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33 Therefore, the loading cases to be studied, location of poten- is every year or more frequently if there are specific problems. tial cracks, degree to which the dynamic effects associated It is not known if efforts beyond what is normally put forth with a fracture are included in the analysis, and fineness of the models and choice of element type should all be agreed upon during the inspection of non-FCBs are actually warranted; by the owner and the engineer. The ability of a particular soft- however, even if there are only a few cases of such efforts pre- ware product to adequately capture the complexity of the prob- venting a major failure, they are most likely worthwhile. 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 Questions do arise with respect to inspection requirements Combinations of Table 3.4.1-1 should be considered as should for newer bridges, which were built using superior steels the number of loaded design lanes versus the number of striped (especially using any type of HPS), subjected to advanced traffic lanes. NDT techniques, and fabricated using higher-quality welding procedures than used in the past. In addition, those fabricated NCHRP Report 406 (34) gives practical requirements for the using the FCP should be of superior quality than bridges built residual capacity of the damaged superstructure. before the introduction of the FCP. Modern steel bridges are also built with a composite deck slab and are inherently more This same type of analysis is being used to evaluate older capable of carrying redistributed loads through alternate paths. structures as well to better direct resources for maintenance Two-girder curved-girder bridges, especially those built since and replacement; for example, if a bridge is not really frac- the early 1980s, almost always contain heavy transverse cross ture-critical then it may not be necessary to replace it as soon. frames capable of carrying a significant load from one girder to another. Other countries do not make a distinction in FCMs; how- ever, scanning tours have noted that three-dimensional refined In addition, based on the survey responses and infor- analysis is more often used in the design and evaluation of mation received during interviews of recognized experts in bridges in Europe and that the inspection interval is often fatigue and fracture, it was found that there is a strong sense based on risk. Some in the United States believe that fracture- that the inspection interval should in some way be related to critical inspection should be less frequent for the modern risk, ADTT, and age of the structure. Possibly, bridges built bridges as a result of the much better detailing and materials, after the implementation of the FCP or with HPS can be and perhaps should be based on truck traffic, fatigue details, inspected at greater intervals. Furthermore, bridges with low and other risk factors. ADTT could be inspected at a frequency somehow related to traffic data. The approach could be similar to that used in ROLE OF INSPECTION FOR the aircraft industry, where airframes are inspected based on FRACTURE-CRITICAL BRIDGES hours of flight, not just years of service. It is recognized that issues related to corrosion, settlement, scour, and so forth, There is no doubt that the hands-on fracture-critical inspec- which can only be quantified through regular inspection, are tions have revealed numerous fatigue and corrosion prob- not related to ADTT. However, these issues are common to lems that otherwise might have escaped notice. Many of all nonredundant bridges and not just FCMs. these problem details are discussed in Appendix A. Twenty- three percent of the survey respondents (see chapter three) Research in risk-based inspection seems justified because indicated that they had found significant cracks and corrosion the payoff can be substantial. For example, there would be that could have become much worse; and in doing so, possi- significant savings if it could just be shown that bridges can bly averting collapses. Similar examples may be found in be inspected every 3 years instead of 2. The potential savings trade magazines [see Zettler (31)]. However, agencies also should be considered when developing the scope and budget report finding these problems on non-FCBs when hands-on for such a research project to ensure that reliable data are col- inspections are performed. Therefore, such inspections are lected and safe recommended changes to the inspection pro- good for all steel bridges, not just FCBs. gram established. However, owners spend a major portion of their budget on It must be emphasized that before inspection intervals are efforts associated with the inspection and maintenance of all adjusted, it is critical that the potential ramifications be thor- structures. Fracture-critical inspections consume a large frac- oughly understood. It is possible that some components need tion of that budget for a comparatively few structures. The cost to be inspected on the schedule currently in place. For exam- of the fracture-critical inspection is typically two to five times ple, the existing inspection intervals might need to remain greater than inspections for bridges without FCMs. Inspec- unchanged to ensure that problems with elements such as tions of closed sections such as tub girders and tie members deck slabs and bearings do not go undetected. It was also sug- are extremely expensive because inspectors must get inside gested by a few of the designers who were interviewed that it such sections. What constitutes a fracture-critical inspection might be important to inspect the deck, for example, more fre- is subject to interpretation and disagreement. The frequency quently than the steel superstructure. Another example would of fracture-critical inspections is actually not specified and be a bridge susceptible to scour, where it might be required to varies up to every 5 years, is typically every 2 years, but often inspect the substructure at a greater frequency than the super-
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34 structures. Nevertheless, it is clear that before changes are result of several factors. First, few academic institutions offer made there are many factors that must be considered. any substantial material in undergraduate coursework on fatigue and fracture. Therefore, recent graduates must learn on the job and often simply follow cookbook specification Education and Training of Inspectors approaches with little understanding of the spirit of the spec- ifications. Second, there has been a major shift in the experi- Overall, training of inspectors seems to be adequately avail- ence level in U.S. DOTs over the last several years. Many of able through the existing courses offered by the National the most experienced engineers are retiring and with their Highway Institute. None of the responding agencies identified departure will be lost the years of experience acquired dur- any problems with the current educations strategies. However, ing the period when most of the issues with fatigue and frac- one area that appears to need additional effort is related to the ture were foremost (19701990). documentation and archiving of previous failures and prob- lems. As discussed in the previous chapter, failures known to To address this, several owners and engineers expressed have occurred in certain states were not always reported in the the need for additional training. Although current National replies to the survey. Therefore, a better method of tracking Highway Institute courses seem adequate for training inspec- such information appears to be needed. tors and providing overall guidance, a more in-depth course appears to be needed for bridge designers. For example, the Education and Training of Engineers Pennsylvania DOT has sponsored the development of a short course on fatigue and fracture design for its engineers and Although not evident from the surveys, discussions with consultants. In addition, FHWA is currently (2005) consid- industry leaders have also revealed that a general knowledge ering the development of a similar course designed to address gap exists with respect to fatigue and fracture design, evalu- this need that would be taught in strategic locations across ation, and behavior in the engineering community. This is the the United States.