National Academies Press: OpenBook

Bridge Rating Practices and Policies for Overweight Vehicles (2006)

Chapter: Chapter Two - Literature Review

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Suggested Citation:"Chapter Two - Literature Review." 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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Suggested Citation:"Chapter Two - Literature Review." 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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Suggested Citation:"Chapter Two - Literature Review." 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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Suggested Citation:"Chapter Two - Literature Review." 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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Suggested Citation:"Chapter Two - Literature Review." 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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Suggested Citation:"Chapter Two - Literature Review." 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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Suggested Citation:"Chapter Two - Literature Review." 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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Suggested Citation:"Chapter Two - Literature Review." 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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5Review of OS/OW permits may require evaluation of the bridges that the permit vehicle would cross before approval is granted. For those who are not familiar with bridge evalu- ation, its concept and process are briefly reviewed here to provide the background needed for the following literature review. Bridge evaluation for permit review is very similar to bridge load rating. Both are used to understand the bridge’s safe load carrying capacity, with the former targeting the per- mit vehicle as the reference and the latter referring to some standard vehicle loads meant to represent the general truck traffic. Because of their common objective to understand bridge capacity, these two phrases or processes are some- times interchanged. Bridge load rating in the United States is guided by the AASHTO specifications Manual for Condition Evaluation of Bridges (MCEB) (2000). The safe load carrying capacity here as the result of load rating refers to a load level that the bridge can safely carry, but not the ultimate capacity. The bridge load rating process is usually analytical (i.e., it does not involve the physical testing of the material or the en- tire bridge system). When estimating the quantities needed, such as the material strength and load effect distributed to the structural component, the AASHTO MCEB refers to the de- sign specifications (Standard Specifications . . . 2002). This also reflects the conceptual consistency between the design and rating processes for highway bridges. The following load rating factor (LRF) is the result of bridge load rating for a bridge component with respect to a specific failure mode (bending moment, shear, etc.), according to the MCEB: Load rating factor = (R − A1 DL)/(A2LL) (1) where R is the bridge component’s resistance for that particular failure mode, and DL and LL, respectively, are the total dead and live (vehic- ular) load effects in that component. A1 and A2 are the dead and live load factors that cover possi- ble uncertainty involved in estimating DL and LL. The spec- ifications (MCEB 2000) give specific values for each de- pending on which limit state (i.e., the load factor method or the service load/allowable stress method) and which rating level (i.e., the inventory or the operating rating) are used. This formula should also be applied to all of the critical cross sections of the bridge component, and the lowest resulting LRF is taken as the LRF for that bridge component. In addi- tion, for a bridge structure with a number of structural com- ponents, the same formula needs to be applied repeatedly to all the components of concern. The lowest of the resulting LRFs for these components and failure modes is typically taken as this bridge’s LRF. This is an important index in the jurisdiction’s inventory for the particular bridge. It is also re- quired by FHWA, as included in the National Bridge Inven- tory. When the LRF of the bridge is found to be 1.0 or higher for a standard vehicular load LL, the bridge is said to be able to carry that standard load. Alternatively, the bridge is also said to have a (safe) load carrying capacity equal to the standard load’s tonnage times the LRF. For example, if a bridge is found to have a LRF of 1.20 for the AASHTO standard HS20 live load (gross vehicle weight or GVW = 36 tons or 72,000 lb) as shown in Figure 1, it is said to have a capacity of 1.20 × 36 tons = 43.2 tons (86,400 lb). As seen in Figure 1, the HS20 load consists of the standard truck and the lane load. The lane load may induce a higher load effect than the truck depending on the span length. When this happens, the larger load effect is taken and used in Eq. 1 as LL for load rating. The H20 load is another AASHTO standard load as shown in Figure 2, which is sometimes also used as a refer- ence when stating the load carrying capacity. Note that the load carrying capacity in tonnage depends on the reference standard vehicle load used, because the (vehicular) live load effect used in Eq. 1 is not proportional between different standard loads. For example, Figure 3 shows the maximum load effects (bending moments and shear forces) of simply supported bridge spans for the AASHTO HS20 load, and Figure 4 for the H20 load for comparison. Both are taken from the AASHTO design specifications (Standard Specifi- cations. . . 2002), which specify the HS20 truck GVW as 72,000 lb and the H20 truck GVW as 40,000 lb. However, the ratio of the two maximum load effects is not always 40/72 for every bridge span. This also illustrates that different per- mit vehicles may induce different load effects in a bridge’s components. Therefore, for heavier truck loads, bridge eval- uation is required to understand their individual effects to the bridge and associated risk of bridge failure. CHAPTER TWO LITERATURE REVIEW

