Previous posts in this series have looked at the analysis of beams where the steel stress is within the elastic range, and the concrete stress is sufficiently low that the assumption of linear elastic behaviour in compression is a reasonable approximation. I now move on to analysis for the ultimate limit state, where some or all of the steel is past its yield stress, and the concrete compression zone may be analysed as having a constant stress, close to its failure stress.
The file ULS Design Functions.zip contains User Defined Functions (UDF’s) that will find the ultimate bending capacity of any reinforced or prestressed concrete section (defined by rectangular or trapezoidal layers) with any number of layers of reinforcement, when subject to a specified axial load. As for the elastic design functions, the analysis uses a closed form solution, rather than an iterative procedure. The current version follows the Australian Standards AS 3600 or AS 5100. Other codes will be added to future versions. Details of the procedure will be given in later posts.