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Earthquakes
Earthquakes radiate seismic energy as body waves and surface waves. Body waves—P- and S-waves—travel through Earth’s interior, while surface waves travel only in the crust and upper mantle, along Earth’s surface. Surface waves cause the most destruction during an earthquake, causing the ground to roll up and down and shake back and forth as they pass through.
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Like water waves, seismic waves can be described by their amplitude and frequency. The amplitude is a measure of how far the ground moves when a wave passes through. The frequency is a measure of how quickly it moves up and down or back and forth. The greater the amplitude, the farther the ground—and all the buildings on the ground—moves up and down or from side to side. The greater the frequency, the faster the shaking occurs.
Can we make buildings Earthquake resistant?
To be earthquake proof, buildings, structures and their foundations need to be built to be resistant to sideways loads. The lighter the building is, the less the loads. This is particularly so when the weight is higher up.
A foundation is a lower portion of building structure that transfers its gravity loads to the earth. Foundations are generally broken into two categories: shallow foundations and deep foundations. A tall building must have a strong foundation if it is to stand for a long time. To make a foundation, we normally dig a trench in the ground, digging deeper and deeper until we come to subsoil, which is more solid than the topsoil that is used to grow plants and crops. When the trench is deep enough, we fill it with any strong, hard material we can find. Sometimes we pour in concrete into the trench, which we strengthen even more by first putting long thin round pieces of steel into the trench. When the concrete dries, the steel acts like the bones in our body to tie the foundation together. We call this reinforced concrete.
If the sideways resistance is to be obtained from walls, these walls must go equally in both directions. They must be strong enough to take the loads. They must be tied in to any framing, and reinforced to take load in their weakest direction. They must not fall apart and must remain in place after the worst shock waves so as to retain strength for the after shocks.
If there are open holes, such as doorways or windows, this can be done with ‘fixed feet’. 'Fixed feet' connects the "feet" or bottom of the building with many bolts into large foundations, or by having a grillage of steel beams at the foundation level able to resist the building movements. Such grillage can also keep the foundation of a building in place.
If the beams in the frame can bend and yield a little at their highest stressed points, without losing resistance, while the joints and the columns remain full strength, then a curious thing happens: the resonant frequency of the whole frame changes. If the building was vibrating in time with shock waves, this vibration will tend to be damped out. This phenomenon is known as ‘plastic hingeing’ and is easily demonstrated in steel beams, though a similar thing can happen with reinforced concrete beams as long as spalling is avoided. Plastic hingeing allows for the beams to bend with the shaking instead of breaking.
Many times during building construction, builders incorporate triangles. Triangles are the building blocks of many structures mainly because of their ability to bear large loads without deformation. They are considered the strongest shape because a triangular structure subject to strong forces only collapses due to material fatigue and not to geometric distortion. It is easier to understand why triangles are so strong when compared to a square or rectangular structure.
Dampers
In addition to structural aids, there are also damping devices. Damping devices act as shock absorbers. Different types work in different ways to reduce the amplitude of the shaking. They help to stabilize a building and decrease the impact of violent motion from surface waves during an earthquake. There are many types of dampers below are some examples of a few of them.
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