Bridges To Brooklyn: Williamsburgh Bridge

 

 
 
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 118 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.

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 tic 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.

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.

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.

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.

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 1/2 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.

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 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 slip-joints, 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.

The contract prices for the bridge were as follows:

New York tower foundation
Brooklyn tower foundation
Anchorages,
Towers and shore spans
Cables and suspenders
Approaches,
Main span suspended system
 $373,463
   485,082
1,570,000
1,221,726
1,398,000
2,411,000
1,123,400

The total estimated cost of the bridge, including land and stations, is $20,000,000.


 

Website: The History Box.com
Article Name: Bridges To Brooklyn: Williamsburgh Bridge
Researcher/Transcriber Miriam Medina

Source:

BIBLIOGRAPHY: Rapid Transit in New York City and in Other Great Cities: Prepared for the New York Chamber of Commerce of the State of New York 1905
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