National Academies Press: OpenBook

LRFD Minimum Flexural Reinforcement Requirements (2019)

Chapter: Chapter 1 - Introduction

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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. LRFD Minimum Flexural Reinforcement Requirements. Washington, DC: The National Academies Press. doi: 10.17226/25527.
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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2019. LRFD Minimum Flexural Reinforcement Requirements. Washington, DC: The National Academies Press. doi: 10.17226/25527.
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4 1.1 Overview To study the minimum flexural reinforcement require- ments for concrete flexural members, analytical and experi- mental programs were used in National Corporative Highway Research Program (NCHRP) Project 12–94. This report doc- uments the scope, objectives, and outcomes of that project. A set of recommendations to modify the current minimum flexural reinforcement requirements in the AASHTO LRFD Bridge Design Specifications, 8th edition (AASHTO 2017), and some design examples are also presented. 1.2 Problem Statement Traditionally, minimum flexural reinforcement has been used to ensure that a structural member would possess suf- ficient strength and ductility. For this purpose, ductility is defined loosely, in order to ensure that a brittle failure would not ensue beyond the flexural cracking limit state. This requirement in codes and specifications is regulated by requiring a member’s minimum flexural strength to be above a certain percentage of the cracking moment. For example, in the current AASHTO LRFD Bridge Design Specifications (2017), this ratio can reach up to 28% for reinforced concrete members. Variables that may affect this requirement include compressive strength of concrete, concrete cracking strength, type of cross section, amount of prestressing in the member, effects of creep and shrinkage, use of unbonded tendons, and load combinations. Therefore, there is a tendency for mem- bers to be designed with unnecessarily excessive amounts of longitudinal reinforcement merely to satisfy this requirement, although the needed reinforcement (based on the governing design load) is less. As a result, the cost of construction could be increased unnecessarily. Also, increasing the longitudinal reinforcement can potentially make the member fail in shear, which is associated with brittle failure. Further, utilizing an excessively conservative amount of minimum mild steel rein- forcement in pre- or post-tensioned concrete members can bring the member to an over-reinforced condition, that is, the member may fail in a compression-controlled mode, despite meeting the minimum flexural reinforcement requirement. From the design perspective, especially for prestressed con- crete members, increasing the nominal capacity of a member can result in increasing its cracking moment. This makes the design process iterative and, in some cases, without conver- gence, so that the current minimum flexural reinforcement requirement for post-tensioned members is difficult to satisfy. This also may lead to less efficient design of pre- and post- tensioned concrete flexural members. In 2005, the modulus of rupture of concrete was increased from 0.24 ′fc (ksi) to 0.37 ′fc (ksi) in the AASHTO LRFD Bridge Design Specifications in response to research find- ings on high-strength concrete, where f ’c is the compressive strength of concrete. Consequently, significantly more rein- forcement and/or prestress was needed to meet the minimum reinforcement requirement. It was argued that the suggested increase in flexural cracking strength was inappropriate, as the test data used for this justification were largely influenced by heat-cured conditions. However, it was demonstrated that up to 20% to 30% additional prestressing steel could be required to meet these provisions in segmental box girders (Holombo and Tadros 2010). In NCHRP Project 12–80, Holombo and Tadros (2010) tackled these concerns, and their work ultimately led to the following findings: 1. The modulus of rupture of 0.37 ′fc (ksi) for estimat- ing the flexural cracking moment in accordance with the AASHTO LRFD was too high. Overestimating the flex- ural cracking moment would unnecessarily increase the minimum reinforcement requirement, so the modulus of rupture was changed back to 0.24 ′fc (ksi). 2. Lightly reinforced concrete members have significant duc- tility and sufficient strength beyond experiencing flexural cracking in a displacement-controlled loading regime. C H A P T E R 1 Introduction

