In the first place we need to define what is meant by the term structure. The reader may well have a mental picture of a bridge, dam or large building; any of these would most certainly be classed as a structure, but they severely limit the use of a term which can be applied in a much wider sense. A car, aeroplane, milk crate or even the chair that you are sitting on are all structures. They are all designed to carry a particular form of applied loading and, what is most important, in the majority of cases only deform by a small amount when the loading is applied. All engineering structures or structural elements are subject to external forces or loads. These will induce other external forces or reactions at the points of support of the structure. Consider the chair: it is designed to carry an external load, provided by your own mass; forces are transmitted through the various members of the chair and the reactions act at the ends of the legs. In the case of an aeroplane in level flight, there must be a vertical component of reaction acting on the wings as a lift force to balance the mass of the plane. The structure of the wing is so designed that it is capable of transmitting this force to the fuselage.
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