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25 provided by the AASHTO LRFD Bridge Design Specifica- The greatest scatter in the responses appeared to be related tions. However, although the definition provided was consis- to three-girder bridges and twin-steel "tub" girders. As indi- tent, how it is applied to various structure types was much cated earlier, because respondents classified the bridge type more variable. A question was posed in tabular format asking based on their opinion owing to the lack of a formal policy by the respondent to classify various types of bridges as fracture- their agency, it is clear that different results would be obtained critical or non-fracture-critical. The question and responses from different people. Interestingly, this implies that the same are summarized in Table 2. Most respondents indicated that structural configuration may be classified as fracture-critical their answers were based both on general policy where in one portion of a state and non-fracture-critical in another. applicable and their own opinion. As indicated in Table 2, agencies were also asked to iden- As is apparent from the summarized data, there is consid- tify and classify other types of bridges as fracture or non- erable inconsistency in how owners apply the definition of fracture-critical. The responses were limited to this question. FCBs. The only structural configurations that all respondents However, approximately one-third of the agencies indicated classified the same were a two-girder system (fracture-critical) that there are other bridge types they classify as fracture- and multisteel tub girder bridges (non-fracture-critical). critical. Some of these included timber bridges, post-tensioned concrete, and steel-tied arch bridges. Suspension bridges The respondents were generally more conservative in their were also typically classified as fracture-critical owing to application of the definition than the AASHTO Manual for the main cables. Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges (3) [identical in this respect to the SUMMARY OF RESPONSES TO PART II-- predecessor Manual for Condition Evaluation of Bridges (54)] INSPECTION AND CLASSIFICATION and the Bridge Inspector's Reference Manual (29). Part II of the survey provided considerable information as Interestingly, truss bridges with two truss lines were clas- to the number of bridge types in the inventories of the respond- sified as non-fracture-critical by a few owners. In discussing ing agencies. Specific structural configurations were identified this issue with one of the respondents, they indicated that they by most agencies. Unfortunately, not all agencies provided data were primarily referring to riveted trusses because individual for this set of questions, because they were not able to extract members were internally redundant and fracture of the entire the information from their databases. tension chord was very unlikely. When asked if they would consider riveted two-girder bridges as non-fracture-critical if As can be seen, approximately 11% percent of the steel there were riveted cover plates, they indicated that they might bridge inventory is classified as fracture-critical, based on if there was more than one cover plate, but that this would have the agencies that responded to this question. The percentage to be considered on a case-by-case basis. Therefore, it appears ranged from approximately 10% to up to 30%, depending that internal member redundancy is sometimes equated with on the agency. However, the majority of these bridges (about load-path redundancy, or it at least lowers an owner's concern 75%) were built before 1975 (around the time the FCP was regarding load-path redundancy. introduced by AASHTO). This is most likely for several reasons, including increases in quality control and the more stringent (and hence more costly) material and fabrication TABLE 2 costs demanded by the FCP. In addition, the lack of apparent SURVEY RESPONSES TO QUESTION 3 (How would you redundancy of FCBs was an obvious undesirable feature and categorize the following bridges?) led many agencies to move away from building such struc- Fracture-Critical tures. Because most of the FCBs in the U.S. inventory were designed before 1975, these bridges contain fatigue details Description Yes No known to be poor and those that are susceptible to out-of- Two-girder bridges 38 0 plane distortion. It should also be noted that the "modern" Three-girder bridges 9 28 fatigue design provisions were introduced into the specifi- Three-girder bridges with girder spacing 10 21 cations beginning around 1973. Hence, most of the bridges Multigirder bridges with girder spacing 3 32 built before 1975 were designed using the older, nonconser- vative fatigue design provisions. Fortunately, the loading used Truss bridges 34 3 for fatigue design and the analytical models were, in most Two-girder bridges fabricated using HPS 70W 31 1 cases, quite conservative. Truss bridges fabricated using HPS 70W 28 2 Single-steel ėt ub" girder bridges 32 5 FCBs designed beginning in the early 1980s were detailed Twin-steel "tub" girder bridges 22 12 to minimize out-of-plane distortion cracking and minimize Multisteel "tub" girder bridges 0 34 the use of low-fatigue resistance details (D, E, and E). In addi- tion, these bridges were built with material having improved Other (post-tensioned, timber, steel cross girders, etc.) 13 3 fracture toughness requirements and shop inspection.
