A Bridge Too Far

It should have been the pride of Paris; a stunning suspension bridge leaping clear across the Seine. It should have been one of the crown jewels of both Paris and the career of Claude Navier, one of 19th-century France’s most brilliant mathematicians and engineers. But things don’t always work out as they should.

This sketch by Navier shows what he planned for his 560-foot (155-metre) span across the Seine, at the site now occupied by the Pont Alexandre III. Suspension bridges were the latest thing, and Navier was the best French bridge analyst of the day. Construction started in 1824 and seemed to proceed well. In 1826 the bridge was not finished, but chain cables had joined the two shores and a roadway was suspended from the cable chains. Then the unthinkable occurred in July 1826: a crack appeared in the cable anchorages.

The anchorages were crucial to the safety and permanence of the bridge. In suspension bridges, the cables from which the roadway is suspended must carry all the weight of the bridge and traffic. At each end of the bridge, the cables pass over the bridge towers and then down into the ground where they are anchored with stone, concrete, and the earth above the abutments.

The first crack could be explained away by normal settling. What happened next could not be explained away so easily.

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