Risk-Based Inspection and Strength Evaluation of Suspension Bridge Main Cable Systems (2023)

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2 C H A P T E R 1 Introduction Background Most suspension bridges in use today have cables composed of thousands of aerially spun, cold- drawn, high-strength steel wires, each less than 0.2 inches in diameter. Though a small percentage of the total number there are some suspension bridges that have cables comprised of preformed helical strands. It is the high strength of these individual wires and preformed strands that makes their support of long span suspension bridges possible. As tension members, main cables and suspenders carry the load resulting from the weight of the deck and the imposed live load to the towers and the anchorages. The use of cold-drawn wire began in the nineteenth century and spans using this technology have grown from nominal lengths of just over 100 feet to spans today with lengths exceeding a mile. Most suspension bridges in use today are aging and carry high volumes of traffic. Deterioration of the elements of the suspension system is a problem, replacement of these elements can be expensive and problematic while failure could be catastrophic. Hence the need for effective inspection and maintenance of these elements. The primary impediment for inspecting and maintaining main cables is the protection systems that are used. The bundles of wires or strands that make up bridge cables are protected from the elements by an external covering. This covering usually takes the form of transverse wrapping wires coated with a paint system or sheets of neoprene or fiber reinforced acrylic fiber. The existence of this “protection” system makes visual inspection much more complicated and costly than for other bridge elements. In 1986, the Federal Highway Administration (FHWA) published a document entitled, “Inspection of Fracture Critical Members – Supplement to the Bridge Inspectors Manual” (Harland, 1986). It contained limited information on how to inspect suspension bridges. Much of the information contained in this document was based on investigations of the Maysville and Portsmouth Bridges that span the Ohio River. Theodore Hopwood II, the principal investigator for these inspections, and James H. Havens published three reports in May of 1984, the second of which contained the first standardized descriptions of corrosion of bridge wires (“Corrosion of Cable Suspension Bridges”, Hopwood, 1984). Their separation of corrosion into four stages was adopted by the FHWA Supplement authors and continues to be used by the bridge engineering community as the visual standard for describing bridge wire. This report will also rely on Hopwood/Havens classification system for corrosion. Driven by the findings of bridge owners in the northeastern United States, numerous meetings seeking a greater understanding of the causes and possible cures for cable deterioration were held. Probably the most important of these meetings was the Workshop on Safety Appraisal of Suspension Bridge Main Cables, sponsored by NCHRP, a division of the Transportation Research Board, under NCHRP Project 20-07/Task 100, held in Newark, New Jersey, on November 16 to 17, 1998. This meeting was attended by bridge owners, bridge cable engineers, corrosion experts and members of various public agencies. They discussed their experiences, as well as areas of concern and uncertainty and the type of research needed.

3 The result of the Newark meeting was an invitation from NCHRP for proposals to develop “Structural Safety Evaluation Guidelines for Suspension Bridge Parallel Wire Cables.” The Guidelines were intended to provide better definitions of the factors that affect cable integrity, improve the means of interpreting field data, and lead to greater rigor in assessing cable strength. This resulted in the publication of NCHRP Research Report 534: Guidelines for Inspection and Strength Evaluation of Suspension Bridge Parallel Wire Cables (Mayrbaurl, 2004). These guidelines were based on the final report from NCHRP Project 10-57, “Structural Safety Evaluation Guidelines for Suspension Bridge Parallel Wire Cables” (Mayrbaurl and Camo, 2004b). In May of 2004, the FHWA-released Report No. FHWA-1F-11-045 – “Primer for the Inspection and Strength Evaluation of Suspension Bridge Cables” (Chavel, 2004). This publication was meant to be a practical supplement to NCHRP 534 and FHWA-PD-96-001 (“Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation’s Bridges”). It was intended to “serve as an initial resource for those involved in the inspection, metallurgical testing, and strength evaluation of suspension bridge cables in addition to providing necessary documentation for recording performed inspections, testing, and strength evaluation.” In addition, the report was intended to provide field inspectors, technicians, and/or engineers with the necessary forms and information they need to perform an inspection. The FHWA report also included an appendix describing an alternative approach to determining the remaining cable strength and service life of bridge cables called the BTC Method. This appendix was not written by the authors of the Primer (FHWA 11-045), and its inclusion does not necessarily constitute an endorsement of the BTC Method by the FHWA or the authors of the Primer. NCHRP 534, has been used nationally and internationally as the standard for the evaluation of suspension bridge cables since its publication in 2004. When written, there was limited data available regarding the inspection of main cables; therefore, much of the approach was based on experience and data collected from just two bridges. Since its implementation, a significant number of suspension bridge cables have been inspected resulting in various “questions/suggestions” for improving the original methodology. Risk-based inspection approaches have also gained widespread adoption since the original publication of NCHRP 534 in 2004. Notably, NCHRP 782 “Proposed Guideline for Reliability-Based Bridge Inspection Practices” (Washer and Connor, 2014) provided a methodology for the rational determination of inspection frequency based on various conditions and characteristics of an individual bridge. This methodology, while intended for typical bridges in the inventory, can also be applied to special structures, such as suspension bridges, albeit with some modifications. With more than a decade of continued research effort and the emerging new technologies on nondestructive examination (NDE) and structural health monitoring (SHM), it is also important to review the information regarding present methodologies to reflect advances in research and technology as well as the recent lessons-learned by the suspension bridge cable inspection and strength evaluation industry. One of the primary driving forces behind the need for the NCHRP Project 12-115 is the desire of owners and stakeholders to improve on the standardized, understandable, and open-source methods of NCHRP 534 by incorporating risk-based and probabilistic methods for the inspection and strength evaluation of main cable systems.

4 Research Objectives The two main objectives of this research were to: 1. Develop Guidelines for risk-based inspection and strength evaluation of suspension bridge main cable systems using probabilistic approaches. 2. Plan and conduct a workshop for suspension cable bridge owners to demonstrate the use of proposed guidelines. Due to the COVID-19 pandemic, this workshop was postponed until the end of the project. The workshop was held on June 29, 2022 at the Keck Center of the National Academies in Washington, DC.