Luyện theo cấu trúc Cambridge IELTS 20 · Listening Part 4: Designing for Earthquakes

Lecturer: Good morning, everyone. In today's lecture in our series on engineering and natural hazards, I'm going to look at how buildings can be designed to survive earthquakes.

Lecturer: Let's begin with a point that surprises many people. In a major earthquake, it is very rarely the shaking of the ground itself that injures or kills people. The vast majority of casualties are caused by the collapse of buildings. So the question engineers ask is not how to stop the ground moving, which is impossible, but how to stop structures from falling down when it does.

Lecturer: The first thing to understand is that the same earthquake can have very different effects just a few streets apart, and this is largely to do with the ground underneath. Solid rock transmits vibrations quite efficiently, but it doesn't amplify them. Soft soil, on the other hand, particularly soil that contains a lot of water, can behave almost like a jelly, and the shaking at the surface may be several times stronger than on rock nearby.

Lecturer: The second factor is the building itself. Every structure has a natural rate at which it sways back and forth. If that rate happens to match the frequency of the ground motion, the movement builds up with each cycle, a little like a child on a swing being pushed at exactly the right moment. This is called resonance, and it helps to explain why, in some earthquakes, medium-height buildings have suffered far more than either very low or very tall ones.

Lecturer: So how do engineers respond? Well, one key principle is ductility. A brittle material, such as unreinforced brick, can't absorb much energy, and when it fails, it fails suddenly. What we want instead is a structure that will bend rather than snap, so that even if it is badly damaged, the people inside have time to get out. That's why concrete in earthquake zones is reinforced with carefully placed steel bars.

Lecturer: Engineers also pay close attention to weak points. One of the most dangerous designs is what we call a 'soft storey'. This is common in older apartment blocks where the ground floor has been left open, supported only by thin columns, to provide parking. In an earthquake, the movement concentrates at that level, and the upper floors can simply drop down onto it.

Lecturer: Now let's turn to some more advanced technologies. The first is base isolation. Here, instead of fixing the building rigidly to its foundations, engineers place it on bearings made from alternating layers of rubber and steel. These bearings are stiff vertically, so they can support the weight of the building, but flexible horizontally, so much of the ground's movement is absorbed before it reaches the structure above.

Lecturer: A second approach uses dampers. These are devices built into the frame which work rather like the shock absorbers in a car, turning the energy of the movement into heat. And in some very tall towers, you'll find what's called a tuned mass damper: essentially a huge pendulum hung near the top of the building. When the tower sways one way, the pendulum swings the other way, reducing the motion.

Lecturer: Of course, all these technologies are expensive, and many of the people most at risk live in regions where they simply aren't affordable. So I want to mention some low-cost solutions. Researchers have found, for instance, that wrapping the walls of simple houses in a plastic mesh, rather like fishing net, and then covering it with plaster can stop the walls from falling apart, even if they crack. This can be done with local labour and inexpensive materials.

Lecturer: Finally, a word on policy. Most countries in earthquake zones now have detailed building codes, and many of them are excellent on paper. But studies of recent disasters suggest that the main weakness isn't the codes themselves; it's enforcement. Inspectors may be too few, poorly paid or under pressure to approve buildings that don't meet the standards. And that, I think, is where the greatest gains in safety could be made in the coming decades. Next week, we'll look at how cities prepare their emergency services for earthquakes.