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26 The two most common types of fracture-critical structural included, however, it can be assumed that they involve a more systems are not surprisingly two-girder bridges and trusses. thorough inspection requiring a greater level of effort. The combination of these two structural systems comprises approximately 83% of all FCBs. About one-half of these struc- Some of the following cost data were briefly discussed in tures (41%) are riveted structures, the remaining being fully chapter two. Costs associated with bridge inspection take up a welded or welded and bolted. considerable portion of each agency's budget. It is believed by many owners that inspection costs associated with FCBs con- Inspection Issues sume a large portion of the budget dedicated to inspection. Questions were asked to determine if this belief is consistent Many agencies (60%) require that inspectors successfully with actual practice. There was considerable variation in the complete the National Highway Institute fracture-critical train- data obtained. This is partly because the survey did not clearly ing course and indicated that experience with various NDT indicate to respondents what cost comparisons should be techniques is helpful. NDT (discussed in detail in Appendix A), included. However, for a given agency, it is reasonable to such as magnetic-particle testing, dye-penetrant testing, or assume that the person completing the survey compared the ultrasonic testing, was required when warranted. These same items for each type of inspection. Therefore, although the techniques were also used depending on the bridge's age, response from different agencies cannot be compared, relative average daily truck traffic (ADTT), stress level, and condi- increases indicated from an individual survey are comparable. tion. No information on the use of special devices such as boroscopes for inspection of details that cannot be accessed Owners were asked to estimate what, if any, additional was provided. Several states rotate field inspectors and shop costs are incurred when inspecting an FCB. Surprisingly, the inspectors and report that the cross transfer of knowledge is answers ranged from 0% to 6,000%, with most agencies indi- beneficial. cating increases of from 200% to 500%. It should be noted that only two owners mentioned that there was none or neg- Many respondents are concerned about shrinking budgets ligible increases in costs associated with inspecting FCBs. and staff, personnel turnover, and lost expertise. Many agen- (Some agencies did not reply to these questions.) The indi- cies are also concerned about contracting out inspections to viduals who indicated that there were significant additional consultants; however, no firm examples to warrant such con- costs provided solid reasons for these increases, the most cern were provided. These concerns are usually coupled with common of which were as follows: the perceived need for improved documentation. Concerns have been expressed about locally owned bridges and bridges · Additional costs associated with the use of special- less than 20 ft (6 m) in span not receiving any or enough atten- ized access equipment such as a snooper, manlift, or tion from skilled inspectors. Consultants are often hired to do rigging. In many cases, non-FCBs can be inspected this work and results are often reported to be inadequate. A from the ground with binoculars. FCBs require "arm- local municipality was contacted as well as two local con- length" access. sultants and there does not appear to be a unified approach to · Additional costs associated with traffic control to close the inspection of local bridges. Some, depending on size, lanes to permit the access equipment to be placed on or location, and use, are inspected more rigorously than others. below the bridge. Indirect costs associated with lane Some states may override the decisions of the local govern- closures were estimated by one agency to be $11,000 ments. Some state agencies have expressed the desire to be per lane per hour of closure. Thus, if inspection required able to review the quality of local inspectors to ensure that two lanes to be closed for 3 h, there would be a cost of they are adequately trained and performing inspections con- $66,000 to the motoring public. sistent with required standards. · Increased costs associated with additional employee- hours required to conduct a detailed hands-on inspection. Although the inspectors' training is considered to be ade- · Additional costs associated with needs to more frequently quate, many engineers have noted how the training in fatigue perform NDT. and fracture is not adequate. Engineers are reportedly not · Many states inspect FCBs at greater frequency than non- learning lessons from fatigue and fracture incidents because FCBs. This in itself may raise costs assuming that there of a lack of understanding. There is concern that they are not are no other increases. able to predict future problem details. Better education in this area in engineering programs as well as short courses for As stated, some agencies inspect FCBs more frequently practicing engineers could lead not only to better new bridge than non-FCBs. As part of the survey, owners were asked to designs but also to more qualified and knowledgeable engi- provide the intervals at which inspections are conducted. neers to participate in maintenance and inspections. There was moderate variation in the responses, with intervals ranging from 2 to 5 years. Certainly, the condition of a given In addition, approximately 65% of respondents indicated structure has a significant influence on the interval between that special procedures were followed when inspecting FCBs. inspections. Based on discussions with some owners, inter- No details were provided as to what these special procedures vals of 6 months or even less are sometimes used in unusual
