Chapter 04: Bridges to Brooklyn
Rapid Transit in New York City and in the Other Great Cities · Chamber of Commerce, 1906
The concentration of business at the lower end of Manhattan, and the development of the district across the East River for residential and later for business purposes, caused attention to be given at an early date to the matter of better transit facilities in that direction.
Brooklyn Bridge With Williamsburg Bridge in Distance.
Brooklyn Bridge. A proposition to build a chain suspension bridge over the East River, with a clear span of 1,500 feet and a length of 2,100 feet between the "toll gates," was proposed as early as 1829. The structure was to have been 28 feet wide, the height of the granite piers from the water line to the roadway 160 feet, and from the roadway to the extreme point of the pier 65 feet. This and similar schemes came to naught, until, in 1867, the Legislature passed an act incorporating the New York Bridge Company. In May of that year John A. Roebling, who had, by designing and erecting the great Niagara Suspension Bridge, earned a high reputation in this branch of engineering, was appointed chief engineer. A survey of the line was made in the summer of 1869, and the Brooklyn tower located. It was while engaged in this work that Mr. Roebling met with an accident that resulted in his death. In the same year his son, Washington A. Roebling, was appointed to the position formerly occupied by his father, and under his supervision the work was carried to completion in May, 1883.
Government Commission. In May, 1869, the War Department, in compliance with an act of Congress, appointed Generals Wright and Newton and Major King a commission of government engineers to examine into the feasibility of the project, and to report whether the bridge would be an obstruction to navigation. This resulted in changing the clear height to 135 feet above mean high tide, and in widening the bridge from 80 to 85 feet, in order to provide a double roadway on each side. In those days it was not contemplated to use the bridge for the passage of ordinary street cars, but to furnish it with a cable system of its own. The importance of the slight increase in width mentioned can hardly be overestimated, since it made provision for the vehicle and trolley traffic of the present time. In June, 1874, an act was passed changing the name to that of the New York and Brooklyn Bridge, and making it a public work to be constructed by the two cities, Brooklyn paying two-thirds of the cost and New York one-third.
Foundations. The foundation of the Brooklyn tower was begun in 1870. The caisson, a rectangular chamber 102 feet wide by 168 feet long and having a solid roof 15 feet thick, was sunk to a depth of 44.5 feet. Brick piers were then built in the air-chamber, which was finally completely filled with concrete. As the caisson was sunk the granite masonry forming the tower proper was built above it. The New York foundation was also carried to solid rock, the caisson in this case being slightly larger than the other, 102 feet wide by 172 feet long. The cutting edges were extended to a depth of 78 feet below mean high water, this being necessary because of the presence of extensive beds of quicksand resting on the rock. The roof was 22 feet thick and was surmounted by a cofferdam reaching to high water, thereby increasing the buoyancy and lessening the pressure on the frames during sinking.
The Towers. The towers are not solid masses of masonry, but each is composed of three buttressed shafts joined together up to the roadway by four connecting walls. In the Brooklyn tower the course of the walls next [to] the caisson is 17 feet thick; the thickness diminishes by offsets until at high water it is but 10.5 feet. This forms two well holes which are filled with concrete below water line and left open from there to the roadway. Spaces are also left from 2 feet above the crown of the arches to within 4.5 feet of the top of the tower. Above the roadway the tower consists of three columns having an oblong section; they are united at the top by arches having a span of 33.75 feet. Each arch is formed by the intersection of two arcs of circles having a radius of 48-1/6 feet. Below the water the masonry is largely limestone, except the facing of the two upper courses, which is granite; the backing from high water to the roadway is granite, which constitutes all the remainder of the tower. At the towers the height of the roadway is 119.25 feet, the top of the towers being 272 feet from the water. The height from the bottom of the foundation to the top is, in the Brooklyn tower, 316 feet, and in the other 349 feet. At high water mark the towers measure 141 by 59 feet, at the roadway 131 by 48, and at the base of the cornice 126 by 43. The greatest pressure at any point is in the tower masonry at the base of the central shaft at the roadway, where each square foot supports about 26 tons.
