On the use of MCFT per AS 5100.5 for the assessment of shear capacities of existing structures

In the recent update of AS 5100 (2017 + Amdt. 1, 2018), the Modified Compression Field Theory (MCFT) was introduced for shear design of members. Part 7 of the code, which deals with the assessment of existing structures, directs engineers to use the MCFT provisions of Part 5 (for design) in assessing the shear capacity of an existing member. The MCFT provisions in Part 5 are intended for shear design, rather than shear capacity assessment. Without modification, false shear capacity assessments can result from their use. This note discusses the background to the design rules and explains how capacity assessment should differ. It shows how, through iteration, the design rules can be adopted to give capacity assessments for existing sections. It concludes by showing the errors that can arise for a range of sections, typical of bridge assessments, from the incorrect application of the shear design rules to an assessment problem. Recommendations are given to remove this ambiguity in future editions of AS 5100.7.