Italy, in addition to having a huge number of classical, middle-ages, and renaissance buildings, has a fair number of earthquakes. Engineers, like everyone else, study the problems at hand, so a lot of Italian engineers study the effect of earthquakes on unreinforced masonry just as a lot of New York engineers study the dynamics of tall, slender steel-frame buildings. Every so often, the study leads somewhere interesting. In this case, it’s an analysis of how the leaning tower in Pisa, which was getting perilously near tipping over under gravity load before its stabilization, has been able to survive earthquakes.
This article in Phys.org – 500-year-old Leaning Tower of Pisa mystery unveiled by engineers – summarizes the study. The soft, weak soil at the tower keeps the tower from reaching resonance with the ground motion. In other words, the soil acts as a shock absorber, so that the tower’s motion during a quake doesn’t sync up with the ground’s motion. This protection provided by the soil is ironic given that the soft, weak soil is the reason that the tower is tilted in the first place.
This shock-absorber action sounds great, doesn’t it? Like something we might be able to use to protect buildings, right? Actually, it already is used, in the form of base isolation. That’s a fancy term for providing shock absorbers at the base of a building to prevent the building from reaching resonance with the ground motion. And while this technique has advanced considerably in recent years, the idea has been around for a while.
Interestingly, base isolation is not really used in New York because it requires some clear space around the building so that the building can move relative to the soil. If there’s one thing not readily available in the city, it’s clear space.
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