Pisa's famous leaning tower is more than just the world's most famous architectural mistake. In fact, if one can ignore the lean of the tower and look at the design itself in conjunction with the basilica and baptistry, the group is one of the most harmonious and beautiful church campuses in the world.
Why does the Leaning Tower of Pisa lean?
The simple answer is that the soil simply could not support the weight of the tall thin structure. Modern architects and engineers rely on extensive soil testing to help them design foundations to support buildings. When the tower was built, in 1174 A.D., soils engineering was not quite as advanced as it is today.
Builders relied primarily on past experience and instinct to design the foundations of their buildings. In the case of the Pisa bell tower, the clay soil behaved differently than expected. Instead of settling straight down, the soil started to slip to the side. The weight of the tower began to squeeze water from the soil, and the soil failed in a shear plane. It became as if the entire tower was sitting on a foundation that was shaped like the bottom half of an egg.
In defense of Bonanno Pisano, the original architect of the tower, the clay layer of soil that failed lies about 10 meters below a layer of sandy soil, and there was no other building at the time that would have required similar foundations. With the technology and experience available at the time construction began, no one could have predicted the failure.
Construction sequence of the tower
The soil failure caused the construction of the tower to halt twice. The tower sat at three and a half stories for nearly one hundred years until work started again in 1272. during this phase of the construction, the tower began to lean to the north, then to the east.
At completion of the sixth story, the tower again showed an alarming tilt, this time to the south.
Construction was stopped again in 1278, at the completion of the seventh story. Again, almost 100 years passed until the construciton of the project resumed. In 1370, the eighth and final story was added. The fifth, seventh, and eighth stories of the bell tower are built wedge shaped, as an attempt to compensate for the lean. This gives the tower a slightly banana shaped form.
Stabalization attempts
In the modern era, there have been several attempts to stabilize the tower and halt the continuous leaning of the tower. The tower continued to lean at a rate of about a milimeter per year throughout its existence. Several attempts were made to stop the leaning.
These efforts range from pumping concrete under the existing foundations, adding lead counter weights to the high side of the foundation, anchoring the tower with cables, and removing soil from under the high side of the building. These efforts have managed to stop the leaning, and have actually reversed the lean by several centimeters.
Design of the Tower
The architectural design of the tower is remarkably consistent, considering the nearly 200 years of accumulated construction stoppage during the project. The composition essentially takes the facade of the nearby cathedral and wraps it around the cylindrical shape of the campanile. The open arcade gives the tower a light and airy feel as it soars up above the ground level.
Together with the cathedral and baptistry, the tower helps to complete one of the most beautiful and harmonious campuses in the world. The baptistry also has a slight lean, but because it is a wider structure, it is much less noticeable than the campanile.
Sources:
Robert Mark, Editor. Architectural Technology up to the Scientific Revolution. The MIT Press. 1993
Trachenberg, Marvin and Hyman, Isabele. Architecture from prehistory to Post-Modernism. Prentice Hall. 1986.
Join the Conversation