Comprehensive Truck Size and Weight Limits Study - Bridge Structure Comparative Analysis Technical Report
Executive Summary
Background
This report documents analyses conducted as part of the U.S. Department of Transportation (USDOT) 2014 Comprehensive Truck Size and Weight Limits Study (2014 CTSW Study). As required by Section 32801 of MAP-21 [Moving Ahead for Progress in the 21st Century Act (P.L. 112-141)], Volumes I and II of the 2014 CTSW Study have been designed to meet the following legislative requirements:
- Subsection 32801 (a)(1): Analyze accident frequency and evaluate factors related to accident risk of vehicles to conduct a crash-based analyses, using data from States and limited data from fleets;
- Subsection 32801 (a)(2): Evaluate the impacts to the infrastructure in each State including the cost and benefits of the impacts in dollars; the percentage of trucks operating in excess of the Federal size and weight limits; and the ability of each State to recover impact costs;
- Subsection 32801 (a)(3): Evaluate the frequency of violations in excess of the Federal size and weight law and regulations, the cost of the enforcement of the law and regulations, and the effectiveness of the enforcement methods;
- Subsection 32801 (a)(4): Assess the impacts that vehicles have on bridges, including the impacts resulting from the number of bridge loadings; and
- Subsections 32801 (a)(5) and (6): Compare and contrast the potential safety and infrastructure impacts of the current Federal law and regulations regarding truck size and weight limits in relation to six-axle and other alternative configurations of tractor-trailers; and where available, safety records of foreign nations with truck size and weight limits and tractor-trailer configurations that differ from the Federal law and regulations. As part of this component of the study, estimate:
(A) the extent to which freight would likely be diverted from other surface transportation modes to principal arterial routes and National Highway System intermodal connectors if alternative truck configuration is allowed to operate and the effect that any such diversion would have on other modes of transportation;
(B) the effect that any such diversion would have on public safety, infrastructure, cost responsibilities, fuel efficiency, freight transportation costs, and the environment;
(C) the effect on the transportation network of the United States that allowing alternative truck configuration to operate would have; and
(D) the extent to which allowing alternative truck configuration to operate would result in an increase or decrease in the total number of trucks operating on principal arterial routes and National Highway System intermodal connectors.
To conduct the study, the USDOT, in conjunction with a group of independent stakeholders, identified six different vehicle configurations involving six-axle and other alternative configurations of tractor-trailer as specified in Subsection 32801 (a)(5), to assess the likely results of allowing widespread alternative truck configurations to operate on different highway networks. The six vehicle configurations were then used to develop the analytical scenarios for each of the five comparative analyses mandated by MAP-21. The use of these scenarios for each of the analyses in turn enabled the consistent comparison of analytical results for each of the six vehicle configurations identified for the overall study.
The results of this 2014 Comprehensive Truck Size and Weight Limits Study (2014 CTSW Study) study are presented in a series of technical reports. These include:
- Volume I: Comprehensive Truck Size and Weight Limits Study – Technical Summary Report. This document gives an overview of the legislation and the study project itself, provides background on the scenarios selected, explains the scope and general methodology used to obtain the results, and gives a summary of the findings.
- Volume II: Comprehensive Truck Size and Weight Limits Study. This volume comprises a set of the five comparative assessment documents that meet the technical requirements of the legislation as noted:
- Modal Shift Comparative Analysis (Subsections 32801 (a)(5) and (6)).
- Pavement Comparative Analysis (Section 32801 (a)(2)).
- Highway Safety and Truck Crash Comparative Analysis (Subsection 32801 (a)(1)).
- Compliance Comparative Analysis (Subsection 32801 (a)3)).
- Bridge Structure Comparative Analysis (Subsection 32801 (a)(4)).
This Volume II: Bridge Structure Comparative Analysis describes the methodology used and presents the results of the an assessment to ascertain the impacts that certain alternative configurations may have on the bridge structures on the National Highway System (NHS), including the Interstate System. It also estimates the impacts that trucks operating at or below current Federal limits have on bridge infrastructure as compared with trucks operating above those limits.
Purpose of the Bridge Structures Analysis
The main objective of this report is to determine and assess the implications of the structural demands on U.S. bridges under six alternative truck configuration scenarios analyzed in the US Department of Transportation USDOT 2014 CTSW Study. The scope of this study includes both the immediate structural effects on the existing bridge inventory (Chapter 3) and the bridge capital cost effects that would accrue over time due to that change (Chapter 5). This study includes an assessment of one-time bridge costs that may be incurred as a result of posting issues (see Chapter 4) and related strengthening or replacement of bridges (see Chapter 6), as indicated by the analysis.
