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2019 N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP RESEARCH REPORT 906 LRFD Minimum Flexural Reinforcement Requirements Sri Sritharan Hartanto Wibowo Michael J. Rosenthal Jacob N. Eull Iowa State UnIverSIty Ames, IA Jay Holombo t. y. LIn InternatIonaL San Diego, CA Subscriber Categories Bridges and Other Structures Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration
NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM Systematic, well-designed, and implementable research is the most effective way to solve many problems facing state departments of transportation (DOTs) administrators and engineers. Often, highway problems are of local or regional interest and can best be studied by state DOTs individually or in cooperation with their state universities and others. However, the accelerating growth of highway transporta- tion results in increasingly complex problems of wide interest to high- way authorities. These problems are best studied through a coordinated program of cooperative research. Recognizing this need, the leadership of the American Association of State Highway and Transportation Officials (AASHTO) in 1962 ini- tiated an objective national highway research program using modern scientific techniquesâthe National Cooperative Highway Research Program (NCHRP). NCHRP is supported on a continuing basis by funds from participating member states of AASHTO and receives the full cooperation and support of the Federal Highway Administration, United States Department of Transportation. The Transportation Research Board (TRB) of the National Academies of Sciences, Engineering, and Medicine was requested by AASHTO to administer the research program because of TRBâs recognized objectivity and understanding of modern research practices. TRB is uniquely suited for this purpose for many reasons: TRB maintains an extensive com- mittee structure from which authorities on any highway transportation subject may be drawn; TRB possesses avenues of communications and cooperation with federal, state, and local governmental agencies, univer- sities, and industry; TRBâs relationship to the National Academies is an insurance of objectivity; and TRB maintains a full-time staff of special- ists in highway transportation matters to bring the findings of research directly to those in a position to use them. The program is developed on the basis of research needs identified by chief administrators and other staff of the highway and transportation departments, by committees of AASHTO, and by the Federal Highway Administration. Topics of the highest merit are selected by the AASHTO Special Committee on Research and Innovation (R&I), and each year R&Iâs recommendations are proposed to the AASHTO Board of Direc- tors and the National Academies. Research projects to address these topics are defined by NCHRP, and qualified research agencies are selected from submitted proposals. Administration and surveillance of research contracts are the responsibilities of the National Academies and TRB. The needs for highway research are many, and NCHRP can make significant contributions to solving highway transportation problems of mutual concern to many responsible groups. The program, however, is intended to complement, rather than to substitute for or duplicate, other highway research programs. Published research reports of the NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM are available from Transportation Research Board Business Office 500 Fifth Street, NW Washington, DC 20001 and can be ordered through the Internet by going to http://www.national-academies.org and then searching for TRB Printed in the United States of America NCHRP RESEARCH REPORT 906 Project 12-94 ISSN 2572-3766 (Print) ISSN 2572-3774 (Online) ISBN 978-0-309-48057-4 Library of Congress Control Number 2019944465 Â© 2019 National Academy of Sciences. All rights reserved. COPYRIGHT INFORMATION Authors herein are responsible for the authenticity of their materials and for obtaining written permissions from publishers or persons who own the copyright to any previously published or copyrighted material used herein. Cooperative Research Programs (CRP) grants permission to reproduce material in this publication for classroom and not-for-profit purposes. Permission is given with the understanding that none of the material will be used to imply TRB, AASHTO, FAA, FHWA, FMCSA, FRA, FTA, Office of the Assistant Secretary for Research and Technology, PHMSA, or TDC endorsement of a particular product, method, or practice. It is expected that those reproducing the material in this document for educational and not-for-profit uses will give appropriate acknowledgment of the source of any reprinted or reproduced material. For other uses of the material, request permission from CRP. NOTICE The research report was reviewed by the technical panel and accepted for publication according to procedures established and overseen by the Transportation Research Board and approved by the National Academies of Sciences, Engineering, and Medicine. The opinions and conclusions expressed or implied in this report are those of the researchers who performed the research and are not necessarily those of the Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; or the program sponsors. The Transportation Research Board; the National Academies of Sciences, Engineering, and Medicine; and the sponsors of the National Cooperative Highway Research Program do not endorse products or manufacturers. Trade or manufacturersâ names appear herein solely because they are considered essential to the object of the report.
