Scientific American. He considered mathematics a spiritual perception, as well as the highest science, and since all engineering questions were governed by “simple mathematical considerations,” the suspension bridge was “a spiritual or ideal conception.”
His new bridge was to be “a great avenue” between the cities, he said. Its over-all width was to be eighty feet, making it as spacious as Broadway itself, as he liked to tell people, and the river span would measure sixteen hundred feet, from tower to tower, making it the longest single span in the world. But of even greater import than length was the unprecedented load the bridge was designed to bear—18,700 tons.
The long river span was
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