Soft-storey failure in a Philippines earthquake

From this video of the Philippines earthquake, it appears that the ground floor had been set back to create additional space for the entrance and retail frontage, while the upper floors extended over it.

This configuration created a classic vertical structural irregularity. More importantly, it may have introduced what engineers commonly refer to as a “soft-storey” mechanism.

In many commercial buildings across Asia, the ground floor is often opened up to maximise retail exposure, customer access and leasable space. To achieve this, columns, walls, bracing, and any other structural elements that would otherwise provide lateral stiffness are reduced, relocated or removed altogether.

As a result, the upper floors remain relatively stiff, while the ground floor becomes significantly weaker and more flexible under lateral loads.

In structural terms, the upper floors effectively transfer loads through a transition zone, while the ground floor becomes the soft storey.

During a major earthquake, these soft-storey levels are often subjected to excessive inter-storey drift and concentrated structural demand. Once the critical columns, walls or transfer elements fail, progressive collapse can occur rapidly.

Based on the footage, the front cantilevered portion appears to have been one of the first areas to fail, which may have initiated the collapse sequence of the entire structure.

This is precisely why I always pay close attention to transfer structures, cantilevered portions and potential soft-storey conditions when assessing existing buildings.

Many structural failures do not start at the roof.

They start at the weakest level.