Based on several years of my practical and research experience in using Strut-and-Tie Model (STM) in design and analysis of D-regions-both simple and complex types, I found that design provisions, such as those in Appendix A of ACI318-08, CEB-FIP, AASHTO LRFD, etc., consist of rules for evaluating the capacity of the load-resisting truss that is idealized to carry the forces through the D-Region. These code rules were primarily derived from test data on simple D-Regions such as deep beams and corbels. However, these STM provisions are taken as being sufficiently general and conservative that they can be used for the design of all possible D-Regions, including those regions in which a highly statically indeterminate and complex truss is selected and designed to carry the imposed loadings. Since STM design provisions are only for capacity assessment and given the wide range of applicability of the method, members designed using these STM provision may not necessarily exhibit satisfactory performance under service load levels or during overloads.

Thus, I can simply point out more particular limitations of current STM design provisions include: (1) no methodology for satisfactorily estimating the complete load-displacement response history, cracking loads, stress and strain states in reinforcement, and failure modes; (2) a lack of guidance for selecting the shape, dimensions, and stress limits of STM components; (3) a lack of guidance for proportioning of forces in statically indeterminate STM designs; and (4) an inability to employ performance-based design concepts.


For more information and details, please refer to my Ph.D. Dissertation.