Historical temperature ranges must also be analysed. Paris has been recording temperatures for more than two centuries, with winter minimums below -20⁰C and summer maximums of around 40⁰C. We should also take into account the effect of solar radiation – metals can reach much higher temperatures in direct sunlight, often exceeding 60⁰C or 70⁰C.
Leaning away from the sun
Now, let’s do the maths. We’ll estimate how much a simple 100-metre-long metal bar expands when the temperature fluctuates by 100⁰C – the approximate range experienced by the Eiffel Tower.
The calculation is simple. If a one-metre bar expands by 0.000012 metres when the temperature rises by one degree, a 100-metre bar expands by 0.12 metres when the temperature rises by 100 degrees. And a 300-metre bar would expand three times as much: 0.36 metres. That is, 36 cm. This is a noticeable difference.
Clearly, a simple bar does not behave the same as a tower made of more than 18,000 pieces of riveted iron oriented in all directions. Furthermore, the sun always shines on one of its sides. This means one of its faces grows more than the others, causing a slight curve in the tower, as if it were leaning away from the sun.
Specialists have estimated that the Eiffel Tower actually grows between 12 and 15 centimetres when comparing its size on cold winter days with the hottest days of summer. This means that, in addition to being a landmark, a communications tower and a symbol of Paris itself, the Eiffel Tower is also, in effect, a giant thermometer.
Federico de Isidro Gordejuela, Profesor adjunto de Construcciones Arquitectónicas, Universidad CEU San Pablo