Mad About Science: The Tower of Pisa

By Brenden Bobby
Reader Columnist

The Tower of Pisa is one of the most unique and inspiring structures in the world. Magnificent in both style and size, the world famous tower also leans with a curious and noticeable tilt. Surely, this thing should have fallen by now, right?

It should have, and that’s why it’s so special.

The Tower of Pisa is part of the Pisa Cathedral complex, which is centered on the Pisa Cathedral and also contains the Pisa Baptistery. The tower’s purpose is to house seven bells, with each designed to ring a specific note on the musical scale. If you think it’s curious to have a completely freestanding bell tower separate from the cathedral, you’d be right. The medieval republics of Italy were equal parts pious and vain, and they were eager to build a large and showy structure as a display of power and wealth. The cathedral complex was notably built outside of PIsa’s city walls in open defiance of the Republic of Florence of the time, showing that Pisa did not fear attack from its neighbors.

Courtesy image.

There are many stories about how the tower came to lean. Some cite an earthquake, others decry a design flaw, while some say it was intentional. In actuality, the foundation of the tower is extremely heavy and the ground upon which it was built is mostly soil and clay, so the immense weight of the tower’s foundation actually caused it to partially sink and lean very early into its construction. While building began in 1172 C.E., it’s believed to have begun leaning around 1178.

Construction of the tower would occur over nearly two centuries and generations of architects, masons and bronze workers. Due to the nature of the leaning foundation, the architects and stonemasons had to adapt the plans and compensate for the structure’s tilt by cutting taller stones and adding additional layers to one side over the other, which caused the top of the tower to curve slightly — though this is virtually imperceptible to the human eye.

As you likely suspect, the tower would not be able to lean so dramatically forever without adverse effect. Throughout the centuries, the denizens of Pisa have worked hard to save the tower in numerous ways, such as excavating dirt from below the taller side to correct the lean, to a more recent solution involving thousands of tons of lead blocks. 

While it may be possible to fully correct the tower’s lean today, the distinct structure has proven to be a major tourist attraction for almost a thousand years precisely because of this unusual quirk, and therefore the challenge has evolved. No longer is it sufficient to keep the tower from falling down, but the tower must be kept leaning without threat of collapse.

Another problem compounds the difficulty of keeping the tower partially upright. As mentioned above, the tower was built taller on one side than the other to compensate for the lean decades before masons built the upper reaches. As the tower’s foundation slipped into the clay substrate, it also began to rotate. This became very problematic as voids were created (accidentally and intentionally) around the base that allowed water to seep in around the foundation. Luckily, water also helps with soil compaction, and may have done some good for the tower in this case.

The most recent efforts to save the tower involved removing 70 metric tons of dirt from beneath the structure, cinching it with cables and adding some stabilizing elements to keep it from toppling over. The most recent estimates suggest that the tower should be stable for at least another three centuries.

At this point, you might be thinking to yourself: “How has the tower survived so long in an area of the world that’s famous for its earthquakes?”

Interestingly enough, a wild interaction happens between the tower and the soil during an earthquake. The soil, which is quite loose, acts in opposition to the rigidity of the tower while buffering it from much of the energy transfer of an earthquake. Stick a toothpick into a bowl of Jello-O and shake it up a little bit if you’d like a visual, or just need an excuse to eat some Jell-O. The soil that caused the tower to lean to begin with is the very thing preserving it from one of the most destructive forces on Earth.

The tower faced two of its greatest challenges around World War II. Italian dictator Benito Mussolini despised the tower and believed its trademark lean to be an embarrassment to Italy. In an attempt to correct it and straighten the tower, he had workers drill into the foundation and pour nearly 80 tons of grout, with the intent of turning its foundation into a ballast. This backfired in a colossal way, and may have caused the tower to lean to a near-catastrophic 5% tilt. 

Near the end of the war, German soldiers were reportedly using the tower as a lookout and sniper’s roost, which put the tower in the crosshairs of Allied forces. Luckily for the world, the tower and surrounding structures were spared explosive destruction, and remain standing for people to enjoy to this day.

Are you looking for an opportunity to exercise some small-scale engineering principles? The library is currently hiring an exploration assistant to help me host programs for community members of all ages with the intention of experiential learning — learning new skills by doing them. This ranges from challenging youths to build towers with limited supplies and unexpected challenges, to assisting in 3-D design classes for adults to hauling the library’s mobile makerspace trailer to events, and even running our virtual reality experiences and working with me to maintain the library’s 3-D printers.

This is a paid position, so if you’d like more information or would like to apply, please check out the library’s employment portal at ebonnerlibrary.org/employment. This job will be closing on Friday, June 23, so if you’re interested, check it out fast!

Stay curious, 7B.