Page:Popular Science Monthly Volume 23.djvu/401

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THE BROOKLYN BRIDGE.

then bent over it, and the socket is filled up with melted lead. Each suspender is tested to sustain 100 tons, but the greatest weight that can come upon any one is 10 tons.

Besides being suspended in this way from the cables, the spans are further secured for 437 feet each way from the towers by stay-ropes, 27 of which start from each saddle-plate, and, spreading out like the sticks of a fan, are attached, at intervals of 15 feet, to the bottom chords of the trusses.

There are six vertical trusses which inclose the five ways into which the roadway is divided. The top chords of the two outer trusses are 912 feet above the roadway, those of the other four are 16 feet. Each truss has a slip-joint in the middle of each span to allow for expansion and contraction of the structure from heat and cold. For a hundred feet out from the towers these trusses are said to be able to support themselves, not adding their weight to the strain on the cables.

The floor-beams, which lie crosswise, at intervals of 712 feet, are 86 feet long, and each consists of two sections spliced end to end. To each beam are attached four suspenders, one from each cable, except for 250 feet out from the anchorages, where the cables dip below the roadway, the floor-beams resting on pillars above them, and in the archways of the towers, where the beams rest upon the masonry. The beams are triangular lattice-girders, having a top and a bottom chord connected by vertical posts and diagonal braces. They are 32 inches wide and 9 inches thick. Alternating with these main floor-beams are lighter beams which are fastened to them by crossties.

The three spans are protected against lateral swaying by four stays from each corner of each tower, which run out just under the roadway, and are attached to the truss at the opposite side at different distances. Beyond these are other similar stays running from side to side. Further, the two outer cables are drawn inward, and the two inner ones outward, as they recede from the towers, so that each opposes its weight in the form of an arch to lateral movement. The suspenders also tend to prevent swaying, for, instead of descending vertically, each is drawn in toward its half of the bridge.

The total length of the bridge and approaches is over a mile and an eighth (5,989 feet). The middle of the main span is 135 feet above high-water mark, at 90° Fahr. This is sufficient for the bulk of the shipping that uses the East River, but the largest ships have to take down their highest spars to clear it. The cable-wire, trusses, and floor beams are all of steel, this being the first steel suspension-bridge ever built. The total weight of the suspended portion, including the cables, is 14,680 tons, and the total load that can come upon it is 3,100 tons. To support these 17,780 tons we have the cables, with a united strength of 54,244 tons, and in addition the trusses and stays, which bear no