Before describing Structural Resonance, we will start by defining a simple resonant system and how it works.  A good example of a simple resonant system would be a plucked guitar string or a pendulum.             Let's take the pendulum for example.  If some force such as a push sets the pendulum in motion it will begin to oscillate.  It oscillates at a certain frequency until ravity or friction causes the amplitude to die away.

Graph of Plucked Guitar String       d     i     s     t     a     n     c     e     Displacement vs. Time

 
 
 
 

       Resonance can be described as a special case when a driven oscillator matches the natural frequency of the vibrating system.  A driven oscillator is one where an external force is periodically applied.  This driving frequency is usually different from the natural frequency of the system.  If these two frequencies coinside in the case of buildings or bridges, it can result in a collapse.
 
 
 
 

     Lets take a look at these springs oscillating at different frequencies for example.  If the purple continues to oscillate at the same frequency and the purple spring gradually slows down until it comes to a stop, then there would be a period of time where both springs would have the same amplitude.  If their movements were in sync at this time (both at the same position) then resonance would occur.