The Approach From The Brooklyn End. In The Center Swing The Temporary Suspended Platforms For The Construction Of The Main Cables. Munsey's Magazine, 1902.

Munsey's Magazine · February 1902; pp. 714-724.

By Frank W. Skinner, C.E.

For Nineteen Years The Brooklyn Bridge Has Been One Of The Wonders Of The World. Now New York Has Added To Herself A New Wonder, More Impressive Than The Old One-The New East River Bridge.

The completion of the new East River Bridge will give to New York City the second longest span in the world.

On opposite sides of the river, one thousand six hundred feet apart, two steel towers rise three hundred and thirty five feet above the water. From their tops four steel cables as large as good sized tree trunks carry a two story platform twice as wide as many city streets, with six railroad tracks, two carriageways, two promenades, and two bicycle paths.

The bridge will be so high above the river that the masts of the largest ships may pass under it. The huge steel beams and girders, calculated for the most enormous traffic of any long span bridge yet built, will look from below like a tracery of delicate lines in the slender perspective of the lofty towers and graceful curves. In order to attain the height over the river required by the United States War Department, the tracks are carried over more than half a mile of streets and houses at each end of the bridge.

The weight of the 1,600 foot span between the towers will be 16,000,000 pounds; it will carry a maximum moving load of 9,000,000 pounds, and will produce a total strain of 10,000,000 pounds. The bearing strain in each of the cables will be 50,000,000 pounds.

The steel in each tower weighs 6,000,000 pounds, and is supported on two masonry piers. Each of these piers, with its load, will produce a foundation pressure of 55,000,000 pounds, nearly twice as much as should be allowed for foundations on the best soils. Each end of each cable is attached to massive chains made of huge steel bars with a combined strength of 60,000,000 pounds, pulling upwards against the bottom of the masonry anchorage pier, weighing 254,000,000 pounds.

The total cost of the bridge and its approaches is now estimated at $9,000,000, besides that of the land, which will be at least $10,000,000 more. Of this sum the cables alone will cost $1,398,000; the 1,600 foot suspended span, $1,120,000; the Brooklyn tower, with its piers and foundation, $780,000, and the Brooklyn anchorage, $725,000.

The eloquence of these figures is apparent without comment.

The Tower Foundations. Each steel tower rests on two separate masonry piers, which weigh thousands of tons. These were built where there was deep, swift water and a river bed composed of soft mud, slime, sand, clay, and gravel; a foundation soil totally unfitted for the purpose of the bridge builder. To insure against any settlement or undermining of the towers, it was evident that their pier foundations must, at any cost, be carried down to the solid rock.

It would have been exceedingly slow, expensive and hazardous, if not impossible, to inclose the piers and to attempt to pump out the water and build the foundations, under water, so the piers were built on pneumatic caissons. That is, enormous rectangular wooden diving bells were floated into position and sunk in pits dredged deep in the river mud.

Each caisson was built to a height of twenty two feet on shore, on four sets of double, inclined launching ways which projected out over the, water. When finished to that extent, it was pushed down the well greased slides by battering rams and powerful hydraulic jacks. In a few seconds the huge mass, weighing nine hundred tons, was afloat, and in readiness for the completion of its upper part. The caisson in its final form weighed nearly four million pounds, requiring about three thousand wagon loads of concrete to sink it.

At high tide the water had a depth sixty feet, and a current of six miles an hour, and it was a serious matter to handle the five thousand ton submerged mass safely and accurately in it. The sides must be exactly in line and not a hand's breadth out of position, and must be held so until the caisson had sunk deep enough into the bottom to be secure.

Heavy docks were built around three sides of the floating caisson and protected by double rows of piles and great masses of broken stone banked up around them. Batteries of steam boilers, large derricks, and concrete mixers were established on the dock; sand, cement, and stone barges were brought up to the open side, and the caisson was filled with concrete until it sank to bottom.

Work in the river bed. Powerful steam compressors were installed in the powerhouse, and pumped air into the working chamber until all the water was expelled beneath the open lower edges. Men then entered and commenced the excavation. Those men loaded part of the material into steel buckets, which were hoisted by derricks through the shafts and air locks. Flexible tubes, of of the caliber of a man's leg, were carried down through the caisson and into the working chamber. When the lower end was opened and put under the surface of the water, mud, clay, and small stones were forced up by the pressure of the air in the caisson and discharged in a stream over its top into the river. This was called the "wet blow off," and gangs of men were kept busy stirring up the mud and feeding and handling the voracious pipe.

The bottom was dug up by picks and shovels, loosened and washed to the blow off by a high pressure hydraulic jet. Boulders and rock were blasted with dynamite, and the caisson descended through mud, sand, clay, and rotten rock at a rate of about four inches a day. The excavation was first carried down eight feet in the center. Then the edges of the caisson were gradually undermined so that it might sink slowly under control. Occasionally the sides tipped out of plumb as much as two feet.

Inside those submerged caissons men worked at the river bottom. Visitors descended in a passenger elevator cage into the air tight steel plate chamber. Thence, by a little trap door, they climbed down a steep ladder through a small hole in the roof to the working chamber. Shadowy figures of brawny men, stripped to the waist, with great rubber hip boots, were digging, drilling, and sledging at the uneven bottom. In the caisson there was little noise, yet no quiet. Voices sounded strange and thin. There was the sound of the rushing of waters in one's ears. Otherwise there was but little discomfort from the pressure of an atmosphere that crushed in upon one with a weight equal to that of a heavily loaded freight car.

The deeper the caisson descended, the fewer hours the men worked and the greater was their pay. At last they received eight times as much per hour as when the descent was first commenced.

