Guidelines for Inspection and Strength Evaluation of Suspension Bridge Parallel Wire Cables (2004)

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T R A N S P O R T A T I O N R E S E A R C H B O A R D WASHINGTON, D.C. 2004 www.TRB.org NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM NCHRP REPORT 534 Research Sponsored by the American Association of State Highway and Transportation Officials in Cooperation with the Federal Highway Administration SUBJECT AREAS Bridges, Other Structures, Hydraulics and Hydrology Guidelines for Inspection and Strength Evaluation of Suspension Bridge Parallel Wire Cables R. M. MAYRBAURL S. CAMO Weidlinger Associates, Inc. New York, NY

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AUTHOR ACKNOWLEDGMENTS The authors wish to acknowledge the contributions of the fol- lowing people who helped in the successful completion of the guidelines and report: Robert G. Easterling, for writing Appendix B of the Report and for his painstaking suggestions regarding sta- tistics; the several subconsultants who supplied detailed drafts in their areas of expertise, including Ronald Latanision of Altran on corrosion mechanisms, Mark Mittelman of Altran on microbially- influenced corrosion, and Robert Torbin and Paul Belkus of Foster- Miller on nondestructive testing; William Moreau and the New York State Bridge Authority for permission to use information regarding inspections of the Bear Mountain Bridge and Mid- Hudson Bridge; Martin Kendall for supplying the actual copies of the Bear Mountain Bridge inspection reports; Chris Gagnon for providing reports on Bridge Z wire testing; Bojidar Yanev and the New York City Department of Transportation for providing the Williamsburg Bridge inspection reports and for granting permis- sion to use information regarding their inspection procedures; Par- sons Transportation Group for providing inspection photographs; all the bridge owners for responding to the original questionnaires; and Jeffrey Hu for managing the questionnaire procedure and the resulting data and Wei Liu for checking the examples. The authors are especially grateful to Helen Goddard for editing both the guidelines and the report, a task that almost exceeded the writing of them. COOPERATIVE RESEARCH PROGRAMS STAFF FOR NCHRP REPORT 534 ROBERT J. REILLY, Director, Cooperative Research Programs CRAWFORD F. JENCKS, Manager, NCHRP DAVID B. BEAL, Senior Program Officer EILEEN P. DELANEY, Director of Publications KAMI CABRAL, Associate Editor NCHRP PROJECT 10-57 PANEL Field of Materials, Construction, and Maintenance—Area of Materials and Construction M. MYINT LWIN, FHWA (Chair) MACIEJ BIENIEK, Columbia University, NY JACKSON DURKEE, Bethlehem, PA CHRISTOPHER HAHIN, Illinois DOT TIMOTHY M. MOORE, Washington DOT WALTER PODOLNY, JR., Burke, VA THOMAS RUT, Moffatt & Nichol, Stockton, CA BOJIDAR YANEV, New York City DOT HAROLD BOSCH, FHWA Liaison Representative STEVEN F. MAHER, TRB Liaison Representative

This report contains recommendations for the inspection and strength evaluation of suspension bridge parallel-wire cables. A companion CD-ROM provides details of the research program undertaken to develop the guidelines and explanations of the guidelines’ recommendations. The material in this report will be of immediate interest to suspension bridge owners and suspension bridge main cable inspectors. There are nearly 50 major suspension bridges in the United States, and more than half of them are more than 50 years old. These bridges represent major investments and are essential transportation links for regional and national commerce and lifelines. As this group of major structures advances in age, the number of in-depth evaluations to determine their condition and load-carrying capacity is expected to increase. The need to estimate remaining service life and to take preventative steps to extend service life will also increase. There has been no reliable and nationally recognized procedure, either practical or theoretical, to inspect and evaluate the condition and strength of suspension bridge parallel-wire cables. At the NCHRP-sponsored “Workshop on Safety Appraisal of Sus- pension Bridge Main Cables” held in Newark, New Jersey, in 1998, the highest prior- ity research needs identified were (1) development of cable inspection, sampling, and testing guidelines and (2) development of models to predict the strength of deteriorated cables. Unreliable methods of inspection and evaluation could result in unnecessary replacement or in unexpected failures. The objective of this research was to develop the needed guidelines for inspection and evaluation of suspension bridge parallel-wire cables. These guidelines provide details of the cable inspection process, including wire sampling and testing. Complete instructions, illustrated with examples, are provided for using the condition and prop- erties of the cable wire, determined by inspection and subsequent laboratory testing, for estimating cable strength. An accompanying CD-ROM (CRP-CD-54) contains a full account of the research leading to the development of the recommended guidelines and provides derivations of all equations. The research was performed by Weidlinger Associates, Inc., with the assistance of Altran Corporation and Foster-Miller, Inc. FOREWORD By David B. Beal Senior Program Officer Transportation Research Board

