Why Doesn't the Leaning Tower of Pisa Fall Despite Its Extreme Lean?
The Leaning Tower of Pisa has long captivated global attention. This historic Italian structure appears to be on the verge of collapse, yet it has remained standing for centuries.
The approximately 56-metre tower was never built perfectly upright. Despite its pronounced lean, the structure has endured to this day.
This phenomenon is influenced by several factors, including the position of the centre of gravity, the soil characteristics beneath the tower, and construction techniques adjusted since the medieval period.
According to the Institution of Civil Engineers (ICE) and CDM Smith, the Leaning Tower of Pisa was originally designed as a freestanding bell tower for the Pisa Cathedral complex.
Problems emerged shortly after construction began in 1173. Around 12 years later, the foundation began to sink due to soft, muddy soil.
The weight of the first three floors caused the underlying clay layer to deform, leading the tower to lean to one side.
One key reason the tower remains standing is that its centre of gravity still lies within the foundation area.
In physics terms, a structure remains stable as long as the line of gravitational force falls within the base. If this point moves outside the foundation, the structure loses balance and risks collapse.
For the Leaning Tower of Pisa, despite its lean, the centre of gravity remains within safe limits, allowing gravity to maintain stability.
Additionally, the soil beneath the tower actually helps preserve the structure.
Soft soil is typically considered risky for tall constructions. However, in this case, the soil layer absorbs vibrations, including seismic shocks.
This phenomenon is known as Dynamic Soil-Structure Interaction (DSSI), the interaction between a structure and surrounding soil characteristics.
The tower’s relatively rigid structure combined with soil capable of absorbing vibrational energy means seismic impacts are not fully transmitted to the building.
Construction of the Leaning Tower of Pisa spanned nearly two centuries, beginning in the 12th century.