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Page 31
Suggested Citation:"Chapter Four - Discussion." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Management of Bridges with Fracture-Critical Details. Washington, DC: The National Academies Press. doi: 10.17226/13887.
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Suggested Citation:"Chapter Four - Discussion." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Management of Bridges with Fracture-Critical Details. Washington, DC: The National Academies Press. doi: 10.17226/13887.
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Page 32
Page 33
Suggested Citation:"Chapter Four - Discussion." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Management of Bridges with Fracture-Critical Details. Washington, DC: The National Academies Press. doi: 10.17226/13887.
×
Page 33
Page 34
Suggested Citation:"Chapter Four - Discussion." National Academies of Sciences, Engineering, and Medicine. 2005. Inspection and Management of Bridges with Fracture-Critical Details. Washington, DC: The National Academies Press. doi: 10.17226/13887.
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Page 34

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31 RESULTS OF FRACTURE CONTROL PLAN—25 YEARS LATER It has been approximately 25 years since the introduction of the FCP, as previously found in the AASHTO/AWS-D1.5 (2). Subsequent to the FCP, there have been relatively few FCBs designed (e.g., two-girder bridges and two-truss systems), when compared with before the plan. This is primarily because of the added costs placed on fracture-critical structures in terms of materials, fabrication, and in-service inspection. Nevertheless, there have been some notable long-span FCBs built. Some states continue to use steel cross girders and twin tub girders. For example, Texas has built approximately 190 FCBs in the last 20 years. Among bridges designed after the FCP, the fracture toughness, detailing, and weld qual- ity has been much better than before, resulting in members with greater reliability than non-FCMs. Approximately 11% of steel bridges have FCMs; however, 76% of these were built before the FCP. Most of these, 83%, are two- girder bridges and two-line trusses, and 43% of the FCMs are riveted. Based on the results of the survey and personal commu- nications with industry and academic experts, there do not appear to have been any failures (certainly none where there was loss of life) of any structures built after the implementa- tion of the FCP. However, it must also be noted that there have been only two such catastrophic bridge failures identi- fied in the past 40 years that are attributed to fracture. Hence, it is difficult to measure the success of the FCP, and the NBIS requirements for hands-on inspection introduced in 1988, in preventing catastrophic failure of bridge structures, because at least one of the two (Mianus River) could have been pre- vented through routine inspection and better maintenance of drainage. Fabricators question the need for much of the test- ing, noting that the procedures become routine and automatic after building FCMs for years. Nonredundant is a broader term than FCM because non- redundant also includes: • Substructures; • Members that may be inherently not susceptible to frac- ture, such as compression members, but still could lead to collapse if damaged by overloading, earthquakes, fire, terrorism, ship or vehicle collisions, and so forth; and • Members made of materials other than steel. Interestingly, extreme loadings of substructure elements such as piers have led to most of the spectacular collapses of both steel and concrete bridges. In addition to prevention of collapse in the event of fracture, redundancy of the superstructure is important for several other reasons. The first is the need to more easily redeck the bridge. Also, events other than fracture can damage and completely destroy members of the superstructure. These are compelling reasons for redundancy. These reasons for redundancy (other than fracture) should not be used to justify unnecessary re- quirements for FCMs, a subset of nonredundant members. The two sets of bridge members, nonredundant members and the subset FCMs, should be addressed separately as appropriate. These reasons for redundancy (other than fracture) should be used to encourage redundancy outright instead of indi- rectly by penalizing FCMs. For example, in Sections 1.3.2 and 1.3.4 of the LRFD Specifications, redundancy is encour- aged. Load factors are modified based on the level of redun- dancy and it is stated that multiple-load-path and continuous structures should be used unless there are compelling reasons to do otherwise. In the AASHTO Manual for Condition Evaluation and Load Resistance Factor Rating (LRFR) of Highway Bridges, redundancy is reflected in system factors that reduce the capacity of each member in nonredundant systems. The sys- tem factors are calibrated so that nonredundant systems are rated more conservatively at approximately the level of re- liability associated with new bridges designed by the LRFD Specifications, called the “inventory” level in former rating procedures. Redundant systems are rated at a reduced re- liability level corresponding approximately to the traditional “operating” level. Redundancy is related to system behavior rather than indi- vidual component behavior and is often discussed in terms of the following three types: • Internal redundancy, also called member redundancy, exists when a member is comprised of multiple ele- ments and a fracture that formed in one element cannot propagate directly into adjacent elements. • Structural redundancy is external static indeterminacy and can occur in a two-or-more-span continuous girder or truss. CHAPTER FOUR DISCUSSION

