The bridge of dreams

Die Eisenbahnbrücke über den Firth of Forth war eine der vielen Glanzleistungen viktorianischer Ingenieurkunst. Obwohl massiger und weniger bekannt als die nur ungleich längere Golden Gate Brücke, gehört sie dennoch zu den schönsten Brücken der Welt.
"Gain the confidence of the public and enjoy a reputation of being not only the biggest and strongest, but also the most stable bridge in the world." This was the main constructional stipulation imposed on the engineers John Fowler and Benjamin Baker when they were commissioned with the project to bridge the Firth of Forth northwest of Edinburgh, Scotland, for a railway line in 1880.
John Fowler and Benjamin Baker came up with a perfect solution: a cantilever bridge. A pair of cantilever arms, or beams stick out from two main towers. These beams are supported by diagonal steel tubes protruding from the top and bottom of the towers. This design made the Firth of Forth Bridge one of the strongest and most expensive in the world. One reason for the high costs was the use of steel. The Forth Bridge was the first large railway structure in Europe to be built with this relatively new material.
The construction of the Forth Bridge fell, more or less, into two parts. The years 1882-1885 were devoted to sinking the caissons and building the foundations and piers which would support the superstructure. Then from about 1886 to 1890 the superstructure itself was constructed. This superstructure is only as stable as the group of four platforms at the base of each cantilever tower. These platforms rest on underwater foundations -- wrought-iron cylinders known as caissons. Filled with concrete, each caisson weighs between 4000 and 20 000 tons.
Fullsize drawings were often made of the superstructure and wooden templates were used, similar to common practices in shipbuilding. First, the skewbacks were assembled, where all the vertical, horizontal and inclined members of the cantilever tower meet on the foundation. The cantilever towers, the main section of the bridge, rise 360 feet above the high-water line from these foundations. These were constructed by extending the tubular members of the skewback. Cage-like riveting machines advanced up the tubes, which were being formed above them on a lifting platform. Reaching outwards from the tower are the cantilever arms. To preserve equilibrium, the arms had to be built from opposite sides of each tower at the same time. The suspended girders, which link each completed cantilever tower and arms together, were constructed in a similar fashion. The working conditions were difficult and dangerous with a minimum of what we today would classify as safety equipment -- 57 of the 4500 workers died during the 8 years of building the bridge.
The remaining procedure of closing the final gap at mid-span depended largely upon temperature. At last, on October 10th 1889, the lower booms closed and were joined.
During the opening of the bridge by the Prince of Wales on March 4th 1890 another famous engineer was present -- Gustav Eiffel, who was probably contemplating the fact that the height of his recently finished tower was only about half the length of the main span of the new bridge. Engineer Benjamin Baker, who received a Knighthood for his work, later said about his most challenging project: ¿If I were to pretend that the building of the Forth Bridge has not been, and continues to be, a source of constant anxiety, no experienced engineer would believe me. Where there's no precedent, the engineer who best succeeds is the one who makes the fewest errors."

The bridge during construction showing the height of the cantilever towers

Confidence Vertrauen
stipulation Bedingung
impose, to auferlegen
commission sb, to jdm einen Auftrag erteilen
cantilever Ausleger
cantilever bridge Auslegerbrücke
beam Träger
protrude, to herausragen
devote, to widmen
superstructure Überbau
wrought-iron Schmiedeeisen
caisson Senkkasten
template Schablone
skewback Gewölbe-Widerlager, Halbpfeiler
inclined geneigt
tubular rohrförmig
rivet, to nieten
equilibrium Gleichgewicht
girder Träger
gap Lücke, Spalte
boom Arm
contemplate, to über etwas nachdenken
pretend, to behaupten
anxiety Besorgnis, Angst
precedent Präzedenzfall
succeed, to Erfolg haben, sich durchsetzen

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