A well studied area is modelling of loading. Depending on the structure of interest, different types of loading will be prevalent. For example, a structure such as a bridge will experience a loading for each vehicle that passes over it, with the magnitude of the loading dependent upon factors such as weight of vehicles, and their time of arrival at the bridge. An aeroplane on the other hand, experiences stresses during flight, as a continuous function, coupled with spikes on take off and landing.
Measurement of the stresses at a particular point in a particular structure yields a trace which can be denoted S(t) representing the stress as a function of time. For different applications, the characteristic shape of this function will differ. It is useful to be able to classify the various stresses that will be experienced by structures, so that these may be taken into account during the design process. These functions may be summarised with statistics, which it is assumed yield a classification of the load experienced.