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27 cases where warranted. However, some agencies did not dis- such as Wyoming, because there are a limited number of per- tinguish between the FCB and non-FCB bridges when deter- sonnel anyway. However, many of the larger states, such as mining the interval between inspections. Texas and Minnesota, also have centralized statewide teams. They noted the advantages of working with a snooper and The survey revealed that there are differences in how continuity in having the same team do the inspections repeat- owners inspect FCBs. When asked if the entire bridge is edly. Other larger states assign inspections to regional divi- inspected or just the FCMs in detail, there appeared to be sions that do the non-FCB inspections as well--or there is a no clear consensus. Apparently, in the training course, mix of some inspections of major or troublesome FCBs done Michael Baker, Jr., has indicated that if there are FCMs the by centralized teams, whereas inspections of smaller, benign entire bridge should be subjected to hands-on inspection. FCBs are done by regions. Some owners see no increase in costs by inspecting the entire bridge in greater detail, whereas others indicated that only Approximately 20% of responding agencies have done the FCMs are included to reduce costs. Some owners noted more rigorous analysis to determine which members or por- that hands-on inspection is encountering significant prob- tions of members are actually in tension and hence consid- lems in non-FCBs as well. ered fracture-critical. The objective of this question was to ascertain if owners actually analyze a structure to determine Inspectors reported that variance of as-built conditions which members, if they were to fail, would lead to the col- when compared with what is shown on the plans is a prob- lapse of the bridge. For example, there may be many tension lem. Documentation of the as-built details could be very use- members in a large truss that are subject to tensile dead and ful. Examples include SR-422 in Pennsylvania (37), the Hoan live load stresses. However, through more advanced analy- Bridge (38), and other bridges with shelfplate details for sis, it can usually be shown that there may only be a few of lateral bracing that were not built as shown in the plans and these critical members that if lost as a result of fracture would were not the same quality welding as expected. This and other lead to collapse. One example is Texas, which has its cen- problem details are discussed further in Appendix A. tralized team do this type of analysis in advance of inspect- ing all of their FCBs. In conversations with owners and bridge engineers it is often expressed that inspection intervals and the level of Unfortunately, this question does not seem to have been scrutiny should be flexible; being determined by the states worded clearly enough to ensure that this is the level of analy- and different for different bridge situations. Many individu- sis being referred to. Thus, it seems that some agencies indi- als have expressed a desire to see such levels based on risk, cating "analysis" is performed were not referring to the which might include ADTT and the type of fatigue details. advanced analysis described previously. Nevertheless, it On the other hand, public and private inspectors want a cook- appears that a very small percentage of owners would perform book procedure, owing to inadequate knowledge, not getting this level of analysis and only in large critical structures. paid to make judgments, and concerns about liability if they do make judgments. A proper balance must be found between In addition to the cost, inspectors and owners see many these two needs and the safety of the bridge. For example, advantages to the hands-on fracture-critical inspections. They John W. Fisher, Professor Emeritus of Lehigh University, reported finding numerous problems with fatigue and corro- expressed similar views during a personal interview con- sion that might not otherwise have been discovered. There ducted in June 2004. Dr. Fisher also stated that efforts related are also reports of finding these problems in non-FCMs; to inspection of a given bridge should be a function of the therefore, hands-on inspection is good for all members of all material used in the fabrication of the bridge. For example, a bridges. These hands-on inspections are also reported to be bridge constructed of HPS 70W material will not need to be useful for the purpose of bridge management; that is, for pri- inspected as often as bridges made from other steel, at least oritizing bridges as part of an overall bridge replacement and with respect to issues of fatigue and fracture. He suggests rehabilitation program. the following inspection scenario for new bridges. After con- struction, a bridge should be inspected every 2 years for the Inspection and Failures first 4 years to identify any critical issues, which are usually manifest early in the life of a bridge. If the condition of the The following question (no. 13) was asked related to inspec- bridge is acceptable after 4 years, then the inspection inter- tion of FCBs and whether or not it has prevented any failures: val could be increased to 5 years for the next 10 to 15 years. Has the inspection of a fracture-critical bridge(s) ever iden- The inspection frequency should be reconsidered and the tified a condition that has clearly prevented a fracture and the inspection interval decreased, if needed, every 15 years. subsequent collapse of the structure? Follow-up questions were asked of some agencies to deter- Respondents were informed that the objective of this ques- mine how many states have centralized teams, including engi- tion was to identify specific cases or examples whereby the neers and NDE technicians, that perform all the FCB inspec- additional inspection efforts dedicated to FCBs prevented a tions in the state. This is common in low population states failure that would have occurred had the inspection not been