Engineers and Soil Scientists May Have Solved Mystery of Pisa's Leaning Tower

The Leaning Tower of Pisa, or simply the Tower of Pisa, is a bell tower of the Cathedral of the Italian city of Pisa, known worldwide for its unintended tilt. Why has the Pisa tower survived the strong earthquakes that have hit the region since the middle ages? This is a long-standing question a team of engineers and soil scientists led by Roma Tre University’s Professor Camillo Nuti has investigated.

The Leaning Tower of Pisa. Image credit: Mark Skillen.

The Leaning Tower of Pisa. Image credit: Mark Skillen.

The Tower of Pisa has eight stories, including the chamber for the bells. Its height is 183 feet (56 m) from the ground on the low side and 186 feet (57 m) on the high side. Its weight is estimated at 14,500 tons.

Its construction began in August 1173 and continued for about two centuries due to the onset of a series of wars.

Despite leaning precariously at a 5-degree angle, the Pisa tower has managed to survive, undamaged, at least four strong earthquakes that have hit the region since 1280.

Given the vulnerability of the structure, which barely manages to stand vertically, it was expected to sustain serious damage or even collapse because of moderate seismic activity. Surprisingly this hasn’t happened and until now this has m`ystified engineers for a long time.

After studying available seismological, geotechnical and structural information, Professor Nuti and co-authors concluded that the survival of the Pisa tower can be attributed to a phenomenon known as dynamic soil-structure interaction (DSSI).

The considerable height and stiffness of the Pisa tower combined with the softness of the foundation soil, causes the vibrational characteristics of the structure to be modified substantially, in such a way that the tower does not resonate with earthquake ground motion; this has been the key to its survival.

The unique combination of these characteristics gives the tower the world record in DSSI effects.

“Ironically, the very same soil that caused the leaning instability and brought the Pisa tower to the verge of collapse, can be credited for helping it survive these seismic events,” said team member Professor George Mylonakis, from the University of Bristol, UK.

The researchers will present their results at the 16th European Conference in Earthquake Engineering in Thessaloniki, Greece.

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