The anchorage piers. The anchorage piers were built in pits, excavated in sand and clay on shore, to a depth below where the ground is saturated with water. The earth was dug out by hand, shoveled into square, three sided wooden boxes, lifted by steam derricks, and filled into dump cars, which were run over scows and emptied into them. As fast As the pits were dug, their sides were lined with solid walls made of heavy, square timbers from twenty five to fifty feet long. These were driven vertically by a steam hammer, assisted by a powerful hydraulic jet scouring out the sand under the end of the pile. In one part of the anchorage hundreds of foundation piles were driven and their heads embedded in concrete. A four foot layer of solid crossed timbers was built all over the bottom of the pit, and on it was spread several feet of concrete. On this in turn was built the stone masonry, more than one hundred feet high. It would have required many months to mix by hand the fifteen thousand cubic yards of concrete used in the foundations, but this was very quickly and economically done by storing the sand and broken stone in great elevated hoppers. From the bottoms of these the material was drawn into cars holding the required proportions of each, was dumped into the cement box and delivered to the steam mixer. In a few seconds the concrete was ready to be conveyed to any part of the work as fast as the cars containing it could be handled.

The steel towers. At the Brooklyn end of the bridge seven great steel and timber trusses were put across the space between the tops of the two piers. On these was erected a fifty by sixty foot platform. A temporary lower wooden tower, as large as could stand inside the posts of the steel tower and about one hundred and twenty five feet tall, was built on this, and powerful derricks were set on top.

With these derricks, huge steel castings weighing nearly fifty thousand pounds apiece were picked up from the deck of a lighter and swung into place on the of masonry pedestals. These were to receive the feet of the tower posts, which they did as easily and as quickly as though they had been pine blocks.

The other columns, struts, and braces were assembled by the derricks, until in forty days 2,500,000 pounds of steel had been lifted up and bolted in position. The tower was thus erected up to the roadway level, about one hundred and twenty five feet above the masonry. Solid steel girders weighing forty thousand pounds each were lifted up to this level in three minutes, and on them a platform was built. The wooden tower was then taken down and re-erected on this higher platform. By means of its derricks, the steel tower was erected about one hundred feet higher.

A house was built on the temporary platform between the masonry piers; and in it were installed four fifty-horse-power boilers, three hoisting engines, and an air compressor. On the hoisting engines were wound the steel cables from the tower derricks. Each hoisting rope was over half a mile in length and reached from top to bottom of the tower in seven parallel lines. By this means, the power the engine was multiplied six fold.

While this work was in progress, diamond drills slowly bored large holes seven feet deep in the hard granite masonry. Into these holes were screwed the thirty two steel bolts by which the tower is anchored to the piers. After the steel tower was practically completed, the wooden tower was removed, and a short, heavy framework was set on top of the permanent tower to accomplish the most difficult feat of all, the hoisting of the two seventy six thousand pound saddle castings whose duty it is to receive the main cables. Two special tackles were attached to each casting so as to multiply the strength of the hoisting engine eighteen times, and correspondingly to decrease its speed. With them the huge masses were quietly and steadily lifted over three hundred feet in fifteen minutes, lowered on to greased rails, and skidded over to place. The great towers were at length substantially ready for the cables.

The great cables. Twelve twisted steel ropes, each three thousand feet long, as thick as a man's wrist, and having a strength of over four hundred thousand pounds, have been carried on barges across the river from pier to pier. Unwinding from their reels, they sank to the bottom. Powerful tackles were then attached to them. The river being cleared of traffic by patrol boats and revenue cutters, the ends of the ropes were hoisted down on the opposite sides. Thus, in about three minutes each, there rose from the muddy depths to a height of one hundred and fifty feet above the water, those stout steel ropes. These were secured to steel beams embedded in the solid masonry. Combined in sets of three, these ropes supported four narrow foot walks, which served as temporary platforms.

Smaller cables are stretched a few feet above the platforms. Suspended pulleys traveling on these pull back and forth the loops of straight wire which form the main cables. The wires are wound on reels. As fast as the end of one coil is reached, another is spliced to it, so as to make an endless thread of steel running back and forth, exactly like a skein of yarn. With delicate instruments and exact measurements, each separate wire is adjusted to hang in the precise curve required. It follows that the wire must then be strained to an exactly uniform tension with the rest. After that is done, all are tightly clamped together in a solid, cylindrical bundle, waterproofed and covered with a plate steel shell.

The approach viaducts. While the great cables are being slowly woven across the river, above the masts of great ships, by men working on the slender platforms, other contractors are building the steel approach viaducts. These rise in long inclines from the street level to the tops of the anchorage piers, higher than the tallest houses there. Clumsy looking travel derricks, with long revolving arms, roll along on top of the three story viaduct. Reaching outward seventy feet in advance, they raise the tall steel columns and massive girders, swing them into place, and so build constantly a track in front of them. On that track they then roll forward to build another section in advance.

The approach spans. The four great three hundred foot spans reaching from the towers over land and water to the anchorages are made with riveted steel trusses. They have been put together on trestles of heavy timber over one hundred feet high. Their massive posts have been braced in every direction, and solidly bolted and spliced together to a height of several stories. Heavy wooden trusses, like railroad bridges, carried the timber over the streets high above the trolley car tracks. On top of the platforms many lines of solid beams carried a tall tower on wheels, which rolled from end to end and with its derricks and hoisting engines swung the steel pieces into place.

It is hard to grasp the strength and magnitude of this vast structure. One gazes down from the top of the completed towers into an awful abyss lined by girders and columns. At the bottom are men as ants walking, little toy houses and Lilliputian ferry boats. It is indeed a triumph of the bridge builders.

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