Architecture

Engineers uncover the Leaning Tower of Pisa's secret to shrugging off earthquakes

Engineers have uncovered how the Leaning Tower of Pisa has managed to withstand several large earthquakes over the centuries
Engineers have uncovered how the Leaning Tower of Pisa has managed to withstand several large earthquakes over the centuries
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Engineers have uncovered how the Leaning Tower of Pisa has managed to withstand several large earthquakes over the centuries
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Engineers have uncovered how the Leaning Tower of Pisa has managed to withstand several large earthquakes over the centuries

With its iconic, unintended tilt, the Leaning Tower of Pisa is probably one of the best known construction blunders in the world. Although the lean was stabilized during restoration work in the 1990s, the tower stood at a precarious angle for more than 600 years before that, surviving numerous large earthquakes without damage. Now, a team of engineers has identified how it managed that feat.

The lean became apparent almost immediately, as the second floor was being built in 1178. The blame was placed on the foundation, which was both too small and sitting atop soft soil – but that didn't stop engineers from continuing their work, with the remaining six stories being added over the next 200 years.

For centuries, the Leaning Tower of Pisa stood at a five-degree angle, so that the top of the 58-m (190-ft) tall building was offset by more than 5 m (16 ft). But despite its apparent vulnerability, the structure has shrugged off at least four strong earthquakes recorded in the area since 1280. Just how it was able to do so has remained a mystery.

So, a research team comprised of 16 engineers, from institutes such as the University of Bristol and Roma Tre University, set out to investigate. The researchers studied seismological, geotechnical and structural information of the tower and its surrounding area, and found that it all comes together to give the Leaning Tower of Pisa a pretty unique set of characteristics.

Essentially, the interaction between the soil and the structure on top of it is far more dynamic than most other buildings. Because the foundation soil is so soft – responsible for the tower's lean in the first place – and the tower is so tall and stiff, the building itself doesn't resonate with the ground motion during earthquakes.

"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," says George Mylonakis, an engineer on the study.

The results are due to be formally presented at the European Conference in Earthquake Engineering in Greece next month.

Source: University of Bristol

3 comments
Jeff Michelson
Interesting that the quality of some contractors hasn't seemed to improve in nearly 1000 years. My guess is the commissioners of this structure took the lowest bid and didn't check reviews on Yelp.
yawood
Well, there you go, I had never heard of Yelp before until I saw your comment!
owlbeyou
I visited the tower in '86, and at the time, people were allowed to go up to the top if they wished. The inside core of the structure, which is a massive cylinder of masonry with a spiral staircase of stone was an unsettling experience because the steps leaned toward the wall and away from it on every floor as you went up. At the top it was even weirder when you were able to look straight down and sense the "lean" dramatically. From the outside, the tower with all its dozens of columns belies its solidity and feels like it's about to come crashing down to a rubble, but miraculously it's still standing. Of course, much has been done since then to stabilize the structure with extensive counterweights, etc. It's been years that the visiting public can no longer go inside it.