The National Academy of Sciences was established in 1863 by an Act of Congress, signed by President Lincoln, as a private, non- governmental institution to advise the nation on issues related to science and technology. Members are elected by their peers for outstanding contributions to research. Dr. Marcia McNutt is president. The National Academy of Engineering was established in 1964 under the charter of the National Academy of Sciences to bring the practices of engineering to advising the nation. Members are elected by their peers for extraordinary contributions to engineering. Dr. C. D. Mote, Jr., is president. The National Academy of Medicine (formerly the Institute of Medicine) was established in 1970 under the charter of the National Academy of Sciences to advise the nation on medical and health issues. Members are elected by their peers for distinguished contributions to medicine and health. Dr. Victor J. Dzau is president. The three Academies work together as the National Academies of Sciences, Engineering, and Medicine to provide independent, objective analysis and advice to the nation and conduct other activities to solve complex problems and inform public policy decisions. The National Academies also encourage education and research, recognize outstanding contributions to knowledge, and increase public understanding in matters of science, engineering, and medicine. Learn more about the National Academies of Sciences, Engineering, and Medicine at www.national-academies.org. The Transportation Research Board is one of seven major programs of the National Academies of Sciences, Engineering, and Medicine. The mission of the Transportation Research Board is to increase the benefits that transportation contributes to society by providing leadership in transportation innovation and progress through research and information exchange, conducted within a setting that is objective, interdisciplinary, and multimodal. The Boardâs varied committees, task forces, and panels annually engage about 7,000 engineers, scientists, and other transportation researchers and practitioners from the public and private sectors and academia, all of whom contribute their expertise in the public interest. The program is supported by state transportation departments, federal agencies including the component administrations of the U.S. Department of Transportation, and other organizations and individuals interested in the development of transportation. Learn more about the Transportation Research Board at www.TRB.org.
C O O P E R A T I V E R E S E A R C H P R O G R A M S AUTHOR ACKNOWLEDGMENTS The research presented in this report was performed under NCHRP Project 12-94 by the Department of Civil, Construction, and Environmental Engineering at Iowa State University (ISU), Ames, Iowa. This project was administered through the Institute for Transportation (InTrans) at ISU. Sri Sritharan of ISU was the principal investigator and author of this report. Hartanto Wibowo of ISU was the senior researcher and coauthor of this report. Other research team members and coauthors included Michael J. Rosenthal and Jacob N. Eull of ISU, who assisted with the experimental and analytical studies, and Jay Holombo of T. Y. Lin International, San Diego, California, who assisted with the develop- ment of the recommendations for revisions to the current AASHTO LRFD Bridge Design Specifications and the design examples. The research team also acknowledges the contribution of Sami Megally of Kleinfelder, San Diego, and Ryan A. Bodendorfer of ISU, in the earlier stage of this research. The experimental work was also assisted by Doug Wood and Owen Steffens, both laboratory personnel at ISU. The epoxy and grout used in the experiment were donated by PILGRIM and BASF, respectively. Finally, the research team appreciates the comments, reviews, and input from the NCHRP Project Panel. CRP STAFF FOR NCHRP RESEARCH REPORT 906 Christopher J. Hedges, Director, Cooperative Research Programs Lori L. Sundstrom, Deputy Director, Cooperative Research Programs Waseem Dekelbab, Senior Program Officer Megan