The Great Bridge Read Online Free Page B

“This part I call the elevated promenade, because its principal use will be to allow people of leisure, and old and young invalids, to promenade over the bridge on fine days, in order to enjoy the beautiful views and the pure air.” There was no bridge in the world with anything like it. And he added, “I need not state that in a crowded commercial city, such a promenade will be of incalculable value.”
    So the roadways and tracks at one level were for the everyday traffic of life, while the walkway above was for the spirit. The bridge, he had promised, was to serve the interests of the community as well as those of the New York Bridge Company. Receipts on all tolls and train fares would, he asserted, pay for the entire bridge in less than three years. To build such a bridge, he said, would take five years.
    Horatio Allen and William McAlpine asked the most questions during the sessions Roebling held with the consultants. The length of the central span and the tower foundations were the chief concerns.
    It had been said repeatedly by critics of the plan that a single span of such length was impossible, that the bridge trains would shake the structure to pieces and, more frequently, that no amount of calculations on paper could guarantee how it might hold up in heavy winds, but the odds were that the great river span would thrash and twist until it snapped in two and fell, the way the Wheeling Bridge had done (a spectacle some of his critics hoped to be on hand for, to judge by the tone of their attacks).
    Roebling told his consultants that a span of sixteen hundred feet was not only possible with a suspension bridge, but if engineered properly, it could be double that. A big span was not a question of practicability, but cost. It was quite correct that wind could play havoc with suspension bridges of “ordinary design.” But he had solved that problem long since, he assured them, in his earlier bridges, and this bridge, big as it was, would be quite as stable as the others. Like his earlier works, this was to be no “ordinary” bridge. For one thing it would be built six times as strong as it need be. The inclined stays, for example, would have a total strength of fifteen thousand tons, enough to hold up the floor by themselves. If all four cables were to fail, he said, the main span would not collapse. It would sag at the center, but it would not fall. His listeners were very much impressed.
    There were questions about his intended use of steel and about the extraordinary weight of the bridge. Then at one long session they had discussed the foundations.
    Roebling planned to sink two tremendous timber caissons deep into the riverbed and to construct his towers upon these. It was a technique with which he had had no previous experience, but the engineering had been worked out quite thoroughly, he said, in conjunction with his son, Colonel Roebling, who had spent nearly a year in Europe studying the successful use of similar foundations. McAlpine could vouch for the basic concept, since he had used it himself successfully, although on a vastly smaller scale, to sink one of the piers and the abutment for a drawbridge across the Harlem River. His caisson for the pier had been of iron and just six feet in diameter. Those Roebling was talking about would be of pine timbers and each one would cover an area of some seventeen thousand square feet, or an area big enough to accommodate four tennis courts with lots of room to spare. Nothing of the kind had ever been attempted before.
    How deep did he think he would have to go to reach a firm footing, the engineers wished to know. Would he go to bedrock? And did he have any idea how far down that might be?
    During the test borings on the Brooklyn side, the material encountered had been composed chiefly of compact sand and gravel, mixed with clay and interspersed with boulders of traprock, the latter of which, he allowed, had “detained this operation considerably.” Gneiss had