In general, these phenomena are very easy to perceive and understand, but difficult to calculate. The danger of failure increases rapidly with decrea… - Eduardo Torroja

[T]here are three different but interconnected conceptions to be considered in every structure, and in every structural element involved: equilibrium, resistance, and stability.

[M]ost of our structures in the past consisted of individual members having relatively small transverse dimensions as compared with their length, and in such members stress analysis is much more simple. ...[M]any modern structures consist of frameworks of this type, and frequently the word "structure" refers specifically to assemblies consisting of linear elements.

The equilibrium... in order to become static—should be steady, permanent, lasting. ...This type of equilibrium... is... independent of any scale. A reduced model will show the same effects as the proper structure. Experiments on models are simple and... instrumental for understanding such structural problems.