The structure known today as the Eiffel Tower was originally dubbed the Tour de 300 mètres, the 300-metre tower. The name was proposed by engineers Maurice Koechlin and Émile Nougier to Gustave Eiffel, who oversaw the tower’s construction. It hinted at the desire to build something extraordinary, a technological feat that would set a new height record.
However, as temperatures rise during the summer months, the Eiffel Tower grows even taller than its original design.
The Eiffel Tower was erected at the 1889 World’s Fair to commemorate the centenary of the French Revolution.
Eiffel chose puddled iron for its construction, a material he knew well and had used in previous projects with good results. This ferrous material can withstand high levels of stress, which allowed for the construction of a large, very light tower that would be safe from horizontal wind forces.
To give an idea of how light the tower is, its weight of 7,300 tonnes is close to the weight of the volume of air contained within it – around 6,300 tonnes.
The Eiffel Tower was intended to be a prime observation point, as well as a base for radio broadcasting. The tower itself is a gigantic triangular lattice structure, much like the Garabit Viaduct (also designed by Eiffel’s office) and the Forth Bridge in Scotland, both from the same period.
All of these structures grow when the temperature of the material increases. However, unlike bridges, which behave in a more complex manner, the Eiffel Tower experiences mainly vertical growth and shrinkage due to changes in temperature. This phenomenon is known as thermal expansion.
We know that most solids expand when the temperature rises and contract when it falls. This is because an increase in temperature causes greater agitation in the atoms, which leads to an increase in the average distance between them.
Depending on the nature of the bond, different kinds of solids experience greater or lesser growth, which engineers have to record with great care. Ceramics and glasses, with stronger bonds, expand less than metals, which in turn expand less than polymers.
So, how can we estimate the amount of movement in a solid? When the elements are straight – as is the case in most public works and architecture, where beams and bars predominate – the movement is proportional to three parameters: the length of the element, the change in its temperature, and the material’s coefficient of expansion.
Many ceramic materials typically have expansion coefficients ranging from 0.5×10⁻⁶ to 1.5×10⁻⁶ (°C) ⁻¹, while metals range between 5×10⁻⁶ and 30×10⁻⁶ (°C)⁻¹, and polymers between 50×10⁻⁶ and 300×10⁻⁶ (°C)⁻¹. These (perhaps strange-looking) numbers indicate the growth of a standard-length unit when the temperature rises by one degree Celsius.
The most expandable materials are polymers, which expand about ten times more than metals, and metals expand ten times more than ceramics.
The puddled iron used in the Eiffel Tower, and its steel components, have a coefficient of around 12×10⁻⁶ (°C)⁻¹, meaning that a one-metre-long iron bar expands by 12×10⁻⁶ metres when the temperature rises by one degree. That is just a dozen microns, less than the thickness of a human hair.
So does heat have any noticeable effect on buildings? Yes, if we take into account that there are two other parameters to consider: the length of the element and the temperature range where it is located.
