The Leaning Tower of Pisa (the world’s most famous construction mistake)

The Leaning Tower of Pisa is the world’s most famous construction mistake. The architect, Bonanno Pisano,
found his bell tower starting to lean almost immediately after he began to build it. And the lean kept increasing through the centuries, a tourist bonanza for Pisa, that would end if the tower leaned too far and fell.
This picture was taken in 1994, prior to a major engineering effort to stabilize the tower and reduce the lean by 10%. The tower was also closed to visitors. I had climbed it back in 1986, great fun, but disorienting.

Why the lean?

The tower is built on unsuitable ground for such a heavy and tall building. It is only about 6 feet above sea level and built on a riverbed. The underlying ground is made up of layers of sand and clay. The layers are not even and the weight of the building has compressed them. Because the layers are not even, as the ground has compressed, it has sunk more in some places than others.

Only 5 years after work on the building began, it was leaning noticeably to the North. The lean was first noticed during construction of the third floor. During the building of the next three floors, the lean was corrected by building the floors parallel to the ground, and not level with the leaning building. During this phase the tower started to lean the other way. Now it was leaning to the South.

Several plans have been tried over the years to stop the tower from falling. Some of them have been almost disastrous. In 1934 an Italian engineer drilled 361 holes into the base and filled them with mortar. The tower promptly leaned over some more. In 1993 650 tons of lead were hung from the North side of the building to try and stop the lean increasing. For a while it worked.

In 1995, they decided to try and increase the foundations under the South side of the building. They froze the ground using liquid nitrogen, to stop it moving, and then started to remove stones, so they could insert metal rods. What they didn’t know was that the stones they were removing were part of the original foundation of the building. That is the nearest the tower has come to disaster. In one night the lean increased as much as it normally increases in two years. They quickly added another 250 tons of lead and decided to rethink the whole thing.

At this point everyone was just about ready to give up. Then a British engineering professor came up with yet another idea. His plan was to remove ground from under the high side, instead of trying to add ground under the low side. In 1999 work began, and was done very slowly, so that the building wouldn’t get a sudden shock. At the beginning of June 2001, the work was complete, and the tower had been straightened up by about 16 inches, which returns it to the position it held in 1838. The engineers believe that it is safe for at least another 300 years.
So, if they know how, why didn’t they just straighten it up all the way? The answer lies in the tower’s name. It is the Leaning Tower of Pisa and just wouldn’t be the same if it didn’t lean! Some of the residents of Pisa say it would be better to let it fall down, rather than to straighten it all the way.

TOWER BASICS



Official Name: TORRE PENDENTE DI PISA
Function: Bell Tower (Campanile)
Original Architect: Bonanno Pisano
Architect who realized that the Leaning Tower could not be straightened:
Tomasso di Andrea da Pontedera (1275)
Years Built: 1173-1350



Latitude: 43.7167 (43° 43′ 0″ N)
Longitude: 10.3833 (10° 22′ 60″ E)
Elevation of Piazza dei Miracoli: About 6 feet, (2 meters) (DMS)
First Construction Stop: 1178 (War with Firenze)
Year in which lean became obvious: 1178 (Third Story)
Height at which lean became obvious: 10.6 meters (35 ft.)
Level at which Tower Straightens to North: 5 (About 110 meters)
Second Construction Stop: 1185 (War with Firenze)
Later Construction Stop: 1284 (War with Genoa, Major Sea Battle Defeat)
Height: 55.863 meters (185 feet). 8 stories.
Outer Diameter of Base: 15.484 meters
Inner Diameter of Base: 7.368 meters
Weight: 14,700 metric tons
Thickness of Walls at the Base: 8 feet
Direction of Lean: 1173-1250 North, 1272-1997 South

7th Floor Completed: 1319
Bell Tower Completed: 1350
First Bells added: 1198 (Third Floor)
Total Number of Bells: 7, tuned to musical scale
Largest Bell: L’Assunta (The Assumption). Three and a half tons, cast in 1655.
Oldest Bell: Pasquarreccia.
Address: Campo dei Miracoli – the “Field of Miracles”, Pisa, Italia
Year cement injected into base, (blamed for lean acceleration): 1934
Steps to Bell Tower: 294
Number of visitors who climbed to top in 1989: 700,000
Date Closed to Public: 7 January 1990
Date Re-opened to Public: 15 December 2001
Weight of Lead added on North side (picture above): 600 tons (1995)
Amount of tilt recorded overnight in September 1995: 2.5 mm (0.07″)
Weight of Lead added after overnight tilt in September 1995: 230 tons
Rate of Fall in 1990: 1.2 mm (1/20″) every year (“Un millimetre per anno”)
Source: The Guardian (London) August 19 1997
Amount of tilt correction from 1990 – 1999: 25 mm (about 1.0″)
Amount of tilt correction from 1999-2001: 43.8 cm (about 17.25″)
Date that Tower was last at current tilt: 1700