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Chapter V


Western Uniformity Scenario Analysis graphic depicting western states and illustration of longer combination vehicle


Damage to highway structures represents the most critical infrastructure cost of allowing larger and heavier trucks on the nation's highways. All of the studies performed by the Federal Highway Administration (FHWA), the Transportation Research Board (TRB), and several universities in the last ten years that examined potential impacts of truck size and weight (TS&W) increases have found that the estimated damage to bridges would be the greatest single infrastructure cost caused by larger, heavier trucks.

Impacts of Truck Loads

Bridges must be strong enough to safely accommodate all vehicular traffic. This accommodation of truck loads is the critical element in the design of highway bridges, except for the few very large bridges where the weight of the bridge itself is so much greater than the traffic loads that the bridge weight is the critical element. The analysis described below examines and compares the bending moments (and therefore the bending stresses) produced by the set of scenario trucks and the base case trucks with the moments caused by the bridge rating vehicle. Before documenting this analysis, bridge behavior when subjected to truck loads is briefly discussed.


In general, bridges must accommodate three forms of stress: bending stress, shear stress and fatigue stress. If a weight were placed at the center of a beam that is supported at each end, the beam would bend, or deflect. Material at the bottom center of the beam would stretch and at the top of the beam it would compress. Truck loads produce a bending moment, which inflicts this stress. A bending moment is a load times a distance; in bridges it is a point or equivalent point load (in cases of uniform or non-point loads) times the distance of that load to the nearest support. There is a direct one-to-one relationship between bending moment and bending stress.

Shear stresses can be thought of as those stresses caused by a force that cuts (i.e., shears) rather than bends the beam. For example, if a very large load were applied very close to the support, there would be no significant bending action (since the distance to the support is very small), however, the beam would resist the "cutting" action, that is, the shear stresses. Fatigue stresses are, most simply, repeated bending stresses. Everyone who has repeatedly bent a paperclip back and forth until it breaks has caused fatigue stresses to the metal of the clip. Although bridge engineers consider and design for all three stresses, in most cases, the bending moment stresses are the critical factor in the design.

Types of Loads

Trucks affect bridges in several ways. When moving across a bridge, they produce static live loads and dynamic live loads. These loads result in the bridge experiencing bending, shear and fatigue stresses. The weight of the vehicle causes the live load stresses; its movement across the bridge, in conjunction with its weight, causes the dynamic stresses; and the movement, weight and the number of repetitions cause the fatigue stresses. When designing bridges, engineers typically increase the static load by a fixed percentage (about 10 to 30 percent) to account for the dynamic load.

Additionally, the bridge must withstand dead loads (the weight of the bridge itself, including the weight of future overlays), wind, thermal, earthquake, and other loads. The AASHTO bridge design manuals provide procedures to account for all these stresses.

Critical Stresses for Analysis

This analysis concentrates on bending moment stresses for several reasons. Generally a bridge designed to accommodate the bending moment stresses caused by the live, dead and dynamic loads, will also accommodate the fatigue and shear stresses. Thermal, wind and seismic stresses are not a function of vehicle weights and dimensions. If the bending stress is excessive, the other stresses usually are excessive as well. This is one reason that bridge replacement often is the best solution for an overstressed bridge. Another important reason is that highway agencies often must improve safety features, alignment, lighting, utilities, and other level of service characteristics if they strengthen a bridge. When costs of these other improvements is added to the cost of strengthening, total bridge replacement often is found to be more cost effective. Strengthening is possible for only some bridge types. Steel girder, some truss and even some prestressed concrete beam bridges can be economically strengthened if they meet all other stress and level of service criteria, but reinforced concrete slab and several other bridge types cannot be easily strengthened.

Bridge analysis for nationwide policy studies must rely on readily available nationwide data. The FHWA's National Bridge Inventory (NBI) is the only such dataset that meets this objective. Unfortunately, the NBI does not contain any detailed data describing the bridge geometry, location of details and the like which effectively rules out the analysis of fatigue, shear or other stresses that require this level of detailed data on the individual bridge design elements. However, the NBI does contain sufficient data describing the bridge length, support type, design type, material, etc., that permits the accurate estimation and computation of the live load and total bending moments. This is an additional reason why previous studies of national TS&W policy issues have either ignored fatigue and other less critical stresses or have handled them in a very simplified manner. But, as noted above, little is gained by considering fatigue or other stresses, since the bending stress is a reasonable proxy for all stresses.

Design Vehicles, Ratings and the Federal Bridge Formula B

An examination of design vehicles, ratings, and the Federal Bridge Formula B (BFB) is necessary in any study of the impacts of TS&W changes, because these three concepts are interrelated with the concept of bridge overstress, which is the measure used to identify bridges that might require improvement if size and weight limits were changed.