Anchorage. At a distance of 930 feet from each tower is an anchorage which rests on timber grillage, and in which the ends of the four cables are anchored. Each anchorage weighs 60,000 tons and is built solid, with the exception of tunnels or openings for the passage of the cables. Near the outside lower angle of each anchorage are four anchor plates (one for each end of each cable), which are held down by the dead weight of masonry piled upon them. Each plate weighs 23 tons, and in shape resembles an enormous wheel having a hub and sixteen spokes. The connection between the cable wires and the plates is made with eyebars, which start in double sets from each plate, one curving over the other, and are vertical for a distance of about 25 feet, when they curve about go degrees on a circle having a radius of 49.5 feet. The bars have an average length of 12.5 feet. The first three sets have a section of 7 by 3 inches, the next three 8 by 3, the next three 9 by 3 inches; the tenth set is double in number and each bar is 1.5 by 9 inches in section. Piercing the center of the anchor plates are two parallel sets of apertures, each containing nine holes. A bar is passed through each hole and a 7-inch pin run through the eyes or holes in the end of each bar. These bolts bear firmly against the under side of the anchor plate, and serve to distribute the strain to every part of the plate. The next series of bars is attached to these by a bolt 5 feet in length and 5 inches in diameter. In this manner the succeeding bars are united, forming a chain having very long links connected to each other by bolts passing through the eyes. At each knuckle of the chains a large block of granite was placed with a heavy cast iron plate inserted as a bearing for the heads of the links. The bars in the last link are increased in number to 38, and are arranged in four courses, one above the other. The wires of the cable are divided into 19 strands and each strand is fastened around a grooved eye-piece which is held between two of the anchor bars.
Cables. The first work connected with cable making was the passing of a rope from one anchorage to the other over the towers. By August, 1876, an endless rope had been placed from a driving engine at the Brooklyn anchorage to and around sheaves at the New York anchorage.
The first operation, preliminary to placing the cables in place, was that of adjusting four wires, one for each cable, to be used as guides in obtaining an exactly uniform deflection of all the others. Four wires of uniform size and weight were selected. These were adjusted to a tangent line for the land spans, whose position had been calculated, and to a level line at the lowest point of the curve for the center of the span. Allowances were made for the temperature prevailing at the time.
Strength of Cables. The cables were made of galvanized steel wires, No. 8, Birmingham gauge. The strength of the cables per square inch of solid section is 160,000 pounds. Each cable is composed of 19 strands, each of which contains 278 wires. The last wire of the cables was run over October 5, 1878. At a distance of 21.5 feet from the anchor bars heavy clamps were put on the cables to draw them to a cylindrical form. This was made necessary as the anchor bars spread so as to cover a space 5 feet square. The cables were finally wrapped with galvanized iron wire, the finished diameter being 15.75 inches.
On top of the towers the cables rest in saddles which furnish a bearing with easy vertical curves. In plan they are rectangular, 13 by 4-1/12 feet, and have an extreme height at the center of 42 feet and a thickness of 4 inches. Each cable passes over the center of its saddle in a groove 19.5 inches wide by 17.5 inches deep. The saddles rest on steel rollers, which in turn rest on planed plates. This permits the cables to move freely backward and forward, and to accommodate themselves to any unequal loading, and also to adapt themselves to changes in temperature.
Passing over the towers alongside of the cables are 100 steel wire rope stays, arranged 25 to each cable, and secured at each end to the trusses carrying the floor system. The longest extend to a distance of about 400 feet each side of the towers, and are spaced 15 feet apart at the trusses.
Suspended Structure. The floor system consists of six longitudinal trusses connected by floor beams, the whole being hung from the cables by suspender ropes. The suspender ropes are of twisted galvanized steel wire, and are from 1-5/8 to 1-3/4 inches in diameter. Each is capable of sustaining about 50 tons, or five times the load it will ever be subjected to. As the floor system is in a continuous line from the top of the anchorages, and as the cables leave the anchorages a few feet below, the floor beams rest on the cables until the latter rise above the grade. The beams are here laid on posts resting on the cables, which vary in height to suit the distances, and are braced by plate brackets. The lower end of each post is bolted to the upper half of a strap encircling the cable. The whole number of suspender ropes is 1,520, and the posts number 280. The floor beams were made in half lengths, and when riveted together at the center formed a continuous beam 86 feet long. They are 32 inches deep, 9; inches wide, and each weighs 4 tons. Each has two top and two bottom chords so united as to form a triangular, latticed girder. The chords are of steel channel bars. The beams are spaced 7.5 feet between centers, and between each pair is placed an I-beam, which rests on the bottom truss chords, so that the planking is supported at every 3.75 feet. The work of placing the floor beans was begun at the towers, and carried each way at the same time, in order to load the cables uniformly.
