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16 CHAPTER FOUR BRIDGE EVALUATION FOR OVERSIZE/OVERWEIGHT PERMITTING As discussed earlier in chapter two, bridge load rating is a fed- procedures, in terms of various factors such as the level of de- eral requirement. The bridge load rating factor, as defined in tail considered and software tools. This issue is observed in Eq. 1, is an important index for the bridge's condition in man- the survey and discussed next. aging the entire United States bridge network. Bridge load rat- ing is currently practiced according to the AASHTO Manual Tables C4-1A and C4-1B in Appendix C offer an for Condition Evaluation of Bridges (2000), which also refers overview of the population of highway bridges with load rat- to the AASHTO Standard Specifications for Highway ings in the United States and Canada. Load rating is a re- Bridges (2002). The result of load rating for a bridge is its ca- quirement of FHWA for all highway bridges in the United pacity to safely carry vehicular load. Therefore, when a vehi- States. The result of load rating a bridge is its safe live (ve- cle exceeds the legal weight limit of the jurisdiction and needs hicular) load carrying capacity with reference to standard a permit to operate, load ratings of the bridges the permit ve- vehicle configurations. They include the HS and H loads dis- hicle planned to cross are often used for examination. Because cussed earlier. In addition, the AASHTO specifications load rating uses standard vehicle loads, such as the AASHTO (MCEB 2000; Guide Manual . . . 2003) also include other HS and the H loads, the load rating results are directly useful standard loads, Types 3, 3S2, and 3-3, as shown in Figure 8. only when the permit vehicle's configuration is close to the The load rating result, as defined in Eq. 1, is the load carrying standard load used. Otherwise, the typical approach of re- capacity of the bridge component as the difference between viewing the permit is to load the bridge with the permit vehi- the rated member's capacity and the total dead load effect, cle and then determine whether the bridge can sustain the with all the safety factors included. There are two levels of load. The latter is referred to as bridge evaluation for permit load rating that are prescribed in the AASHTO MCEB (2000); review, as discussed in chapter two. the inventory and the operating ratings. The inventory rating refers to the "normal" load carrying capacity (or normally It was also noted that a number of different terms have allowed load) and the operating rating to the maximum load been used in the practice and literature to refer to the bridge carrying capacity (or maximum allowed load). The existence evaluation process. These terms are identified in chapter two. of load ratings for a bridge indicates that some information is available about the bridge's capacity, although sometimes the Although bridge load rating is guided by the AASHTO load rating is estimated based on engineering judgment. MCEB (2000), when applied to bridge evaluation for permit When a permit vehicle is reviewed for a particular bridge, the review there is ample room for interpretation and thus existence of the load rating itself can mean that a detailed and nonuniformity (this was discussed briefly in chapter two). quantitative analysis is possible for a relatively small amount With respect to this issue, this chapter presents relevant in- of additional work, because some information is already formation collected from U.S. and Canadian transportation available about the bridge. agencies. Bridge load rating and bridge evaluation for permit review are closely related actions as already discussed. For Table C4-1A shows that all the responding agencies many steps of the two processes and procedures the same have more than 60% of the bridges within their jurisdictions concepts and quantities are used. Therefore, the survey for with a load rating, except for Massachusetts, North Dakota, this synthesis study attempted to gather information on the Ohio, Puerto Rico, and Tennessee. As to what percentage state practices in both, and the results are presented and dis- of the bridges have an electronic model available, the dif- cussed here. ference between the state-level agencies is much more significant, varying from 0% to 95%, as shown in Table C4-1A. An electronic model here refers to a model that can VARIATION IN EVALUATION AND be repeatedly used, but requires minimal updating for some RATING PROCESS of the input data, such as corrosion-induced section loss, re- duced strength owing to aging, the loading vehicle, etc. It has been observed that there is a variation in the manage- When such a model is available for a bridge, the bridge ment of the bridge evaluation and load rating process among evaluation for permit review can be readily done and the re- state-level highway agencies in the United States. This varia- sults will be more consistent, compared with manual calcu- tion may result in different bridge evaluation and load rating lations. These electronic models may use the concept of
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17 FIGURE 8 Three other standard vehicle loads of AASHTO (Types 3, 3S-2, and 3-3) (MCEB 2000). girder line, 2-dimensional grillage analysis, 3-dimensional Also, many fewer bridges have electronic models available finite-element analysis, etc. When asked whether availabil- for repeating and updating load rating. ity of such electronic models for the bridges has had an im- pact on the uniformity in permit review, 25 respondents Tables C4-2A and C4-2B show the responses of the agen- said yes and 14 no. Of those agencies that answered yes, cies to the question of who provides the service of rating (i.e., 23 explained why. It shows that more state-level agencies bridge evaluation) when needed, for United States and agree that electronic modeling is an effective approach in Canada, respectively. The responses show mostly that state improving uniformity in permitting. personnel undertake this function in permit review. This indi- cates that efforts to improve uniformity in this area can be ef- Table C4-1B shows the same data as C4-1A, but for fective when mostly only state personnel are involved, because Canada. It can be seen that considerably lower percentages more resources are available to state personnel compared with of bridges in the Canadian jurisdictions have a load rating. other levels.