August 11, 2002

GREAT BRIDGE, A GREAT BOOK :

First Chapter of The Great Bridge by David McCullough (Denver Post)
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 not to be perfectly horizontal, but would bow gracefully, gently upward. It would pass through the tower arches at an elevation of 119 feet, but at the center it would be 130 feet over the water. This, as Roebling pointed out, was thirty feet higher than the elevation fixed by the British Admiralty for Robert Stephenson's Britannia Bridge over the Menai Strait, built nearly twenty years earlier. Before long, sailing ships would be things of the past, he declared. His bridge therefore would be no obstruction to navigation, only possibly "an impediment to sailing." As it was, only the very largest sailing ships afloat would have to trim their topmasts to pass beneath the bridge.

But because of the great elevation of the river span and the relatively low-lying shores, the rest of the bridge, sloping down to ground level, would have to extend quite far inland on both sides to provide an easy grade. The bridge would have to descend back to earth rather gradually, as it were, and thus the better part of it would be over land, not water. Those inland sections of the bridge between the towers and the two anchorages were known as the land spans, and were also supported by the cables, by suspenders and diagonal stays. The ends of the bridge, from the anchorages down to ground level, were known as the approaches. In all, from one end to the other, the Great Bridge was to measure 5,862 feet, or more than a mile.


Don't know if maybe it's being republished or something, but the Denver Post has the first chapter posted online. If you've never read the book, it's terrific. Posted by Orrin Judd at August 11, 2002 12:14 PM
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