Abstract :
The theory of structural design was crystallized in 1826, the year of publication of Navierʹs Lecons. Navier showed how the internal forces in an elastic hyperstatic structure could be calculated, the most critical section identified, and sections assigned so that the corresponding stresses did not exceed a proportion of their limiting values, it was only over 100 years later that tests on full-scale structures showed that "actualʹʹstresses were very different from those predicted by elastic theory, and this has led directly to plastic methods of design. Paradoxically, those methods lead to the conclusion that the "actual" working state of a structure is essentially unknowable, since it depends crucially on very small imperfections of manufacture and construction. While these imperfections can have no effect on ductile collapse of the structure, some estimate of the working state must be made in order to design against possible instabilities. This aspect of structural design remains largely unexplored