Design Vehicles

Bridge engineers developed the concept of design vehicles prior to World War II. They are hypothetical vehicles intended to represent the entire truck fleet in the vehicle stream. Use of the design vehicle allows the engineer to design bridges to safely withstand live load stresses caused by a single envelop vehicle rather than having to estimate stresses for each of the many different types of trucks on the road. Most States use one type of design vehicle, the HS vehicle. The HS vehicle is a three-axle vehicle with the load on the steering axle of X tons, a load on the second axle of 4X tons 14 feet behind the steering axle, and a load on the third axle also of 4X tons spaced 14 to 30 feet from the other non-steering axle. The engineer tests several axle spacings for the distance between the second and third axles to determine which axle spacing produces the maximum stresses. In most cases, the HS vehicle with the short 28-foot wheelbase is most critical. The number immediately following the HS is the total weight of the vehicle in tons divided by 1.8. Consequently, the HS vehicle weighing 72,000 pounds would be the HS20 vehicle, since 36 tons (72,000 pounds) divided by 1.8 is 20. This vehicle would have a 4-ton load on the steering axle and loads of 16 tons on each of the other two other axles.

Bridge Ratings

States report two bridge ratings to the FHWA for inclusion in the NBI, the inventory rating and the operating rating. The inventory rating is effectively 55 percent of the yield15 stress of the bridge and the operating rating is 75 percent of the yield stress. The design stress level for new bridges is effectively the same as the inventory rating, 55 percent of the yield stress. The FHWA requires that states report these ratings in terms of the hypothetical HS vehicle.

To determine the inventory rating of a bridge the analyst will compute the heaviest HS vehicle that can traverse the bridge such that the weakest structural member is effectively at 55 percent of its yield stress. In a well-designed bridge, once loaded, all the designed members will be at or near 55 percent of their yield stress. Generally, that produces a safety factor of 1.8 (1÷0.55). Most States allow full and legal operation of trucks that produce bending moments on a particular bridge less than or equal to the moment caused by this Inventory Rating Vehicle

The operating rating is computed in a fashion similar to the inventory rating except that the maximum stress is set at 75 percent of the yield stress of the weakest structural bridge member. Generally, this produces a safety factor of 1.33 (1÷0.75). Most States do not allow vehicles with or without a permit to travel on bridges that would be stressed beyond their operating rating. The only exception may be for special non-divisible loads for which a detailed engineering analysis of the bridge confirms that a single passage will not measurable harm the bridge.

The FHWA requires States to use a consistent analysis methodology to compute the rating and to report this rating in the HS rating system. This provides consistency across all States. For example, if the heaviest HS design vehicle that can traverse a bridge without exceeding the bridge inventory rating weighs 62,000 pounds, the bridge is rated at HS17.2, since 62,000 pounds is 31 tons, and 31 divided by 1.8 yields 17.2.

Federal Bridge Formula B

Every truck has a different HS rating, and that rating is different for every bridge. Consequently, a standard had to be developed that would provide an easily enforceable method to regulate the weight of all types of trucks to protect the nation's bridges. Consequently, the current standard, which is used in virtually all States for all highways, is the Federal Bridge Formula B (BFB). While most States provide some exceptions, usually for a few "grandfathered" trucks or for some economically important truck type (log carriers, grain carriers, etc.), the vast majority of States require trucks to meet BFB. A detailed discussion of BFB is in Appendix V-A.

Description of Bridges in Western Uniformity Scenario States

Table V-1, in columns 2 through 4, shows the number of records in the NBI and the number and percent of "actual truck-relevant" bridges for each of the Western Uniformity Scenario States. Not all records in the NBI are bridges that would be affected by TS&W policy changes. There are a number of duplicate records, especially bridges over an Interstate Highway that appear twice in the NBI, once for the Interstate highway and once for the route traveling over the highway. Also there are bicycle and pedestrian bridges, railroad bridges, culverts, tunnels, and some structures less than 20 feet long that generally are not considered bridges.

Of the almost 92,000 actual truck-relevant bridges in the 13 States, almost 25 percent are on the National Truck Network for Large Trucks (NN)16 on which it is assumed scenario vehicles would operate. The numbers of bridges, actual truck-relevant bridges, and the percent of actual truck-relevant bridges on Interstate and other NN highways for each State are shown in columns 5 through 7 in Table V-1.

Analysis Methodology

The methodology used to estimate bridge costs for this scenario is similar to the method used to estimate bridge costs in the Comprehensive Truck Size and Weight (CTS&W) Study. It compares the bridges "overstressed" by the scenario vehicles with the bridges overstressed by the current fleet. Costs of improving or replacing bridges in the former set that are not in the latter set represent the incremental costs associated with the scenario.

