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If we tell you that the Eiffel Tower is moving, you probably won’t believe us. And for good reason, this majestic building seems immutable. Gustave Eiffel has indeed built a sacred baby for us: in total, the monument weighs no less than 10,100 tonnes. Impossible to make him wobble a centimeter.

“And yet, she moves”, would surely have said Galileo in front of a court of Inquisition 2.0, accusing him of blaspheming on the one we call the Iron Lady. Yes, the Eiffel Tower is moving. Above all, she is growing. Or get smaller, it depends on the seasons.

What is this mysterious phenomenon that changes the size of this good old iron structure? When does this funny change happen? Why is the Eiffel Tower going to change size soon?

Thermal expansion

When it was presented to the world in 1889, on the occasion of the tenth edition of the Universal Exhibition held in Paris, the Eiffel Tower was called the “300-meter tower”. At the time, it was clear and, until 1930, the monument was the tallest in the world, before the Chrysler Building in New York came to dethrone it. Okay, nowadays “300 meters”, that’s not much: Dubai’s Burj Khalifa, the world’s tallest human structure today, is 828 meters tall. Just that.

However, let’s not spit in the soup. And then, the Eiffel Tower was even a little bigger than that at the time. Its exact height was then 312 meters (with flag, but we will say that it is okay). Why “was”? Because, today, the tower has grown, stretched towards the heavens by the addition of antennas of all kinds. Today, the monument measures 324 meters. Finally, it depends on the moments.

The Eiffel Tower under construction. | Piqsels

It all really depends on the season. Or temperatures, if you prefer. The metal frame of the Eiffel Tower (we are talking about 7,300 tons here anyway) actually reacts to a physical phenomenon called thermal expansion. This phenomenon means that at constant pressure, the volume of a body increases when its temperature increases, and vice versa. However, the entire structure of the building is made of puddled iron: a metal which is very sensitive to heat and, therefore, subject to the variations induced by thermal expansion. In other words, when it is hot, mainly in summer, the tower grows, and when it is cold, when winter comes, it shrinks.

As you can see, as the temperatures drop, our dear Iron Lady is getting ready to make herself smaller. Don’t expect it to disappear behind the rooftops of Paris, either. The variation in the size of the building is only ten centimeters. It is estimated that the Eiffel Tower loses about 10 centimeters in winter, and gains up to 20 centimeters when summer is in full swing and temperatures exceed 30 ° C.

In addition to causing its size to fluctuate, thermal expansion also has another consequence on the Parisian building: it can cause it to tilt to one side or another. The explanation? When the sun heats only one of the four sides of the structure, that part inevitably expands and the tower is unbalanced. It then tilts slightly to the side opposite the sun, explains Futura Science. Again, this change is only a handful of inches.

Why iron?

In view of the reaction of puddled iron to temperature variations, one may wonder why the French engineer chose this material. It is in fact the only reliable one he had at his disposal. Can you imagine a wooden tower of 300 meters? As for a stone tower, it would have probably crashed in no time under its own weight. Iron, meanwhile, was used at the time, produced in large quantities and had been mastered for centuries, unlike reinforced concrete, for example, which had only just emerged. Steel, which was beginning to make a place for itself on construction sites, was obviously not favored by genius.

It must be said that Gustave Eiffel had absolute confidence in iron: many of his previous constructions were made with this material, such as the Maria Pia viaduct in Porto or the Eiffel footbridge in Bordeaux. But the iron of the tower is very special. Its metal is purified, it is rid of excess carbon. A treatment that gives it the name of puddled iron, a guarantee of large quantities and solidity.

Today, old infrastructure could face a new challenge: climate change. Whether the Eiffel Tower takes up 10, 20 or 30 centimeters in the heat does not seem to be a big problem. Its structure can recover from these variations without eyebrowing. But this is not the case with all man-made buildings.

Our dear Iron Lady, over 132 years old, is therefore not expected to curl up any further in the future, but, on the contrary, may well gain a few more inches each summer under the influence of climate change. On closer inspection, many old infrastructures are at the mercy of high temperatures.

Popular Science, an American quarterly magazine for the popularization of science, reports for example the case of the DuSable Bridge in Chicago, whose construction was completed in 1920 and which, in the summer of 2018, was completely swollen during a wave heat. Like the Eiffel Tower, the joints of the bridge, here in steel, increased in volume with the heat, thus preventing the structure from opening to let boats pass. One example among many others.

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