Chamberlain, Senior Program Assistant Eileen P. Delaney, Director of Publications Natalie Barnes, Associate Director of Publications Janet M. McNaughton, Senior Editor NCHRP PROJECT 12-94 PANEL Field of DesignâArea of Bridges Hussam Z. âSamâ Fallaha, Florida DOT, Tallahassee, FL (Chair) Geetha Chandar, Texas DOT, Austin, TX Elmer E. Marx, Alaska DOT and Public Facilities, Juneau, AK Edmund H. Newton, South Darmouth, MA Don L. Nguyen-Tan, California DOT, Sacramento, CA Chuanbing Sun, Louisiana Tech University, Ruston, LA Teddy S. Theryo, Florida DOT, Tallahassee, FL Benjamin A. Graybeal, FHWA Liaison Stephen F. Maher, TRB Liaison
F O R E W O R D By Waseem Dekelbab Staff Officer Transportation Research Board This report presents proposed revisions to the AASHTO LRFD Bridge Design Specifica- tions minimum flexural reinforcement provisions for load and resistance factor design (LRFD) with detailed design examples illustrating the application of the proposed revi- sions. The proposed revisions are based on comprehensive analytical and large-scale testing programs for investigating the minimum flexural reinforcement requirements with consideration to scale effects. The material in this report will be of immediate interest to bridge engineers. 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 AASHTO LRFD minimum flexural reinforcement requirements, especially when applied to pre- tensioned or post-tensioned concrete flexural members. Increasing the nominal capacity of a member can result in increasing the cracking moment of the same member. This makes the design process iterative, and the current minimum reinforcement requirement for post- tensioned segmentally connected members is difficult to satisfy. This also may lead to less efficient design of pre-tensioned/post-tensioned concrete flexural members. Provisions for the design of minimum flexural reinforcement must be suitable for all structure and reinforcement types covered by the AASHTO LRFD Bridge Design Specifications and should be consistent with the LRFD design philosophy. Under NCHRP Project 12-94, Iowa State University was asked to propose revisions to the AASHTO LRFD Bridge Design Specifications minimum flexural reinforcement provisions. The research considered reinforced, pre-tensioned, and post-tensioned concrete flexural members (e.g., bonded and unbonded tendons for segmentally and other than segmentally constructed bridges). A number of deliverables for the project are provided in three appendices to the contrac- torâs final report that are available on the NCHRP Project 12-94 web page on the TRB website (trb.org): â¢ Appendix A: Test Girder Drawings, â¢ Appendix B: Design Examples, and â¢ Appendix C: Parametric Study Results.
C O N T E N T S 1 Summary 4 Chapter 1 Introduction 4 1.1 Overview 4 1.2 Problem Statement 5 1.3 Research Objectives and Scope 5 1.4 Organization 6 Chapter 2 State of Knowledge on Minimum Reinforcement Requirements 6 2.1 Introduction 6 2.2 Philosophical Background 7 2.3 Flexural Member Behavior with Minimum Reinforcement 15 2.4 Methodologies and Code Approaches 21 2.5 Past Research on AASHTO Minimum Reinforcement Requirements 22 2.6 Factors Affecting Minimum Reinforcement Requirements 24 2.7 Summary of Findings 25 Chapter 3 Experimental and Analytical Investigations 25 3.1 Introduction 25 3.2 Experimental Test Matrix 25 3.3 Experimental Test Setup 49 3.4 Experimental Test Results 71 3.5. Analytical Study 85 Chapter 4 Specifications Update 85 4.1 Introduction 85 4.2 Criteria for Evaluation 85 4.3 Specification Update Evaluation 91 4.4 Parametric Study of Proposed Updates to Specifications 94 4.5 Recommendations 94 4.6 Proposed Revisions to the AASHTO LRFD Specifications 98 Chapter 5 Conclusions 98 5.1 Introduction 98 5.2 Review of U.S. and International Practice 98 5.3 Experimental Investigation 99 5.4 Analytical Study 99 5.5 Recommended Changes to AASHTO Specifications
100 List of Notations 103 Abbreviations 104 References Note: Photographs, figures, and tables in this report may have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.trb.org) retains the color versions.