**Resonance**

**Resonance ** is a phenomenon which occurs when the vibration of a system tends to reach its maximum magnitude. The frequency corresponding to resonance is known as the resonance frequency of the system. When the damping of the system is small, the resonance frequency is approximately equal to the natural frequency of the system.

- The resonance frequency is related to the damping ratio, the input (loading or ground motion) and the selected measurement parameter (relative or absolute movement, displacement, velocity or acceleration).
- Increasing damping will effectively reduce the response of the structure at resonance.

For a single degree-of-freedom (SDOF) system subject to a harmonic input, such as a direct load or a base motion, there are three characteristics:

- The maximum dynamic displacement is close to the static displacement for a given load amplitude if the load frequency is less than a quarter of the natural frequency of the system.
- The maximum dynamic displacement is less than the static displacement if the load frequency is more than twice the natural frequency of the system.
- The maximum dynamic displacement is several times the static displacement if the load frequency is close to or matches the natural frequency of the system.