As mentioned earlier, bridge evaluation for permit re- view also has the same purpose of understanding the bridge’s load carrying capacity, but for a specific target of the permit vehicle. Namely, it is to answer the question whether the bridge is able to carry the particular permit ve- hicle. Thus, bridge evaluation for overweight permit review typically replaces the standard vehicle load’s load effect LL with the permit vehicle’s load effect in Eq. 1. Similarly, if the bridge’s LRF is 1.0 or higher, after all the components 6 and critical cross sections are taken into account, the permit vehicle is then considered to be permissible for that evalu- ated bridge. In practice and literature, there have been many differ- ent phrases used to refer to bridge evaluation for permit re- view. For example, “bridge load rating” is one term for the obvious reason of using the same equations and identical quantities. Other terms that were found to refer to bridge FIGURE 1 AASHTO standard HS20 vehicle live load: (top) truck load; (bottom) lane load.

7evaluation for permit review include “bridge review,” “structure review,” “bridge study,” “engineering study,” “engineering analysis,” “engineering evaluation,” and “en- gineering review.” These phrases have all been used by dif- ferent states in the responses to the questionnaire, which will be discussed later. It is also important to point out the following issues that have been the focus of discussions regarding bridge evalua- tion for permit review. They are relevant and possibly attrib- utable to the observed nonuniformity in permit review. LIVE LOAD FACTOR In Eq. 1, the live load factor is A2. For example, the AASHTO specifications (MCEB 2000) prescribe this factor as 1.3 for the operating rating and 2.17 for the inventory rating when the LFR is used. The operating rating refers to the maximum load level the bridge is allowed to carry. The inventory rating is a load level the bridge is allowed to carry without a time limit (MCEB 2000). There have been discussions on whether using these live load factors for permit review is appropriate, because the bridge evaluation for permit focuses on the FIGURE 2 AASHTO standard H20 vehicle live load: (top) truck load; (bottom) lane load.

particular permit vehicle and the load rating process consid- ers general truck traffic loads. These two groups of loads have very different probabilities of occurrence (Fu and Moses 1991; Fu and Hag-Elsafi 1996). The latest AASHTO bridge evaluation specification [Guide Manual for Condition Evalu- ation and Load Resistance Factor Evaluation (LRFR) of Highway Bridges 2003] has adopted a probabilistic concept of prescribing different load factors for the standard bridge load rating and the bridge evaluation for permit review. LIVE LOAD DISTRIBUTION FACTOR In Eq. 1, LL is defined as the live load effect distributed to the particular component being evaluated. The load distribution de- pends on how many vehicles are used to load the bridge, the material types of the structural components involved, and their structural arrangement. It also depends on how the wheel lines are arranged in the transverse direction, namely the vehicle’s 8 gage width. The AASHTO evaluation specifications (MCEB 2000) refer to the AASHTO design specifications (Standard Specifications . . . 2002) to guide how load distribution should be done in bridge evaluation for the standard gage width of 6 ft. Although these guidelines may be adequate for bridge load rating of general truck traffic, they are unable to cover all bridge evaluation scenarios for permit review. There are many situa- tions where these guidelines are not applicable. For example, permit loads may not simultaneously appear in two or more lanes on the bridge, as likely as nonpermit vehicles. In addition, the gage widths of permit vehicles may not be the standard 6 ft. These factors leave ample of room for interpretation and alter- natives, which could lead to different results of permit review. IMPACT FACTOR In Eq. 1, LL also includes the so-called impact factor meant to cover the dynamic amplification of the vehicle load. According FIGURE 3 Maximum load effects of AASHTO HS20 live load for different bridge span lengths.

9to the AASHTO bridge design specifications (Standard Speci- fications . . . 2002) this factor can be as high as 1.3 or 30% above the static load effect. Many transportation agencies adjust that factor in bridge evaluation for permit review, particularly when the bridge capacity is otherwise below the required level. When a lower impact factor is used in LL, the rating factor in Eq. 1 can become higher and therefore more likely to reach the 1.0 level to allow issuance of the permit. This also imposes a requirement for the permit vehicle’s operation to control the driving speed, braking, and/or acceleration to limit impact when crossing the bridge. Apparently, different jurisdictions used different prac- tices with respect to this factor. In this study, a literature search was undertaken with re- gard to bridge evaluation for permit review and other possi- bly related subjects. The identified previous research efforts reported in the literature are reviewed next. Some of the issues possibly causing nonuniformity are addressed in these research reports and papers, with respect to bridge evaluation for permit review. NCHRP Synthesis of Highway Practice 143: Uniformity Efforts on Oversize/Overweight Permits 1988. This synthesis study focused on the uniformity efforts in OS/OW permit issuance. The report summarized the reasons for nonuniformity in permit procedures as follows. From the states’ perspectives, the difficulty in common permit proce- dures includes concerns about physical, safety, economic, legal, and political factors. More specifically, the following factors were identified as contributors to the observed nonuniformity: inadequate funding and staffing, continuing changes in state policies, inadequate data for analysis, pres- sure from the trucking industry, concern about federal pre- emption, a lack of constituency, concern about reducing standards, and national effort having little chance for suc- cess. From the federal perspective, only a limited degree of intervention was considered possible; therefore, the federal government preferred to have the states take the lead toward a higher level of uniformity. FIGURE 4 Maximum load effects of AASHTO H20 live load for different bridge span lengths.

