Levels-of-Approximation Approach in Codes of Practice

Abstract Civil engineering projects typically involve a number of design phases, such as preliminary design, tender design and executive design. The required degree of accuracy for the analysis of the behaviour and strength of the structural members increases as a project evolves. Codes of practice, however, seldom reflect this situation and propose a single design expression to be used at all phases of a project. This is not convenient in a number of situations, leading to lengthy analyses for initial design phases and sometimes not allowing to consider all potential load-carrying mechanisms or strength reserves for advanced analyses (design of complex structures and assessment of critical existing structures). In opposition to this approach, codes of practice can privilege a design strategy named the levels-of-approximation (LoA) approach. It is based on the use of physically sound theories where the accuracy of the mechanical parameters used in the design expressions can be refined, if needed, in successive LoA. In the initial design phases, simple and safe hypotheses allow performing preliminary design tasks within limited time, checking principal dimensions and identifying critical regions and failure modes. Refinements on the values of the mechanical parameters in successive LoA thereafter allow to increase the accuracy of the estimate of the strength and behaviour. In this paper, the main ideas and advantages of the LoA approach are introduced and explained. An example of how this approach can be used with reference to the shear design of bridge deck slabs is also presented and discussed.