One of the most fundamental question that an Engineer has to ask him/herself is what is how does it deform, and when does it break. Ultimately, it its the answer to those two questions which would provide us with not only a proper safety assesment of a structure, but also how to properly design it. Ironically, botht he ACI and the AISC codes are based on limit state design, yet practically all design analyses are linear and elastic. On the other hand, the Engineer is often confronted with the task of determining the ultimate load carying capacity of a structure or to assess its progressive.