5 As a result of this study, a reliability-based approach for quantifying minimum reinforcement was proposed, and the suggested changes were adopted in Section 5.7.3.3.2 of the AASHTO LRFD Bridge Design Specifications, 6th edition (2012), which defined minimum reinforcement differently for concrete members, prestressed girders, and segmentally constructed beams. However, NCHRP 12–80 did not include experimental verification to support the analytical find- ings. The main emphasis of NCHRP 12–94 was, therefore, to provide experimental verification of the current AASHTO provisions and establish appropriate refinements to the spec- ifications for minimum reinforcement. 1.3 Research Objectives and Scope The goals of this study were to examine the current AASHTO LRFD specifications with regard to minimum flex- ural reinforcement and to improve the effectiveness of the minimum flexural design requirement. These goals were accomplished by achieving the following objectives: • Complete comprehensive analytical and experimental studies utilizing reinforced, pre-tensioned and post- tensioned concrete flexural members constructed with, precast, cast-in-place concrete and segmental construction practices, and verify the current AASHTO specifications on minimum flexural reinforcement. • Predict the performance of test units and improve the analysis capabilities appropriately. • Develop recommended changes to the AASHTO LRFD Bridge Design Specifications and commentary to reflect the new knowledge gained from the project with regard to the minimum reinforcement requirement. Five phases with a total of 15 tasks were completed in this project to achieve the aforementioned objectives. The five phases of the project and the tasks within each phase were as follows: Phase I. Research: 1. Critical review of relevant specifications, technical litera- ture, and owner/industry experiences; 2. Development of analytical and testing programs to inves- tigate minimum flexural reinforcement; and 3. Preparation of Interim Report No. 1. Phase II. Analytical Investigation: 4. Execution of the approved work plan for the analytical program; 5. Identification of areas of the AASHTO LRFD Bridge Design Specifications that might require modification; 6. Finalization of the testing program to validate the findings of the analytical program; and 7. Preparation of Interim Report No. 2. Phase III. Testing Program and Analytical Program Validation: 8. Execution of the testing program according to the approved work plan; 9. Validation and finalization of the analytical program on the basis of the test results; and 10. Completion of Interim Report No. 3. Phase IV. Proposed Modifications to the AASHTO LRFD Bridge Design Specifications: 11. Development of specifications and commentary lan- guage for proposed changes to the AASHTO LRFD Bridge Specifications on the basis of the analytical and testing investigations, and supported with examples; 12. Quantification of the potential economic impact of the proposed revisions; and 13. Preparation of Interim Report No. 4. Phase V. Final Products: 14. Update of the proposed modifications to the AASHTO LRFD Bridge Specifications after consideration of Interim Report No. 4 comments and preparation of ballot items for the consideration of the AASHTO Highway Sub- committee on Bridges and Structures and 15. Completion of the final report. 1.4 Organization This report is organized in five chapters. Chapter 1 sum- marizes the project objectives. Chapter 2 includes the rel- evant literature review on past research and various codes, standards, and specifications worldwide regarding minimum flexural reinforcement requirements. Chapter 3 presents the experimental and analytical results. Chapter 4 provides recom- mendations for changes to the current specifications and com- mentaries on the minimum reinforcement requirements in the AASHTO LRFD Bridge Design Specifications. Finally, Chapter 5 presents a summary and conclusions from this project. A number of deliverables for the project are provided in three appendices to the contractor’s final report that are available on the NCHRP Project 12–94 web page on the TRB website (trb.org). Appendix A: Test Girder Drawings contains detailed drawings of the test units; Appendix B: Design Examples presents design examples based on the recommended revisions to the AASHTO specifications; and Appendix C: Parametric Study Results provides more infor- mation on the results of the parametric study carried out in this study.

Next: Chapter 2 - State of Knowledge on Minimum Reinforcement Requirements »
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TRB’s National Cooperative Highway Research Program (NCHRP) Research Report 906 includes proposed revisions to the AASHTO LRFD Bridge Design Specifications minimum flexural reinforcement provisions for load and resistance factor design (LRFD) with detailed design examples illustrating the application of the proposed revisions.

According to the AASHTO LRFD Bridge Design Specifications, minimum reinforcement provisions are intended to reduce the probability of brittle failure by providing flexural capacity greater than the cracking moment. There was a concern with the current American Association of State Highway and Transportation Officials (AASHTO) LRFD minimum flexural reinforcement requirements, especially when applied to pretensioned or post-tensioned concrete flexural members.

A number of deliverables for the project are provided in three appendices to the contractor’s final report that are available online. They include the following:

Appendix A: Test Girder Drawings,

Appendix B: Design Examples, and

Appendix C: Parametric Study Results.

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