Structural dynamics is the engineering discipline concerned with the behaviour of structures subjected to dynamic loading (excitation).
Whereas it is possible to accurately predict the behaviour of structures under static loading, dynamic loading presents certain parameters that are difficult to model such as the damping of the structure. Therefore, to accurately determine a structure's response to a dynamic loading, it is necessary to perform some form of structural dynamic testing such as modal testing.
Modal testing allows you to experimentally determine the modal parameters of a structure. These include resonant frequency, damping and mode shape. The resonant frequency is the frequency at which any excitation produces an exaggerated response. This is important to know, as excitation close to a structure's resonant frequency will often produce adverse effects. These generally involve excessive vibration leading to potential fatigue failures, damage to the more delicate parts of the structure or, in extreme cases, the structure breaking up. As the resonant frequency relates to the structure's stiffness, it can be compared to the analytical calculations of the structure's stiffness, and thus used to correct the analytical model of a structure. The damping of the structure gives information on how quickly the structure dissipates vibrational energy and returns to rest when the excitation force is removed. Lightweight structures exhibit less damping than heavy structures, and the system's damping cannot be accurately determined analytically, it must be obtained by measurement. The modeshape is a representation of the structure's vibration pattern at a given resonant frequency.
Other structural dynamic measurements include ODS (Operational Deflection Shapes) which shows the vibration pattern of a structure while it is in operation or under operational conditions.