Analysis Tool

The model used in this analysis is the Bridge Analysis and Structural Improvement Software (BASIC) model. The model computes the live load and total load bending moment for any truck configuration, for any span length, for most bridge types, and for both continuous and simple span bridges. Additionally it computes the ratio of those moments to the NBI-reported inventory or operating rating. The model computes live load moments directly for each truck configuration and weight and representative dead loads for each bridge type (reinforced concrete, steel girder, prestressed concrete T beams, etc.) and span length. It assumes a vehicle in every lane of the bridge; lane loadings "kick in" according to AASHTO procedures. Although the model can handle the most prevalent bridge types, it cannot analyze suspension, movable, and timber bridges. BASIC applies State-specific unit construction costs to estimate replacement costs. It computes the square footage of the replacement bridge and then applies the unit cost to estimate the replacement cost.

Table V-1
Numbers of Bridges on Various Systems by State
State Entire Highway System Interstate and National Network Highways
Number of Records Number of "Actual" Bridges Percent of Records that are Actual Bridges Number of Records Number of "Actual" Bridges Percent of Records that are "Actual" Bridges
Colorado 8,933 6,435 72.0% 2,861 1,774 62.0%
Idaho 4,557 3,872 85.0% 999 747 74.8%
Kansas 27,276 18,602 68.2% 5,699 2,967 52.1%
Montana 6,042 4,836 80.0% 2,164 1,682 77.7%
North Dakota 4,823 3,788 78.5% 670 321 47.9%
Nebraska 16,293 12,920 79.3% 3,006 1,622 54.0%
Nevada 1,635 843 51.6% 784 444 56.6%
Oklahoma 24,748 16,290 65.8% 2,119 1,069 50.4%
Oregon 8,037 6,971 86.7% 2,136 1,496 70.0%
South Dakota 6,502 5,123 78.8% 5,136 4,241 82.6%
Utah 3,584 2,232 62.3% 1,272 852 67.0%
Washington 9,228 7,279 78.9% 4,095 2,766 67.5%
Wyoming 3,331 2,620 78.7% 1,573 1,270 80.7%
TOTAL 124,989 91,811 73.5% 32,514 21,251 65.4%

Overstress Identification Criterion

The stress level that should be used to estimate bridge replacement or major rehabilitation needs has been controversial. The U.S. Department of Transportation, in all of its TS&W studies, has used the inventory rating as the basis for determining whether bridge improvements would be needed if larger, heavier trucks were allowed to operate. The Transportation Research Board, on the other hand, has used the operating rating to identify bridge replacement needs. 17 This has resulted in much lower estimates of bridge improvement needs, but many analysts and bridge engineers believe the use of the operating rating underestimates bridge improvement needs. While States may allow vehicles that would stress bridges up to the operating rating to travel on a limited basis under special permits, many would not allow those vehicles to travel routinely at weights that would stress bridges to their operating rating.

Significant cost differences result from the choice of rating. To test the sensitivity of bridge investment needs to assumptions about the level of stress at which bridge improvements would be made, this study estimates investment needs for several stress levels between the inventory and operating ratings.

Use of the lower stress level (inventory rating) results in many more bridges being identified as needing to be upgraded to accommodate increased weights. This is as expected, since the design rating is effectively the same as the inventory rating on a new bridge. Bridge designers have used the HS20 vehicle as the design standard for most bridges built in the last 50 years, although some States have begun to use the HS25 design vehicle so that the new bridges better accommodate heavier trucks. Use of the HS20 design vehicle resulted in bridges being over-designed for the truck fleet of 50 years ago. However, over time, as trucks were allowed to become heavier, this extra factor of safety has evaporated.

Today, while the HS20 vehicle still envelops most of the current truck fleet (except for LCVs and a few other very heavy trucks in States with "grandfather" rights), it does so with little margin of error. Consequently, small increases in truck weight will result in trucks having stresses greater than the HS20 design vehicle for most bridges. However, since the operating rating stresses are 36 percent greater than the inventory rating stresses, only large increases in truck weight and length will overstress bridges when the operating rating is used as the threshold in defining "overstress."


The term "overstress" is figurative and does not necessarily mean that a bridge is in danger of failure. The NBI contains an inventory rating for each bridge that represents a stress effectively equivalent to 55 percent of the lowest yield stress of the primary bridge members. The rating is expressed in terms of a standardized vehicle, e.g., the HS20 vehicle. If a bridge has an HS20 inventory rating as reported in the NBI, it means that an HS20 vehicle on each lane produces an acceptable stress for the bridge; any vehicle that creates a greater moment than the HS20 vehicle "overstresses" the bridge. States regularly allow small overstresses, but large overstresses could cause premature deterioration or, if truly excessive, failure of key bridge members.

