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