The Leaning Tower of Pisa: Flawed Beauty

The leaning tower and its associated cathedral. (Copyright Lee Krystek, 2018)

In 1990 an international team of engineers, mathematicians, and historians met on the Azores Islands in the middle of the Atlantic Ocean. Their task was to figure out how to save an 800-year-old historic building that was close to collapsing. The structure was then tilted by 5.5 degrees to one side. If something wasn't done soon, the world famous Leaning Tower of Pisa would come crashing to the ground.

The structure was originally meant to simply be a free standing bell tower (or campanile) for the city of Pisa's cathedral. Such towers were commonly built in 10th century Italy to symbolize the town's power and economic wealth. Historians aren't sure who the structure's original architect might be. For many years it was thought to be Bonanno Pisano, a Pisa resident and artisan noted for creating a number of ornate bronze doors for cathedrals. However, scholars now suspect that Diotisalvi, who was the architect of Pisa's San Nicola bell tower and the city's baptistery, might actually be responsible.

Seven Quick Facts
Height: 183 feet, 3 inches (55.8m)
Angle of tilt: 3.97 degrees - 12 feet, 10 inches (4m) off vertical
Steps to top: 296
Construction: Started 1173, finished 1372
Function: Bell Tower for the Cathedral at Pisa, Italy
Built of: Masonry and Marble
Other: Contains 7 bells tuned to the major musical scale

In either case we know that construction on the tower started in 1173 AD near the city's cathedral on a piece of land known as Piazza dei Miracoli ("Square of Miracles"). The tower was round and 52 feet (16m) in diameter at the base with a projected height of 191 feet (58m). The first level was to be taller than the rest and have an elaborate entry portal decorated with sculptures of monsters and animals. The second story would have open marble arcades with many columns. On top of that there would be five more levels with facades similar to the second story. Finally, the roof would be capped with a bell chamber designed to contain 7 bells.

Imperfect from the Beginning

By the time the third floor was finished in 1178, it was clear that the tower was leaning slightly to the northwest. The problem was that the foundation was only 10 feet (3m) deep and the soil underneath was soft and unstable. The citizens of Pisa decided to stop construction for a while and let the tower settle a bit with the hope that it would straighten itself out, or at least stop moving. In any case, several wars with neighboring towns required their attention.

The tower now leans 3.97 degrees off of true.(Photo by Softeis. Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license)

It was almost a hundred years before they got back to doing more work on it. In 1272, under the direction of Giovanni di Simone, construction resumed. To try and compensate for the tilt, he designed the upper stories so that they had one side slightly taller than the other. By the time the 7th floor was completed, it became apparent that the building was moving again and the lean increasing, this time to the south. In 1284 work was halted again due to another war. Finally, the bell tower was finished in 1372. Over the next few centuries the 7 bells would be installed in the bell chamber, one for each note of the musical major scale.

The tower continued to move and by the 16th century it was a full 3 degrees off vertical. Pisa's campanile, however, wasn't the only European tower that was crooked. Over the years many of these collapsed or were replaced with newer structures. Pisa's tilted bell tower, however, survived and eventually became a major landmark not despite its lean, but because of it.

In 1911 engineers began careful measurements of the tower's angle and realized that it was still moving at the rate of a 1/20 of an inch per year. In 1934 engineers working for the dictator Benito Mussolini - who found the tilt an offense to his Fascist ideals - attempted to correct the problem by injecting 200 tons of concrete under the foundation. This, however, just made the problem worse. By 1989 the tower had reached a tilt of 5.5 degrees and its top was out of plumb by 17 feet (5m). Its predicament was underscored when a similar bell tower in the town of Pavia unexpectedly collapsed. Officials decided to close the Pisa's tower to visitors, evacuate the area under it and put together an international team to figure out how to save it.

Fixing the Tower

One of the seven bells housed in the tower. (Copyright Lee Krystek, 2018))

Team member John Burland, a soil mechanics specialist from Imperial College London, wondered if removing soil from under the tower's northern foundation might help correct the tilt. After running a number of computer simulations, the team decided that such a strategy was their best bet. To keep the building from coming apart while they worked on it, a number of temporary measures were put in place.

First, steel bands were wrapped around the first floor to support the masonry there. Then, 827 tons of lead weights were placed on the northern side of the building to act as a counter-balance. A concrete ring was also placed around the building at the third level connected to heavy steel cables anchored in the ground to keep the structure from any further southern movement. Finally the massive bells were removed to decrease the total weight of the building.

With these measures in place, the team drilled a hole at an angle under the northern side of the tower to remove some earth. Over a number of years they repeated this procedure while measuring what it did to the tilt of the tower. A total of 41 holes were made removing 77 tons of soil causing the tower to move backwards toward a more stable position. By 2001 the tower had moved back to the same lean it had in the 1930's and officials decided it was safe to reopen the structure to visitors.

The intention was never to straighten the tower completely. It still leans 3.97 degrees out of true with the top being 12 feet, 10 inches (4m) from vertical. The cables and other temporary measures have been removed. Engineers believe they have stabilized the tilt so that the tower should be safe for at least several hundred years, barring a major incident like an earthquake.

Looking up at the tower. (Copyright Lee Krystek, 2018)

That's a relief to the citizens of Pisa for whom the well-loved, but off-kilter tower is a world recognized icon for their town and a source of tourist dollars. The structure isn't only known for the great beauty of its Romanesque architecture but also for its historical significance. Galileo Galilei, the astronomer, is said to have dropped a cannon ball and a musket ball from the tower to prove that weight does not affect how fast an object falls. While his conjecture indeed proved true, historians have called into question whether he actually did his test from the top of the tower at Pisa. Even so, the tower remains strongly associated in people's minds with the great scientist.

Why Does it Stand At All?

One of the mysteries of the tower is not just why it leans, but why it continues to stand at all. The area around Pisa is subject to earthquakes and the tower has lived through at least four major quakes during its life, yet even as off-kilter as it is, it still stands. In 2018 a study led by Roma Tre University's Professor Camillo Nuti suggests that the reason it leans is the exact same reason it still stands.

"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," noted Professor George Mylonakis, from the University of Bristol, UK, who was a member of the team that worked on the study. According to the report the softness of the soil, combined with the stiffness of the tower, prevents it from resonating, or vibrating along, with the earthquake's ground motion and protects the structure from damage.

In 1987 the tower, along with the associated cathedral, baptistery and cemetery, was declared a UNESCO World Heritage Site. The tower has also been called one of the Seven Wonders of the Medieval World. Ironically, while the preservation efforts have saved the tower, they have also caused it to lose the title of the most crooked structure in the world, This distinction now goes to the leaning tower of Suurhusen, in Germany, which is 5.3 degrees out of true.

The baptistery, cathedral and bell tower.(Photo by WeEnterWinter. Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license)

Copyright Lee Krystek 2012-2018. All Rights Reserved.