There are several factors that allow some bridge overstress without compromising safety. First, using the inventory rating as the basis for determining the level of overstress, provides a large measure of safety since it represents stresses of only 55 percent of the yield stress a bridge can withstand. Secondly, bridges have some unmodeled redundancy. The method used by the States to compute the bridge ratings reported to the FHWA do not consider the strength contributed by unmodeled members of the bridge superstructure; consequently, ratings are inherently conservative. Third, the rating methodology considers a truck with a moment equivalent to the rating vehicle in each lane of the bridge. This rarely occurs, especially on low volume roads, and thereby contributes to a considerable factor of safety.

Except in unusual cases, the dead load and the truck live load (times a multiple to account for dynamic stresses) are the prevailing factors in the design of the bridge, and in decisions concerning whether bridge loadings associated with particular vehicle configurations would necessitate bridge replacement or repairs.

Base Case Vehicles

The first step in the analysis of the base case was to identify the vehicles in the current fleet and the highway systems on which they operate. Determining the vehicles currently operating in each study State is difficult because most State permit practices allow widespread use of vehicles that are heavier than Federal weight limits. Base case vehicles include not only vehicles operating at Federal and State weight limits without special permits, but also vehicles operated under monthly or annual permits that allow unlimited trips. In many cases these vehicles operate almost as freely as legal vehicles.

Motor vehicle laws and regulations were examined to discover what the legal and permitted loads and vehicle lengths were in each State. Although every State in the study uses Federal Bridge Formula B to determine truck axle loads and spacings, most States eliminate the 80,000 pound cap for grandfathered trucks, the multi-trip permit trucks, and for trucks operating on most of the non-Interstate highways. In addition, each State had unique overall length and trailer length restrictions.

The next step was to identify a small group of the critical vehicles from the current fleet for actual analysis. The objective was not simply to identify the heaviest trucks, but rather, to identify those trucks that would produce the greatest bending moment, and therefore the greatest bending stresses, on all types of bridges and spans lengths. It was not necessary to analyze every truck in the current fleet, but only the heaviest set of trucks representative of the current fleet. This usually means the heaviest of both short and long trucks. The base case trucks for each State are described in Table V-2.

The specific highway systems on which each set of trucks can operate were identified so that bridges subjected to overstress by the current vehicle fleet could be identified. Since not all overstresses are cause for immediate action, the number of bridges subjected to various levels of overstress was estimated. Bridges on the Interstate System and other parts of the NN were separated from those on other highway systems since the majority of LCV travel is on those higher-order systems.

Table V-2
Base Case and Uniformity Vehicles
State System SU2 SU3 SU4 5 CS5 CS6 CS6 7 DB5 7 DB5 7 DB8 DB8 DB9 DB9 DB10 DB11 TRP
WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips) WB (ft) GVW (kips)
CO All         22 54     36 74.5 67 80 72 80                                
Non Int         22 54     36 85 67 85 72 80                                
Some Int                         75 94 97 110 109 126 121 110                    
ID All 9 38     26 59.5 67 80 55 80     70 80                                
Arterials 13 42 13 55.8 20 59.5 67 80 55 91.5     72 80                                
Special Arterials                 55 91.5                                     105 105.5
Special Arterials                 55 91.5             61 105.5     83 105.5                
Special Arterials                 55 91.5                     103 105.5                
KS All 9 38     26 59.5 80 80 43 80     65 80 80 80                            
Except Interstate                 53 85.5 67 85.5 65 85.5 80 85.5                            
Turnpike                         65 92             115 120         70 120 98 120
MT All         26 59.5     43 80 43 90.6 65 89.6     43 90.6 77 108.4 91 123.2 99 127.6            
ND All         26 59.5 69 80 43 80     56 80 75 80                            
All, less Interstate 10 40     19 60             86 105.5             60 105.5                
NE All         25 58.5 69 80 43 80     56 80 65 80                            
All, less Interstate 10 40     19 60         70 95 85 95 60 95                            
Harvest, non-Int         19 65         60 109 93 109                                
NV All         25 58.5 69 80 43 80     56 80 75 80                            
Int and arterials 10 44.5     19 54         74 103.7     86 114             60 91 101 115     101 128
OK All         25 58.5 69 80 43 80     56 80 65 80                            
All, less Interstate                 68 90 52 90         49 90     111 90                
Toll roads RINT,ROPA                                             111 108 54 108     117 108
OR All 10 40     19 60 51 80.5     74 101 86 105             60 105             117 105.5
SD All 10 40     19 60         74 101                         65 113 96 127.5    
UT All 10 40     26 59.5 67 80 43 80     65 80 80 80 80 80                        
Inter & Arterials         19 60         74 101                         65 113 96 127.5 102 126
Specific Arterials         19 60         74 101                         65 113 114 136    
WA All 10 40     27 65     43 80     63 80                                
All, less Interstate         23 62 48 83     73 107.5                 88 117                
WY All         23 64 48 83     73 105.5                 88 117                

Truck Descriptions: SU = Single Unit; CS = Combination Truck with Semi-Trailer; DB = Combination Truck with One Semi-Trailer and One Full Trailer; TRP = Combination Truck with One Semi Trailer and Two Full Trailers. The number after the letter designation is the number of axles.

