Inspection and Management of Bridges with Fracture-Critical Details (2005)

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5BACKGROUND The inspection and maintenance of all types of bridges is critical to the safety of the public and often critical to the economy of a region. A bridge as defined in the AASHTO LRFD Bridge Design Specifications (LRFD Specifications) (1) is “a structure, including supports, erected over a depres- sion or an obstruction . . . having an opening measured along the center of the roadway of more than 20 feet” (6 m). Fail- ure of a bridge as a result of inadequate inspection and main- tenance can lead to loss of life as well as incalculable user costs. To minimize the potential for such problems, signifi- cant public funds are spent inspecting and maintaining our nation’s bridges. These funds should only be expended if • There is a payoff in lower future maintenance costs or • The reliability (a measure of the relative safety) of the bridge is inadequate and the inspection and maintenance will sufficiently improve the reliability. SCOPE AND OBJECTIVE The focus of this report is on the inspection and maintenance of bridges with fracture-critical members (FCMs). The LRFD Specifications (1) define an FCM as a “component in tension whose failure is expected to result in the collapse of the bridge or the inability of the bridge to perform its function.” Slight variations of this definition can be found in the AASHTO/ AWS-D1.5 Bridge Welding Code (2), the AASHTO Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges (LRFR Manual) (3), and FHWA’s National Bridge Inspection Standards (NBIS) (4). Note that by this definition substructures and superstructure members made of concrete and other materials can also be classified as FCMs, although in practice, typically, only steel superstructure members are so classified. Although the definition indicates that failure of an FCM may lead to collapse, it is not clear what loading would be necessary to cause the collapse. The definition leaves much to engineering judgment, and consequently there is disagree- ment about what type of members should be classified as FCMs, as is discussed in detail in chapter two. The LRFD Specifications note that designers are required to clearly indicate FCMs in the contract documents. Note that although the term “member” is often used to refer to an entire girder, the definition of an FCM refers only to the critical ten- sion element of a member. In the case of an ‘I’ or box girder, the tension flange and the web plate(s) would be FCMs, as well as the weld of the tension flange to the web. However, the compression flanges and the weld of the compression flanges to the webs would not be FCMs. According to the LRFD Specifications, longitudinal attachments welded to the FCM and greater than 4 in. (100 mm) in length in the lon- gitudinal direction are also considered as FCMs. Bridges con- taining FCMs are commonly referred to as fracture-critical bridges (FCBs), although there are large parts of the bridge that are not fracture-critical. FHWA has proposed that states create FCM inspection plans. There is therefore much interest in the current practices of various states for inspecting and managing FCBs. How- ever, before the development of such a plan, a more compre- hensive and unambiguous method of classifying bridges and members as fracture-critical must be developed to ensure consistent application of such inspection standards. There is also a need for other information related to FCMs that may be used to update the LRFR Manual and the current NBIS. Finally, for obvious reasons, many owners are also concerned about terrorist threats against FCBs. Therefore, NCHRP initiated this synthesis project to focus on inspection and management of bridges with FCMs. The objectives of this synthesis project were to • Survey and identify gaps in the literature; • Determine practices and problems with how bridge owners define, identify, document, inspect, and manage bridges with fracture-critical details; and • Identify research needs. The scope of the project included studying • Inspection frequencies and procedures; • Methods for calculating remaining fatigue life; • Qualification and training of inspectors; • Available and needed training; • Experience with FCM fractures and problem details; • Examples of where inspection programs prevented failures; • Cost of inspection programs; • Retrofit techniques, including emerging technologies; • Nondestructive evaluation (NDE); CHAPTER ONE INTRODUCTION

• International experience and practice (largely from scan- ning tours); • Fabrication methods and fabrication inspection; and • How owners are dealing with fracture-critical details that cannot be easily inspected. ORGANIZATION The following chapter (chapter two) presents results from the literature review, including a brief summary of the develop- ment and impact of specifications related to fatigue and frac- ture of steel bridges, the performance of bridges in the event of fractures, and classification of FCMs. Additional detail on these subjects is also presented in Appendix A, along with details on fatigue life prediction, NDE, repair and retrofit techniques, and the impact of high-performance steel (HPS). In general, the literature review confirmed that research con- ducted to evaluate the robustness of bridges deemed to be fracture-critical demonstrated that most bridges have consid- erable reserve strength. Studies evaluated robustness using different methods including case histories of actual fractures that occurred in service, field testing, finite-element simula- 6 tion, and reliability theory. An annotated bibliography of the most relevant research is included in Appendix D. Chapter three reports on the results of the survey of bridge owners and consultant inspectors, and discusses targeted interviews. The survey was sent to 72 state DOTs, agen- cies, and provinces, such as those in Canada. Useful replies were received from 40 agencies. (It should be noted that more than 40 surveys were returned; however, a few surveys were not filled out or the agency did not have any FCBs in their inventory.) One of the objectives of this survey was to gather information on fracture-critical structures and how bridge owners define, identify, inspect, and manage FCBs. Information related to structural failures was gathered along with repair and retrofit policies and strategies. In addition, input was solicited from owners on their perceived research needs related to FCBs. Useful information was gained related to inspection, failures, classification of FCBs, and costs asso- ciated with field inspection. Chapter four includes discussion of the findings, and chapter five provides the conclusions and proposes future research needs.