Leaning Tower of Pisa
Surviving an Earthquake
At least four strong earthquakes hit the region since 1280, but the apparently vulnerable Tower survived. The reason was not understood until a research group of 16 engineers investigated. The researchers concluded that the Tower was able to withstand the tremors because of dynamic soil-structure interaction (DSSI): the height and stiffness of the Tower, together with the softness of the foundation soil, influence the vibrational characteristics of the structure in such a way that the Tower does not resonate with earthquake ground motion. The same soft soil that caused the leaning and brought the Tower to the verge of collapse helped it survive.