The Guidelines were written as a general resource for all persons who have respon- sibility for assessing cable integrity and to clarify and standardize the process of eval- uating a cable in service for an extended period of time. They will not make a suspen- sion bridge expert out of a novice, but are intended for use by engineering professionals who are already versed in the design and analysis of suspension bridges and have some knowledge of bridge cables. A cable evaluation team should consist of the following: • Chief investigator, a professional engineer with expertise in suspension bridges who leads all phases of the project • Chief inspector, a professional engineer with experience in bridge inspections, preferably cable inspections • One or more cable inspectors, graduate engineers with two to three years of design or inspection experience • Office staff, graduate engineers with experience in computer analysis, spread- sheets and applying mathematical equations. • Qualified testing laboratory • Metallurgical and corrosion consultants • Statistician (as needed) The Guidelines present a series of orderly steps that define a thorough cable eval- uation from planning inspections through strength estimation. They concern parallel- wire cables only, although portions of the text may be applied to helical strand cables. If inspection is limited to only a few panels, the Guidelines cannot provide information about the strength of the weakest point in the cable. However, they do contain recom- mendations for when more thorough investigations are needed. The Guidelines are arranged in two columns, as are other specifications and man- uals published by AASHTO. In general, a description of the required tasks is on the left, pertinent comments and background information are on the right. The columns are merged in Section 1, which is introductory. Figures and tables appear at the end of each section. They are numbered consecutively, prefixed by the article numbers in which they are described (e.g., Figure 1.4.2.1-1). Section 1 is mostly a general description of bridge cables, including their internal construction, connections to the bridge structure, and protection systems. Figures are used to illustrate the various parts of the cable system. The causes of corrosion are dis- cussed, as well as investigative techniques to locate corrosion. Of special interest are photographs showing the visual rating scale for corroded wires. The section also includes a list of persons who should use the Guidelines, a glossary of technical terms, and information on health and safety requirements. Section 2 presents three levels of inspection: routine visual inspections by mainte- nance personnel, biennial inspections, and internal inspections that expose the wires inside the cable. The section also contains instructions about the data to be recorded PREFACE

