EXPLAINER: How are buildings made earthquake-resistant?

This is the aftermath of the magnitude 7.8 earthquake that struck Mindanao: a school building collapsed, a commercial establishment crumbled, and debris fell from several structures.

The General Santos City Hall was also not spared. In Davao City, a high-rise building appeared to sway as the ground shook during the quake.

Data from the Office of Civil Defense as of June 9 showed 47 infrastructure damage incidents in affected areas, while damaged houses reached 1,889.

Meanwhile, estimated infrastructure damage in Regions 11 and 12 has reportedly breached P562 million.

Engineer Ronaldo Ison, former president of the Association of Structural Engineers of the Philippines, said earthquake resistance is a key standard in building construction. This means structures are designed to withstand strong earthquakes, potentially up to magnitude 7 to 8.2.

“‘Yung pinakamalakas na lindol na puwede nating maranasan is between 7 to 8.4 magnitude, so ‘yung mga buildings natin, dapat nakadisenyo dito," he said.

(The strongest earthquakes we could possibly experience are between magnitude 7 to 8.4, so our buildings must be designed for this.)

"So whether bahay ‘yan o kaya building o kaya bridge, dapat nakadisenyo ito sa ganoong kalakas na magnitude ng earthquake,” Ison added.

(Whether it is a house, a building, or a bridge, it must be engineered to withstand that level of shaking.)

Structural design calculations are based on the National Structural Code of the Philippines (NSCP).

What exactly are the factors considered in making buildings earthquake-resistant?

In computing the materials needed for construction, engineers first determine gravity loads—the weight the structure carries based on its design and usage.

“‘Di ba merong panahon na walang earthquake? So ang tinatawag namin doon ‘yung gravity loads. ‘Yun ‘yung self-weight ng building saka ‘yung occupants, saka yung other elements na nakapatong sa building. Ang tawag doon, gravity loads,” said Ison.

(There are times when there are no earthquakes, right? So we refer to that as gravity loads. That includes the self-weight of the building, the occupants, and other elements resting on the structure.)

From gravity loads, engineers then factor in earthquake forces, where seismic variables are assigned values. One of the first is the near-source factor, or the building’s distance from an active fault line.

“Kapag mas malapit ka sa active fault, mas malakas ‘yung earthquake force na i-apply mo sa building… Habang lumalayo ang isang gusali sa isang active fault, humihina ‘yung epekto ng earthquake sa gusali na ‘yun,” said Ison.

(The closer you are to an active fault, the stronger the earthquake force applied in design. As a structure gets farther away, the impact of the earthquake decreases.)

To ensure structural integrity, soil type is also a critical consideration, as it directly affects foundation design.

“Kapag malambot ‘yung soil, kunyari maputik o kaya hindi siya matigas, kailangan natin ng mas malaking pundasyon.

(When the soil is soft—for instance, muddy or loose—we need larger foundations.)

"Kapag nasa rock or bato ‘yung mga foundation, mas maliliit yung mga pundasyon na kailangan natin. May tinatawag kang mga liquefiable soils,” said Ison.

(If it sits on a rock, smaller foundations are sufficient. There are also liquefiable soils.)

“‘Yung liquefiable, yun yung parang nagtutubig or lumalambot na parang tubig kapag nagkakaroon ng lindol; in most cases, kailangan mo ng ibang type of foundation.”

(Liquefiable soil behaves like water or becomes soft during an earthquake. In most cases, this requires a different type of foundation.)

Soil type also influences how much shaking a building experiences, he said.

“‘Yung type ng lupa nakakadagdag sa paggalaw ng isang building, so ‘pag mas malambot ‘yung lupa, mas naa-amplify yung earthquake. So ‘pag mas matigas, mas hindi naa-amplify yung earthquake,” Ison said.

(The type of soil contributes to a building’s movement; softer ground amplifies shaking, while harder ground reduces it.)

Life safety remains the top priority in structural design, Ison said. Buildings must be designed to avoid collapse during strong earthquakes, even if minor damage such as cracks may still occur.

For older buildings, proper maintenance and regular structural assessment are essential to determine if retrofitting is needed.

“Kapag hindi properly maintained, nagde-deteriorate ‘yung mga biga, yung mga poste, so maaaring mag-contribute ito sa pagiging mahina ng structure,” said Ison.

(If a structure is not properly maintained, beams and columns deteriorate, which can weaken the structure.)

During structural evaluation, engineers assess whether a building still meets updated NSCP standards. If it no longer does, retrofitting is recommended.

“‘Yung retrofitting, aayusin mo yung building para mag-conform siya or maging malakas uli siya para kargahin ‘yung ganitong mga requirements – earthquake force, gravity force, gravity loads,” said Ison.

(Retrofitting means modifying a building to meet standards again or restoring its strength to carry required loads such as earthquake and gravity forces.)

Structural inspections are especially important when buildings show signs of aging or alteration.

“If ikaw ‘yung may-ari ng building, dapat check-in mo yung building at least yearly… titingnan mo kung may pagbabago sa itsura ng structural elements, kung may signs na nagde-deflect or lumulundo, may corrosion o kalawang, o may cracks. So from time to time, dapat chine-check yun,” said Ison.

(If you own a building, you should inspect it at least yearly… check for changes in structural elements such as deflection, sagging, corrosion or rust, and cracks. It should be checked regularly.)

Ison reminded the public that construction standards must be strictly followed, stressing that safety should never be compromised.

“Huwag nating tipirin yung pag-construct ng isang building at bawasan yung mga requirements natin sa bakal, sa poste, sa semento para maging safe ‘yung ating structure,” he said.

(We should not cut corners in construction or reduce requirements for steel, columns, or cement if we want our structures to be safe.)—Vince Angelo Ferreras with research by GMA DigiLab/MCG, GMA News