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An approach to improve the use of Strut-and-Tie Model (STM) for better structural performance design

posted Oct 14, 2009, 11:11 PM by Domain Admin   [ updated Sep 30, 2012, 3:41 AM ]
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.

To overcome these limitations, the new approach is proposed based on an experimental and computational program conducted during my PhD research at University of Illinois. In addition, my PhD research also aimed to generate the data needed for evaluating and validating the STM approach, for advancing STM shape selection techniques, and also to develop more reliable computational models for predicting the response of cracked structural concrete. These research studies result in a new approach for design and analysis D-Regions for better structural performance of D-Regions. In addition, the approach enables automated nonlinear FEA design validation that integrates the current STM design provisions with two additional components: (1) the shape selection technique of STM based on topology optimization and (2) the new automated nonlinear FEA tool that was specialized for modeling and analyzing complex D-Regions.

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