In structural analysis, the actions are considered quasi-static, so that the calculation and design of structures can be carried out without considering inertial forces by means of equilibrium considerations on the structural system.
The assumption of quasi-static actions is often no longer justified since dynamic actions act on structures. The calculation and assessment of dynamically stressed structures is the subject of structural dynamics, which considers the temporal variation of the dynamic actions and the inertial forces activated in the structure. The dynamic actions excite the structure to vibrate, whereby the actions either act directly on the structure or are introduced into the structure via the subsoil.
A typical example of a time-varying dynamic load is found in bridge structures because of traffic excitation by road traffic.
Common high-rise structures such as houses, bridges, etc. are "static" structures. People who spend time in them assume that these structures do not move. In reality, every structure moves, only the movements are so small that they are not felt.
If this perceptibility limit (~ 0.1 mm/s) is exceeded, vibrations are very quickly perceived as unpleasant. If the vibrations become even stronger, not only the comfort of people can be impaired, but also a "reduction" of the serviceability of the building can occur: Cracks appear, objects shake, etc. In extreme cases, it can even endanger the stability of the entire building.
Depending on the vibration velocity, the effects can be divided into four classes according to the figure below. These four classes are described in detail in the subsections.