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The Brooklyn  Bridge

The Brooklyn Bridge

World’s First Steel-Wire Suspension Bridge

Spanning the East River between the boroughs of Manhattan and Brooklyn, the Brooklyn Bridge is the first hybrid cable-stayed/suspension bridge in New York City.

The Brooklyn Bridge was opened on May 24, 1883. At the time of its opening, it was the longest suspension bridge in the world, with a main span between the two suspension towers is 1,595.5 feet (486.3 m) long, 85 feet (26 m) wide and a deck 127 ft (38.7 m) above mean high water (MHW).

Suspension Span

The bridge widens and contracts between the extremes of temperature from 14 to 16 inches. Navigational clearance is 127 ft (38.7 m) above MHW.

There is 930 feet (280 m) long gap between each suspension tower and each side’s suspension anchorages.

Structure containing six trusses supports the main span and side spans. This structure runs parallel to the roadway, each of which is 33 feet (10 m) deep. The Brooklyn Bridge is able to hold a total load of 18,700 short tons (16,700 long tons) with the help of the trusses. These trusses are supported by suspender ropes, which hang downward from each of the four main cables. Crossbeams run between the trusses at the top, and diagonal and vertical stiffening beams run on the outside and inside of each roadway.

An elevated pedestrian and cycling promenade that is 10 to 17 feet (3.0 to 5.2 m) wide, runs in between the two roadways and 18 feet (5.5 m) above them. It typically runs 4 feet (1.2 m) below the level of the crossbeams, except at the areas surrounding each tower. Here, the promenade rises to just above the level of the crossbeams, connecting to a balcony that slightly overhangs the two roadways.

Approaches

The approach ramps are connected to each of the side spans. The 1,567-foot (478 m) approach ramp from the Manhattan side is longer as compared to the 971-foot (296 m) approach ramp from the Brooklyn side. The Renaissance-style arches made of masonry supports the approaches; the arch openings themselves were filled with brick walls, with small windows within. The approach ramp contains nine arch or iron-girder bridges across side streets in Manhattan and Brooklyn.

Construction

Construction of the Brooklyn Bridge began on January 2, 1870. The first work entailed the construction of two caissons, upon which the suspension towers would be built.

Caissons

The caissons are made of southern yellow pine on which the towers rest on underwater. Both caissons contain interior spaces that were used by construction workers. The Manhattan side’s caisson is slightly larger as compared the Brooklyn side’s caisson. The Manhattan’s side caisson which is located 78.5 feet (23.9 m) below high water & Brooklyn’s side caisson located 44.5 feet (13.6 m) below high water, measures 172 by 102 feet (52 by 31 m) and, measures 168 by 102 feet (51 by 31 m) respectively. The caissons were designed to hold at least the weight of the towers which would exert a pressure of 5 short tons per square foot (49 t/m2) when fully built, but the caissons were over-engineered for safety.

Compressed air was pumped into the caisson, and workers entered the space to dig the sediment until it sank to the bedrock. On March 6, 1871, the caisson had reached its final depth of 44.5 feet (13.6 m). At its final depth, the caisson’s air pressure was 21 pounds per square inch (140 kPa).

The Manhattan caisson was lined with fireproof plate iron. It was launched from Webb & Bell’s shipyard on May 11, 1871. After the Manhattan caisson reached a depth of 78.5 feet (23.9 m) with an air pressure of 35 pounds per square inch (240 kPa), Washington Roebling deemed the sandy subsoil overlying the bedrock 30 feet (9.1 m) beneath to be sufficiently firm, and subsequently infilled the caisson with concrete in July 1872.


Cables

There are four main cables that supports the Brooklyn Bridge, which descend from the top of the suspension towers and help support the deck. Two are outside the bridge’s roadways and two are in the median of the roadways. The main cables measures 15.75 inches (40 cm) in diameter and contains 5,282 parallel, galvanized steel wires wrapped closely together in a cylindrical shape. These wires are bundled in 19 individual strands, with 278 wires to a strand.

The first temporary steel wire which was provided by the Chrome Steel Company of Brooklyn, stretched between the towers on August 15, 1876. 

The wire was then stretched back across the river, and the two ends were interwoven to form a traveler, a lengthy loop of wire connecting the towers, which was driven by a 30 horsepower (22 kW) steam hoisting engine at ground level.The wire was one of two that were used to create a temporary footbridge for workers while cable spinning was ongoing. A second traveler wire was stretched across the span, a task that was completed by August 30 1876.Anchorages

The bridge consists of an anchorage for the main cables. The Manhattan anchorage rests on a foundation of bedrock while the Brooklyn anchorage rests on clay.