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

33 Therefore, the loading cases to be studied, location of poten- tial cracks, degree to which the dynamic effects associated with a fracture are included in the analysis, and fineness of the models and choice of element type should all be agreed upon by the owner and the engineer. The ability of a particular soft- ware product to adequately capture the complexity of the prob- lem should also be considered and the choice of software should be mutually agreed upon by the owner and the engineer. Relief from full factored loads associated with the Strength I Load Combinations of Table 3.4.1-1 should be considered as should the number of loaded design lanes versus the number of striped traffic lanes. NCHRP Report 406 (34) gives practical requirements for the residual capacity of the damaged superstructure. This same type of analysis is being used to evaluate older structures as well to better direct resources for maintenance and replacement; for example, if a bridge is not really frac- ture-critical then it may not be necessary to replace it as soon. Other countries do not make a distinction in FCMs; how- ever, scanning tours have noted that three-dimensional refined analysis is more often used in the design and evaluation of bridges in Europe and that the inspection interval is often based on risk. Some in the United States believe that fracture- critical inspection should be less frequent for the modern bridges as a result of the much better detailing and materials, and perhaps should be based on truck traffic, fatigue details, and other risk factors. ROLE OF INSPECTION FOR FRACTURE-CRITICAL BRIDGES There is no doubt that the hands-on fracture-critical inspec- tions have revealed numerous fatigue and corrosion prob- lems that otherwise might have escaped notice. Many of these problem details are discussed in Appendix A. Twenty- three percent of the survey respondents (see chapter three) indicated that they had found significant cracks and corrosion that could have become much worse; and in doing so, possi- bly averting collapses. Similar examples may be found in trade magazines [see Zettler (31)]. However, agencies also report finding these problems on non-FCBs when hands-on inspections are performed. Therefore, such inspections are good for all steel bridges, not just FCBs. However, owners spend a major portion of their budget on efforts associated with the inspection and maintenance of all structures. Fracture-critical inspections consume a large frac- tion of that budget for a comparatively few structures. The cost of the fracture-critical inspection is typically two to five times greater than inspections for bridges without FCMs. Inspec- tions of closed sections such as tub girders and tie members are extremely expensive because inspectors must get inside such sections. What constitutes a fracture-critical inspection is subject to interpretation and disagreement. The frequency of fracture-critical inspections is actually not specified and varies up to every 5 years, is typically every 2 years, but often is every year or more frequently if there are specific problems. It is not known if efforts beyond what is normally put forth during the inspection of non-FCBs are actually warranted; however, even if there are only a few cases of such efforts pre- venting a major failure, they are most likely worthwhile. Questions do arise with respect to inspection requirements for newer bridges, which were built using superior steels (especially using any type of HPS), subjected to advanced NDT techniques, and fabricated using higher-quality welding procedures than used in the past. In addition, those fabricated using the FCP should be of superior quality than bridges built before the introduction of the FCP. Modern steel bridges are also built with a composite deck slab and are inherently more capable of carrying redistributed loads through alternate paths. Two-girder curved-girder bridges, especially those built since the early 1980s, almost always contain heavy transverse cross frames capable of carrying a significant load from one girder to another. In addition, based on the survey responses and infor- mation received during interviews of recognized experts in fatigue and fracture, it was found that there is a strong sense that the inspection interval should in some way be related to risk, ADTT, and age of the structure. Possibly, bridges built after the implementation of the FCP or with HPS can be inspected at greater intervals. Furthermore, bridges with low ADTT could be inspected at a frequency somehow related to traffic data. The approach could be similar to that used in the aircraft industry, where airframes are inspected based on hours of flight, not just years of service. It is recognized that issues related to corrosion, settlement, scour, and so forth, which can only be quantified through regular inspection, are not related to ADTT. However, these issues are common to all nonredundant bridges and not just FCMs. Research in risk-based inspection seems justified because the payoff can be substantial. For example, there would be significant savings if it could just be shown that bridges can be inspected every 3 years instead of 2. The potential savings should be considered when developing the scope and budget for such a research project to ensure that reliable data are col- lected and safe recommended changes to the inspection pro- gram established. It must be emphasized that before inspection intervals are adjusted, it is critical that the potential ramifications be thor- oughly understood. It is possible that some components need to be inspected on the schedule currently in place. For exam- ple, the existing inspection intervals might need to remain unchanged to ensure that problems with elements such as deck slabs and bearings do not go undetected. It was also sug- gested by a few of the designers who were interviewed that it might be important to inspect the deck, for example, more fre- quently than the steel superstructure. Another example would be a bridge susceptible to scour, where it might be required to inspect the substructure at a greater frequency than the super-

