Historical Tribute
SUSPENSION BRIDGES MAY DATE BACK TO ANCIENT TIMES, BUT THE FIRST TRULY MODERN SUSPENSION BRIDGE WAS CREATED BY JOHN AUGUSTUS ROEBLING, IN NEW YORK.
Although modern suspension bridges dazzle the eye as awesome feats of technology, the engineering form is nearly as old as man himself. Vines were the source of cables on the earliest suspension bridges. In the fourth century A.D. plaited bamboo and iron cables were used on bridges in India.
The first truly modern suspension bridge was constructed in the mid-nineteenth century by John Augustus Roebling, a German born American engineer. His bridges, which still stand, have towers supporting massive cables, tension anchorage for the stays, a roadway suspended from the main cables, and—a vitally important innovation—a stiffening deck below of beside the road deck to prevent oscillation. Roebling’s first grand success, was a bridge with four suspension cables and two decks, that spanned Niagara Falls in 1855. The determined engineer soon undertook a still more ambitious task: construction of the Brooklyn Bridge connecting Long Island and downtown Manhattan, a distance of 1,600 feet across the East River.
Historical Tribute
JOHN ROEBLING HAD A TRAGIC ACCIDENT AND DIED IN 1869. HIS SON, WASHINGTON AUGUSTUS ROEBLING TOOK OVER THE RESPONSIBILITY FOR THE CONSTRUCTION.
In 1867, Roebling had a revolutionary idea of using steel wire, more resistant than iron, for the cables. Six iron trusses would run the length of the bridge’s floor for stability. Aesthetics, too, were a consideration, as evidenced by the beautiful stays and broad walkway. Roebling had a tragic accident and died in 1869, but his son Washington Augustus Roebling assumed responsibility for the construction, which was not completed until 1883.
In order to dig the foundations and sink the towers of the Brooklyn Bridge, Roebling used pneumatic caissons, a method still very in the experimental stage in his day. A pneumatic caisson is a huge box or cylinder that has a lower cutting edge, closed at the top and is filled with compressed air to prevent soil and water from entering. The vessel contains an internal, airtight deck, with a pressurized chambers below, where workers excavate the waterbed. Mud, rocks and rubble are hauled out through another air lock, and concrete is lowered in. The pneumatic caissons today are now constructed of reinforced concrete; Roebling’s caissons (which measure approximately 100 by 160 feet) were made of yellow pine, and coated with pitch on the inside of it, with tin on the outside of it. Furthermore, no one in the mid-nineteenth century understood the necessity of decompressing slowly after working in the chamber of the caisson; more than 100 bridge
1866
1867
1868-1869
Legislation passed for construction of New
John Roebling presents design for 1,600 foot
Council approves $1.5 million for the costs of
York-Brooklyn Bridge.
bridge across East River.
construction. As surveying nears completion, Roebling is injured in freak accident. He dies of tetanus 17 days later. His son, Washington Roebling becomes project engineer.
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Roebling himself suffered severe cases of “the bends.” Today, the length of time one spend underwater and the rate of decompression are regulated by law. Roebling’s two pneumatic caissons suffered a series of disasters— floods, fires, and blowouts—as the cutting edges gradually sank into the riverbed. Working three shifts of eight hours each for more than two years, 360 men remove mud and gravel, exploded the hard clay bottom, and removed trap rock and gneiss—all by the light of gas burners and calcium lights. Erecting the cables occupied another twenty–six months. After the 271.5 foot towers were constructed, the first wire connecting the banks was towed the way across by scow and hoisted into position
THE CAISSONS SUFFERED A SERIES OF DISASTERS—FLOODS, FIRES, AND BLOWOUTS—AS THE CUTTING EDGES GRADUALLY SANK INTO THE RIVERBED.
between the two towers. A second rope was dragged back and the two ends spliced together, forming a continuous rope, or
“traveler rope.” On each bank the rope was looped around driving and guiding wheels attached to the anchorage. (In all suspension bridges, anchorages secure the ends of the cables and may be made of masonry, concrete, or natural rock.)
