Our planet is covered by tectonic plates that are slowly moving around, pushing into or sliding past one another along boundaries called faults. Friction sometimes causes two of these plates to get stuck to each other at spots along a fault. Tension builds up over years, decades or even centuries until suddenly the fault snaps. The two sides lurch past each other, unleashing an earthquake.
From the place where the fault ruptures, seismic waves ripple outward in all directions. When they reach Earth’s surface, they can set buildings or any other structures shaking—violently and destructively if the quake is strong and close enough, as were the two massive temblors that struck Turkey and Syria on February 6, which was followed by a large aftershock on the same day.
These quakes killed more than 45,000 people, many of them in collapsed buildings. Though earthquakes can’t be prevented or predicted, science does have some ways to protect buildings—and the people inside them. Scientific American spoke with several earthquake engineering experts to learn more about how using the right building methods can prevent homes, offices and other structures from succumbing to the capricious movements of the Earth.
What happens to a building during a quake?
Imagine you’re driving a car down the road, and you suddenly need to stop. As you slam on the brakes, those groceries sitting on the passenger seat (and anything else not strapped down) will fly through the air in the same direction and at the same speed as the car was originally going. This is because of inertia—an object’s tendency to stay at rest or to maintain a uniform speed and path until some other force acts on it. That same tendency is what puts a building at risk during an earthquake.
During a quake, the ground beneath a building moves quickly back and forth. But because the building has mass, it has inertia. “The earthquake is shaking the ground, and the building is trying to stay put,” says Ertugrul Taciroglu, a structural engineer at the University of California, Los Angeles. But once it does start moving, the building wants to keep going in whatever direction the earthquake has pulled it—essentially, it is always lagging behind the ground motion. These lags generate horizontal inertial forces on the building, causing any vertical columns and walls to deform at an angle (creating a parallelogram shape if one were looking at a side view of a rectangular building). When a building has multiple stories, each story is holding up the weight of those above it. That means lower stories have to bear larger inertial forces than those above. If walls and columns are not properly designed or reinforced, they may not be able to support the weight they once held.
The larger an earthquake is and the closer it is to the surface —and the nearer a building is to the fault rupture—the larger the inertial forces will be on that building during a quake. The type of ground a building is sitting on can also play a role: compared with hard rock, looser soils magnify ground motions.
How do we build buildings so they don’t collapse during an earthquake?
To keep a building intact when an earthquake hits, it needs to be constructed to resist horizontal inertial forces. Exactly how that can be done depends on the building material being used. Let’s focus on two of the most common: concrete and steel. Much of the building stock in the affected area of Turkey used these materials.
Under normal circumstances, concrete is a great material for holding the weight of a building because it performs well under what engineers call compression. A concrete building can easily last for decades if it only has to support its own weight. Yet the quake-generated inertial forces that set vertical walls and columns swaying put the concrete into tension, the opposite of compression. Although the forces are trying to stretch the concrete out, “it doesn’t give. It doesn’t let the building form move but tries to hold on really tight, and it generates these inertial large forces,” says Perry Adebar, a structural engineer at the University of British Columbia. The stressed concrete columns and walls can eventually crack and fail because they can no longer support the weight above them.
Concrete is still one of the most widely used building materials in the world, in part because it is cheap and abundant and because it has an ability to bear structural weight. To make concrete more suitable for seismically active areas, engineers add steel (in the form of rebar), which is much more flexible. “You have to put steel in wherever you’re going to have tension,” Adebar says.
Oman Observer is now on the WhatsApp channel. Click here
