ABSTRACT
The representation of physical systems by means of discrete models in which properties like inertia, stiffness and damping are localized and identified with different elements like masses, springs and dampers is often very convenient and leads to satisfactory results in many circumstances. In reality, however, one has to deal with, say, aircraft structures, pipelines, car bodies, various types of buildings, etc.; in other words, with structures which generally comprise cables, rods, beams, plates and shells, all of which are neither rigid nor massless. Every material portion of the system may possess mass, stiffness and damping properties at the same time, and these properties may vary from point to point. In these cases, whenever possible, one can resort to continuous models (we already encountered some examples of such models in Chapters 3 and 5), where the displacement is a continuous function of both space and time and we are in presence of an infinite number of degrees of freedom.
