WHEN designing a structure in earthquake-prone areas, the structural engineer’s main goal is to ensure the survival of its occupants.
Anthony Pimentel, structural engineer and president of Pimentel & Associates Engineering Consultants, said earthquakes damage buildings because of several failures or weaknesses in the way they were built. Engineers who are either about to build a new structure or assess an existing one post-quake will look at several factors to determine its quake resilience.
First is the “failure of foundation.” When the foundation is not able to withstand the seismic stresses imposed by an earthquake, this can cause a building to collapse or sink. These buildings would shake from their original position and cannot hold the structure above.
Then, the engineers determine soil profile to address the soil’s potential failure.
Pimentel said seismic waves generated by earthquakes can turn soft soil into a loose mass of sand particles, reducing its ability to bear the weight of structures built upon it. Softer soil amplifies more seismic waves and induces more ground movements. This can lead to the collapse of buildings, even those that are well-built to withstand tremors.
Ruel Ramirez, principal of structural engineering consulting firm RBRA, said that the soil has to be tested to see if it is “liquifiable. It’s like water, it’s not solid. If you shake that soil, it will sink and come down because it’s normally sand. If your land rests on liquifiable soil, no matter how strong your building is, it can collapse.” During quake-induced liquefaction, the ground loses its strength during an earthquake. Any structure built on the affected area will sink.
Checking for frailties
Pimentel also cited the “failure of soft floors,” which are defined as levels of a building that have large spaces, minimal shear walls and additional floor-to-floor height. These are usually found in a building’s lower-level floors. This is why if a building collapses from an earthquake, the upper floors tend to remain intact while the lower levels crumble since seismic forces are at their highest on the ground level. This effect is called the “pancake collapse” as the building is flattened.
Ramirez said a soft story that has weight or mass irregularity is more prone to collapse because it does not have a sense of balance. An example is one that has placed rigid heavy hollow blocks at the top and then used glass at the bottom.
Pimentel said another important factor is the use of the appropriate materials. Interviewees emphasized the need for the building owner to invest in the right kind of strong, long-lasting material, instead of resorting to cost-cutting measures to buy inferior concrete or wood. For load-bearing concrete hollow blocks, Ramirez recommends that the strength should be at least 350 PSI to 750 PSI; steel bars or rebars should have a minimum grade of 40.
A building’s alteration or changes from the original design can also affect its resilience, said Ramirez.
Adding a new floor to an old building, for example, will increase its weight, he said.
Then there is design and technology. Aside from load-bearing blocks, engineers should have buildings fitted with steel bars to reinforce thin walls and shock absorbers or dampers at the base to dissipate seismic waves. Instead of making a building withstand an earthquake by reinforced materials alone, the idea is to make it sway with the forces generated to keep it from toppling.
Another innovation, BASE isolation technology, has the building bear on the foundation through flexible bearing pads that absorb earthquake energy, allowing the ground to shake beneath the structure.
This drastically reduces seismic forces as the flexible isolators flex and absorb kinetic energy. Miranda likened this to a building on roller skates. The building may move during an earthquake, but it retains its composure.
Jose Miranda, chairman of the Committee on Resilient Architects, cautioned against buying and using sub-par materials, which can increase the structural risk of a building. As an example, he said one- or two-story buildings damaged by an earthquake in northern Cebu in 2025 had undersized steel bars and improper confinement of hollow block walls. The use of unwashed salty beach water with its high level of salinity on the reinforcements caused corrosion and lessened the bond between the steel bars and the concrete.
Building codes
The government has enacted several building codes that architects, structural engineers and other allied professionals must adhere to. According to Pimentel, structural engineers follow a criterion or guideline when designing a structure. Perhaps the most important is the National Building Code of the Philippines (Presidential Decree 1096), which, upon its enactment in 1977, has governed all building design, construction, alteration and maintenance in the country.
Still, the building owners should not be happy with mere compliance with the minimum requirements of PD 1096, said Ramirez. Adherence to the very basic conditions of the law indicates a performance level that brings about “life safety,” but it does not necessarily guarantee building resilience. It can help increase the chances that the individuals located in that structure at the time of the earthquake will not perish. However, the building can still suffer extensive damage.
The National Structural Code of the Philippines (NSCP), published by the Association of Structural Engineers of the Philippines (ASEP), is the official set of guidelines governing the design, construction and maintenance of structures in the country. It provides the minimum structural requirements to ensure public safety against natural disasters like typhoons and earthquakes. The NSCP has “performance-based provisions... that would provide the parameters that would allow [a building] to attain resistance to earthquakes with Intensity 7 to 8.4,” says Miranda.
Ramirez encourages building owners to “go beyond” the code’s minimum requirements and ask their structural engineers for “a better structure” and determine the “performance level” of the building in case of an earthquake. Property developers of high-rise buildings are using this kind of “performance-based design,” with some asking that their buildings take on only 20 percent of the total damage that the earthquake can inflict. This demand started when businessmen and insurance companies realized that their “economic losses will be huge” if they just complied with the code’s minimal requirements.
Prevention is always better than the cure, even in construction. Some local government units mandate yearly building inspections. The tests done by structural engineers on new buildings happen every 15 years.
However, there is no time limit on having a building assessed for earthquake resilience. Ramirez said a structural engineer can determine the extent of the quake’s damage [on the building] from a certain magnitude after gathering and analyzing pertinent information like post size and soil quality. “We can predict the extent of damage and the amount of losses to the building owner,” he said. And should the owner learn that an earthquake can damage 70 percent of his building, he can ask the engineer to make improvements to reduce that number.
The industry players, like the ASEP and the MMEIRS authors, are also updating their reports and guidelines, as most of their initial data and recommendations were completed decades ago.
Felino “Jun” Palafox encourages all building owners to become more proactive when it comes to their building’s earthquake resilience. “It’s 90-percent less expensive to address the hazards before they become disasters, aside from saving human lives, buildings and infrastructure,” he said.