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18 VARIATION IN EVALUATION AND day of travel, and altering the vehicle's configuration to dis- RATING PROCEDURE tribute the load to more members. These restrictions all can change the bridge evaluation result for permit review. The AASHTO MCEB (2000) has been used extensively in guiding bridge evaluation for permit review. This has also left ample room for the engineer to decide on many issues Computer-Aided Modeling and aspects in the evaluation. In addition, the newly adopted AASHTO LRFR Bridge Design Specifications (2004) offers Bridge evaluation for permit review and routine load rating another alternative for bridge load rating and bridge evalua- now are largely done using computer software programs. tion for permit review. Table C4-3 displays the responses of Therefore, which program is used may have a strong impact U.S. transportation agencies to the survey regarding their on the result, although conceptually the differences should practice on some of these aspects of permit review. It shows not be significant between the different programs. Table that only one of the states (Pennsylvania) is using the LRFR C4-5 shows the models and corresponding software pro- specifications, most states use LFR, and many use both the grams used by the U.S. agencies. In addition to the finite- Allowable Stress Rating and LFR. As to which load rating element analysis (FEA), grillage, and girder line methods level is used, it can be seen almost uniformly that the operat- given in the questionnaire, the following methods were also ing level is used for permit review. The reasons for using this mentioned in the responses: load testing and in-house meth- level are mainly as follows: (1) the operating rating allows ods and programs. These results show that the girder line higher loads, so that the probability of having a permit ap- method is the most often used by the responding agencies. plication approved can be maximized and (2) the operating Therefore, if uniformity in analysis methods and software level is rational for infrequent loads with higher certainty. programs is a goal, more emphasis should be placed on this Note also that Canada uses different specifications for load method and its associated software. rating. Therefore, the differences between the United States and Canadian practices are more noticeable. Permit Screening Approach Load Placement As discussed earlier, many U.S. state agencies take a two-step approach in permit review: (1) screening the permits into two How to place the load on the bridge in bridge evaluation for groups, one requiring bridge evaluation and the other not, and permit review and how to determine the associated load dis- (2) performing bridge evaluation if required. Various screen- tribution factor is one of the most important elements affect- ing concepts and approaches were cited in the responses. ing the result. Tables C4-4A and C4-4B contain the responses Table C4-6 summarizes the findings in this area. A large ma- of the U.S. and Canadian agencies, respectively, regarding jority of the responding agencies use comparison with the de- this issue. Seventeen U.S. agencies load only one lane with sign vehicle and/or acceptable axle spacing and axle weight the permit vehicle, whereas another 15 load other lanes in ad- for this screening. Other approaches are also being used dition to the lane loaded with the permit vehicle. One agency including (1) comparison with the standard rating vehicles, uses both methods. For comparison, the Canadian agencies (2) agency-specific formula (e.g., as used by Indiana), (3) mostly use multiple-lane loading (Table C4-4B). Note that comparison with the Federal Bridge Formula, and (4) com- quantifying the probability of multiple vehicle presence on a parison with previously approved permits. The comparisons bridge is still a subject for further research, because it has not may be done using charts, maps, and/or computer programs. been scientifically proven which way(s) is more appropriate. It is interesting to note that Kansas, Nebraska, and North The right-hand portions of Tables C4-4A and C4-4B Dakota use a rather unique approach that examines every show the responses of the agencies regarding possible re- bridge on the selected route, so that no screening is needed. strictions for the permit vehicle on the bridge. These restric- It appears that the systems include electronic models for all tions are usually imposed on the vehicle to reduce stress and the bridges in the jurisdiction. This approach is believed to therefore the risk of failure, which in turn increase the prob- be able to maintain a high level of uniformity, at least within ability for the permit application to be approved. These the respective states. restrictions include the loading position of the permit vehicle on the bridge, vehicle speed, whether other vehicles are allowed simultaneously on the bridge, and whether acceler- Bridge Condition and Material Properties ation or deceleration is allowed on the bridge. The first three options have been used by most of the agencies that re- Bridge load rating requires quantified estimation for bridge sponded (88%), whereas the fourth option is less frequently components' material properties. When existing old bridges used (45%). In addition, other measures have been men- are involved, this estimation may not be uniformly done. tioned to permit heavy loads: restricting traffic under the Table C4-7 shows that approximately one-third of the U.S. bridge to be crossed by the permit vehicle, restricting time of agencies do not have specifications or guidelines as to how