Promenade and Roadway. The bridge is divided by six longitudinal trusses into five passage ways, the trusses being of the following heights, measured from the top of the floor beams: The two outside ones 7.5 feet, and the four intervening ones 15 feet 7.5 inches. Across the central opening is a system of light beams supporting the promenade, which is 12 feet above the floor beams. The roadways at the outside are 18.75 feet wide in the clear, and although only designed for vehicles, each now has a trolley track. The two remaining divisions are 12-2/3 feet wide in the clear, and are used for passenger cars. As the foot passenger approaches the towers he ascends a few steps, the walk dividing and passing through each of the tower arches on a flooring laid on the beams over the car tracks.
Vibration. To prevent horizontal vibrations and to resist the force of the wind, there are wind braces placed beneath the floor beams. These are steel wire ropes from 2 to 3 inches in diameter, and are anchored at the four facing corners of the towers to eye bolts set in the masonry. From the corners to which they are attached they pass diagonally across the floor beams to the opposite side of the bridge, where they are secured. The longest ones reach about half way across. Similar braces are provided on the short spans. As a further precaution, and particularly to secure stability at the center of the span, where the braces are of little effect, the outside cables are drawn in a short distance toward the center. To allow for expansion and contraction of the trusses, slip joints are formed between the towers and anchorages and in the main span. The aggregate weight of the suspended structure, including cables, trusses, suspenders, braces, timber flooring, and rails, is 14,680 tons; the estimated transitory load is 3,100 tons, making the total weight of the superstructure 17,780 tons.
Approaches. The Brooklyn approach is 971 feet long on the center line, and is 100 feet wide throughout. It spans several streets by plate girders, and has one curve at about 200 feet from Sands street. The New York approach is 1,562.5 feet long, begins at grade at Park Row, and rises 3.25 feet per hundred to the rear of the anchorage. It is 100 feet wide for about 500 feet of the distance, and 85 for the remainder. At Franklin Square is an opening measuring 210 feet on one side and 170 on the other, which is spanned by a bridge. The other streets are crossed by semi-circular brick arches. Both approaches consist of arches resting on massive piers, the fronts being entirely of granite. The cornice over the arches has a dentil course below, surmounted by a heavy projecting coping course. The whole is capped by an ornamental granite parapet. The arches are used as stores and warehouses.
Operation of Cars. When the bridge was opened the cars were moved by an endless cable operated by engines located beneath the Brooklyn approach. This service soon proved to be inadequate, and the third-rail electric system was introduced and is now in effect. As a further improvement, and in order to accommodate the travel using the elevated roads of Brooklyn, connection was made with these loads so that the passage of the bridge could be made without change. In addition to this a trolley track was laid along each roadway in order that all the trolley lines in the Borough could cross the bridge without interruption. All these changes made necessary the complete re-designing and re-construction of the Brooklyn approaches, and also the changing of the station at the western end.
Cost. The financial condition of the bridge on March 31, 1883, shortly before it was opened to traffic, was stated as follows:
Cash received from New York,....... $4,871,900.00 Cash received from Brooklyn,....... 9,423,692.73 Cash received from rents, interest, sale of material, &c.,........ 391,463.93 Total ....................... $14,687,056.66 Still due from the City of New York.. 216,666.66 Still due from the City of Brooklyn.. 433,333.34 Total cost of bridge,........ $15,337,056.66
Administrative Integrity. During the period occupied by its erection New York was in the clutches of the Tweed ring, an audacious and unscrupulous gang of thieves. Yet, when the accounts were finally audited, every dollar of the appropriations was found to have been expended in wages or material, and its actual face value was represented in the completed structure. The ring had proposed otherwise and the belief was general that the bridge treasury had been looted. Mayor William C. Havemeyer appointed a committee to investigate the matter. Abram S. Hewitt was a member of that committee. In speaking of the results of this inquiry, Mr. Hewitt said:
Mr. Hewitt's Testimony. "The duty was performed without fear or favor. The methods by which the ring proposed to benefit themselves were clear enough, but its members fled before they succeeded in reimbursing themselves for the preliminary expenses which they had defrayed. With their flight a new era commenced, and during the three years I acted as a trustee I am sure that no fraud was committed, and that none was possible. Since that time the board has been controlled by trustees, some of whom are thorough experts in bridge building, and the others men of such high character that the suggestion of malpractice is improbable to absurdity."
