In August 1173, the construction for the bell tower of the Duomo of Pisa, Tuscany, began. Unfortunately, five years later the yet unfinished building began to tilt. Construction was immediately stopped and for almost a century the tower remained an unfinished stump. In 1275, the work was resumed and it was completed around 1372.
At the time, some precautions were adopted to avoid a collapse of the tower. Longer columns on one side of the tower reduced the inclination of the upper floors and asymmetrical decoration hides the tilt. Also, the tower’s final height was lowered considerably, measuring 183 feet from the ground on the low side and 186 feet on the high side. Over the following centuries, the tower still slightly tilted and in 1990- 2001 restoration work was done, reinforcing the foundations of the building and using 800 tonnes of lead counterweights to reverse the axial tilt by almost one degree. Today, the eye-catching inclination has an angle of 4 degrees, even if it’s sometimes exaggerated like in this figure from 1816. The tilt is without doubt one reason why the tower is so popular among tourists.
It is not only surprising that the tower is still leaning after 600 years, but also because it has survived a series of strong earthquakes. At least four have hit the town of Pisa since its construction. A paper, to be published in the June edition of the journal Earthquake Spectra by a team of Italian engineers claims that it solved the mystery of how the Tower has survived. As it turns out, it is actually the inclination that saved it from collapsing.
A look underground reveals the problem. Alternating layers of sand and clay in the underground are compressed by the 14-ton tower unevenly: as the foundations of the building tilt, the tower follows. Such soft soil is generally bad for buildings, as it amplifies the shaking of an earthquake and buildings start to oscillate until the collapse.
In the case of the leaning tower of Pisa, when an earthquake shakes the ground the tower doesn’t oscillate around a vertical axis, as in an upright standing building, but instead rotates around the inclined central axis. The rotary motion dissipates most of the energy that otherwise could damage or break the columns supporting the building.
Experts think that with the restoration work done before 2001, the tower should last at least another 300 years - even with earthquakes.
