How does the Leaning Tower of Pisa survive earthquakes

Despite the hazardous inclination of approximately five degrees, the Leaning Tower of Pisa (in Italy) has managed to survive four strong earthquakes since 1200 A.D.

The construction of the Tower began in 1173 and was completed in 1372. Inclination to the north side was detected from the beginning of the project. However, by the next 100 years the building had shifted to the south. The differential subsidence of the ground lead to an offset of 5 meters at the top of the -58 meters height- tower.  

The leaning of Pisa tower is a result deriving from the foundation soil. Clay and sand formations alternate down to almost 70 meters while the foundation depth is less than 3 meters. Nevertheless, the most significant hazard for the tower is the earthquakes that occur systematically in the region. Considering the sensitivity of the construction, a seismic event was expected to cause serious damage to the building. Until today, the response of the tower has proved the expectations wrong.

To investigate this phenomenon, a team of 16 engineers from the UK and Italy was founded. After expended research, the scientists concluded that the survival of the building is attributed to the beneficial interaction of the construction and the soil beneath. The stiffness of the tower combined with the softness of the foundation ground causes the characteristics of a seismic vibration to be mitigated. This effect of the Dynamic Soil-Structure Interaction (DSSI) functions in such a way that the tower does not resonate with the ground motion and therefore the forces acting on the structural elements of the construction are diminished.

Professor Mylonakis, Chair in Geotechnics and Soil-Structure Interaction, and Head of Earthquake and Geotechnical Engineering Research Group in the Department of Civil Engineering at the University of Bristol, states: "Ironically, the very same soil that caused the leaning instability and brought the Tower to the verge of collapse, can be credited for helping it survive these seismic events."

Furthermore, results from the study will be formally announced at the 16th European Conference in Earthquake Engineering in Thessaloniki, Greece next month (18 to 21 June).