The task of the engineer entrusted with the design of a structure is two-fold. First, he must clearly appreciate and value the external forces which his structure must
resist. And, second, he must dispose and proportion the parts of his structure so that each is able to withstand the several forces it will be subjected to. In dealing with this latter, the consideration of how the material will behave under stress immediately arises. What is going on in material of his structure, as well as what is going on in the structure as a whole, becomes of importance.