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.

1866 Legislation passed for construction of New

York-Brooklyn Bridge.

1868-1869

Council approves $1.5 million for the costs of

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.

1867

John Roebling presents design for 1,600 foot

bridge across East River.

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

JOHN ROEBLING HAD A TRAGIC ACCIDENT AND DIED IN 1869. HIS SON, WASHINGTON AUGUSTUS ROEBLING TOOK OVER THE RESPONSIBILITY FOR THE CONSTRUCTION.

Historical Tribute

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1870

Construction for the wooden caisson, on

the Brooklyn-side, begins in January. A fire

damages caisson in December.

1871

Construction begins on New York–side

caisson and the construction is completed

on Brooklyn-side caisson.

1872

Washington Roebling is incapacitated by the

“caisson disease,” making him an invalid. He

continues to direct the project with the help

of his wife, Emily.

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

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.)

THE CAISSONS SUFFERED A SERIES OF DISASTERS—FLOODS, FIRES, AND BLOWOUTS—AS THE CUTTING EDGES GRADUALLY SANK INTO THE RIVERBED.

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Historical Tribute

1875

Construction of New York–side caisson

is completed. The construction of both

anchorages are in process. The towers

are completed on both sides.

1876-1877

Manufacturing of steel–strand cables begins

for bridge.

1878

Small strand in bridge support cable snaps,

leading to an investigation of J. Lloyd Haigh

company, supplier of inferior bridge cables.

A temporary footbridge is opened and the

construction of roadway begins.

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.

A TOTAL OF 286 WIRES, BOUND PARALLEL, WENT INTO EACH STRAND, AND NINETEEN STRANDS, IN TURN, MADE UP EACH OF THE FOUR MASSIVE CABLES.

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1882

By a narrow 10-7 vote, the bridge company

retains Roebling as project engineer, after a

dispute over delays and cost overruns.

1883

Bridge roadway is completed. Roebling’s

wife, Emily, becomes first person to travel

across completed bridge by carriage. The

bridge opens to traffic on the 24th of May.

President Chester Arthur attends opening

ceremony of the bridge.

1880

J. Lloyd Haigh is imprisoned for fraud. The

road construction continues.

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 cross-

timbers 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.

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