Our planet is covered in tectonic plates that slowly move around, push against each other, and slip through boundaries called faults. Friction can cause two of these plates to stick together along the fault. Years, decades, or centuries of tension build up and suddenly the fault breaks. The two camps collide and an earthquake occurs.
Seismic waves ripple in all directions from where the fault breaks. When an earthquake reaches the surface, it can violently shake buildings and other structures. Like the two large quakes that struck Turkey and Syria on February 6, he was followed by violent aftershocks if the quakes were close enough and strong enough. On the same day.
These quakes killed more than 45,000 people, many trapped in collapsed buildings. Although earthquakes cannot be prevented or predicted, science has several ways to protect buildings and the people inside them. Scientific American I spoke with several earthquake engineering experts to learn more about how proper building methods can help prevent homes, offices, and other structures from succumbing to the whims of the earth.
What happens to buildings in an earthquake?
Imagine you are driving a car and suddenly have to stop. If you hit the brakes hard, the groceries (and anything loose) sitting in the passenger seat will be blasted through the air at the same speed and in the same direction the car was originally going. This is due to inertia. That is, the tendency of an object to remain stationary or maintain a constant velocity and path until some other force acts on it. The same tendency is what makes buildings dangerous during an earthquake.
During an earthquake, the ground beneath the building moves quickly back and forth. But since buildings have mass, they have inertia. Ertugrul Taciroglu, a structural engineer at the University of California, Los Angeles, said: But once it starts moving, the building will try to keep moving in the direction it was pulled by the earthquake. In other words, the building is always lagging behind the movement of the ground. These delays generate horizontal inertial forces on the building, causing vertical columns and walls to deform diagonally (creating a parallelogram shape when a rectangular building is viewed from the side). When a building has multiple floors, each floor supports the weight of the floor above it. This means that the lower floors have to withstand greater inertial forces than the upper floors. If walls and columns are not properly designed or reinforced, they may not be able to support the weight they once held.
The larger the earthquake and the closer it is to the ground, and the closer the building is to a fault rupture, the greater the inertial force on the building during the earthquake. The type of ground on which the building is located can also play a role. Loose soil magnifies ground movement compared to hard rock.
What can be done to prevent buildings from collapsing in an earthquake?
To keep buildings intact in the event of an earthquake, they must be constructed to resist horizontal inertial forces. How it is done depends on the building materials used. Let’s focus on the two most common: concrete and steel. Much of the building stock in the Turkish-influenced areas used these materials.
Under normal circumstances, concrete is an excellent material for holding the weight of a building because it performs well under what engineers call compression. Concrete buildings can easily last for decades if they just support their own weight. However, the inertial forces generated by the earthquake cause the vertical walls and columns to sway, exposing the concrete to tension as opposed to compression. The military is trying to stretch the concrete, but “you can’t do that. It’s trying to hold the shape of the building firmly without moving it, and it creates these large inertial forces,” said British, a structural engineer at his Columbia University. Perry Adever says. A stressed concrete column or wall can eventually crack and fail because it can no longer support the weight on it.
Concrete is still one of the most widely used building materials in the world. One reason is that it’s cheap, plentiful, and able to withstand structural weight. To make concrete more suitable for earthquake-prone areas, engineers add much more flexible steel (in the form of rebar). “Wherever there is tension, you have to put a steel in,” says Adever.
Steel behaves elastically under constant tension. Imagine pulling gently on the bottom of a wire coat hanger and letting it go back to its original shape. But under greater tension, such as a very strong earthquake, steel “plasticizes and deforms,” he explains Adebar. Think about pulling hard on the bottom of a coat hanger and bending it out of shape. For buildings during an earthquake, “that’s exactly what you want,” Adebar says. This is because deformed steel can effectively absorb these inertial forces and still support weight.
Doesn’t that mean the building is broken?
A big earthquake, yes. Reinforced concrete buildings can suffer considerable damage after an earthquake, perhaps to the point of rendering them unusable. This has to do with how governments set building codes. Building codes tell engineers how to design buildings to withstand a certain level of seismic shaking. Codes, including those in the United States and Turkey, generally require buildings to achieve so-called “life safety” under the largest earthquakes expected in a particular area. “Our seismic standards are only minimum requirements,” said Sissy Nikolaou, a seismic research engineer at the National Institute of Standards and Technology. “Under the premise that these buildings could be seriously damaged, I just want them to at least have a chance to survive and escape when a large-scale building occurs.” It resembles a crumpled car. The vehicle absorbs the impact and protects the occupants, but is completely destroyed.
Of course, there are different criteria for buildings and other infrastructure (such as hospitals) that are considered critical and must remain functional after an earthquake. Experts such as Nikolaou are also beginning to rethink life safety standards so that more structures can be used after an earthquake. That way, you can avoid situations where people are stuck at home for months or years. Many in Turkey are now facing this possibility, with tens of thousands of buildings believed to be at risk of collapsing due to the damage caused by the February 6 earthquake.
There are ways to keep buildings habitable after an earthquake. Some methods involve smarter designs using common materials such as reinforced concrete. Others may require a more technical approach, such as “seismic isolation”. With this technology, the building is not rigidly attached to its foundation. Instead, it rests on a flexible structure and is detached from the foundation and thus detached from the shaking ground. However, this type of system is expensive to build, and some building owners are unable or unwilling to pay for it. In the United States, it is used to protect critical structures such as hospitals and to refurbish historic buildings while preserving their original architecture. Some hospitals in Turkey have basic isolation systems that have survived recent earthquakes.
Why is there a possibility of collapse even if the building was built according to earthquake resistance standards?
Buildings are designed to withstand a certain level of shaking based on the seismic risk of the location. For example, buildings in Los Angeles are built to withstand larger earthquakes than New York City. However, seismologists don’t always know exactly how large an earthquake a fault will cause. “The main difficulty in engineering design is the uncertainty about future earthquakes, because we don’t know exactly what will happen,” he says. The higher the magnitude, the rarer the earthquake. The largest can occur every hundreds or thousands of years, but the most recent seismic measurements go back only decades. Many seismologists believed that the East Anatolian Fault (the fault involved in the earthquakes in Turkey and Syria) was likely to produce maximum magnitudes of 7.4 or 7.5. However, the February 6 quake was magnitude 7.8, about four times as large on the logarithmic scale of magnitude. As such, some structures built for cryptography in Turkey may have experienced more forces than were simply built to withstand, Taciroglu said.
Building codes also evolve as science’s understanding of seismic risk and engineering change advances, so a building that was built to the code when it was built may not meet the updated code. Renovating such buildings is often prohibitively expensive. Tasiroglu says this is likely the reason why many buildings in Turkey have been badly damaged or collapsed.
Human error can also occur. It can range from deliberate, profit-driven omissions that can occur at various points in the design and construction process, to honest mistakes that are not apparent until something like a major earthquake occurs. There is a nature.
