Leaning Tower of Pisa: Flawed Beauty
leaning tower and its associated cathedral. (Photo
by Johann H. ADdicks. Licensed under the Creative Commons
Attribution-Share Alike 3.0 Unported license)
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.
183 feet, 3 inches (55.8m)
of tilt: 3.97 degrees - 12 feet, 10 inches (4m) off
to top: 296
Started 1173, finished 1372
Bell Tower for the Cathedral at Pisa, Italy
of: Masonry and Marble
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.
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.
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.
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.
of the seven bells housed in the tower. (Photo
by LoneWolf1976 Licensed under the Creative Commons Attribution-Share
Alike 3.0 Unported license)
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.
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
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.
baptistery, cathedral and bell tower.(Photo
by WeEnterWinter. Licensed under the Creative Commons
Attribution-Share Alike 3.0 Unported license)
Lee Krystek 2012. All Rights Reserved.