It was also concluded in this study that among all the ef- forts aimed at achieving better uniformity only the NETC has been able to succeed in developing relatively uniform per- mitting procedures. (An update of the NETC activity is given in chapter five.) The reasons were summarized as follows: • Recognition of the importance of the issue by the chief administrative officers of the involved state DOTs. • A set of issues was selected for resolution that all par- ticipating DOTs believed were critical and for which the probability of achieving success was very high. • Full cooperation and participation was achieved by the technical individuals of the DOTs who were responsi- ble for issuing permits. • Within that framework of mutual cooperation, each of the states was willing to drop its “jurisdictional barriers.” • The participating states presented a uniform position to the trucking industry. • The NETC did not attempt to include all permit re- quirements within the regional agreement; therefore, each state can deal with the exceptions in the usual way and no situation is excluded. • A concerted, centralized staff effort was funded to de- velop and implement this program. • Every state gained and none lost anything from this agreement. • The participating states believed that it was inevitable that uniform procedures will be required by the federal government, and that it is much more efficient for the states to take the lead before they are preempted. It was also concluded that the NETC experience illustrates that it is possible to enhance better uniformity. However, it appears that it cannot be accomplished initially on a national scale. Rather, it should begin on a regional or even a subre- gional basis as the NETC was able to do. Then, it would re- quire that the appropriate policy and political as well as tech- nical interaction take place within and between regions. Note that in view of contemporary concerns about a fast re- sponse to natural and terrorist driven disasters, harmonization must be accomplished readily to facilitate permit reciprocity across multistate areas in time of disaster and to eliminate the conflicts in OS/OW permitting. NCHRP Synthesis of Highway Practice 108: Bridge Weight-Limit Posting Practice 1984 For overweight permit issuance, weight limit owing to bridge capacity (load rating) is often a critical factor. Understanding how the bridge weight limits are determined is therefore rel- evant to this study. NCHRP Synthesis 108 summarizes the practice of bridge weight-limit posting in the United States as of 1984. Besides the administrative aspects of weight-limit posting, engineering practices, which are more relevant to the current study, were also addressed in that study. 10 In practice, bridge weight-limit posting is typically de- termined based on bridge inspection and bridge load rating. Bridge inspection, in the context of its relation to weight- limit determination, is necessary to obtain the information for properly evaluating the strength of the bridge and its be- havior and performance under the load. On the other hand, this synthesis emphasized that bridge inspectors are often not involved in the following structural load rating, possibly using the inspection results. This “discontinuity” may con- tribute to nonuniformity or inconsistency in bridge weight- limit determination, which in turn affects the uniformity in permit issuance. In the issue of bridge load rating, the synthesis noted that the relevant AASHTO specifications for the practice allow for considerable leeway for the use of engineering judgment in evaluating or posting bridges. This leeway has resulted in considerable variation in the ways different states evaluate and post bridges. This issue will be discussed further in chap- ters three and four. “Overload Permit Checking Based on Structural Reliability” (Fu and Moses 1991) and “New Safety- Based Checking Procedure for Overloads on Highway Bridges” (Fu and Hag-Elsafi 1996) Currently, a common practice in issuing overweight permits is to use the bridge load rating formula Eq. 1 provided ear- lier, prescribed in the AASHTO specifications (MCEB 2000 and its previous editions). Note that these bridge rating for- mulas are intended for use in evaluating bridges against typ- ical truck traffic loads, and not necessarily for very heavy and occasional permit loads. These two papers represent the first research efforts focusing on the issue of different probabili- ties of occurrence of legal vehicle traffic and permit vehicles above the legal load level done for the Ohio and New York DOTs, respectively. The research projects reported in these publications ana- lyzed data of the respective states for normal truck traffic and permit truck traffic and developed different live load factors (A2 in Eq. 1) specifically for bridge evaluation in permit re- view, to maintain the same bridge safety as for bridge load rating intended to cover general truck traffic. These live load factors are different for single trip permits and multitrip (e.g., annual) permits. In general, the live load factor can be smaller for less frequent permit loads. This concept has been adopted in the latest AASHTO bridge evaluation specifica- tions (Guide Manual . . . 2003). Overload Permit Procedures (Noel et al. 1992) As reviewed earlier, overweight vehicle review uses the same load rating concept of the AASHTO specifications (MCEB 2000); namely, the vehicle requesting a permit is placed on a bridge structure in a mathematical model, replacing the

