WITH MEN WHO DO THINGS
BY A. RUSSELL BOND
Author of “The Scientific American Boy” and “Handyman’s Workshop and Laboratory”
Chapter VII
SPINNING A WEB ACROSS THE RIVER
“You mean the towers,” I ventured.
“Oh, no, they could n't begin to stand up against the pull of those cables. We just put in the towers to raise the bridge high enough above the river—something after the fashion of the clothes-poles with which a washerwoman props up her clothes-line. Why, you have no idea what a strong pull there is on the bridge cables. We have to build great masses of stonework, and imbed in the masonry enormous steel bars linked together like giant chains, to which the cables are fastened. The anchorages of this bridge are each as long as a city block (225 feet), and 175 feet wide, and when they are finished, they will be built up as high as an eight-story building.”
With this impressive introduction, Mr. Blanchard led the way up to the anchorage, and let us see for ourselves the huge chain of eye-bars. They were stringing the cables in separate strands, and each strand was fastened to a separate pair of eye-bars.
As Mr. Blanchard was anxious to inspect the work at the other end of the bridge, he did not stop to explain this just then, but started with us up one of the temporary foot-bridges that ran up, under each cable, to the nearer tower.
It was quite a climb, particularly as we neared the top, where the slant of the footwalk was very steep. The towers, reaching up to a height of 350 fect above the water, had looked very slender from a distance, and hardly strong enough to sustain the weight of a heavy double-deck bridge; but we found on closer inspection that they were made of massive steel, rising 322 feet above the masonry pier.
“They 're tremendously strong, are n’t they? I should think they would stand up under almost any load,” remarked Will.
“They ‘ll carry the load,” said Mr. Blanchard, “but we expect them to sway some, back and forth. the top may move one way or the other, as much as two feet from the upright position.”
“Why, how 's that?” I queried.
“When the summer sun beats upon the cables, they will grow so hot that it would be uncomfortable to put your hand on them, and you will find that they will have expanded considerably. On the other hand, when the bitter cold winds of winter chill them down below zero, they will contract appreciably. We expect the cables to be twenty or thirty inches shorter in winter than summer. The total change will be greater in the long span between the towers than in the shorter shore spans from the towers to the anchorages, and so the towers will have to bend to accommodate themselves to this variable pull. In the Brooklyn Bridge, the cables pass over cradles on rollers, so that they can travel back or forth with the cable to allow for these variations in the length of the spans, In this bridge, we are going to let the cables rest directly on the towers, and let the towers themselves bend back and forth, to allow for differences in length of the cable. I don’t suppose they will ever bend much more than six inches one way or the other, but we have allowed for a flexure of twenty-four inches.”
We followed Mr. Blanchard down one of the steep foot-bridges and up the opposite tower. It was quite a long walk, over a quarter of a mile in a straight line, and considerably more following the curve of the cables, as we had to, The foot-bridges were merely continuous platforms, about nine feet wide, and supported on temporary cables under the four main cables that were being strung. I kept strictly to the center line of that platform, and did n’t pay much attention to the boats that were plying back and forth beneath us. The foot-bridges were connected in pairs every five or ten feet, by means of beams, and at various intervals there were cross-walks connecting the south pair of bridges with the north pair. It made my flesh creep to see the workmen walk across the narrow beams between the platforms.
All the time, the wire carriers were traveling back and forth over our heads, just like spiders spinning their threads across the river. The carriers were merely large pulley-wheels connected to traveling-cables. The wire was looped over the pulley-wheel, and as the wheel traveled across, it would string two lengths of wire at once.
When we had reached the opposite side of the river, Mr, Blanchard explained the wire-string-