"The bridge has not only been honestly built, but it may be safely asserted that it could not now be duplicated at the same cost. Much money might, however, have been saved if the work had not been delayed through lack of means and unnecessary obstacles interposed by mistaken public officials. Measured by its capacity and the limitations imposed on its construction by its relation to the interests of traffic and navigation, it is the cheapest structure ever erected by the genius of man."
Success of the Bridge. In a certain sense this bridge was an experiment. That it would largely eliminate the East River as an obstacle to travel to and from Brooklyn, and that it would place the outlying districts of that city at the door of New York, were facts known and appreciated by its promoters; but whether the people would avail themselves of the increased facilities afforded was a matter of freely expressed doubts. To convince oneself that the bridge has more than fulfilled all the expectations of its projectors, it is only necessary to view the vast multitudes that are continuously passing across it. But the real work it has accomplished can best be ascertained by an examination of the sections of Brooklyn, formerly waste spaces, that are now covered with the homes of people, a large portion of whom resort daily to New York.
Its Importance. We have extended our notes on the Brooklyn Bridge to considerable length because of its vast importance in providing easy transit between sections of the city that were separated by a natural barrier, because it was the first municipal undertaking on the line of rapid transit, and because the bridge is beautiful to a degree as well as useful. It is said to be the most inspiring example of suspended bridge construction in the world. It is doubtful whether it will be duplicated anywhere in the future. Its lofty towers and its graceful span are visible to everyone who enters our harbor. It is a notable monument to the genius of the engineers who planned it and to the public spirit of those citizens who, with untiring zeal in the face of great obstacles, so worked that it was carried to completion. It typified union between nearby centers of unrelated population. It has led to the conception of that political union which has made New York the second city on the globe. It brought the men of both sections into collaboration for transportation facilities of far wider scope and usefulness.
Capacity. A second suspension bridge over the East River was begun in 1896 and formally opened in December, 1903. It spans the river between the foot of Delancey street, Manhattan, and the foot of South Fifth and South Sixth streets, Brooklyn, and has a total length, from the entrance at street grade in Manhattan to the entrance in Brooklyn, of 7,200 feet. Through its entire length it has a clear width of 18 feet, and provides for two elevated railway tracks, four street railway tracks, two 18-foot roadways, two footpaths, and two bicycle paths. It is very remarkable in its capacity to carry traffic.
Caissons. The foundation piers, two for each tower, were sunk to bed rock, about 70 feet below mean high water, by means of timber caissons similar to those used in the old bridge, but different in one essential point. The entire caisson was stiffened with a series of massive plate-steel riveted trusses, eight in all, which extended entirely across it from wall to wall. The working chamber was also strengthened with two solid bulkheads built across it. Level with the bottom of the walls was a framework of 16-inch timbers bolted to the side walls with tie rods. At each intersection vertical posts reached from this frame to the roof, and the whole system was tied together and stiffened against lateral distortion by diagonal struts and tie rods. The object of this bracing and truss work was not merely to enable the roof to carry the superincumbent load of masonry, but to enable the whole caisson to endure without distortion the heavy transverse strains to which it would be subjected should it become "hung" upon any projecting point of the uneven rock bottom. Each caisson was built upon launching ways and floated to its destination.
The piers are built of limestone up to low water level, above which they consist of a granite facing with a limestone backing. They are finished with two heavy coping courses of simple and pleasing design, and one pedestal course of granite blocks.
Anchorage. The anchorages measure 182 feet in width, 158 feet in depth, and 120 feet from the foundation to the coping. Forty feet of the mass is below the street level, above which it rises some 80 feet. The total pull of the four cables is 20,250 tons. The anchorage could only be moved by being rotated upon its "toe" as an axis, or by sliding bodily forward. To resist rotation the masonry is massed at the rear, most of it being directly above the anchor plates to which the cables are secured, the forward half being of hollow construction. Sliding is resisted by the mass of earth at the toe and by the frictional resistance between the masonry itself and the earth upon which it rests; this is also increased by the stepping of the bottom of the foundation.
