If the size of a body be diminished, the strength of that body is not diminished in the same proportion; indeed the smaller the body the greater its relative strength. Thus, a small dog could probably carry on his back two or three dogs of his own size, but I believe that a horse could not carry even one of his own size.Galileo (1638)IntroductionThe testing of small-scale models is indispensable for complex structural systems which are difficult to analyse theoretically and numerically or to study experimentally. The dynamic response of underground structures, impact of nuclear fuel capsules missile impact of nuclear power installations and collision protection of ships illustrate several areas which have been studied with the aid of small-scale models.Dynamic tests are conducted on a small-scale model in order to obtain the response characteristics of a geometrically similar full-scale prototype which is the actual system of interest. This procedure is known as scaling, modelling or similitude and is governed by certain principles. Apart from the obvious purpose of relating the behaviour of a model to that of a prototype, these principles also predict various dimensionless combinations of the governing variables which are valuable for the planning of experimental investigations and the choice of numerical calculations.Introduction to geometrically similar scalingSome features of geometrically similar scaling for the dynamic response of structures are introduced in this section using an elementary approach. It is assumed, for simplicity of presentation, that a small-scale model and a full-scale prototype are made from an elastic material with the same values of Young's modulus of elasticity (E), mass density (ρ) and Poisson's ratio (ν).