1173
Pisa, Italy
Can there be anyone in the world who doesn’t recognize what is probably the most famous engineering disaster of all time? Its image has appeared in movies, books, and magazines so often that it almost seems a little unreal—like a unicorn or a dragon. And when people see the Leaning Tower of Pisa in person, they’re often tempted to stick around a while, just to be there when it finally does fall over.
But it doesn’t—and it hasn’t for more than 800 years. Other mistakes bite the dust within months of appearing on the world scene. The Leaning Tower of Pisa, on the other hand, just leans and leans and leans, remaining a favorite for tourists and engineers alike. Is it defying gravity? Do normal rules of science not apply? Why did it stop where it did and not tilt farther? Is it made of marshmallows or something? This very familiar building turns out to be quite a mystery after all. . . .
What Went Wrong?
The city of Pisa lies on Italy’s west coast, at the mouth of the River Arno. By the 12th century, Pisa had become a powerful trading, artistic, and military center. In that period, just a couple of centuries before Italy gave birth to the Renaissance, notable cities like Pisa, Florence, Venice, and others demonstrated their importance by constructing marvelous religious buildings.
In the mid-1100s, the Pisans set about building a collection of those buildings, all to be finished in brilliant white marble. The largest was the cathedral, flanked by the baptistery and the campanile (bell tower). Work on the bell tower, which was to become the most famous of the three, began in 1173. Like the other buildings, it marked the transition from the heavier Romanesque style of architecture—full of curved arches—to the lighter Gothic style, with slender columns rising high toward the heavens.
Renaissance
A period in the late Middle Ages when Europeans rediscovered some of the artistic traditions of the ancient Greeks and Romans and began applying them to their own art, architecture, and culture.
The only problem was that instead of soaring heavenward, the tower began sinking earthward. And it didn’t even sink straight. First it tilted one way, and builders tried to mask the problem by building unevenly (to make it look straighter). Then it began tilting the other way, and they tried the same trick on the other side. Nothing seemed to be going right!
By the 14th century, it was clear that Pisa’s bell tower had a major problem. It was already known as the Torre Pendente (Leaning Tower). Nevertheless, work continued on the tower until it reached its full 8-story height in 1370. It was complete, but still had its tilt of about 3 degrees off vertical.
That leaning tower eventually became world-famous—it is as recognizable as the Empire State Building or the Taj Mahal, and is the subject of thousands of joke photos each year (with people pretending to push it back). But would this beautiful example of Italian architecture be half as famous if it had turned out straight?
Going, Going, Gone!
The inhabitants of Pavia, about 125 miles north of Pisa, could have told the Pisans a thing or two about falling towers. On the morning of March 18, 1989, bricks started falling from the town’s 236-foot-high Civic Tower. Within minutes, the tower simply collapsed, killing four people and injuring 15. The exact cause of the collapse is still a mystery—the tower had stood since 1060 and never even tilted.
Two big factors almost certainly account for the famous tilt in Pisa’s bell tower: unstable soil and inadequate foundations beneath the tower. Builders probably brushed aside concerns about either of these, because two large religious buildings already stood on the level ground where work began on the tower in 1173. The cathedral and baptistery are both massive structures, and their tiny tilting can’t be noticed with the naked eye.
So it must have come as a shock, soon after construction began, to see the tower beginning to lean to the north when the builders had reached only the third of the planned eight stories. Despite being interrupted by numerous wars, workers
undertook the 840-year process to save the tower. At first they tried optical illusions: They lengthened the columns on the north side of the third floor to make the tower look more level. By 1272, the tower was leaning to the south (the direction it still points), so builders began making the south columns longer on upper floors.
Dangerous Distractions?
Italy in the Middle Ages wasn’t a single, united country as it is now. Instead, it was a patchwork of city-states and small kingdoms that always seemed to be fighting each other. Pisa was at war off and on for centuries, and the tower’s construction was halted and resumed several times as a result. Could that type of distraction have played a part in the whole business? Well, think of the times you’ve heard your parents call, “How can you do your homework with that music on?!”
