DURING the thirty seconds the earth shook in General Santos City, facades broke and whole structures collapsed. However, some of the older buildings, built with deep foundations, stood the shaking but were still considered unsafe, and skyscrapers in Davao were literally swaying and flexing, triggering public concern but were still allowed to be occupied. To many observers, visible movement appears to be evidence of impending collapse.
In a chat with infrastructure experts of Bentley Systems, a building infrastructure management software and digital twin firm, said that modern earthquake engineering is based on a principle not simply of rigidity but also of flexibility.
The common misconception is that the safest building is the strongest and most rigid. Structural engineers know the opposite can be true. A completely rigid structure has little ability to absorb and dissipate the enormous forces generated when the ground beneath it suddenly shifts.
Earthquakes do not directly push against buildings. Instead, they accelerate the ground, which in turn transfers energy into a structure through its foundation. The challenge for engineers is not to prevent all movement but to manage it.
If a building were designed to remain perfectly still during a major earthquake, the forces generated by ground motion would concentrate in critical structural components. Without a mechanism to absorb that energy, columns, beams and joints could fail suddenly and catastrophically.
One of the most important concepts in earthquake engineering is ductility — the ability of a material to deform without breaking. Structural steel and properly reinforced concrete can bend, stretch and absorb energy while remaining intact. This is a hallmark of modern seismic design and relies on controlled flexibility.
Building codes around the world, including the National Structural Code of the Philippines, incorporate this principle by requiring structures to withstand significant deformation while maintaining their ability to support occupants and avoid collapse. The requirement to stand a magnitude 8 earthquake demands different application of this ductility based on the land a building stands on and the height of the structure itself.
To achieve this, engineers design buildings with systems that can absorb and redistribute seismic forces. Moment-resisting frames, for example, allow beams and columns to flex at their connections. Rather than resisting every force, the structure is permitted to sway, reducing stress concentrations and preventing brittle failure.
Some buildings employ base isolation systems that physically separate the structure from the ground using specialized bearings. During an earthquake, these devices reduce the amount of energy transmitted into the upper floors, limiting both structural damage and occupant discomfort.
Engineers may also deliberately designate certain areas of a building to absorb damage first. These zones function much like sacrificial components, dissipating energy through controlled deformation while protecting critical load-bearing elements.
Another important consideration is resonance. Every structure has a natural period of vibration determined by its height, mass and stiffness. Tall buildings generally sway more slowly than shorter structures. Problems arise when the frequency of seismic waves closely matches a building's natural frequency.
When this occurs, a phenomenon known as resonance can amplify movement, much like pushing a playground swing at precisely the right moment. The resulting increase in motion can place significant demands on the structure.
To counter this effect, engineers use advanced computer modeling and, in some cases, install tuned mass dampers. These large counterweights move in opposition to a building's motion, helping reduce excessive sway and improve stability during earthquakes and strong winds.
This is why post-earthquake images can sometimes be misleading. Cracked finishes, broken glass and localized structural damage may appear alarming, but they do not necessarily indicate failure. In many cases, they are evidence that the building absorbed and dissipated seismic energy as intended.
The ultimate objective of earthquake engineering is not to eliminate movement. It is to prevent collapse and protect human life. Modern buildings are designed to yield to nature's forces rather than fight them completely.
