What information is there about earthquakes?

There is too much information about earthquakes, so let me talk about why buildings are knocked down.

Earthquakes have always been a terrible phenomenon. As the scale of our cities gradually expands, earthquakes also cause more deaths. Because one of the biggest dangers is collapsed buildings.

Why do buildings collapse during earthquakes? How can you prevent building collapse?

If you've watched many disaster movies, you probably know that buildings collapse because the ground beneath them shakes violently or even cracks.

But this is not the case.

First, most buildings are not on fault lines, and the moving plates are much deeper than the building's foundations.

So what happened?

In fact, the principles of earthquakes and the consequences of earthquakes on buildings are a bit complicated. To understand their relationship, architects and engineers use models to conduct studies.

For example, use two-dimensional lines to represent columns and beams, or use a line with a round object on it, such as a lollipop, to represent the mass of a building. Even such a simple model is useful for predicting how buildings will respond to earthquakes.

It is mainly a physical effect.

Most collapses during earthquakes are not actually caused by the earthquake itself. Instead, when the ground beneath the building moves, it shifts the foundation and lower floors, and then the shock waves are transmitted throughout the structure. Causes the structure to vibrate front and back.

The force of the shock is mainly due to two factors: the mass concentrated at the base of the building and its stiffness, the latter being the force that causes some degree of displacement, plus the building materials and columns shape.

The hardness is mainly related to the height. Lower buildings are harder and less mobile, while taller buildings have higher mobility. You might think that the solution is to build a lower building. They will shake less.

But the 1985 Mexico City earthquake was an example that proved otherwise. In that earthquake, many buildings from 6 to 15 floors collapsed. Strangely, nearby lower buildings did not collapse, and buildings above 15 floors also suffered less damage.

The sway of those collapsed medium-sized buildings was greater than the amplitude of the earthquake. Why?

The answer is [natural frequency].

In a swinging system, the number of cycles is the number of times it swings back and forth in one second, which is the opposite of the number of seconds it takes to swing back and forth. A building's [natural frequency] is determined by its mass and stiffness and is the frequency at which its vibrations are concentrated. As the mass of a building increases, the natural frequencies decrease. As the hardness increases, the vibration will also increase.

A formula is used to express the relationship between these. Hardness and natural frequency are directly proportional, and mass and natural frequency are inversely proportional.

The cause of the collapse of a large number of medium-height buildings in Mexico City was the result of earthquakes! The frequency of seismic waves is exactly the same as the natural frequency of medium-sized buildings. Just like a push at the sweet spot of a swing, each subsequent seismic wave amplifies the vibrations of the building in the same direction, causing it to swing further in that direction, eventually achieving a larger displacement than before.

Today, engineers work with geologists and seismologists to predict the frequency of seismic movements at building sites to prevent vibration-induced collapse. They consider soil types and fault types, as well as data from past earthquakes.

Low-frequency vibrations cause more damage to taller and more resilient buildings. On the contrary, high-frequency vibrations threaten the structure of relatively low and highly rigid buildings. Engineers are also trying to find ways Methods of absorbing vibrations and preventing building deformation with innovative systems. Foundation isolation uses elastic layers to isolate the rest of the building from the movements of the foundation, allowing a system of tuned dampers to cancel out vibrations. , eliminating vibration at natural frequency to reduce vibration.

The conclusion is: not the strongest, but the smartest buildings can stand.

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