Most collapses that occur during an earthquake aren’t actually caused by the earthquake itself. Instead when the ground moves beneath a building it displaces the foundation and the lower levels sending shock waves through the rest of the structure causing it to vibrate back and forth. The energy released during an earthquake generates strong lateral forces that travel along the surface of the earth and under the structure. When the ground shakes, buildings respond to the accelerations transmitted from the ground through the structure's foundation. The inertia of the building (it wants to stay at rest) can cause shearing of the structure which can concentrate stresses on the weak walls or joints in the structure resulting in failure or perhaps total collapse.
A building’s natural frequency is determined by its mass and stiffness. Increasing the building’s mass slows down the rate at which it naturally vibrates while increasing the stiffness makes it vibrate faster. If the frequency of the earthquake seismic waves happens to match the natural frequency of the structure, each additional seismic wave amplifies the building vibration making it reach farther displacement and resulting in collapse.
IT IS NOT THE STURDIEST THAT REMAIN STANDING, BUT THE SMARTEST ONES.
Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. It is a collection of mitigation technique for Earthquake engineering. It is of utmost importance for historic monuments, areas prone to severe earthquakes and tall or expensive structures.
The extensive damage of bridges all over the world in recent earthquakes has been the motivation in significant advancement in the earthquake resistant design and retrofitting of bridges. The bridges constructed prior to 1970 were not designed for adequate seismic resistance as the ductility provisions were not incorporated in the seismic codes till then. As a result the bridges designed before this year lack in earthquake resistance and ductility and may be vulnerable to significant damage even from moderate earthquakes.
It has also been revealed from damage surveys that many of the damages that occurred in bridges and flyovers could be prevented by proactive measures of seismic retrofitting prior to earthquakes.
The major steps in retrofitting of bridges are as follows:
The retrofitting of bridge may be required in each of the components: superstructure, bearings, substructure and foundations. The extent of retrofitting will be based on detailed seismic assessment following dynamic methods of analysis.
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