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Problem Statement |
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The Strut-and-Tie Method (STM) is an emerging and rational design procedure that has the potential to revolutionize the way that engineers design D- (Discontinuity) Regions in structural concrete. This is of great significance as most structural problems occur in D-Regions due to the inadequacy of empirical design provisions and detailing practices. D-Regions are those parts of structures in which concentrated forces or discontinuities, such as a change in section depth or opening, induce a complex state of strain in the surrounding region. A substantial portion of structures, including joints, corbels, deep beams, and pile caps, are D type regions. The STM involves envisioning that an internal truss, consisting of concrete struts and steel tension ties connecting in nodal zones, carries the load through the D-Region to its supports or boundaries. With this idealization, structural design involves detailing reinforcement to serve as the ties and checking that the capacity of the struts and joints (nodal zones) are sufficient to carry the design loads. Unfortunately there are barriers to the STM becoming the dominant design methodology for D-Regions. They include unresolved and fundamental design procedure issues, an unnecessarily time consuming design process, and the unfamiliarity of the practicing and educational communities with this design methodology. The following three activities are proposed to help overcome these barriers.
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Experimental and Analytical Research to Address Critical and Fundamental STM Issues |
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The objective of the proposed experimental research is to develop relationships and values that describe the effective dimensions, stiffness characteristics, and strength of struts, ties, and nodal zones. This includes quantifying the effect of different boundary conditions and the benefits of reinforcement products that provide anchorage, confinement, and ductility. The development of these relationships will be made possible through the acquisition and use of an advanced non-contact instrumentation system that can very accurately and quickly measure the position in 3-dimensional space of hundreds of small markers mounted on the surface of the test specimens. A non-linear finite element program will be used to help plan the experimental program, expand the scope of the investigation, and predict displacements. The measured displacements will be published on the web as a service to help facilitate the validation and calibration of all non-linear finite element tools for reinforced and prestressed concrete structures.
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Development and Dissemination of Educational Resources for Instructors, Students, and Designers |
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The majority of US educators and practitioners are not familiar with the STM. This presents a special opportunity to develop educational resources that will make the educational process efficient, interesting, and complete. Beyond CAST, instructional resources to be developed will include an educational video, presentation slides, an instructional topic booklet, an interactive web-based teaching module, worked examples, and unsolved problems. These materials would be promoted at conferences and be made available from a designated strut-and-tie web site.
The PI is well qualified to undertake this project due to his extensive experimental research background and active participation on ACI committees charged with developing STM design provisions and examples. To ensure that the proposed work meets its objectives, a team of 14 collaborators has been established that well represent the educational, academic, industrial, and practicing communities. Funding of this proposal would help the PI develop as a leading researcher in understanding the behavior of cracked reinforced concrete subject to a complex state of stress (D-Regions) and as a developer of practical tools for education and design.
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Continued Development of the Computer Aided Strut-and-Tie (CAST) Education and Design Tool |
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It has been the PI’s experience that students are drawn to the simplicity and versatility of the STM. However, this beauty is often encumbered by the need to make repetitive calculations of truss geometry, member dimensions, and stress values. Over the last 2 years, program CAST has been under development by the PI to provide a graphical tool to make the design process more efficient and transparent. It currently enables users to draw the boundaries of a structure, specify loading, draw an internal supporting truss, calculate member forces values, select reinforcement, and make on-screen adjustments of design variables. Funding of this proposal would enable the addition of the many features that are necessary to make this program a comprehensive education and design tool. These features include libraries of common types of D-Regions and trusses, automatic generation of node forces from section force values, automesh generation, illustration of principal elastic stress trajectories, the ability to combine adjacent struts, the ability to evaluate multiple load cases, a reinforcement detailing utility, CAD input/output features, and tutorials. This program will be designed to serve the needs of students, instructors, and practicing engineers. |
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