Presently, bridges and members classified as fracture-critical consume a significant portion of inspection resources due to the mandatory 24 month, arms-length inspection. The indirect costs associated with these inspections (equipment, highly trained personnel, user costs, etc.) can be substantial. For example, TxDOT estimated that in urban areas, the indirect cost to the public is about $11,000 per lane per hour of lane closure. The policies revolving around fracture-critical bridges are over 30 years old and date to a time when little was known about fracture mechanics and system behavior, and steel was not manufactured to the same quality as it is today. The ¿Fracture Critical Inspection Techniques for Steel Bridges¿ course offered by the National Highway Institute strictly teaches attendees that fracture critical members are members either partially or wholly in tension that have no load path redundancy, ignoring the role of structural and internal member redundancies. According to NCHRP Synthesis 354, at least some of the surveyed states recognize the role of internal redundancy, but a great many do not. There is a strong need to evaluate the role of internal member redundancy in the evaluation of fracture-critical members for proper classification and application of inspection protocol.

The Federal Highway Administration currently has the authority to allow owners to forego fracture-critical inspection for low redundancy bridge structures on a case-by-case basis if supported by a rigorous damage analysis as per AASHTO LRFD. However, since there is limited experimental data specifically focused on this issue, using internal redundancy as the sole measure of redundancy cannot be approved by FHWA at this time. Hence, specific research is needed that can be used to evaluate the potential for a fracture propagating from one mechanically fastened element to another in a built-up member as well as the effects on the fatigue resistance of the faulted member. Furthermore, there is no known research that has quantified the energy release (and resulting loads on the remaining section) that is likely to occur during such a fracture event. This project will explore whether internally redundant members (either mechanically fastened built-up members, or those with parallel elements) do posses the internal arrest mechanisms to safely carry loads during and after a fracture event considering one of multiple elements are fractured. The project will be primarily based on full-scale specimens subjected to various fracture simulations for the purposes of gaining deeper understanding of the energy release, load redistribution, and subsequent fatigue resistance of damaged section. However, analytical studies will also be conducted to assist in the development of code-ready assessment methodologies. The project will also assess the role of inspection technique and frequency for internally redundant members considering the research may show they do not fit the AASHTO definition of fracture-critical. Obtaining experimental data in a controlled fashion will provide the needed evidence to establish if internal member redundancy is a reliable redundancy measure to use for collapse prevention in a fracture-critical assessment. This will improve the ability to properly allocate limited bridge inspection resources and improve overall safety of the nation¿s infrastructure.

¿ Perform a literature review to document past internal redundancy research and all case studies of bridges that sustained fractures that were arrested by an internal redundancy mechanism. In addition, the approaches used in other industries, which use similar redundancy concepts, will be examined. ¿ Develop a test program that can be used to assess the static, dynamic, and fatigue strength of built-up members with fractured elements. The project will seek to obtain components from decommissioned bridges to use in the testing. This could include, but not be limited to riveted and bolted built-up I-shapes, boxes, or tubs. Large fatigue cracks will be grown in one or multiple elements of the member and then suddenly overloaded to induce the fracture event. If entire bridge components cannot be attained, specimens will have to be fabricated using either modern steel, or large web plates removed older bridges. Testing may also include small-scale testing that can be performed in a universal testing machine with or without a temperature chamber. The need for low-temperature testing will also be considered. ¿ Develop analytic techniques for assessing the static, dynamic, and fatigue strength of fractured built-up members. The models will be calibrated using the data obtained from the experimental studies. Parametric studies will be performed to investigate parameters not included in the experimental matrix. The parametric studies will cover fastener pretension, scale effects, load/shear transfer between elements/fasteners and associated local stress amplifications, etc. ¿ Evaluate the need for future hands-on ¿FC¿ in-service inspection as well as guidance on the interval and scope of such inspections. This evaluation will include and consider the potential for future fatigue cracking in the faulted state. Deliverables: The research will result in the following: ¿ Draft ballot-ready language, equations, and commentary for inclusion into the AASHTO LRFD and Manual for Bridge Evaluation. The objective is to provide guidance on how to design, analyze, and detail new bridges and evaluative existing bridges to ensure member level redundancy is a reliable technique for collapse prevention in bridges and members traditionally classified as fracture-critical. It is presently envisioned that new and existing bridges will need to be treated differently. ¿ Recommendations regarding the need for future hands-on ¿FC¿ in-service inspection as well as guidance on the interval and scope of such inspections for bridges with built up members. This evaluation will include and consider the potential for future fatigue cracking in the faulted state.

Estimated cost: $500,000 Requested funding is $25,000 per year, for 3 years. One-time contributions will also be accepted.