Analysis Results

Sensitivity Analysis

The BASIC program computes and compares bending moments of different vehicles. For this study bending moments of scenario vehicles are compared to moments produced by the inventory-rating vehicle. As noted above, this vehicle was chosen because bridges typically are designed based on the inventory rating and the Bridge Formula weight limits for different axle groups are derived from the inventory rating. Since States typically would not replace most bridges subjected to stresses that just exceed the inventory rating, a sensitivity analysis was performed to estimate the number of bridges that would be overstressed at various stress levels. Specifically, the number of bridges that would be overstressed by stresses 5 percent, 10 percent, 15 percent, 20 percent, 25 percent, 30 percent, and 36.4 percent greater than the inventory-rating vehicle was estimated. State responses to the various levels of overstress would vary depending on the particular bridge and the traffic volumes it carries, but this sensitivity analysis provides a basis for estimating the likely range of impacts rather than simply assuming that States would take actions at a single overstress level.

Analysis Results - Base Case

Table V-3 shows the aggregate number and percent of bridges on the Interstate System and other NN highways that are estimated to be subjected by vehicles in the current fleet to bending stresses equal to or greater than the stresses caused by the inventory-rating vehicle.

In addition, it presents the number and percent of bridges subjected to stresses greater than or equal to 1.05, 1.10, 1.15, 1.20, 1.25, 1.30 and 1.364 times stresses caused by the inventory-rating vehicle. The percent of bridges estimated to experience bending moments greater than the moments caused by the inventory-rating vehicle varies greatly by State, from 92 percent for Colorado to 44 percent for Wyoming. This percentage is a function of the size and weight of the vehicles in the current fleet as well as the strength of the bridges in each State.

Following completion of the CTS&W Study, many comments were received indicating that States would not have to replace all structurally-deficient bridges, as was assumed in that study, but rather could strengthen some bridges. As noted above, not all types of bridges can be strengthened, and it would not be cost effective to strengthen others if significant other improvements were required to bring them up to current safety and geometric standards. To reflect the fact some bridges perhaps could be strengthened rather than having to be replaced, a second set of costs is estimated for each set of overstressed bridges. These lower costs are based on the assumption that one half the deficient bridges could be strengthened rather than replaced and that the cost of strengthening would be one-third the replacement cost. These costs and the assumptions upon which they are based are purely illustrative. The number of bridges that could be strengthened rather than having to be replaced cannot be estimated in a study such as this, and the costs to strengthen various types of bridges can vary widely.

Table V-3
Analysis of the Base Case Trucks on the Interstate and National Network Highway Systems
State Number and Percentage of Actual Bridges Experiencing "Overload" for Given Thresholds1
0% 5% 10% 15% 20% 25% 30% 36.4 % 2
Colorado 1,626 91.7% 1,574 88.7% 1,288 72.6% 919 51.8% 680 38.3% 532 30.0% 426 24.0% 323 18.2%
Idaho 562 75.3% 337 45.1% 173 23.2% 82 10.9% 45 6.0% 29 3.8% 19 2.6% 11 1.5%
Kansas 1,974 66.5% 1,494 50.4% 1,123 37.9% 865 29.2% 663 22.3% 515 17.4% 396 13.3% 287 9.7%
Montana 1,290 76.7% 862 51.3% 345 20.5% 295 17.6% 243 14.4% 142 8.5% 123 7.3% 86 5.1%
N. Dakota 197 61.2% 114 35.5% 59 18.2% 29 9.1% 20 6.2% 14 4.2% 8 2.6% 4 1.3%
Nebraska 1,167 72.0% 1,038 64.0% 619 38.2% 376 23.2% 277 17.1% 201 12.4% 168 10.3% 122 7.5%
Nevada 403 90.7% 275 61.9% 94 21.1% 23 5.1% 6 1.4% 1 0.2% 1 0.2% 0 0.0%
Oklahoma 747 69.9% 341 31.9% 237 22.2% 172 16.1% 144 13.4% 133 12.5% 132 12.4% 122 11.4%
Oregon 1,194 79.8% 736 49.2% 348 23.3% 146 9.8% 80 5.3% 55 3.7% 31 2.1% 19 1.3%
S. Dakota 3,803 89.7% 2,723 64.2% 2,187 51.6% 1,669 39.3% 1,254 29.6% 1,053 24.8% 947 22.3% 858 20.2%
Utah 392 46.0% 81 9.5% 23 2.7% 11 1.3% 7 0.9% 7 0.9% 6 0.7% 6 0.7%
Washington 1,840 66.5% 1,134 41.0% 648 23.4% 410 14.8% 293 10.6% 220 7.9% 155 5.6% 126 4.6%
Wyoming 553 43.5% 331 26.1% 171 13.4% 80 6.3% 44 3.5% 28 2.2% 19 1.5% 11 0.9%
TOTAL 15,749 74.1% 11,041 52.0% 7,315 34.4% 5,079 23.9% 3,756 17.7% 2,931 13.8% 2,431 11.4% 1,975 9.3%

1. "Overload" simply means that the vehicles produce a greater moment (and therefore a greater bending stress) than the bridge's inventory rating computed by the State.
1. Effectively, this represents the operating rating.

