Popular Science Monthly/Volume 23/July 1883/Constructive Elements of the East River Bridge
|CONSTRUCTIVE ELEMENTS OF THE EAST RIVER BRIDGE.|
By F. A. FERNALD.
THE New York Bridge Company, having for its object the building of the suspension-bridge between New York and Brooklyn across the East River, was chartered by the State Legislature April 16, 1867. The work was begun by this company, and continued until its control was transferred to the two cities, June 5, 1874, from which time until the completion of the structure it was carried on by a board of trustees. John A. Roebling was at first chosen chief-engineer, but, dying two years later, he was succeeded by his son and associate, Colonel Washington A. Roebling.
The engineer's plans and estimates were submitted in September, 1867, and were finally approved in the spring of 1869. The mechanical work was begun on the site of the Brooklyn tower January 3, 1870. The finished tower rises 278 feet above high-water mark, and measures from top to foundation 316 feet. It is faced above water with granite, but is built partly of blue limestone. At the water-level the tower is 140 feet wide and 59 feet thick; the roadway passes through it at a height of 119 feet 3 inches by means of two archways each 117 feet high, and 33 feet 9 inches wide at the base. Where this tower stands, the river-bed is a compact conglomerate of clay, sand, and bowlders, in which its foundation rests at a depth of 442 feet below high-water mark. The lowest course of masonry rests on a layer of pine-beams 15 feet thick, i. e., the roof of the caisson used to carry down the foundation. Under the roof of the caisson are built 72 brick pillars, 91⁄2 feet high, and the rest of the space is filled in with a solid concrete. The Brooklyn tower was finished in May, 1875. The New York tower differs from this in being three feet wider, and in extending down for 781⁄2 feet below high-water mark, where it reaches some spurs of the bed-rock, making the total height of the tower 350 feet. The roof of the caisson was made 22 feet thick, so as to support the greater weight of masonry to be built upon it during its descent. This tower was finished in July, 1876. Neither has as yet settled two inches.
The four cables are each 154 inches in diameter, over two thirds of a mile long (3,5782 feet), and each consists of 5,282 galvanized steel wires, not twisted as in a small wire rope, but lying parallel from end to end. No. 7 wire was used, which is a little over one eighth of an inch thick, and each cable was made in nineteen strands. The coils of wire for one strand were spliced together, so that each strand consists of a continuous wire running back and forth across the river, and at each end passing around a grooved piece of iron called a shoe. The running out and regulating of the wires occupied a year and four months. After the strands of each cable were made, they were united in one bundle, which was wound from end to end with wire. All the wire used had received five coats of oil, and the bundle received another coat before the wrapping; finally, the finished cable was painted with white-lead and oil. Where the cables pass through the tops of the towers, they rest in grooves on iron plates, called saddles, which are 13 feet long, 4 feet 1 inch wide, and 4 feet 3 inches thick in the highest part. The saddles lie lengthwise under the cables, and their tops are rounded so as to afford an easier bearing. Each saddle is supported on 40 wrought-iron rollers, 32 inches in diameter, which rest in grooves on an iron saddle-plate.
The nineteen shoes, around which the separate strands of a cable are looped, are bolted to as many iron bars, which are 12 feet long, 9 inches wide, and 3 inches thick. These bars are laid side by side in two courses, ten in the lower and nine in the upper. They are bolted to another set of similar bars by means of pins running through eyes in the ends of all the bars in each course. In this way chains of bars are formed, each consisting of ten links, which reach backward and downward to the anchor-plates, both plates and chains being imbedded in the masonry of the anchorage. The anchor-plates are elliptical, star-shaped masses of iron, measuring 172 by 16 feet. There is a series of holes in the middle of each, through which the last links of the chains are passed and fastened by bolts. Four of these plates lie horizontally beneath and close to the rear wall of each anchorage. The mass of stone which holds these plates down measures 129 by 119 feet at the base, is 89 feet high on the front and 85 feet at the back. The site of the Brooklyn anchorage was dug down to the water-level, and a platform of timber was laid under water, upon which the first course of stone was laid. The soil on the New York side was so loose that piles had to be driven in order to secure a firm foundation.
From the terminus the roadway rises to the top of the anchorage over a series of arches which gradually increase in height. Streets run through some of the archways, under the approaches, and the others are to be fitted up with fronts and floors, and let as warehouses. The length of the New York approach is something over a quarter of a mile (1,5622 feet), that of the Brooklyn approach is 971 feet.
From the anchorages to the towers stretch the landward spans of the bridge, each nearly one fifth of a mile (930 feet) long, while the central span, from tower to tower, measures nearly a third of a mile (1,5952 feel). These three spans are attached to the cables by means of wire ropes. The suspender-ropes vary in length from 170 feet next the towers to 3 feet in the middle of the main span, and each is attached to a wrought-iron band 5 inches wide, which encircles the cable. The band holds a socket into which the end of the suspender is set, and fastened by a pin driven down between the wires, which are 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 92 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 72 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 small part of the weight. Engineer Roebling says that the cables are strong enough to pull up the anchorages, but, as each of these weighs 60,000 tons, they probably will never be called upon to perform that feat.
The roadway on the approaches is 100 feet wide, on the spans 85 feet. The outside avenues are for vehicles, the next two are car-tracks, and a footway 16 feet wide occupies the middle. This latter is paved with asphalt as far as the anchorage, and is only three feet above the driveways. Here it rises by a flight of steps to a plank walk twelve feet above the driveways, from which a clear view over both sides of the bridge can be had. To prevent danger in case the brakes should fail to control a train coming down the incline of the roadway, the car-tracks are kept at the same level for the last 600 feet on each approach, which brings them out at about the level of the elevated roads. The cars are to be propelled by means of an endless wire rope, which will run between the rails of each track, over grooved wheels, set upright 221⁄2 feet apart. Motion is communicated to the rope by two stationary engines located on the Brooklyn side. It is calculated that the cars can take 80,000 passengers across in an hour, that 50,000 more can cross on the promenade, while the driveways will accommodate nearly 1,500 vehicles an hour.
The roadway is lighted by 70 electric lights, set on posts upon the trusses that run between the car-tracks and carriage-ways.
The bridge, which had been fourteen years in building, was formally opened May 24, 1883. Up to April 1st there had been paid out on the work $14,429,003.25, and the expenses then remaining to be met will bring the cost up to nearly $16,000,000. In length of span the Brooklyn Bridge surpasses every other bridge in the world. The span of Roebling's Niagara suspension-bridge is 821 feet, a little more than half as great; the span of the suspension-bridge at Fribourg, built in 1832, the longest in Europe, is 870 feet; while Roebling's Cincinnati bridge, which has the second longest span in the world, measures 1,057 feet between the towers, or about two thirds the length of the East River span.