Towers. At each corner of each of the tower foundations, or piers, is a large block of dressed granite upon each of which rests a casting forming the base of a leg, or column, of the tower. Each half of each tower is composed of four columns which are 8 feet square at the bottom and taper to a square of 4 feet at a height of 20 feet, the latter section being then maintained throughout their full height. The columns are 310 feet in height, and are built up of two thicknesses of plate riveted together. The base is stiffened by diaphragms, but in the upper 4-foot section there are eight built-up Z-bars, two on each inside face of the column. All the columns are vertical up to the level of the roadway, above which they have a batter toward each other of 14 feet in a height of 215 feet. The four columns are strongly united by bracing, and just below the floor a system of lattice bracing is placed entirely around each tower and also between the towers. Above the roadway the towers are tied together by latticed and diagonal members. The saddle castings upon which the cables rest are located immediately above the legs of the towers, the weight being distributed and the structure stiffened at this point by a system of deep girders.
Cables. Each of the four cables consists of 37 strands of No. 8 wire, and each strand is made up of 281 wires, so that in each cable there are 10,397 wires. The specifications required a tensile strength of 200,000 pounds per square inch of section, and an elongation of at least 5 per cent. in a length of 8 inches. Instead of wrapping the cables with wire in order to protect them from the atmosphere, as was done with the Brooklyn Bridge cables, they were enclosed in 1/16-inch sheet steel which reaches from one suspender band to another. The suspenders, which are 20 feet apart, are steel wire rope; they are attached to the stiffening trusses at their point of intersection with the floor beams.
Saddles. The saddles weigh over 32 tons each. The cable rests in a groove struck in a plane parallel with the axis of the bridge and on a radius of 21 feet 6.5 inches. The saddle is supported upon 22 steel channel beams, and movement of the saddle is provided for by 40 steel rollers placed between the saddle casting and the beams.
Suspended Structures. In order to compensate for the vertical distortion produced by unequal loading, and to distribute such loads, it was necessary to stiffen the floor system. In the old bridge this was accomplished by four longitudinal trusses; but in this case there are only two trusses, each 40 feet deep, which extend entirely across the bridge. The bottom chord is built into the floor system and is of the same depth. The floor of the bridge is composed of a series of transverse plate girders, 5 feet in depth, which extend all the way across. These are spaced 20 feet apart, and are bridged longitudinally by lines of plate-steel stringers. There are 20 of these lines of stringers which extend through the structure from end to end. The roadways are carried by the overhanging ends of the floor beams. The central portion of the floor beams is supported at two points from overhead trusses, which are built in between opposite panel-points of the upper chords of the stiffening trusses. This construction reduces the weight and admits of the use of much shallower floor beams than would otherwise be necessary. Wind pressure is resisted by a horizontal truss between the top chords of the stiffening trusses, and by the manner in which the longitudinal stringers are riveted intercostally between the floor beams; the tensional stresses, due to a wind blowing across the bridge, are resisted in the leeward half of the floor by the stringers and the bottom chord of the stiffening truss, and the compressive stresses are similarly provided for by the stringers and bottom chord of the windward half of the floor system.
The Trussed Structure. The suspended portion of the structure occupies only that portion lying between the towers, the land part of the cables carrying no load whatever. Between the anchorages and towers are parallel-chord trusses with their centers resting upon steel piers. The main trusses are not provided with slipjoints, as are those of the Brooklyn Bridge, but are continuous from anchorage to anchorage; neither are they rigidly united to the towers or anchorages. They are furnished with roller bearings at the anchorages and with rocker bearings at the main towers; this construction permits of their free expansion from the center toward each anchorage.
The bridge was designed by L. L. Buck, whose work in renewing the original Roebling suspension bridge at Niagara had already attracted attention.
Cost. The contract prices for the bridge were as follows:
New York tower foundation..... $373,463 Brooklyn tower foundation..... 485,082 Anchorages.................... 1,570,000 Towers and shore spans........ 1,221,726 Cables and suspenders......... 1,398,000 Approaches.................... 2,411,000 Main span suspended system.... 1,123,400
The total estimated cost of the bridge, including land and stations, is $20,000,000.
