National Academies Press: OpenBook

Bridge Rating Practices and Policies for Overweight Vehicles (2006)

Chapter: Chapter Six - Conclusions and Future Research Needs

« Previous: Chapter Five - Efforts Possibly Contributing to Improved Uniformity of Bridge Rating for Oversize/Overweight Vehicles
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Suggested Citation:"Chapter Six - Conclusions and Future Research Needs." National Academies of Sciences, Engineering, and Medicine. 2006. Bridge Rating Practices and Policies for Overweight Vehicles. Washington, DC: The National Academies Press. doi: 10.17226/13954.
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Page 26
Page 27
Suggested Citation:"Chapter Six - Conclusions and Future Research Needs." National Academies of Sciences, Engineering, and Medicine. 2006. Bridge Rating Practices and Policies for Overweight Vehicles. Washington, DC: The National Academies Press. doi: 10.17226/13954.
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Page 27

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27 Most states developed tools or methodologies years ago that determined the allowable weight limitations and vehicle di- mensions for permit vehicles in their jurisdictions. Some of the approaches that states have used are: • Determine allowable permit axle and/or axle group weights by increasing Federal Formula B weights by some amount. • Develop a permit design and analysis vehicle and set the allowable axle group weights based on the load effects of this vehicle. • Use locally developed methodologies to extrapolate al- lowable permit weights from the bridge design loading. • Use weight limitations that are legislated rather than de- rived based on engineering concepts. • Use basic weigh policies of adjacent state(s) with modifications. Note that these approaches not only establish the maximum axle and/or axle group weights, but may also affect the axle spacing or number of axles required to carry the weight. For instance, Federal Formula B considers the number of axles in a group, whereas in some states the spread of the axles is used as an important factor in permit review. These weight policies have evolved over the years to make accommodations for local industry needs and the needs of specialized vehicles and industries. The policies may be difficult to modify for the following reasons: • The permit trucking industry has made an economic in- vestment based on these policies. Changing the policies would change the interstate and intrastate competitive playing field. • The personnel who developed these polices are no longer employed at the agencies. The present personnel are re- luctant to make significant changes without additional studies or electronic computer modeling of the bridges that would support the considered changes. Resources are frequently not available for the studies or computer models. Some states now have various electronic computer mod- els, which allow them more flexibility to make changes. This also contributes to the nonuniformity in permit weight policies and practices. Apart from the policy differences, the specifications for bridge evaluation also allow ample room for variation because they do not have specific enough provisions. Several conclusions can be drawn to summarize this syn- thesis study. • In the United States, overweight vehicle permitting is operated through a highly complex system that involves many agencies at the state and local levels. The gov- erning policies and regulations vary extensively and significantly in terms of permit type and processing op- eration. The industry has a strong interest in enhanced uniformity in this area. • Bridge evaluation for permit review as a step in the permitting system also varies noticeably among the state-level agencies, primarily as a result of variation in interpretation of the AASHTO Manual for Condi- tion Evaluation of Bridges (2000) and the referenced AASHTO Standard Specifications for Highway Bridges (2002), and possibly also as a result of the dif- ferences in the computer software programs used. The Southeastern Association of State Highway and Trans- portation Officials (SASHTO) study discussed in chapter four shows that the difference in the allowable vehicle weight for one single case can be as high as 40%. Note that the lack of specific provisions in these specifications for the involved quantities has allowed for the various observed interpretations. • Having electronic models of the bridges that can repeatedly be used for bridge evaluation is an effective approach to enhanced uniformity for permit review. It can reduce turnaround time for permit review, allow for more effort on examining other options for approv- ing the permit, and provide more consistent and accu- rate results. • The multistate permit programs of the New England Transportation Consortium and SASHTO represent a successful model for improved uniformity in oversize/overweight permitting. In these programs, one permit can be applied in all participating states for inter- state trips if the vehicle falls within the defined spec- ifications for dimensions, GVW, axle weights, and/or other parameters. As a result of this synthesis effort, the following needs were identified for further research in the area of improved uniformity. CHAPTER SIX CONCLUSIONS AND FUTURE RESEARCH NEEDS

• Further understanding of the technical basis for issuing those permits that do not require bridge evaluation cur- rently practiced in many states is needed. This effort should cover, for example, the vehicle configuration mod- els used in determining the thresholds for not requiring bridge evaluation, axle and axle group load limits, and ex- periences of the states with different thresholds. Next, it would be helpful to develop rational methods and the needed electronic tools to perform screening, which could easily be implemented in most states. This research effort should include review of the computerized systems used by transportation agencies in Alabama, Nebraska, Ohio, South Carolina, South Dakota, and West Virginia, which can analyze every bridge on any route given in the re- spective state. • Acquire quantitative understanding of the differences in the approaches, procedures, and software tools used for bridge evaluation in permit review, followed by the de- velopment (or identification) of those that can achieve 28 enhanced uniformity. For example, they could address the differences in estimating the load distribution fac- tor, dynamic impact factor, and live (vehicular) load po- sitioning. This effort could cover those bridges consid- ered not ratable for a variety of reasons. It is also suggested that the details collected in the SASHTO sur- vey be fully understood in the context of this research effort, to identify the causal factors for nonuniformity in bridge evaluation for permit review. • Develop a national envelope vehicle based on the en- velope vehicles of the New England Transportation Consortium and SASHTO, and the uniform vehicle of the Specialized Carriers & Rigging Association. This vehicle can be a candidate for a national permit pro- gram, which should cover a majority of permit vehicles in the country. Resolution of these issues could improve the uniformity of permit review for oversight/oversize vehicles.

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TRB's National Cooperative Highway Research Program (NCHRP) Synthesis 359: Bridge Rating Practices and Policies for Overweight Vehicles explores overweight vehicle permit processes. The report includes information on state and provincial bridge rating systems, bridge evaluation practices, and permit policies as they relate to overweight and oversize vehicles. The report is designed to help in the understanding of the reasons for nonuniform permitting practices. The report reviews specifications, software types, treatment of nonstandard configurations, and allowance for in-place dead loads; processes of permit review; and personnel assigned to permit review.

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