and measured, and the requirements for removal of wire samples. Recommendations for frequency and locations of internal inspections are based primarily on the data from a limited number of cable inspections in which a significant number of panels was opened. As more cables are inspected, the results can be combined with existing data to justify or modify these recommendations. Acoustic monitoring of wire breaks, a recent development, is referred to briefly in Section 2. The sound of a wire as it breaks inside the cable is recorded and the catalog of recent breaks is used to decide when, and especially where, to perform the next inspection. This is very helpful with older cables, because timing of inspections and selection of the best locations are critical. Section 3 lists the requirements for the physical and chemical tests that are made on samples removed from the cable, including tensile tests, and tests to determine the chemical composition of wires and the condition of the zinc coating. Section 4 focuses on the techniques used to catalog the damage inside the cable and the statistical analysis of test results. The calculations for obtaining the mean val- ues and standard deviations of wire properties that are needed for strength estimation (tensile strength and, in some cases, ultimate elongation) do not require advanced knowledge of statistics, but can be performed using standard spreadsheet programs, or even by hand. A slightly more sophisticated statistical analysis is used to estimate the probable minimum value of these properties in a given length of wire, which is much the same as estimating the strength of the weakest link in a chain. The graphs that are provided reduce the complexity of the latter analysis substantially. It has been noted during internal cable inspections that friction among the wires introduces tension back into a broken wire as the distance from the break location increases. A method of estimating the force that is reintroduced as the wire passes through a cable band is presented in Section 4. In this analysis, the effects of wrapping wire, which are not negligible, are conservatively ignored. Three models for estimating cable strength are given in Section 5 based on the fol- lowing assumptions. All wires are subject to the same elongation between cable bands. An individual wire breaks at its ultimate elongation and thereafter ceases to share in the cable tension. Only after some wires have broken does the cable attain its strength, which is smaller than the product of its area and the mean tensile strength of the wires. These suppositions are borne out by strand efficiency tests performed during the design of the Bear Mountain Bridge and Benjamin Franklin Bridge. Section 5 presents methods for assigning wires to groups that differentiate increas- ing levels of deterioration. Wherever appropriate, graphs are inserted to assist in esti- mating the effects of deterioration in panels adjacent to the evaluated panel, including the effects of broken wires. The statistics in this section are more advanced than in the previous section. The Weibull distribution of the ultimate strain or tensile strength of the wires, included in some spreadsheet programs, simplifies the calculation. The equa- tions for the distribution are given in Appendix A, as well as an iterative method for calculating the parameters of the distribution. An alternative method using Weibull paper is not presented here, but can be found in statistical texts, Rao [1] for example. The equations used for estimating the cable strength are also included in Appendix A. Section 6 lists the points to be covered in written reports for all three levels of inspection. Appendix B shows the rationale for calculating the effects of deterioration in adja- cent panels on cable strength in the evaluated panel. The calculation is extremely tedious if all the cable panels have been inspected; therefore, it should be reserved for the worst panel found in an inspection, or eliminated, unless it is essential to take all sources of cable strength into account. The assumption that all panels are in the same condition as the evaluated panel leads to a lower estimate of cable strength; graphs are provided to simplify the calculation.

Three examples of a strength calculation are given in Appendix C, all using the same inspection data. The first two examples use the Simplified and Brittle-Wire Mod- els, with the assumption that all panels are in the same condition. The third example assumes that all panels have been inspected and employs, in part, a very large spread- sheet that requires 16 pages to print and is tedious to set up, demonstrating the reasons for not using it except in extreme cases. The method that is commonly used to replace broken wires or sample wires is pre- sented in Appendix D. REFERENCE 1. Rao, S.S., Reliability-Based Design. 1st ed, ed. R. Hauserman. 1992: McGraw-Hill, Inc. 569.

1-1 SECTION 1 General 1.1 Introduction, 1-3 1.2 Health and Safety Requirements, 1-8 1.3 Suspension Bridges, 1-9 1.4 Causes of Corrosion, 1-15 1.5 Investigative Techniques, 1-18 1.6 Figures for Section 1, 1-19 1.7 References, 1-25 2-1 SECTION 2 Inspection 2.1 Introduction, 2-6 2.2 Inspection Intervals and Locations, 2-6 2.3 Internal Inspections, 2-12 2.4 Inspection and Sampling, 2-20 2.5 Figures for Section 2, 2-40 3-1 SECTION 3 Laboratory Testing 3.1 Introduction, 3-2 3.2 Tests of Wire Properties, 3-2 3.3 Zinc Coating Tests, 3-5 3.4 Chemical Analysis, 3-6 3.5 Corrosion Analysis, 3-6 3.6 Figure for Section 3, 3-8 3.7 Reference, 3-8 4-1 SECTION 4 Evaluation of Field and Laboratory Data 4.1 Introduction, 4-3 4.2 Notation, 4-3 4.3 Mapping and Estimating Damage, 4-5 4.4 Wire Properties, 4-10 4.5 Wire Redevelopment, 4-14 4.6 Figures for Section 4, 4-17 5-1 SECTION 5 Estimation of Cable Strength 5.1 Introduction, 5-3 5.2 Notation, 5-3 5.3 Estimated Cable Strength, 5-6 5.4 Figures for Section 5, 5-21 6-1 SECTION 6 Inspection Reports 6.1 Introduction, 6-2 6.2 Maintenance Personnel Inspection, 6-2 6.3 Biennial Inspection, 6-2 6.4 Internal Inspection, 6-3 A-1 APPENDIX A Models for Estimating Cable Strength B-1 APPENDIX B Effect of Deterioration in Adjacent Panels on Estimated Cable Strength C-1 APPENDIX C Illustrative Examples D-1 APPENDIX D Splicing New Wires CONTENTS