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19 bridge condition is taken into account in load rating or bridge · A method cited in "A Rational Procedure for Over- evaluation for permit review, and seven reported that no weight Permits" (Bakht and Jaeger 1984); and specifications or guidelines are used for material property es- · A method described in Bridge Analysis Simplified timation for bridge evaluation and load rating. This situation (Bakht and Jaeger 1985). may have contributed to nonuniformity in bridge evaluation for permit review, because individual and possibly inconsis- It would clearly be valuable to compare these methods and tent decisions may have been made regarding these issues. to eventually develop a more widely acceptable method (that can be one or a combination of these methods) to have con- sistent practice. Vehicle Gage Width For multilane loads, columns 3 and 4 of Tables C4-8A and The gage width here refers to the center to center distance be- C4-8B provide additional details on this subject. Cranes actu- tween dual wheel tires in the direction of the axle (see Fig- ally can be viewed as a special case of OW vehicles; however, ures 1 and 2). Gage width is important in bridge evaluation many transportation agencies single out them for special treat- because it directly affects the lateral distribution of the axle ment, as can be seen from these tables. load over the bridge superstructure. In the case of beam-type bridges, the gage width directly affects the fraction of a wheel load to be transferred through the deck to the beams Other Details in Load Rating supporting the deck. Tables C4-9 and C4-10 exhibit the responses of the agencies The AASHTO vehicles used in the design of bridges are to the questions regarding various details in bridge load rating. based on trucks that have a gage of 6 ft. Consequently, a They include dead load distribution, span length definition, gage of 6 ft is considered to be the standard gage width by treatment of rebar cutoffs in concrete members, determination virtually all permitting agencies in the United States. Gage of dynamic impact factor, load effects considered as limit widths that vary significantly from the standard 6 ft gage states, lateral load distribution factor, and inclusion of nontra- are often referred to as nonstandard gage widths. Although ditional additional loads and environmental factors. many OS/OW vehicles have standard or near standard gage widths, many do not, and typically the very heavy According to Tables C4-9 and C4-10, the following factors OS/OW vehicles (i.e., superloads) have nonstandard gage appear to be relatively uniformly treated: span length defini- axles. tion, dynamic impact factor, and dead load distribution. The most nonuniformly treated factors are, in order of decreasing In bridge evaluation for permit review, there is consider- severity, environmental factors, bar cutoff, additional dead able disparity between the states regarding the lateral distri- loads, limit state, and lateral distribution factor (with the gage bution of the nonstandard gage axle loads. This is because length issue excluded). there is no nationally recognized specification for the distri- bution of nonstandard gage axles. As seen in Table C4-8, the Consideration of the listed environmental factors varies most frequently used specifications for lateral distribution is from "not considered at all" to "as detailed as in design." The by far the AASHTO Standard Specifications for Highway same is observed for bar cutoffs. Bar cutoff here refers to steel Bridges (2002), followed by the AASHTO LRFD Bridge De- reinforcement discontinued in concrete members. It causes a sign Specifications (2004) and the AASHTO Guide Specifi- sudden reduction in load carrying capacity at the cutoff sec- cations for Distribution of Loads for Highway Bridges tion. These types of sections may require checking in bridge (1994). However, these specifications are all based on the evaluation as critical sections. As seen in Table C4-10, the standard gage axle and do not have specific provisions for the most commonly concerned limit states are moment, and then distribution of nonstandard gage axle loads. shear. The most commonly used specifications for lateral load distribution factors by far is the AASHTO Standard Specifi- Tables C4-8A and C4-8B show the responses of United cations for Highway Bridges (2002), followed by the States and Canadian agencies, respectively, as to how they AASHTO LRFD Bridge Design Specifications (2004) and the deal with this issue in permit review. Sixteen of the respond- AASHTO Guide Specifications for Distribution of Loads for ing agencies indicated that gage width is not taken into account Highway Bridges (1994). in permit review at all, and those who do take into account gage width use the following methods or concepts: Local Bridge Evaluation · Empirical distribution factors as functions of gage Of the total numbers of bridge structures in this country, width, such as the effective width concept of McLelland there are more locally owned bridges than there are bridges (2003); owned by state-level agencies. Also, local bridges have · Lever rule; a higher percentage of their inventory with a lower load