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New York's New Finger
By Frederic Williams
on the North British Railway system, in Scotland. The total length of the latter structure, including piers, is 8,296 feet, or a little over one and a half miles. The total length of the Blackwell's Island Bridge, including approaches, will be 7,449 feet.
Y no means content with its nar row quarters on Manhattan Island, New York is forever reach
ing out to clutch more territory in its grasp. Each of the great bridges, stretching across the Hudson, has been compared to a finger of the greedy hand with which the metropolis takes firm hold of the mainland.
Of these, the first Brooklyn Bridge, completed in 1883, at a cost of $17,000,000; the Williamsburg Bridge, completed late in 1903, at a cost of $12,000,000; and the Manhattan Bridge, which will be finished in 1907, at an estimated cost of $15,000,000, are suspension bridges.
The Blackwell's Island Bridge, which is to be completed in 1907, is a cantilever structure. Its estimated cost will be $12,500,000, and it will be the heaviest and most capacious long-span bridge ever constructed. With a single exception, it will also be the longest truss span bridge ever built, being exceeded only by the famous bridge across the river Forth,
A Modern Marvel No more remarkable illustration of the marvelous development of Greater New York—and of the advance of engineering science as well—could be afforded than the building of these great bridges. Twenty-two years ago, slow-going ferries supplied the only means of travel between New York and Brooklyn. Twenty years earlier, not an engineer in the world could have made the plans and computations necessary for the building of any one of the bridges now constructed or being constructed; nor were there machine shops in which the parts required could be manufactured.
Building bridges and digging tunnels tion to its own weight (estimated at have undergone greater changes in their 100,000,000 pounds), a load of 6,300 processes and methods than any other pounds per linear foot, regular traffic, operations in the field of engineering and 12,600 pounds of conjested traffic; work. All the mining and metallurgical on each elevated railroad track, a load processes known at the present time, all of 52 tons on four axles ; on each streetthe rolling-mill and machine-shop opera- car track, a load of 26 tons on two axles; tions, are involved in the production and on every part of the wagon roadway, 24 putting together of the various parts that tons on two axles ; on the foot-walks, a go to make up the finished structure. load of 100 pounds per square foot. The
DISTANT VIEW OF BLACKWELL'S ISLAND BRIDGE AS IT WILL APPEAR WHEN COMPLETED.
determination of all this—of the heaviest loads to be carried, and the greatest allowance for stresses and strains, etc.depends upon a number of purely technical considerations of the forces which may act upon every piece in the entire bridge, and requires months of work by the engineering staff.
Cantilever Construction The method of construction of the Blackwell's Island Bridge differs, of course, from that of the suspension bridges over the same river. bri
A cantilever is a bracket, a structure overhung from a fixed base. A simple illustration
Blackwell's Island Bridge, for example, will contain about 9,000,000 pounds of nickel-steel I-beams and pins, 6,400,000 pounds of I-beams and pins of structural steel, and 71,000,000 pounds of other structural steel—a total of over 86,000,000 pounds of steel. The fabrication of all this material, its transportation from long distances to the bridge site, its hoisting into position, and the accurate placing of each part, require a degree of engineering skill which in the simple pier-and-arch construction of former times was unknown.
In the modern bridge, the lengths of the different members which enter into the structure must be computed to the thousandth part of an inch ; the widths must be made to fit perfectly into one another; the steel pins, turned to the required diameter in the machine shop, must enter holes bored with minute accuracy to accommodate them; andthis has been determined in advance by the engineer—the size of each member must be such that the greatest possible strain to which it will be subjected when in service shall not be more than from one-fifth to one-third of the total weight which it can carry.
In the particular case of the Blackwell's Island Bridge, the structure has been proportioned to support, in addi
upon the piers at either shore. The total amount of masonry in these piers is about 50,000 cubic yards, and they rest on solid rock below the earth's surface. Thiey are 125 feet in height above mean water level, 130 feet wide at the base, and 40 feet thick, tapering gracefully toward the top. They represent 22,000 yards of granite face, 14,000 yards of limestone backing, 17,000 yards of concrete, 14,000 yards of earth excavation, and 275,000 pounds of steel. The four main piers will carry the weight of the bridge, and the anchor piers will act as counterweights to keep the spans from overturn
PIER AND SUPERSTRUCTURE AS IT WILL APPEAR WHEN
of the cantilever system is that of two trees which, separated by some distance, send out branches that touch. In the same way, a cantilever bridge consists of two independent parts thrust forth, as it were, from towers, which meet over the midstream, and which, ordinarily, are joined simply by a roadway, while a space of about sixteen inches is allowed for the expansion of the steel in their structure. The Blackwell's Island Bridge, however, is not a perfect cantilever type, as the two cantilever arms meet at the center of the span, instead of carrying a short suspended structure between them.
Dimensions and Capacity The new bridge will span the river at the same elevation as both the Brooklyn and the Williamsburg structures-namely, 135 feet. The lengths of the spans are: the Queen's shore arm, 459 feet; the easterly cantilever river span, 984 feet; connecting the span across Blackwell's Island, 630 feet; the westerly cantilever river span, 1,182 feet; the Manhattan shore arm, 459.5 feet. Thus the length of the main structure, between centers of anchorage piers, is 3,714.5 feet. The lengths of the approaches at either end are, together, 3,735 feet.