Cost of Replaced/Strengthened Bridges

Bridge replacement costs are based on the unit costs per square foot for each State as reported by the State to FHWA. The length and width of the bridge as reported in the NBI are multiplied together to get the area, that area is increased by 25 percent, and the result is multiplied by the unit cost per square foot to estimate the replacement cost. The increase of 25 percent is because FHWA data shows that replacement bridges, for reasons of safety and horizontal and vertical alignment, average about 25 percent longer than the bridges they replace.

Table V-4 below presents the costs associated with each level of overstress for total replacement and for the assumed less-than-full-replacement scenario described above. Based on assumptions in this analysis, base case bridge improvement costs in the scenario States could range from nearly $13 billion to slightly more than $0.5 billion. This is a large range, but as noted above it is unlikely that States would replace or improve many bridges subjected to stresses no greater than those of the basic bridge design vehicle, and it is also unlikely that States would allow bridges to be repeatedly subjected to stresses equivalent to the bridge operating rating without making plans to replace or improve those bridges. If one were to assume that on average bridges would be replaced or improved when stresses exceeded design stresses by from 15 to 20 percent (about half way between the inventory and operating rating), the range of base case bridge improvement costs would be between $3,257 million and $1,586 million. All of those improvements would not have to be made immediately. If costs were spread over a 20-year period, the average annual cost would be between $163 million and $79 million.

Table V-4
Base Case Cost Associated with Full Replacement and Less than Full Replacement for Eight Different Overstress Thresholds
Overstress Threshold (Percentage of Inventory Rating) Number of Actual Deficient Bridges Full Replacement Costs ($ millions) Less Than Full Replacement Costs ($ millions)
1.00 15,749 $12,922 $8,614
1.05 11,041 $8,628 $5,746
1.10 7,315 $5,317 $3,544
1.15 5,079 $3,257 $2,171
1.20 3,756 $2,379 $1,586
1.25 2,931 $1,656 $1,104
1.30 2,431 $1,294 $ 863
1.3664 1,975 $839 $ 559

Analysis Results - Western Uniformity Scenario

The analysis of the scenario trucks follows identically the procedure for the base case vehicles. Earlier in this Chapter, Table V-2 describes the scenario vehicles. Because some of the vehicles are assumed to operate only on the Interstate System and others on both Interstate and non-Interstate portions of the National Network, each set of highways/vehicles was analyzed separately and the results combined to prevent double counting of overstressed bridges. Table V-5 shows estimates of the number of bridges that would be overstressed at various assumed thresholds relative to the inventory rating, and Table V-6 contains cost estimates to replace or improve those bridges. Again the third column represents the Less Than Full Replacement scenario based on the same assumptions as were used in the Base Case analysis.

The two alternative cases of the Western Uniformity Scenario - the high-cube allowing a combined trailer length of 101-feet and the low-cube that only allows a combined trailer length of 95-feet - do not make a difference in the analysis of bridge impacts.

Table V-5
Analysis of the Uniformity Scenario Trucks on the Interstate and National Network Highway Systems
State Number and Percentage of Actual Bridges Experiencing "Overload" for Given Thresholds1
0% 5% 10% 15% 20% 25% 30% 36.4 % 2
Colorado 1,572 88.6% 1,402 79.0% 1,150 64.8% 889 50.1% 687 38.7% 495 27.9% 360 20.3% 230 13.0%
Idaho 463 62.0% 346 46.3% 237 31.7% 167 22.4% 108 14.5% 71 9.6% 52 7.0% 36 4.8%
Kansas 2,314 78.0% 1,986 66.9% 1,623 54.7% 1,308 44.1% 1,055 35.6% 841 28.3% 689 23.2% 522 17.6%
Montana 1,173 69.7% 610 36.3% 397 23.6% 336 20.0% 268 15.9% 138 8.2% 124 7.4% 99 5.9%
N. Dakota 228 71.0% 188 58.6% 158 49.2% 106 32.9% 74 23.1% 52 16.3% 36 11.1% 26 8.1%
Nebraska 1,421 87.6% 1,132 69.8% 819 50.5% 654 40.3% 484 29.9% 331 20.4% 243 15.0% 182 11.2%
Nevada 371 83.5% 289 65.2% 179 40.4% 106 23.9% 77 17.4% 47 10.7% 43 9.7% 41 9.3%
Oklahoma 625 58.4% 478 44.7% 418 39.1% 368 34.5% 290 27.2% 172 16.1% 140 13.1% 132 12.4%
Oregon 1,181 79.0% 997 66.6% 761 50.9% 526 35.2% 435 29.0% 339 22.7% 307 20.5% 246 16.5%
S. Dakota 3,333 78.6% 3,030 71.4% 2,794 65.9% 2,331 55.0% 1,992 47.0% 1,701 40.1% 1,518 35.8% 1,367 32.2%
Utah 680 79.8% 571 67.0% 281 33.0% 167 19.6% 100 11.7% 54 6.3% 46 5.4% 31 3.6%
Washington 1,815 65.6% 1,463 52.9% 1,162 42.0% 893 32.3% 690 25.0% 487 17.6% 374 13.5% 302 10.9%
Wyoming 1,007 79.3% 793 62.5% 627 49.4% 408 32.1% 269 21.2% 169 13.3% 93 7.3% 38 3.0%
TOTAL 16,183 76.2% 13,287 62.5% 10,604 49.9% 8,260 38.9% 6,530 30.7% 4,899 23.1% 4,023 18.9% 3,254 15.3%