It wasn’t until the 20th century that engineers really got to work trying to stop the tilt—or at least protect the tower from toppling. Scientists in 1911 measured the movement of the top of the tower at 0.05 inch a year, and the rate was getting worse. After a similar tower in Pavia collapsed in 1989, the tower was closed to the public.
Scientists and engineers from around the world worked out a long-term plan, beginning in 1990, to remove soil carefully from the “high” end of the tower bit by bit. It was a complicated maneuver, but rested on the basic principle of center of mass. The tower was reopened in 2001, and engineers confidently predicted that it would be safe for the next 300 years. Time will tell. . . .
center of mass
An imaginary point either inside an object or near it where the mass of the object could be considered to be most concentrated.
Remember, the scientific term “center of mass” means the point at which the mass of an object could be said to be concentrated. What does that mean? Well, in an object that has the same mass throughout—like a hockey puck—it would be right in the middle. But in a hammer, which has unevenly distributed mass, the center of mass would be way over by the head of the hammer.
Why is all this important, especially for the Tower of Pisa? Simple. If something’s center of mass is within the base of the object (the part touching the ground), then the object will stay upright. If not, well, hold on tight, because it’s going to tip. This experiment shows how the center of mass moves as something tips. That was happening for centuries in Pisa. Watch what happens when it goes a little too far.
YOU WILL NEED
➤ Empty soda can
➤ Floor, table, or counter
➤ Water
METHOD
1. Hold the empty can upright and place it on the floor.
2. Tilt the can so it’s resting on just a small bit of its base and let go—it will tip over.
3. Try tilting it at different angles and letting go—same result.
4. Fill the can about one-third full with water.
5. Repeat Step 3—if you’re careful, the can will stay standing (and still tipped) when you remove your hand.
WHAT’S UP?
When you started out, the center of mass for the empty can was pretty much in the center of the can, but not above the small base of the tilted can. When you added water, the center of mass moved downward within the can (because of the extra mass of the water), adding stability. That new center was now above the base, so the can stayed up. So far, luckily, the Leaning Tower’s center of mass has remained above its base. Now you know to expect trouble if those repairs haven’t worked.
Why aren’t the world’s biggest, most massive (heavy) buildings leaning, too? One good reason is that they’re built on solid ground—really solid. New York’s lofty skyscrapers sit on tough granite that’s billions of years old. Many of Europe’s castles are built on hills, which tend to be made of the toughest rock that hasn’t been worn down the way the stuff all around it has.
Meanwhile, the Leaning Tower of Pisa has a problem. The builders only dug down about 6 feet for its foundation, and that subsoil wasn’t too solid—the big, heavy tower was built on soil that couldn’t support it. Engineers have a word to describe a building’s sinking into weak, uneven soil: subsidence. You can see its effects close-up in this experiment.
YOU WILL NEED
➤ 10 sheets of scrap paper
➤ Large dictionary (or telephone book)
➤ 10 other books (preferably hardbacks)
➤ 2 empty egg cartons (dozen-size)
TAKE CARE!
It’s best to do this one on a patio or other hard outdoor surface so you don’t break anything inside.
METHOD
1. Scrunch up enough scrap paper (into balls) to cover a 1-foot-square area on a hard floor (wood, tile, or concrete).
2. Starting with the dictionary, pile books on carefully, keeping the heaviest down low.
3. Continue until the books fall or the paper balls collapse, and count how many books you used.
4. Lay down the 2 empty egg cartons side by side on the paper balls and repeat Steps 2 and 3.
5. Repeat Steps 2 and 3 again, but lay the books down directly on the floor.
WHAT’S UP?
You’ve seen how your book “tower” stands up—literally—to different subsoils, or in this case paper, egg cartons, and a hard floor. If you can imagine Manhattan or the Scottish Highlands having a solid rock base like your floor, then you can see how tall buildings can be supported without swaying or tilting. Poor Pisa has a loose base that matches that scrunched-up paper. However, it’s not quite the same all over that Cathedral Square. Remember that the two other, even larger, buildings next door are secure. The bell tower builders just ran out of luck—or solid stone to build on. And that’s when the subsidence took over.
