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32 Load-path redundancy is internal static indeterminacy I-girders owing to the torsional capacity of the intact box arising from having three or more girders or redundant girder and the alternate load paths available within each box truss members. One can argue that the transverse mem- girder; however, most agencies consider these FCMs. These bers such as diaphragms between girders can also pro- structures contain four webs and are fully composite with vide load-path redundancy. the concrete deck and unlikely to collapse catastrophically. It seems unreasonable to consider these structures fracture- Internal and structural redundancies are often neglected, critical. However, the AASHTO LRFD Bridge Design Spec- but this is clearly oversimplifying and possibly overconserva- ifications do not specifically address what types of super- tive. Retrofits are described that have been used to add redun- structures have FCMs. dancy to bridges with FCMs. Common assumptions that fractures in certain superstruc- Ultimately, it is the target level of reliability that design- ture types will lead to collapse are too simplistic. Numerous ers and raters of bridges should strive to achieve; they should bridges have had a full-depth fracture of an FCM girder and not focus exclusively on redundancy. Redundancy has a major did not collapse, usually owing to the alternative load carry- impact on the risk of collapse and this impact is accounted ing mechanism of catenary action of the deck under large for appropriately for all types of structures in both the LRFD rotations at the fracture. It is apparent that other elements of Specifications and the LRFR Manual. Using these LRFD and these two-girder bridges, particularly the deck, are sometimes LRFR procedures, it is possible to achieve the target level of able to carry the loads and prevent collapse in these FCBs. reliability without redundancy in a bridge that is more conser- These alternate load paths were so robust in some of these vatively designed, by only approximately 17%. For example, failures that there was little or no perceptible deformation a nonredundant bridge designed for an HS-25 loading would of the structure. For example, the I-79 Neville Island Bridge have greater reliability than a redundant bridge designed for over the Ohio River, which would have been classified as HS-20 loading. fracture-critical, did not collapse and actually carried traffic for a considerable period. Therefore, was this bridge actually The cost premium for higher toughness is lower than it was fracture-critical? Furthermore, it was fabricated before the 30 years ago. Even ordinary bridge steel typically has much implementation of the FCP; therefore, it serves as an exam- greater than minimum specified notch toughness, except per- ple of the robustness of structures that were not fabricated to haps for Zone 3. The new HPS provide a toughness level that the higher requirements. far exceeds the minimum requirements. This has significantly altered the economic factors that were considered in setting There are many other examples of bridges classified as the current AASHTO toughness requirements. There is a rea- fracture-critical or containing FCMs that have exhibited com- sonable argument for increasing the notch toughness require- plete fractures in girders, cross girders, hangers, and other ments. Extremely large cracks, greater than 14 in. (350 mm), members that continued to perform until the failure was could be tolerated in mild steel if the temperature shifts were discovered, sometimes by accident. Therefore, the question zero. An increase could be combined with some loosening of becomes, should these bridges or members have been classi- the definition of FCMs that would, for example, allow two- fied as fracture-critical in the first place because collapse did girder bridges. If full advantage were taken of the toughness not occur? of HPS, research suggests it would be possible to: Clearly, one of the greatest research needs is related to Eliminate special in-service inspection requirements how to determine if a bridge should be considered fracture- for fracture-critical structures for HPS, critical. This finding is consistent in two ways with the sug- Reduce the frequency and need for hands-on fatigue gestions noted on the survey. First, there was considerable inspections for HPS, variation in how individual agencies classify bridges (i.e., as Eliminate of the penalty for structures with low redun- fracture-critical or non-fracture-critical). Second, those indi- dancy for HPS. viduals who identified areas of needed research selected top- ics related to this area as being the most needed. IDENTIFYING FRACTURE-CRITICAL BRIDGES The capacity of damaged superstructures (with the FCM "damaged" or removed from the analysis) may be predicted Assuming that the FCP is serving its purpose, the literature, using refined three-dimensional analysis. However, there is survey, and several failures appear to suggest that it is not a strong need to clarify the assumptions, extent of damage, the FCP that needs to be revisited. Rather, how individual load cases and factors, and dynamic effects in these analyses. bridges are characterized as fracture-critical in the first The commentary of the AASHTO LRFD Bridge Design Spec- place may require further refinement. There are varying clas- ifications states that: sifications of superstructure types as having FCMs and con- sequently there is wide disagreement. For example, twin box The criteria for refined analysis used to demonstrate that part of girders would be expected to perform even better than twin a structure is not fracture critical has not yet been codified.