Tower finds self on firm ground

The Leaning Tower of Pisa and its 5.5-degree lean has vexed engineers for centuries.

Partially constructed on unexpectedly soft soil, the ancient bell tower began to lean before it was even finished, a historical goof that went on to become one of the world’s historical oddities — and made the tower a UNESCO World Heritage site.

How can something so obviously structurally unsound endure in an earthquake-prone region for hundreds of years?

Professor George Mylonakis wanted to know why.

The tower in northwestern Italy has managed to survive two world wars and at least four strong earthquakes that have hit the region since 1280, according to Phys.org. One of those quakes was greater than magnitude 6.0.

Mylonakis, an engineering professor who studies geotechnics and soil-structure interaction, and more than a dozen researchers came up with an answer that involves that famous soft soil and a term called “dynamic soil-structure interaction.”

According to Phys.org, the engineers determined that the tower’s height and stiffness, “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.”

So during a quake, the tower doesn’t shake as much as the earth beneath it, in further defiance of gravity.

“Ironically, the very same soil that caused the leaning instability and brought the tower to the verge of collapse can be credited for helping it survive these seismic events,” Mylonakis told the website.

The researchers have only released some of their findings. They expect to release the rest this month at the European Conference on Earthquake Engineering in Greece.