Potential modal shifts associated with six different truck size and weight policy options (scenarios) are addressed in this report, but for a more thorough analysis and discussion, please see Volume II: Modal Shift Comparative Analysis.
Table ES-1 shows the vehicles that would be allowed under each scenario as well as the current vehicle configurations (the control vehicles) that operate within the 80,000 lb. maximum gross vehicle weight (GVW) allowed under Federal limits.
The first three scenarios assess tractor semitrailers that are heavier than generally allowed under currently Federal law. Scenario 1 assesses a 5-axle (3-S2) tractor-semitrailer operating at a GVW of 88,000 pound, while Scenarios 2 and 3 assess 6-axle (3-S3) tractor semitrailers operating at GVWs of 91,000 and 97,000 pounds, respectively. The control vehicle for these scenario vehicles is the 5-axle tractor-semitrailer with a maximum GVW of 80,000 pounds. This is the most common vehicle configuration used in long-haul over-the-road operations and carries the same kinds of commodities expected to be carried in the scenario vehicles.
Scenarios 4, 5, and 6 examine vehicles that would serve primarily less-than-truckload (LTL) traffic that currently is carried predominantly in five-axle (3-S2) tractor-semitrailers and five -axle (2-S1-2) twin trailer combinations with 28 or 28.5-foot trailers and a maximum GVW of 80,000 pounds. Scenario 4 examines a five-axle (2-S1-2) double trailer combination with 33-foot trailers with a maximum GVW of 80,000 pounds. Scenarios 5 and 6 examine triple trailer combinations with 28.5-foot trailer lengths and maximum GVWs of 105,500 (2-S1-2-2) and 129,000 (3-S2-2-2) pounds, respectively. The five-axle twin trailer with 28.5-foot trailers (2-S1-2) is the control vehicle for Scenarios 4, 5, and 6 since it operates in much the same way as the scenario vehicles are expected to operate.
At this point it is important to note that for the purposes of the study the control double has an approved GVW of 80,000 pounds; however, the GVW used for the control double in the study is 71,700 pounds based on data collected from weigh-in motion (WIM)-equipped weight and inspection facilities and is a more accurate representation of actual vehicle weights than the Surface Transportation Assistance Act (STAA)-authorized GVW. Using the WIM-derived GVW also allows for a more accurate representation of the impacts generated through the six scenarios.
| Scenario | Configuration | Depiction of Vehicle | # Trailers or Semi-trailers | # Axles | Gross Vehicle Weight (pounds) |
Roadway Networks |
|---|---|---|---|---|---|---|
| Control Single | 5-axle vehicle tractor,53 foot semitrailer (3-S2) |  |
1 | 5 | 80,000 | STAA 1 vehicle; has broad mobility rights on entire Interstate System and National Network including a significant portion of the NHS |
| 1 | 5-axle vehicle tractor, 53 foot semitrailer (3-S2) |  |
1 | 5 | 88,000 | Same as Above |
| 2 | 6-axle vehicle tractor, 53 foot semitrailer (3-S3) |  |
1 | 6 | 91,000 | Same as Above |
| 3 | 6-axle vehicle tractor, 53 foot semitrailer (3-S3) |  |
1 | 6 | 97,000 | Same as Above |
| Control Double | Tractor plus two 28 or 28 ½ foot trailers (2-S1-2) |  |
2 | 5 | 80,000 maximum allowable weight 71,700 actual weight used for analysis 2 | Same as Above |
| 4 | Tractor plus twin 33 foot trailers (2-S1-2) |  |
2 | 5 | 80,000 | Same as Above |
| 5 | Tractor plus three 28 or 28 ½ foot trailers (2-S1-2-2) |  |
3 | 7 | 105,500 | 74,500 mile roadway system made up of the Interstate System, approved routes in 17 Western States allowing triples under ISTEA Freeze and certain four-lane PAS roads on East Coast 3 |
| 6 | Tractor plus three 28 or 28 ½ foot trailers (3-S2-2-2) |  |
3 | 9 | 129,000 | Same as Scenario 5 3 |
1 The STAA network is the National Network (NN) for the 3S-2 semitrailer (53 feet) with an 80,000-lb. maximum GVW and the 2-S1-2 semitrailer/trailer (28.5 feet) also with an 80,000 lbs. maximum GVW vehicles. The alternative truck configurations have the same access off the network as its control vehicle. Return to Footnote 1
2 The 80,000 pound weight reflects the applicable Federal gross vehicle weight limit; a 71,700 gross vehicle weight was used in the study based on empirical findings generated through an inspection of the weigh-in-motion data used in the study. Return to Footnote 2