1870
1871
1872
Construction for the wooden caisson, on
Construction begins on New York–side
Washington Roebling is incapacitated by the
the Brooklyn-side, begins in January. A fire
caisson and the construction is completed
“caisson disease,” making him an invalid. He
damages caisson in December.
on Brooklyn-side caisson.
continues to direct the project with the help of his wife, Emily.
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Historical Tribute
A TOTAL OF 286 WIRES, BOUND PARALLEL, WENT INTO EACH STRAND, AND NINETEEN STRANDS, IN TURN, MADE UP EACH OF THE FOUR MASSIVE CABLES.
Next, other ropes were drawn across and a planked footbridge is built, to regulate the placement of the cables. Extemely large wheels of wire were positioned near the anchorages on the Brooklyn side. A loop of two wires was hung around the light wheels of a “traveling sheave,” which in turn was fastened to the traveler rope. Thus one trip of the sheave carried two wires from the Brooklyn to the New York bank, where they were fastened to a horseshoe-shaped structure called a shoe. The sheave brought two more wires across. A total of 286 wires, bound parallel, went into each strand, and nineteen strands, in turn, made up each of the four cables. The modern method of suspending a suspension bridge, is not all that different. Although faster and much more efficient. Tower and anchorage foundations are laid first by underwater excavating, driving piles, the use of pneumatic caissons, or cofferdam (a wall that isolates the area of work) depending upon the exact condition of the waterbed. Next, the concrete pier tops are leveled. Steel slabs nearly 5 inches thick are attached to the piers with steel dowels, and bottom sections of the towers are welded to the slabs. Steel platforms, equipped with cranes and other hoisting gear, are slung between the lower towers, which are gradually built up. Cables of high-tensile steel, with individual parallel wires or twisted wires, may be as massive as 1 reels bearing nearly 30 miles apiece, the ends of the cables being carefully spliced together. A carriage motivated by a pulley system bears the reel over a fixed cable, from one anchorage.
1875
1876-1877
1878
Construction of New York–side caisson
Manufacturing of steel–strand cables begins
Small strand in bridge support cable snaps,
is completed. The construction of both
for bridge.
leading to an investigation of J. Lloyd Haigh
anchorages are in process. The towers
company, supplier of inferior bridge cables.
are completed on both sides.
A temporary footbridge is opened and the construction of roadway begins.
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CHORDS OF THE STIFFENING TRUSS ARE ADDED ONLY WHEN THE BRIDGE IS NEARLY COMPLETE. UNTIL THAT TIME IT REMAINS VULNERABLE TO STRONG WINDS.
In order for the construction crew to have access to cables, a temporary catwalk of crosstimbers is erected. Another machine binds the cables together with wire. Finally, a coat of corrosion-resistant paint is added. All of this to insure the stablilty and longevity of the suspention bridge. The road deck is built out gradually from both sides of the river. Or sections of it that may be floated out and raised into place. There is always a certain degree of distortion, so chords of the stiffening truss are added only when the bridge in nearly completed. Until that time it remains vulnerable to strong winds.
The main span (the length between the two towers) of the Verrazano-Narrows Bridge, connecting Staten Island and Brooklyn, New York, reaches 4,260 feet—the longest in the world—and the beautiful Golden Gate Bridge in San Francisco stretches a total of 8,981 feet, with a main span of 4,200 feet. The bridge that is the strongest is, New York’s George Washington Bridge, designed by P. H. Ammann and constructed in 1931. Each of the four original cables, 1 yard in diameter, containing 26,474 galvanized steel wires, giving a live load strength of 5,080 pounds per foot. A second road deck was added in 1962, and the bridge now accommodates fourteen lanes of traffic.
1880
1882
1883
J. Lloyd Haigh is imprisoned for fraud. The
By a narrow 10-7 vote, the bridge company
Bridge roadway is completed. Roebling’s
road construction continues.
retains Roebling as project engineer, after a
wife, Emily, becomes first person to travel
dispute over delays and cost overruns.
across completed bridge by carriage. The bridge opens to traffic on the 24th of May. President Chester Arthur attends opening ceremony of the bridge. 1
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