RESONANCE
"The principle cannot fail. It is as powerful when applied to the earth as it is when applied to a [violin note shattering a] wineglass, a [boy pushing a man on a] swing, or a steel link. Anyone who doubts should only bear in mind the illustration of the swing. A small boy, by each time adding a pound to the force with which a 200-pound man swings, can soon set the man swinging with the force of 500 pounds. It is necessary only to keep adding a little force at the right time."
-Nikola Tesla .
The above statement by Tesla very well suites for Earth quake engineering
Starting with Natural frequency, if a system or structure oscillates at a frequency under no external force or driving force it is known as Natural Frequency. Also the frequency under when system vibrates freely is also called as natural frequency.
Resonance is when Forced Vibration(forced frequency) matches or corresponds with the natural frequency of the structure resulting in the increase of amplitude of the vibration to great limits.
When a Mechanical system (say building) at a particular frequency has high maximum amplitude it is known as a Resonant Frequency. And at resonant frequencies, small periodic driving forces have the ability to produce large amplitude oscillations, as the energy gets stored in the system due to the storage of vibrational energy in the mechanical system, leading to resonance disaster.
Many resonant objects have more than one resonance frequency. It will vibrate easily at those frequencies, and less so at other frequencies. So we as a engineers must find the natural frequency of a structure before building it which is done with the help of a simulation softwares or with calculations and formulas (with whichever you find it easy). Even the ground also has some specific resonance,
Hard bed rock has higher frequency than Soft sediments. if the period of the ground motion (forced frequency) matches the natural frequency of buildings it would cause large oscillations.
In those days it was thought that if a building has more strength or more stronger the more it can withstand Seismic forces without collapsing. But, if a superstructure is rigidly attached to a foundation the seismic waves will be directly transmitted to the structure so the building should be less rigid survive a quake, which was made possible by the introduction of BASE ISOLATION TECHNIQUE
which is nothing but Disengaging or Decoupling a Superstructure from its foundation during a seismic wave. If the earthquake has natural frequencies with high energy that match the natural frequencies of the building, it will cause the building to oscillate violently in harmony with the earthquake frequency. However, if the natural frequency of the building can be changed to a frequency that does not coincide with that of earthquakes, the building is less likely to fail”. The base isolated foundation minimizes the stiffness of the superstructure reducing its frequency. So the structure receives very less seismic force.
Damping is one of the most important techniques developed to prevent or minimize damage to buildings during an earthquake. Damping also plays a crucial role in minimizing the seismic force. It actually reduces the force amplitude of vibrations .Great example is shown by 630-60 mass tuned damper in Taipei 101 which modifies the resonance of the structure and resonates at a frequency which usually does not occur.
Great example for Resonace disaster is given in this video.
But many would think that TACOMA NARROWS BRIDGE is a great example for resonance disaster but it isn't true it was actually caused by AEROSTATIC FLUTTER.
Hope U have got a clear idea on resonance.......
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