11 standard vehicle, such as the HS20. If the induced load effects (moment, shear, etc.) do not exceed the capacity allowed, the vehicle will then be allowed to cross the bridge and, oth- erwise, not. Although there may be many bridges needing evaluation for a permit type (e.g., annual permits without specified routes), analyzing every bridge in the jurisdiction is costly and can become particularly difficult. Therefore, there have been some models suggested and developed to cover a group or population of bridges within a jurisdiction. Noel et al. (1992) represents a typical effort in this direction for bridges designed to the AASHTO H15 load. Hereafter, these bridges are referred to as H15 bridges. Figure 2 shows the H20 load whose 75% proportional reduction is the H15 load (15/20 = 75%). The reason for focusing on H15 bridges in this study was that they were considered to be the bottleneck in approving overweight permits in Texas. These bridges typically have the lowest capacity, because the H15 design load has been obsolete for some years and it induces lower load effect than the HS20 design load. As a result, the study developed a formula of maximum gross weight as a function of the permit vehicle’s wheel- base (the distance between any two axles) and the bridge’s span length. A similar formula was also proposed as a func- tion of the wheelbase only. It is more restrictive than the first one (with span length as another variable), because it uses the most restrictive gross weight for all span lengths. Furthermore, an empirical modification factor was also suggested to cover the gage width of the permit vehicle other than the standard 6 ft. It should be noted that the analysis used in this study included only one permit vehi- cle in one lane (not multiple vehicles in all lanes available) on the bridge. The proposed formulas can be useful for a first screening of permit vehicles if the formulas’ validity is confirmed. This type of screening can reduce the work load required, because otherwise every permit vehicle needs to be analyzed. It also should be noted that those permit vehicles that fail the screening may still be permissible; however, they will need to be analyzed on a case-by-case basis. Sometimes additional requirements (such as reduced speed for reduced dynamic impact) may be needed on the permit vehicle’s operation. Bridge Analysis Simplified (Bakht and Jaeger 1984) In this paper, Bakht and Jaeger proposed an overweight per- mit review method. The method’s concept is that the safe permit load can be the worst combination(s) of the maximum vehicle loads the bridge is likely to have sustained during its lifetime. The procedure was derived using theoretical as- sumptions regarding the probability distribution of the truck 0 500000 1000000 1500000 2000000 2500000 3000000 3500000 4000000 1997 1998 1999 2000 2001 2002 2003 0 500000 1000000 1500000 2000000 2500000 3000000 2001 2002 2003 Non-Divisible Single Trip Permits Non-Divisible Annual Permits Divisible Single Trip Permits Divisible Annual Permits Overwidth Permits FIGURE 5 Number of overweight/overwidth permits issued by states. FIGURE 6 Overweight/overwidth permits issued by states by type.

loads that have been experienced. Although the probabilistic concept appears to be reasonable, it is not clear whether the theoretical model is applicable to all real bridges, because the truck load spectrum for each bridge can be very much dif- ferent. It has not been reported since then that this procedure has been applied to real cases of permit review. “National Commercial Motor Vehicle Size & Weight Enforcement Trends” (2004) FHWA routinely monitors the practice of OS/OW vehicle op- eration. Apart from the earlier FHWA statistics published in 1991, these statistics derived in 2003 may currently be the latest presented. Figure 5 shows the number of overweight/ overwidth permits issued in the states from 1997 through 2003. An annual increase in permit issuance can be seen. 12 Figure 6 shows these permits broken down by type for 3 years (2001–2003), indicating nondivisible trip permits dominating the population. Nondivisible loads here refer to those that cannot be cost-effectively divided, such as a large transformer, a house, a piece of construction equipment, etc. It should be pointed out that the figure shows “head counts” for the permits, not indications of how frequently each per- mit type of vehicles appears on the road. Statistically, the an- nual permits represent significantly more trips than the sin- gle trip permits. Therefore, the annual permit vehicles may appear much more often on the roads. FHWA (LRFD Bridge Design Specifications 2004) also indicated that “Overweight permit issuance continues to increase, with annual or multi- ple trip permits becoming more commonplace.” In addition, “Freight tonnage moved by truck is forecast to continue to in- crease.” This has been a concern of FHWA for some time.

Next: Chapter Three - Nonuniformity in Permitting Systems »
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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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