The anchorages are trapezoidal limestone structures situated inland of the shore, measuring 129 by 119 feet (39 by 36 m) at the base and 117 by 104 feet (36 by 32 m) at the top. Each anchorage weighs 60,000 short tons (54,000 long tons).

The anchorages both have four anchor plates, one for each of the main cables, which are located near ground level and parallel to the ground. The anchor plates measure 16 by 17.5 inches (410 by 440 mm), with a thickness of 2.5 inches (64 mm) and weigh 46,000 pounds (21,000 kg) each. Each anchor plate is connected to the respective main cable by two sets of nine eyebars, each of which is about 12.5 feet (3.8 m) long and up to 9 by 3 inches (229 by 76 mm) thick. The chains of eyebars curve downward from the cables toward the anchor plates.

Towers

The bridge’s two suspension towers are 278 feet (85 m) tall with a footprint of 140 by 59 feet (43 by 18 m) at the high water line. They are built of limestone, granite, and Rosendale cement. The Manhattan tower contains 46,945 cubic yards (35,892 m3) of masonry and the Brooklyn tower has 38,214 cubic yards (29,217 m3) of masonry.

Each tower contains a pair of Gothic Revival pointed arches, through which the roadways run. The arch openings are 117 feet (36 m) tall and 33.75 feet (10.29 m) wide. The tops of the towers are located 159 feet (48 m) above the floor of each arch opening, while the floors of the openings are 119.25 feet (36.35 m) above mean water level, giving the towers a total height of 278.25 feet (84.81 m) above MHW.

After the caissons were completed, piers were constructed on top of each of them upon which masonry towers would be built. The towers’ construction was a complex process that took four years. Since the masonry blocks were heavy, the builders transported them to the base of the towers using a pulley system with a continuous 1.5 – inch (3.8 cm)-diameter steel wire rope, operated by steam engines at ground level. The blocks were then carried up on a timber track alongside each tower and maneuvered into the proper position using a derrick atop the towers.

Construction on the suspension towers started in mid-1872, and by the time work was halted for the winter in late 1872, parts of each tower had already been built. By mid-1873, there was substantial progress on the towers’ construction.

The arches of the Brooklyn tower were completed by August 1874. The tower was substantially finished by December 1874 with the erection of saddle plates for the main cables at the top of the tower. The Manhattan tower was completed in July 1876. The saddle plates atop both towers were also raised in July 1876.

Costing

The project depleted its original $5 million budget in the year 1875, while it was under construction. So, both the bridge commissioners, one each from Brooklyn and Manhattan, petitioned New York state lawmakers to allot another $8 million for construction. At the end, the legislators passed a law authorizing the allotment with the condition that the cities would buy the stock of Brooklyn Bridge’s private stockholders.

The bridge had cost US $15.5 million in 1883 dollars to build, of which Brooklyn paid two-thirds. Rehabilitation of the bridge cost about $153 million. As part of the project, the bridge’s original suspender cables installed by J. Lloyd Haigh were replaced by Bethlehem Steel in 1986, marking the cables’ first replacement since construction.

Use of Steel

A permanent contract was awarded, the builders ordered 30 short tons (27 long tons) of wire in the interim, 10 tons each from three companies, including Washington Roebling’s own steel mill in Brooklyn. In the end, it was decided to use number 8 Birmingham gauge (approximately 4 mm or 0.165 inches in diameter) crucible steel. In January 1877, a contract for crucible steel was awarded to J. Lloyd Haigh, who was associated with bridge trustee Abram Hewitt.

Opening
The Brooklyn Bridge was opened for use on May 24, 1883. Thousands of people attended the opening ceremony, and many ships were present in the East River for the occasion. Officially, Emily Warren Roebling was the first to cross the bridge. The bridge opening was also attended by U.S. President Chester A. Arthur and New York mayor Franklin Edson.
Renovation

The first major upgrade to the Brooklyn Bridge commenced in 1948, when a contract for redesigning the roadways were awarded to David B. Steinman. The renovation was expected to double the capacity of the bridge’s roadways to nearly 6,000 cars per hour, at a projected cost of $7 million. The renovation included the demolition of both the elevated and the trolley tracks on the roadways, the removal of trusses separating the inner elevated tracks from the existing vehicle lanes and the widening of each roadway from two to three lanes, as well as the construction of a new steel-and-concrete floor.

Conclusion

The Roeblings family are largely responsible for the conception, design, development and execution of the bridge.

This bridge was designated a National Historic Civil Engineering Landmark in 1972.

At the time of construction, contemporaries marveled at what technology was capable of, and the Brooklyn Bridge was seen as a symbol of optimism and aspiration in the field of engineering upon its completion in 1883 and it remains an inspiration till date.

 

Reference
  1. https://en.wikipedia.org/wiki/Brooklyn_Bridge
  2. https://www.britannica.com/topic/Brooklyn-Bridge

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