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. As historian Edna Kranakis writes in Constructing a Bridge:

On the night of September 6 [1826], an accident occurred. A water main that passed close to the underground anchorage buttresses on the Champs-Elysées side of the bridge ruptured and flooded the area around the buttresses. Some of the supporting earthwork ceased to bear against the anchorages. As a result, the two fissures on the Champs-Elysées side suddenly widened … and the buttresses suffered some upturning and displacement… This movement in turn caused the towers on the Champs-Elysées side to tilt toward the river.

The bridge had not fallen down, although it was listing. And other bridges had had similar problems during construction, been repaired and then served for many decades, even centuries. At first, professional opinion stated the bridge could be rescued at a total cost of only 1 to 2% of the contract price. But in the end, the project was cancelled in spring 1827 and the Pont des Invalides (as it was known at the time) was dismantled, leaving in its place much more than an empty space.

What happened? What went wrong?

The story starts with Claude-Louis-Marie-Henri Navier (let’s just call him Claude). He was born in 1785 in Dijon. His father died when he was still quite young, but an uncle took him in and looked after his education. The uncle, Emiland Marie Gauthey, was professor of mathematics at the École des ponts et chaussées and a practising engineer.

Navier studied at the École polytechnique and then at the École des ponts et chaussées. Upon graduation in 1807 he started work for the Département de la Seine and remained there until 1822. In addition to his practical engineering work, Navier edited several engineering books and in 1821 became an associate professor of Applied Mechanics at the École des ponts et chaussées. In 1823 he published his first theoretical work on bridges and in 1830 joined the faculty of the École polytechnique. He held both posts until his death in 1836.

As historian Tom F. Peters writes in Transitions in Engineering, Navier “was not primarily a practitioner, although he had successfully built four large bridges and several canals in Italy.” However, he was thoroughly engaged in the major suspension bridge design and construction debates of the day and a leading engineering theoretician.

The French love theory. A colleague of mine once told me a story about two bureaucrats discussing a new economic project. The French one said, “Yes, we can see that it works in practice. But does it work in theory?” Navier would have understood this question.

Navier’s proposed suspension bridge was largely intended to show the importance of mathematical analysis to bridge building. Otherwise, nobody actually wanted a bridge in that location. Navier, perhaps somewhat unwisely, wrote:

There exists no urgent necessity to construct a bridge [from the Esplanade of the Hôtel des Invalides] to the Champs-Elysées; there is no obligation to build a suspension bridge in Paris. But if it is desired that one be built, let it be made into a monument; let the character of grandeur be given to this work that the style of construction admits of; let its disposition be calculated with the idea of an edifice approved by artists, agreeable to the public, honourable to the administration.

He must have known about the many arguments against it. It would block sightlines to the Hôtel des Invalides. The Egyptian decorative motifs were inappropriate. The beauty of the Champs-Elysées would be destroyed by building a major thoroughfare across it. It would create too much traffic.

Nevertheless, the exquisite drawings and the beauty of the mathematical calculations impressed the bureaucrats in the Corps des Ponts et Chaussées. They recommended only small changes. However, they disagreed with Navier on one point. Navier thought that something so magnificent needed to be state-financed. The bureaucrats insisted that the bridge had to be financed and built privately, and that investors should be given the right to charge tolls for 55 years.

On April 7, 1824, the project was opened to bidding. By August the successful bidders, headed by Alain Desjardins, started construction. But it was a curiously one-sided contract. The bridge had to be built according to a detailed set of specifications provided by the Corps. No change could be made without the written consent of the director of the Corps. There was no clause authorizing the contractor to suggest modifications, even though such a clause was a standard feature of this type of contract.

So if the contractor foresaw the problems, he could do nothing to prevent them. When the cracks appeared and the Corps des Ponts et Chaussées demanded that a new bridge be built at contractor expense, Desjardins argued that his company had built the bridge precisely as the contract had stipulated.

Stalemate. The ruined incomplete bridge became a symbol of what happens when theory and practice do not match.

Eventually the conflict was resolved with payouts to contractor and investors. Desjardins was given permission to build three more bridges in Paris, and could use the materials from the failed bridge. And perhaps most galling to Navier and others, Desjardins had a free hand in the bridge designs, so long as they could withstand the load tests the Corps would supervise.

Desjardins’s designs were far from elegant. At least one of his replacements was so hideous that it was described in one engineering publication in 1830 as “truly a villainous thing.”

But Navier’s career as a practising engineer was over. He bore the brunt of public criticism, was overlooked for further promotions, and died aged 51 in 1836. Posterity has been kinder, however, and his is one of the 72 names inscribed on the Eiffel Tower, for his contributions to mathematics.

It is difficult to know what to make of Navier and his disastrous bridge. Clearly he was a brilliant theoretician. But his story contains an element of arrogance; he was departing from proven practice in the way the abutments bore the weight of the entire structure, an approach that had been arrived at without sufficient testing.

Navier was an inspiration to those who wanted to see a greater role for mathematical analysis in engineering, design and construction. But he was caught up in political manoeuvring that let an untested structure go ahead and fail spectacularly.

“Trust me, I know what I’m doing” is what people usually say just before disaster strikes. Perhaps “Trust me, the theory works perfectly,” is what Navier said before his bridge wobbled, listed, and then was demolished. A lesson to us all.

Text by Norman Ball. Illustrations from Structurae, Wikipedia, and University of Houston.

For more on the chequered history of suspension bridges in France, read: The mystery of the missing suspension bridges of Paris.