foundation system of burj al arab

Ground Motion During Earthquake

Earthquake are the sudden, rapid release of energy stored in rocks and becomes perceptible by the constant motion of the earth's surface. The earth's rock layer is broken into large pieces. These pieces are in slow but constant motion. They may slide by each other smoothly and almost imperceptibly. From time to time, the pieces may lock together and energy that accumulates between the pieces may suddenly released. The energy that is released travels through the earth in the form of waves. People on the surface of the earth than experience an earthquake.


Ground Motion

The energy released at each material point within the three dimensional volume that ruptures at the fault, results seismic waves which produce motion to earth’s surface. The motion at any site on ground is random in nature with its amplitude and direction varying randomly with time as these waves arrives at various instant of time having different amplitudes and carry different levels of energy. Knowing how, fast, for how long, and how much the ground moves during an earthquake is important for estimating how ground motion will affect the built environment. Seismologist use several concepts to express these measurements:

1) Acceleration
2) Duration
3) Velocity
4) Displacement

Acceleration

A series of vibrations are created when seismic waves move through the ground. These movements are transmitted into dynamic loads or inertial forces that cause the ground and any thing attached to it (i.e., the built environment) to vibrate in a complex manner. These inertial forces cause damage to buildings and other structures. Inertial forces are created when an outside force tries to make an object move or change its rate of travel.
Acceleration is the rate of change of motion. The variation of ground acceleration with time recorded at a point on ground during an earthquake is called an accelerogram. The nature of accelerogram may vary depending on

- Energy released at source
- Type of slip at fault surface
- Geology along the travel path from fault rupture to the earth surface
- Local soil

They carry distinct information regarding ground shaking; peak amplitude, duration of strong shaking, frequency content ( amplitude of shaking associated with each frequency) and energy content (i.e., energy carried by ground shaking at each frequency) are often used to distinguish them.

Normally acceleration is not associated with buildings since building is not expected to move. During an earthquake, however, inertial forces may cause the upper part of the building to sway while the foundation remain stationary, or they may cause whole building to “move”. Structures built in seismically active areas must be built to withstand predicted acceleration levels.


Duration


The duration of ground motion is very important because the destructive effects increase greatly with increase in length of duration. The damage will occur the whole time the ground is moving. So more damage is likely to occur the longer an earthquake lasts. In predicting the amount of potential damage that could occur in a specific should include the duration of ground motion.


Velocity


Velocity is mathematically related to acceleration. Velocity is the speed of an object at an instant in time. Velocity is quickly becoming as important as acceleration in determining the building damage. Consider an example: if your car decelerate suddenly, the inertial force may cause your head to hit windshield. The velocity at which your body id traveling at the instant your head hits the windshield determines whether you get a little bump or a fractured skull. For a building, this could mean the difference between superficial damage and building collapse.


Displacement

Displacement is the distance an object is moved from a resting position, such a how far a building is moved or displaced from its foundation. Seismologist use measurements of displacement to judge the impact of an earthquake on a community.


Conclusion

None of the scales (The Richter Magnitude Scale and The Mercalli Intensity Scale ) used to measure earthquake, are not enough to develop a seismic resistant design method or further development of scientific study. The Richter scale does not give ground motion information that is important for designers. The Mercalli scale is subjective and does not cover many new kinds of construction used today. Together with theses information, scientists can define an earthquake having following information:
- Where seismic events takes place
- How large it was
- What its impact was on the built environment.

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