3 The triple network is 74,454 miles, which includes the Interstate System, current Western States’ triple network, and some four-lane highways (non-Interstate System) in the East. This network starts with the 2000 CTSW Study Triple Network and overlays the 2004 Western Uniformity Scenario Analysis, Triple Network in the Western States. There had been substantial stakeholder input on networks used in these previous USDOT studies and use of those provides a degree of consistency with the earlier studies. The triple configurations would have very limited access off this 74,454 mile network to reach terminals that are immediately adjacent to the triple network. It is assumed that the triple configurations would be used in LTL line-haul operations (terminal to terminal). The triple configurations would not have the same off network access as its control vehicle–2S-1-2, semitrailer/trailer (28.5 feet), 80,000 lbs. GVW. The 74,454 mile triple network includes: 23,993 mile network in the Western States (per the 2004 Western Uniformity Scenario Analysis, Triple Network), 50,461 miles in the Eastern States, and mileage in Western States that was not on the 2004 Western Uniformity Scenario Analysis, Triple Network but was in the 2000 CTSW Study, Triple Network (per the 2000 CTSW Study, Triple Network). Return to Footnote 3
Methodology
There are several aspects to impacts to bridges caused by heavy trucks: structural impacts, fatigue impacts and bridge deck wear. Comparative analyses for two of the three cited areas were completed. The lack of a bridge deck impact model suited for estimating the bridge deck wear caused by commercial motor vehicles of various gross vehicle weights limited the ability of the USDOT study team to evaluate the consumption of bridge deck service life attributed to specific configurations and alternative GVWs.
As a result, this report assesses bridge structures and fatigue in the context of the two, 80,000 lb. control vehicles; the six proposed alternative truck configuration scenarios; two Regions, as best defined for bridge analysis purposes; and two primary Highway Networks, again as defined for bridge analysis purposes.
The study team first screened the National Bridge Inventory (NBI) to determine both the total bridge count and the relative number of bridges by bridge type that are on the two subject highway networks: the Interstate System (IS) and other NHS roadways. The 12 most common bridge types were chosen for inclusion in the structural portion of the study, representing 96 percent of all bridges. AASHTO’s AASHTOWare Bridge Rating® (ABrR, formerly VIRTIS) structural analysis program was used to analyze more than 500 representative bridges. The study team used the load resistance factor rating (LRFR) method (AASHTO 2011, 2013) in conformance with the latest design/analysis methodology. The study team obtained ABrR bridge models proof-tested using the LRFR method from the Federal Highway Administration (FHWA, NCHRP Rpt 700, 2012) and from the States. The only exceptions were for through-trusses and girder-floor-beam bridges, for which there is not yet any LRFR capability in ABrR. The load factor rating (LFR) method was employed for those few bridges. The bridge models were selected for analysis in proportion to the number of bridges in the NBI by bridge type on the subject highway networks. The bridges were further screened to assure that they were representative in terms of age, condition, and span length. The results of the analysis were recorded for maximum moment and shear, and the rating factors (RF) for the alternative truck configurations were compared to (i.e., normalized relative to) the 80,000 lb. GVW control vehicles. In physics, moment is a combination of a physical quantity and a distance. Moments are usually defined with respect to a fixed reference point or axis; they deal with physical quantities as measured at some distance from that reference point or axis. For example, a moment of force is the product of a force and its distance from an axis, which causes rotation about that axis. In principle, any physical quantity can be combined with a distance to produce a moment; commonly used quantities include forces, masses, and electric charge distributions. A shear stress, denoted 
(Greek: tau), is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section. Normal stress, on the other hand, arises from the force vector component perpendicular to the material cross section on which it acts.
This is the basis of a statistical assessment of the increase in the gross number of the bridges that would have structural/posting issues potentially requiring strengthening or replacement as a result of the alternative truck configurations. From this assessment, the one-time costs resulting from structural and posting related issues were derived. These one-time costs could pertain to either superstructure strengthening or superstructure replacement triggered by the need to increase live load capacity. The choice of strengthening versus replacement would depend on superstructure type and whichever is the more economical alternative.
