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35 CHAPTER FIVE CONCLUSIONS The fracture control plan (fabrication provisions and Charpy members, but still could lead to collapse if damaged by over- V-notch requirements) and new fatigue specifications have loading, earthquakes, fire, terrorism, ship or vehicle colli- necessitated substantial changes in the design and fabrication sions, and so forth; and members made of materials other of bridges. As a result, modern bridges are much less suscep- than steel. Substructures such as piers are often nonredundant tible to fatigue, corrosion, and fracture than bridges designed and have been responsible for most of the spectacular col- before 1975 (and 1985 for web-gap cracking). lapses of both steel and concrete bridges. In the United States, the fatigue-cracking problem in steel The cost premium for higher toughness is lower than it was bridges is essentially confined to bridges designed before 30 years ago. Even ordinary bridge steel typically has much 1975, expect for web-gap cracking in bridges designed up greater than minimum specified notch toughness, except per- to 1985. Approximately 11% of steel bridges have fracture- haps for Zone 3. The new high-performance steels (HPS) pro- critical members (FCMs) and 76% of these were built before vide a toughness level that far exceeds the minimum require- the implementation of the fracture control plan. Most of these ments. This has significantly altered the economic factors that (83%) are two-girder bridges and two-line trusses, and 43% were considered in setting the current AASHTO toughness of the FCMs are riveted. requirements. However, there is a reasonable argument for increasing the notch toughness requirements. Extremely There are numerous examples of FCMs where one girder large cracks, greater than 14 in. (350 mm), could be tolerated in a two-girder bridge has fractured but the bridge does not in mild steel if the temperature shift was zero. An increase even partially collapse. It is apparent that the deck deflects could be combined with some loosening of the definition of and begins to act as a catenary to carry the load. Other coun- FCMs that would, for example, allow two-girder bridges. If tries do not have additional provisions for FCMs and their full advantage were taken of the toughness of HPS, research associated costs and continue to use fracture-critical designs suggests it would be possible to: with no apparent problems. European countries use three- dimensional analysis to design and assess their bridges, and · Eliminate special in-service inspection requirements the inspection interval is often based on risk. for fracture-critical structures for HPS, · Reduce frequency and need for hands-on fatigue inspec- Very few traditional FCM bridges (e.g., two-girder bridges tions for HPS, and or two-truss systems) are now being built. In some cases, · Eliminate the penalty for structures with low redun- these systems are potentially more efficient; however, there dancy for HPS. is additional initial cost and added inspection cost. The capacity of damaged superstructures (with the FCM Ultimately, designers and raters of bridges strive to achieve "damaged" or removed from the analysis) may be predicted the target level of reliability. Redundancy has a significant using refined three-dimensional analysis. However, there is impact on the risk of collapse, and this impact is accounted for a strong need to clarify the assumptions, extent of damage, appropriately for all types of structures in both the AASHTO load cases and factors, and dynamic effects in these analyses. LRFD Bridge Design Specifications and the Manual for Con- NCHRP Report 406: Redundancy in Highway Bridge Super- dition and Load and Resistance Factor Rating (LRFR) of structures gives practical requirements for the residual Highway Bridges. It is possible to achieve the target level of capacity of the damaged superstructure. This type of analy- reliability without redundancy in a bridge that is more conser- sis and associated waiver of the FCM provisions is presently vatively designed. For example, a nonredundant bridge being done on a case-by-case basis. This same type of analy- designed for an HS-25 loading would have greater reliability sis is being used to evaluate older structures as well to better than a redundant bridge designed for HS-20 loading. direct resources for maintenance and replacement; for exam- ple, if a bridge is not really fracture-critical then it may not There could be a useful distinction between the subset of be necessary to replace it as soon. FCMs and the encompassing set of all nonredundant mem- bers, which includes substructures; members that may be Owners are not consistent in classifying bridges as fracture- inherently not susceptible to fracture, such as compression critical. In the LRFR Manual (which is identical to the earlier
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36 Manual for Condition Evaluation of Bridges) and the Bridge Training of inspectors seems to be adequately available Inspector's Reference Manual, it is stated that twin box or tub through the existing National Highway Institute courses. girders are not considered FCM. It is also stated that: One area needing additional training is fatigue and fracture for design engineers. Also, additional effort is required for · Only welded tie members for arches or single-box gird- national documentation and archiving of previous failures ers are considered FCMs, whereas riveted or bolted built- and problems. up tie members or single-box girders would not be FCMs. This would appear to be giving credit to the internal Recently, FHWA proposed that the states create FCM redundancy of the bolted or riveted built-up members. inspection plans. However, survey data suggest that before the · Not all two-I-girder bridges are considered FCMs; in the development of such a plan, a more comprehensive method Bridge Inspector's Reference Manual, even simple-span of classifying bridges as fracture-critical could be developed two-I-girder bridges are not FCMs unless they have cover to ensure consistent application of such inspection standards. plates, shelf plates, or a suspended span. Innovative retrofits have been developed and implemented The AASHTO LRFD Bridge Design Specifications are not successfully to improve the redundancy of FCBs, including clear about what types of superstructures have FCMs. using post-tensioning and bolted redundancy plates. It has been conclusively determined that hands-on, frac- Owners identified the following as the most important ture-critical inspections have revealed numerous fatigue and areas for future research as related to FCBs: corrosion problems that otherwise might have escaped notice. Twenty-three percent of survey respondents indicated that · Develop load models, criteria for the extent of damage, they found significant cracks and corrosion that could have and guidelines related to advanced structural analysis become much worse, possibly averting collapses. However, procedures to better predict service load behavior in transit agencies also reported finding these problems on non- FCM bridges and the behavior after fracture of an FCM, FCMs when hands-on inspections are done. Therefore, inspec- including dynamic effects from the shock of the frac- tions such as these are good for all steel bridges, not just FCMs. ture and, if necessary, large deformations. · Develop advanced fatigue-life calculation procedures, The cost of a hands-on, in-service inspection for FCMs is taking into account a lack of visible cracks for fracture- estimated to be two to five times that of such inspections of critical bridges. non-FCMs and may not always be required. Inspection fre- · Investigate field monitoring for fracture-critical bridges. quency of existing bridges could be based on risk factors such · Develop rational risk-based criteria for inspection fre- as truck traffic, type of details, and date of design; that is, a dis- quency criteria and level of detail based on average tinction could be made between (1) bridges designed before daily truck traffic, date of design, and fatigue detail 1975, (2) bridges designed between 1975 and 1985, and (3) categories present. modern bridges (designed after 1985) with only Category C · Evaluate fracture-critical issues related to sign, signal, details or better. and light supports.