Size. The new Manhattan Bridge, the foundations and piers of which have been completed, will extend from near the intersection of the Bowery and Canal Street in New York, to Willoughby Street, between Prince and Gold Streets, in Brooklyn. It will be the longest of the bridges across the East River, measuring about 10,000 feet between terminals. The floor of the bridge will be 120 feet wide over all; the center span will be 1,470 feet from center to center of the towers, and each land span will be 725 feet in length. The two steel towers will be 400 feet above high water.
The proposition to use steel eye-bars for this bridge was rejected by the Commissioner of Bridges in 1904, and it was decided to return to the wire cable type. The arrangement of spans, capacity and loadings proposed for the eye-bar bridge and recommended by the commission of engineers appointed during the preceding year was, however, retained in the new wire cable design. The specifications for the cables and suspenders called for an ultimate strength of not less than 215,000 pounds per square inch before galvanizing, and an elongation of not less than 2 per cent. in 12 inches of observed length.
Towers. The principal novelty in the design is found in the towers. Each is composed of four columns standing in a transverse plane, the columns being in the same vertical planes as the chain cables. A side view of the columns shows that, from their greatest width of 22 feet at the platform, they taper to 14 feet where they are supported upon a steel pin 2 feet in diameter, which rests upon a cast-steel footing. This construction distributes the load evenly over the masonry pier. In theory the towers are free to rock upon these pin bearings, but a movement of a few inches at their tops, caused by live load or temperature changes, would be taken care of by the elasticity of the towers themselves.
The anchorages will be formed with arches for street traffic, and will be provided with stairways and elevators, so that access can be had to the roadway from these points. The large interior space will be utilized as an assembling hall.
Cost. The estimated cost of the tower and anchorage piers is about $3,000,000, and of the superstructure $10,000,000. These figures do not include land damages, or stations. The bridge was designed by Gustave Lindenthal, working in connection with H. Hornbostel as consulting architect.
Blackwell's Island Bridge
Design. In 1884 a franchise was granted for the bridging of the East River at Blackwell's Island, but no steps toward actual construction were taken until 1898, when the Commissioner of Bridges prepared plans. These provided for a bridge having its western terminus on the block bounded by Fifty-ninth and Sixtieth streets and Avenues A and B, and its eastern terminus in Long Island City. Work was commenced in 1901, and was carried forward so slowly that in 1902 only about $42,000 had been expended. The plans were then revised, the changes affecting the superstructure chiefly, although provision was made for elevators to the roadway from the island. These called for two cantilever bridges having the following spans: A shore span on the Manhattan side, 469.5 feet in length; a river span of 1,182 feet; a central span across the island of 630 feet; a second river span of 984, and a shore span of 459 feet on Long Island. The length of the bridge, including approaches, will be 7,636 feet.
Towers. The towers will rest upon masonry piers which will extend up to the roadway. Each tower will consist of two steel legs of box section, spaced 93 feet from center to center at the base and 60 feet at the top, transversely of the bridge. The height between the chords at the towers will be 185 feet. The superstructure will be made up of two lines of trusses placed 60 feet from center to center. The top chord, being the tension member, will consist of eye-bars of nickel steel having a tensile strength of 90,000 pounds per square inch and an elongation of 18 per cent. in 8 inches. The bottom chord will be of standard box construction. The roadway will be carried on transverse floor beams which will extend beyond the trusses a sufficient distance to provide a line of trolley cars at each side. The central portion of the bridge will be two-decked, the upper floor having two elevated railway tracks and two footwalks, each 11 feet wide. Beneath this will be two more trolley tracks, between which will be a roadway with a clear width of 36 feet.
The estimated cost of the bridge, including land damages, is $12,548,500. It is expected to be finished in 1906.
The three bridges last described will open sections of Brooklyn that cannot be conveniently reached by the Brooklyn Bridge. They will connect with the elevated system of that Borough, and thereby serve the territory to the northeast of that covered by the old bridge. In addition they will do much toward relieving the crowded condition of the old bridge, since the termini of two of them in New York are in the districts that supply most of the traffic for that bridge.