Resonance Vibration Test Of SDOF System
Resonance occurs when external frequency of the system (in this case of the shaking table) matches with the natural frequency of the system (which depends on natural properties of structure, in this case all three structures have different natural frequencies). As a result amplitude increases as shown in the video.
video courtesy: mstkwon
Frequency at which resonance occurs is also known as resonant frequencies. When we look at the video structure having lower stiffness will first reach resonance because of lower natural frequency. At this stage other two structure are in in phase vibration. After some time 2nd structure starts resonating leaving 3rd one in out of phase and 1st one in in-phase vibration. Finally 3rd one starts resonating when frequency of shaking table is increased enough to match its natural frequency.