Making buildings earthquake resistant
THE recent earthquake in Turkey and Syria has reminded the world once more of the devastating effects of earthquakes, particularly in earthquake-prone areas like the Philippines, which has had its share of tremors.
The country is also preparing for “The Big One,” a powerful earthquake that would originate at the Marikina Valley Fault that could devastate Metro Manila. Measures are being carried out in preparing for this unfortunate event. One of which is to ensure the integrity of buildings, and their ability to withstand such an earthquake.
Why do structures collapse during earthquakes?
One such reason is geography or what is called “failure of the soil.” The seismic waves generated by earthquakes can turn soft soil into a loose mass of sand particles, abandoning its ability to bear the weight of structures built upon it. This can lead to the collapse of buildings, even those that are well built to withstand tremors.
Anthony Pimentel, structural engineer and president of Pimentel & Associates Engineering Consultants, said the closer structures are built to fault lines where earthquakes originate the greater the risk.
The second one is the “failure of foundation.” When the foundation is not able to withstand the seismic stresses imposed by an earthquake (since it can come from various directions), this can cause a building to collapse or sink. These buildings would shake from their original position and cannot hold the structure above.
Another result of an earthquake is liquefaction where the ground loses its strength during an earthquake. Any structure built on an affected area will sink.
The third is the “failure of soft floors.” soft floors 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 lower level floors of a building. 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 its highest on the ground level. This effect is called the “pancake collapse” as the building is flattened.
The fourth is a rather obvious reason “failure of building,” which can be related to failure of soft floors. it is very likely these buildings were not built according to building codes and are made of unsuitable or substandard materials, or poor architectural/ engineering design.
In the case of the recent earthquake in Turkey, the collapsed buildings were “soft story structures.”
These were buildings built to address overcrowded cities. Because of the need for expediency, they were made of materials that are not reinforced. As a result, these buildings easily collapsed when the earthquake struck.
Following an earthquake that struck Taiwan in 2016, it was discovered that one collapsed building in Tainan was made of shoddy materials — empty paint cans and styrofoam used as filler for wall beams.
“Needless to say, non-engineered buildings are the most vulnerable to earthquakes,” said Pimentel.
What can be done?
Given that earthquakes are likely to occur in countries like the Philippines, the government has enacted several building codes architects and structural engineers, and other allied professionals must adhere to.
Pimentel said structural engineers for their part follow a criteria or guideline when designing a structure. First, they determine the proximity of the nearest fault lines on where they will build. They would always consult Phivolcs (Philippine Institute of Volcanology and Seismology) for such information.
Second, they determine soil profile to address the failure of soil. softer soil amplifies more seismic waves and induces more ground movements.
Third is to take into account the geometry of the building. It is here structural engineers and architects need to meet halfway and compromise each design intent. Architects are usually the ones who have their way and engineers find ways to reconcile their ideas with the proposed design.
Lastly, use the appropriate materials, followed by inspection and quality control to ensure structures comply with building codes.
Pimentel gives an example of building a house. One way to make it resilient to earthquakes is to use load-bearing blocks using high strength concrete masonry units.
Another example he gave is to use concrete and steel for the frames of houses to enable them to withstand earthquakes, as well as reinforce the walls to prevent them from crumbling.
For high-rises
What about skyscrapers? Besides earthquakes, another concern is strong winds that could topple a skyscraper if it is not well-designed and engineered.
Besides load-bearing blocks, engineers would have buildings fitted with steel bars to reinforce thin walls and shock absorbers or dampers at the base to dissipate seismic waves. The idea is instead of making a building withstand an earthquake by reinforced materials alone, but to make it sway with the forces generated to keep them from toppling. Buildings in Japan utilize this technology considering they are also earthquake-prone.
Nowadays, when structures are designed and built in earthquakeprone areas, the main goal every structural engineer keeps in mind is to ensure the survival of its occupants, and their designs are based on that idea.