1. "Overload" simply means that the vehicles produce a greater moment (and therefore a greater bending stress) than the bridge's inventory rating computed by the State.
2. Effectively, this represents the operating rating.

Table V-6
Western Uniformity Scenario Cost Associated with Full Replacement
and Less than Full Replacement for Eight Different Overstress Thresholds
Threshold as Percent Greater than Inventory Rating Number of Actual Deficient Bridges Full Replacement Costs($ millions) Less than Full Replacement Costs ($ millions)
0 16,183 $13,507 $9,004
5 13,287 $11,561 $7,707
10 10,604 $9,472 $6,314
15 8,260 $7,382 $4,921
20 6,530 $5,872 $3,914
25 4,899 $3,881 $2,587
30 4,023 $3,113 $2,075
36.64 3,254 $2,543 $1,695

Differential Costs Attributable to the Western Uniformity Scenario Trucks

Subtracting the numbers of bridges and the replacement costs of the Western Uniformity Scenario results from the results of the base case analysis yields the costs attributable to the scenario vehicles. The number of bridges is presented in Table V-7, and the costs in Table V-8.

Table V-7
Analysis of Additional Overstressed Bridges on Interstate and National Nework Systems
State Number and Percentage of Additional Actual Bridges Experiencing "Overload" for Given Thresholds1
0% 5% 10% 15% 20% 25% 30% 36.4 % 2
Colorado -54 -3.0% -172 -9.7% -138 -7.8% -31 -1.7% 7 0.4% -37 -2.1% -66 -3.7% -92 -5.2%
Idaho -99 -13.3% 9 1.2% 63 8.5% 86 11.5% 63 8.5% 43 5.7% 33 4.4% 24 3.3%
Kansas 340 11.5% 491 16.6% 499 16.8% 443 14.9% 393 13.2% 326 11.0% 293 9.9% 235 7.9%
Montana -117 -7.0% -252 -15.0% 51 3.1% 41 2.4% 25 1.5% -4 -0.2% 1 0.1% 13 0.8%
N. Dakota 31 9.8% 74 23.1% 99 30.9% 76 23.8% 54 16.9% 39 12.1% 27 8.5% 22 6.8%
Nebraska 253 15.6% 94 5.8% 199 12.3% 278 17.1% 207 12.8% 130 8.0% 76 4.7% 60 3.7%
Nevada -32 -7.2% 14 3.2% 86 19.3% 83 18.8% 71 16.0% 46 10.4% 42 9.5% 41 9.3%
Oklahoma -122 -11.4% 137 12.9% 181 16.9% 196 18.3% 147 13.7% 39 3.6% 7 0.7% 10 1.0%
Oregon -13 -0.9% 261 17.4% 413 27.6% 380 25.4% 355 23.7% 284 19.0% 276 18.4% 227 15.2%
S. Dakota -470 -11.1% 306 7.2% 607 14.3% 662 15.6% 737 17.4% 647 15.3% 571 13.5% 509 12.0%
Utah 288 33.8% 490 57.5% 258 30.3% 155 18.2% 92 10.8% 47 5.5% 39 4.6% 25 2.9%
Washington -26 -0.9% 329 11.9% 514 18.6% 483 17.5% 397 14.4% 268 9.7% 219 7.9% 176 6.4%
Wyoming 454 35.8% 462 36.4% 456 35.9% 328 25.8% 225 17.7% 140 11.1% 74 5.8% 27 2.1%
TOTAL 435 2.0% 2,245 10.6% 3,289 15.5% 3,182 15.0% 2,773 13.0% 1,968 9.3% 1,592 7.5% 1,278 6.0%

1. "Overload" simply means that the vehicles produce a greater moment (and therefore a greater bending stress) than the bridge's inventory rating computed by the State.
2. Effectively, this represents the operating rating.