With respect to the structural analysis element (see Chapter 3), the study team assessed how many bridges of the bridges selected from the NBI for this study had posting issues and would potentially require either strengthening or replacement based on the derived rating factors for each alternative truck configuration in each scenario. A threshold Rating Factor (RF) value of 1.0 establishes a potential need for bridge strengthening or replacement.
With respect to the fatigue element, the study team investigated load-induced steel fatigue as a result of truck loadings. Four steel bridges of various span lengths, configurations (simply supported and continuous), and fatigue category details were investigated using a comparative analysis approach.
As noted above, the USDOT study team was not able to identify a bridge deck impact model suited for estimating the type of bridge deck wear assessed under this study. While attempts were made to produce a modeling protocol that might be useful for the purposes of conducting a national analysis as undertaken in this study, a modeling approach of suitable scale and based on generally accepted procedures and sound engineering principles was not available, and this aspect of the analysis was not included in the set of results otherwise produced for the study.
Finally, estimation of the cost responsibility assigned to each of the Scenario vehicles was conducted as part of the study, albeit not as thoroughly as originally intended. As noted above, the structural analysis that was conducted fully evaluated the impacts of the scenario vehicles and provides estimates of one-time costs to substantially rehabilitate or replace bridges unable to accommodate each of the alternative configuration vehicles. The evaluation of fatigue attributed to the various truck configurations was also completed and is included in the study. Results produced in these two areas are presented by Scenario with their associated implications on cost. For the purposes of isolating one-time costs, the study assumed a complete end state of each alternative configuration for 2011 freight volumes, where all bridges on the highway systems needing substantial rehabilitation or replacement would be replaced instantaneously. If any of these alternative configurations were to be introduced in the United States, infrastructure owners would make the upgrades gradually over the course of a number of years and likely prioritize necessary bridge rehabilitations or replacements on the system.
Assumptions and Limitations
The USDOT study team performed this bridge structural comparative analysis based on the following assumptions:
- Annual bridge capital costs are based on 2011 (base year) cost summaries from the USDOT’s Fiscal Management Information System (FMIS) and include both the State and Federal shares.
- Bridge damage costs are equated to the total related repair and replacement project costs (inclusive of design, construction inspection, etc.).
- Maximum legal weights for each truck class are used for structural analysis and for fatigue analysis.
Similarly, the following limitations further define the parameters of this study:
- Costs derived for both the one-time structural related issues are independently investigated for each scenario. The costs for multiple scenarios are by the nature of the analysis not additive.
- The reported one-time structural related costs represent an extreme upper bound.
- Distortion induced fatigue in steel members is not included in the study. I-beams, hollow channels and other bridge superstructure elements made of steel are considered steel members.
- While an extensive literature search was conducted and expounded upon, study schedule and time constraints only supported the detailed analysis of representative bridges.
- Load and Resistance Factor Rating (LRFR) capability was not available in AASHTO’s ABrR software for the structural analysis of trusses and girder-floor-beam bridges (consequently, LFR was used for those bridge types).
- Outputs from the modal shift modeling effort produced data that did not distinguish between intra-modal (truck-to-truck) and inter-modal (between modes) shifts.
Summary of Results
Based on the derived rating factors for each of the alternative truck configurations in each scenario, an assessment was made on the number of bridges that had posting issues and would potentially require either strengthening or replacement. A threshold Rating Factor (RF) value of 1.0 establishes a potential need for bridge strengthening or replacement. Table ES2 shows both the projected percentages and the number of bridges that would have posting issues for each scenario assessed.
Based on these findings, Table ES-3 contains a summary of what is considered the upper bound of the projected one-time costs to strengthen or replace these bridges for each alternative truck configuration scenario.
The findings generally indicated that relatively heavier axle loads and axle groupings tend to negatively affect fatigue life when compared to the control vehicles. However, any overall reduction in bridge fatigue life depends on the number of relatively heavier trucks that are in the traffic stream. In general, fatigue-related costs in steel bridges are small compared to the total bridge program cost.
Bridge deck repair and replacement costs and bridge deck preservation and preventative maintenance were initially investigated together since the topics are intimately linked.
Bridge Deck limit states include the ultimate deck strength limit and the deck durability service limit. AASHTO design criteria (AASHTO 2002, 2011) provide bridge decks with adequate strength to carry the potentially heavier alternative truck configuration axle loads; however cyclic axle loadings diminish deck service life or durability.
As noted above, the impact on the annual cost of maintaining bridge decks was not completed due to the lack of a generally accepted modeling regiment. A complete estimate of cost responsibility associated with each of the Scenario vehicles could not be completed for this reason.
Note: Costs are in 2011 dollars.
