Thursday, August 14, 2008

Pisa Cathedral: The Campanile (Leaning Tower)

The city of Pisa stands on the River Arno in the Tuscan region of northern Italy. Its Piazza dei Miracoli is graced by the most beautiful group of Romanesque buildings in the country: the white marble basilican cathedral (begun 1063); the circular, domed baptistery (begun 1153); and the highly original bell tower (campanile), situated between the apse and the southeastern end of the cathedral’s transept and now famous as the Leaning Tower of Pisa. Quite apart from its innovative cylindrical form, the location is remarkable, because bell towers usually stood near the west front of churches. Of course, the tower was designed to be vertical, but it started to lean very early in its construction. Since 1183—only a decade after it was started—repeated and inventive attempts have been made to correct the incline. They continued for 800 years, until the very end of the twentieth century, when modern technology seemed to halt the incremental incline—by then it had reached 10 percent—and save the life of the tower. Taken together, those remedial actions represent a considerable architectural and engineering feat.
The authorship of the tower remains uncertain. Tradition identifies Bonanno Pisano as the architect, but later scholars name Diotisalvi, Biduino, or Guidolotto. Other sources suggest the German Guglielmo of Innsbruck. The Pisa campanile, essentially a hollow cylinder of just over 50 feet (15.5 meters) outside diameter at the base, is 180 feet (55 meters) high. The structure consists of an external wall faced with gray and white San Giuliano limestone ashlar; between it and an inner wall of dressed limestone, 293 steps wind to the top. The continuous facade is divided into six by elegant arcades and crowned (of course) by a cylindrical belfry, a little smaller in diameter than the tower. Throughout, the wall surfaces are inlaid with patterned, colored marble in the Tuscan Romanesque manner, reducing the large building to an appreciable human scale and creating a harmonious unity with the cathedral and the baptistery. There are earlier cylindrical bell towers elsewhere in Tuscany and Umbria, and even in Ravenna, but Pisan historians claim that the tower of Pisa is unique and locally inspired.

Building work commenced on 9 August 1173 under the auspices of the Opera Campanilis Petrarum Sancte Marie (Stonework of St. Mary’s Bell Tower). The foundations were set only 10 feet (3.36 meters) deep, on a bed of dry stones. There is no question that poor foundation soil—clay and sand strata interlayered with pockets of underground water—could not support the highly concentrated loads imposed by the tower; this, together with differential sinking of the soils, was the major contributor to the failure of the tower. The problem first appeared within about ten years, probably when the fourth arcade was reached. The building had sunk by more than 1 foot (30 centimeters), causing a lean of about 2 inches (5 centimeters). It was not unique in that: similar problems would be experienced in Holland, for similar reasons, Church towers still standing at Zierikzee and Leeuwarden have substantial leans; both were abandoned, and the architect of the latter committed suicide.

It seems that the Italian builders were more resolute than their Dutch counterparts, and after some delay, work on the tower of Pisa resumed, probably in the first decades of the thirteenth century. Remedial action was taken in two stages as attempts were made to reduce the lean. It is not known how high the building was when in the early 1270s Giovanni di Simone began to correct the inclination by raising one side of the galleries. By 1284, the six gallery levels were completed: then 156 feet (48 meters) high, the tower inclined about 3 feet (90 centimeters) from the vertical. The work was again suspended but the main part of the building was finished by 1319. The belfry, designed by Tommaso Pisano, was in place by 1350.

Major works undertaken in 1838 to save the tower of Pisa changed the proportion of groundwater and resulted in accelerating the inclination; it was only after some time that it settled to become about a millimeter a year. For over a century, eccentric suggestions were made to correct the problem, including a proposal to landscape the surrounding area to slope so that the tower would appear to be vertical. Moved to action by the 1989 collapse of the campanile of Pavia Cathedral, the Consorzio Progetto Torre di Pisa (Tower of Pisa Project Consortium) commissioned engineers to stabilize the Leaning Tower, then inclining more than 15 feet (4.6 meters) from true. It was closed to the public in 1990, and rescue work began. After some unsuccessful experiments, the three-part “final” solution was reached in July 1998. The tower was restrained by steel cables while 990 tons (900 tonnes) of lead were stacked against its base away from the direction of lean. Then excess water and mud were pumped from under the tower, allowing it to settle and in effect correct itself. As the tower straightened, the lead counterweights were regularly reduced. The project, with an overall cost of 54 billion lire (U.S.$27 million), was completed by the end of 2000. The result was that the lean was corrected by about 1 foot (30 centimeters), bringing the tower of Pisa to the condition it was in about three centuries earlier.

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