structures. Nevertheless, it is clear that before changes are made there are many factors that must be considered. Education and Training of Inspectors Overall, training of inspectors seems to be adequately avail- able through the existing courses offered by the National Highway Institute. None of the responding agencies identified any problems with the current educations strategies. However, one area that appears to need additional effort is related to the documentation and archiving of previous failures and prob- lems. As discussed in the previous chapter, failures known to have occurred in certain states were not always reported in the replies to the survey. Therefore, a better method of tracking such information appears to be needed. Education and Training of Engineers Although not evident from the surveys, discussions with industry leaders have also revealed that a general knowledge gap exists with respect to fatigue and fracture design, evalu- ation, and behavior in the engineering community. This is the 34 result of several factors. First, few academic institutions offer any substantial material in undergraduate coursework on fatigue and fracture. Therefore, recent graduates must learn on the job and often simply follow cookbook specification approaches with little understanding of the spirit of the spec- ifications. Second, there has been a major shift in the experi- ence level in U.S. DOTs over the last several years. Many of the most experienced engineers are retiring and with their departure will be lost the years of experience acquired dur- ing the period when most of the issues with fatigue and frac- ture were foremost (1970–1990). To address this, several owners and engineers expressed the need for additional training. Although current National Highway Institute courses seem adequate for training inspec- tors and providing overall guidance, a more in-depth course appears to be needed for bridge designers. For example, the Pennsylvania DOT has sponsored the development of a short course on fatigue and fracture design for its engineers and consultants. In addition, FHWA is currently (2005) consid- ering the development of a similar course designed to address this need that would be taught in strategic locations across the United States.

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TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 354: Inspection and Management of Bridges with Fracture-Critical Details explores the inspection and maintenance of bridges with fracture-critical members (FCMs), as defined in the American Association of State Highway and Transportation Officials’ Load and Resistance Factor Design (LRFD) Bridge Design Specifications. The report identifies gaps in literature related to the subject; determines practices and problems with how bridge owners define, identify, document, inspect, and manage bridges with fracture-critical details; and identifies specific research needs. Among the areas examined in the report are inspection frequencies and procedures; methods for calculating remaining fatigue life; qualification, availability, and training of inspectors; cost of inspection programs; instances where inspection programs prevented failures; retrofit techniques; fabrication methods and inspections; and experience with fracture-critical members fractures and problems details.

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