Theoretical issues in ultimate limit state design

This letter proposes a fundamental theoretical framework for undertaking ultimate limit state design based on the use of an interaction diagram or yield surface. Conceptually (using Eurocode terminology), the design state is obtained from the characteristic state, not by applying partial factors, but by applying a perturbation force or stress probe within this yield surface until factored actions become equal to factored resistances. This robustly ensures mechanical rigour at all times. The method gives clarity as to how calculations should be carried out and removes ambiguity about factoring actions not in the direction of the perturbing force. Several examples are given to illustrate its application.