An examination of these tables reveals some very interesting results. From Table V-7 one sees several negative numbers in the "greater than zero percent" and "greater than 5 percent" overload column. This means that the Western Uniformity Scenario vehicles produce greater moments than the inventory vehicle (or inventory vehicle plus 5 percent) on fewer bridges than the base case vehicles. This is not surprising since it became clear early on in the study that a few States allow some very large and heavy trucks on their systems through monthly or annual permits. Consequently, if those States fully allowed the scenario vehicles to operate in lieu of, not in addition to, the currently operating vehicles, fewer bridges would be overstressed. However, as the overstress threshold increases to the Inventory rating plus 10 percent or more, then the scenario vehicles overstress more bridges than the current vehicles. This occurs for several reasons including the behavior of continuous bridges and the varying effects of long versus short trucks.

Table V-8
Incremental Cost Differences between Base Case and Western Uniformity Scenario with Full Replacement and Less than Full Replacement for Eight Different Overstress Thresholds
Threshold As Percent Greater than Inventory Rating Number of Actual Deficient Bridges Full Replacement Costs ($ millions) Less Than Full Replacement Costs($ millions)
0 435 $585 $ 390
5 2,245 $2,933 $1,955
10 3,289 $4,155 $2,770
15 3,182 $4,125 $2,750
20 2,773 $3,494 $2,329
25 1,968 $2,224 $1,483
30 1,592 $3,113 $2,075
36.64 1,278 $2,543 $1,695


Many western States already allow operations of vehicles that produce stresses exceeding the inventory rating of many bridges on the National Network in those States. States recognize there is a substantial safety factor built into bridge design when deciding which bridges might need to be replaced or strengthened because of truck loadings, and typically would not consider a bridge stressed only to its inventory rating to require replacement or strengthening. If there were questions about the strength of particular bridges, inspection schedules on those bridges might be accelerated.

Analysis done for this study indicates that fewer than 2,000 bridges currently are subjected to stresses that exceed their operating rating, which typically represents the greatest loads that States allow, even for single trip permits. This analysis assumes that vehicles that are allowed to operate under multi-trip permits may utilize every route on the NN. This assumption may overstate the number of bridges that are subjected to stresses exceeding their operating rating because some permits may contain route restrictions to prevent operations on roads with inadequate bridges. It is unlikely that States would allow widespread operations of trucks that stressed bridges to their operating rating without putting those bridges into their bridge improvement programs for either replacement or strengthening.

In the long term, States likely would replace or strengthen bridges subjected to stresses falling between the inventory and operating rating. Decisions on improvement needs for specific bridges would depend on a variety of factors including the volume and characteristics of truck traffic using those bridges, the availability of alternative routes, and the degree to which the bridge is being overstressed. For purposes of estimating bridge investment needs associated with the Western Uniformity Scenario, it is assumed that bridges overstressed by 15 to 20 percent compared to the inventory rating would require eventual replacement or strengthening because of those stresses.

Without a detailed structural analysis of each bridge, it is impossible to determine, on a national basis, which bridges States might strengthen rather than replace. Furthermore, the cost to strengthen various bridges could vary widely. The CTS&W Study did not consider the potential to strengthen some bridges, but comments from some States indicated that strengthening would be an option for some bridges. For purposes of this analysis it is assumed that 50 percent of the bridges might be able to be strengthened and that the cost to strengthen the bridges would be one-third the cost to replace the bridge.

Based on these assumptions the incremental bridge costs attributable to the Western Uniformity Scenario would be between $2.329 billion and $4.125 billion. In some cases the States could open some bridges to the larger and heavier vehicles assumed in this scenario without having to make the improvements first. States could be expected to determine the priority and timing of needed bridge improvements based on the volumes of traffic and the degree to which the bridge was being overstressed. In some cases States might not allow larger, heavier trucks to use all segments of the network immediately but rather would open segments only when the infrastructure was adequate to accommodate the new vehicles.

15 The limiting or yield stress is defined as the stress at which steel will undergo permanent deformation.
16 The National Network for Large Trucks is a 260,000 mile network of highways designated in 23 CFR 658. It includes virtually all the Interstate systems and other arterials that are used relatively extensively by trucks.
17 The TRB Special Reports 225, Truck Weight Limits: Issues and Options and 227, New Trucks for Greater Productivity and Less Road Wear: an Evaluation of the Turner Proposal estimated the bridge costs of the TS&W changes under study based on the operating rating of 75 percent of yield stress, whereas reviewers of those reports found much higher bridge costs resulting from the use of the inventory rating of 55 percent of yield stress.

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