Question

1. Seismic resonance is responsible for the damage caused to building by earthquakes. Apply the concepts of wave energy and resonance to explain how earthquakes damage the man -made structures .

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Answer 1

All buildings have a natural period, or resonance, which is the number of seconds it takes for the building to naturally vibrate back and forth. The ground also has a specific resonant frequency. Hard bedrock has higher frequencies softer sediments. If the period of ground motion matches the natural resonance of a building, it will undergo the largest oscillations possible and suffer the greatest damage.

What are the key features addressed in this animation?

• Frequency of a wave refers to the number of waves that pass through a point in one second
• Period is the amount of time it takes one wave cycle to pass the given point
• Resonance is the tendency of a system to oscillate with greater amplitude at some frequencies than at others
• Resonant frequency of any given system is the frequency at which the maximum-amplitude oscillation occurs.
• All buildings have a natural period, or resonance, which is the number of seconds it takes for the building to naturally vibrate back and forth.

Over the centuries, some towns and cities have repeatedly been struck — and sometimes devastated — by major earthquakes.

The difference between damage and devastation depends not only on the magnitude of the earthquake, but also on local geology and on building techniques.

Ground conditions

Soft ground, based mostly on sediments such as those in flood plains, reclaimed land or former landfill, amplifies the effect of the earthquake vibrations, while harder rocks limit the amount of shaking.

This effect contributed to the huge amounts of damage caused in the 1906 San Francisco earthquake, the 1923 Kanto (Tokyo) earthquake and the 1985 Mexico City earthquake, for example. In some cases, the land may even liquefy, behaving like quicksand and causing buildings to sink or topple over.

Building design and construction

Poor construction technique, where slab walls and floors are not tied together correctly, for example, makes buildings far more vulnerable to earthquake damage; buildings where the bricks have been held in place with the correct mortar tend to survive much better.

However, there is more to the issue of building design than simply the difference between careful and shoddy construction.

Materials

Firstly, there is choice of materials. Where buildings do collapse, the occupants are more likely to survive when the walls and roof are made of lightweight materials rather than heavy ones; rubble-masonry buildings, or brick buildings with low quality mortar, do not withstand earthquakes well; wooden or steel-framed buildings are generally much better, providing they are correctly braced.

Design

Next, there is the choice of building design. Buildings shake when the frequency of the seismic waves is close to the natural frequency of vibration of the building, an effect known as resonance.

In the Mexico City earthquake in 1985, the majority of buildings with severe damage were those with 6–15 storeys; the resonant frequency of these buildings was the frequency most amplified by the subsoils in the city.

Resonant frequency

The resonant frequency depends on the height of the building: low frequency shaking might cause tall buildings to shake violently while having little effect on low-rise buildings nearby, although higher frequencies of vibration might have the opposite effect.

Damage to tall buildings is often concentrated on the upper storeys, where the motion is greater. Another common cause of damage is 'pounding' by collisions with adjacent buildings.

Tall structures can survive low frequency vibrations if they are designed to do so, however. This typically involves making sure that lower floors are stronger and heavier than upper floors, and avoiding large, unsupported spaces; it may also include extra reinforcement with steel cables, or even placing the building on a foundation which reduces the amount of shaking transmitted to the building structure.

Large, unsupported spaces make buildings with ground floor car parks or viaducts particularly vulnerable.

Seismic waves and energy

Seismic activity that results in earthquake generates two types of seismic waves: body and surface. Body waves move through the interior layers of earth’s. Body waves include primary waves (known as P-waves) and secondary waves (also called as S-waves). P-waves generate sequential push (or compression) and pull (or tension) in soil as shown in below Figure 8a. P waves have relatively little damage potential. On the contrary, S-wave propagates horizontal and vertical motion. S-waves produce shear stresses in the soil along their paths

Seismic waves and energy

Surface waves include Love (L) and Rayleigh (R) waves that propagate through the outer layers of the crust. These waves are generated by body waves move through parallel to the ground surface and various underpass the layer boundaries. These waves cause large displacements. These types of waves take various forms at a further distance away from the earthquake source. Surface waves are occurred during shallow earthquakes; on the other hand, body waves take place at all depths. Surface waves cause serious damage to structures due to their long duration