The bridge will have two decks; and its capacity will be one 36-foot wide roadway, which will permit four threehorse teams to pass abreast; four trolleycar tracks, and two elevated railroad tracks, with two promenades, each eleven feet wide, on the outside, permitting passengers in case of accident to disembark from the trains. The six tracks across the bridge will have an estimated capacity of 150,000,000 passengers a year. Teams and passengers can descend from the bridge to Blackwell's Island, 125 feet below, by means of elevators in the four towers of the two island piers. There will be public comfort stations in these towers; and similar stations are arranged for in the two anchorage piers, where elevators will make it possible to reach the bridge without passing over the long approaches.
The floor system of the bridge will be supported upon massive transverse beams, which will be carried out a dis
In Reality two Bridges The Blackwell's Island Bridge will consist of four distinct units, being in reality two bridges, for Blackwell's Island, in the middle of the river, will furnish two central piers, while each branch of the East River will be crossed by the steel structures which will rest
tance of thirteen feet beyond the main extraordinary contrivance which so suctrusses. Between the floor beams, run- cessfully takes the place of muscle and ning in longitudinal lines through the sinew in the lifting of great weights. In entire length of the structure, will be place of the sweating laborers armed worked in a series of fifteen parallel with leather palm-guards, who formerly plate-steel stringers; and above these lifted with much toil and discomfort the will be placed a continuous steel buckle- masses of iron and steel entering into plate floor. The 36-foot roadway for the construction of a bridge, a queervehicles will be between the main trusses looking, circular body now drops down between the overhead trolley tracks from the traveling crane, and, rising This arrangement of the vehicle road again, lifts with it a half dozen or a way will be a departure from the plans score of the parts needed to form the adopted in other New York bridges, the framework of the structure. custom heretofore having been to sep- No chains or hooks are required to arate them on opposite sides of the axis bind these objects together; they simply of the bridge. The new plan will add hang on, in seemingly the most incomto the dignity of the structure by provid- prehensible manner possible, while the ing a very broad and impressive thor- crane runs along the overhead tracks unoughfare.
til it reaches the point desired. There Each tower of the bridge will consist the operator places his finger on the butof a pair of massive legs of a general ton, whereupon the hold on the objects box section, each leg being battered to is released, and they drop-one by one give greater lateral stability against wind pressure. The two legs of the tower will be heavily sway-braced, and, at the top of the towers, will be connected by a deep latticed truss and by an arch designed to harmonize architecturally with the general construction of the bridge. The I-beams used in the bridge are to have an ultimate strength, annealed, of 90,000 pounds to the square inch.
Work of Construction Ground was broken for the new bridge in September, 1901 ; but the work at first was carried along with that indifferent speed that sometimes characterizes great municipal undertakings. By January, 1902, only $42,000 had been expended on the construction. Of late, considerable energy has been manifested, and good progress is now being made.
In putting the false-work in place for erecting the Island span, the same plan and methods have been employed as are followed in the erection of ordinary truss spans. Because of the enormous weights to be moved, however, an immense electric crane, or "traveler," has been constructed, which runs on two four-wheel trucks and is pulled along the bridge as fast as the bridge is built by means of it.
Throughout all this work the great derrick will handle most of its loads by
und means of the lifting electro-magnet—that
BLACKWELL'S ISLAND BRIDGE IN PROCESS OF CONSTRUCTION.
thebye tons masteat weightsters and the
if they are one above another—to the Blackwell's Island structure, for New platform, ready for the adjusters and the York, having outgrown somewhat its riveters. As great weights as ten or early crudities of taste, now has an eye twelve tons may be raised at a time by to art as well as to mere utility. The the electro-magnets. Even if the objects services of a competent architect were are plates standing on edge, they may be secured by the Commissioner of Bridges picked up and carried by the attraction during the administration of Mayor Seth of the magnet held against the edge, Low, to coöperate with the engineering though the edge may be no more than staff of the department in the design of five-eighths of an inch or thereabout in the great bridge structures crossing the width. No scratch or chafe is left on East River; and the office has been conthe object thus supported, and the hold tinued under the present régime. It is of the magnet is as secure as would be no doubt difficult to obtain a graceful that of the best chain cable.
outline for some structures; yet it is
entirely feasible to obtain results that Making an Electro-Magnet
will be agreeable to the cultivated eye, The electro-magnet is, in principle, without sacrificing anything in strength most simple. Any piece of soft iron, or carrying capacity. regardless of size or shape, may be The old Brooklyn Bridge, with its taken; and if the wire of an ordinary combination of stone towers and its iron hanging incandescent bulb be wound structure, is a decidedly graceful prodabout it, the iron, when the electro cur- uct. But the necessarily small openings rent is turned on, will become a magnet. in its towers of masonry limited its caTurn off the current, and the magnet is pacity to accommodate traffic. The Wilgone. Different arrangements of the dy- liamsburg Bridge was built with towers namos will produce different kinds of of steel for the express purpose of currents, either high-tension or low-ten- obviating this handicap. The result is sion; and the engineers in charge are hideous, for its great, towering mass of able to calculate to a nicety the amount iron is unrelieved by any adornment. of pull each magnet is to exercise, and Blackwell's Island Bridge, however, will the most effective manner of assembling have both the impressive grace that huge them.
piles of masonry convey, and a traffic The æsthetics of bridge building have capacity fully adequate to the demands not been overlooked in the plans for the which will be made upon it.