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20 CHAPTER 2 FINDINGS In accordance with the research approach, a review and There is also general agreement in the research community evaluation was conducted of existing models and approaches that the concrete contribution to shear resistance results prin- for shear design. This study revealed that there are dramati- cipally from a combination of interface shear transfer across cally different methods and bases for shear design provi- cracks in the body of the beam and shear in the compression sions. In Section 2.2, a comparison of relationships used in zone. However, because of the many different ways used to codes and suggested by researchers is made. This led to the calculate the angle of diagonal compression and the many identification of positive attributes of different shear design factors influencing interface shear transfer and shear transfer methodologies. Section 2.3 presents an evaluation of the in the compression zone, the existing forms of shear design accuracy of prominent shear design provisions. Section 2.4 provisions differ greatly. presents the results of a survey conducted to evaluate the For example, in determining the angle of diagonal com- experience of practitioners in using the LRFD Sectional pression it is traditional U.S. design practice to assume a Design Model and the AASHTO shear design provisions. 45-degree angle because this approach has been considered Using the findings from Sections 2.1 through 2.4, criteria to always lead to conservative designs. By contrast, in Euro- were developed for the simplified provisions. See Section pean practice the angle of diagonal compression is taken as 2.5. This led to the development of the proposed changes to low as 18 degrees while in the LRFD Sectional Design the LRFD Sectional Design Model and the Proposed Simpli- Model this angle is determined by considering the calculated fied Provisions presented in Chapter 3. longitudinal strain at mid-depth of the member. These Chapter 2 summarizes the findings. More comprehensive different approaches for determining the contribution of the results are presented in the following appendixes: shear reinforcement then lead to different approaches in calculating the concrete contribution to shear resistance Appendix A: Models for Shear Behavior because Vc = Vtest - Vs. Appendix B: Shear Design Provisions Before presenting and discussing the different shear Appendix C: Shear Database design relationships in codes of practice, it is useful to fur- Appendix D: Evaluation of Shear Design Provisions ther classify shear design approaches by the information on Appendix E: Field Performance Data and Practitioner which they are based: empirical test data, an equilibrium Experience model for the condition of a beam in its ultimate limit state, Appendix F: Recommended Revisions to Shear Provi- a comprehensive behavioral model for shear resistance, or sions of AASHTO LRFD Concrete Provisions some combination of the above. Relying on each of these Appendix G: Evaluation of the Proposed Simplified three types of information has its advantages and limitations Provisions with Selected Shear Database as discussed below. Appendix H: Examination of Proposals Using Design Database 2.1.1 Type 1: Empirical Relationships Appendix I: Utilization of NCHRP Process 12-50 Designed to Fit Test Data Appendix J: Examples of Shear Design Empirical provisions are those based primarily on experi- These appendixes are available in NCHRP Web-Only mental test data. Because of the complexity of how shear is Document 78. carried in structural concrete and the lack of a universally accepted model for shear behavior, this approach has many 2.1 DIFFERENCES IN UNDERLYING BASES clear advantages. No consensus is needed from any commit- OF CODE PROVISIONS tee and no selected model for behavior will bias the resulting provisions from accounting for the complexity of shear As discussed in Chapter 1, the 100-year-old parallel chord behavior. truss model is the predominant model for describing the flow The primary problem with this empirical approach is the of shear forces in a reinforced or prestressed concrete beam. deficiencies in the experimental test data that are available