Effect of Truck Weight on Bridge Network Costs (2003)

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1CHAPTER 1 INTRODUCTION 1.1 PROBLEM STATEMENT AND RESEARCH OBJECTIVE TRB Special Reports 225 and 227 (“Truck Weight Limits: Issues and Options” and “New Trucks for Greater Produc- tivity and Less Road Wear: An Evaluation of the Turner Pro- posal”) noted that trucks produce significant damage to high- way bridges (TRB 1990a, 1990b). A truck’s gross weight, axle weights, and axle configuration (collectively referred to as “truck weight” in this study) directly affect the useful life of highway bridge superstructures. Such damage typically occurs in the bridge deck and in the main superstructure ele- ments, including floor beams and girders, diaphragms, joints, and bearings. The severity of damage is a function of the struc- tural element and its material. Bridge costs associated with increased truck weights are the result of the accelerated main- tenance, rehabilitation, or replacement work that is required to keep structures at an acceptable level of service. Highway agencies require a network-level methodology for determin- ing these costs. A concern of agencies is the fatigue damage caused by the increasing population of heavy vehicles. Many of the details used in older steel bridge girders are particularly prone to fatigue failures directly related to truck weight. Repetitive loading may cause fatigue cracking in these steel members and limit the service life of a bridge. Truck-weight frequency distributions by vehicle type (i.e., truck weight histograms, or TWHs) are needed to estimate reliably the effects on remaining life and the costs caused by changes in legal and permit truck weights. Changing truck weights can affect the truck weight histograms. Because car- rying higher payloads can reduce the operating costs of truck operators, the possibility of a growing share of freight trans- portation shifting (e.g., from rail to truck) needs to be consid- ered in estimating the future truck weight distribution and truck traffic. The objective of this research is to develop a methodol- ogy for estimating the bridge network costs associated with changes in the limits on legal and permit gross weight, axle weights, or axle configurations. 1.2 SCOPE OF STUDY This research project included the tasks listed below. This report documents the process, findings, and the product of the research effort. Note that bridge network costs herein refer to the costs to the highway agency only. Other costs—for exam- ple, costs to highway users—are beyond the scope of this study. Further note that the objective methodology is to esti- mate the incremental (or additional) costs resulting from truck weight limit changes, as opposed to the total costs for accom- modating heavy trucks. Task 1. Review relevant practice, performance data, research findings, and other information related to the effects of truck weight on bridge costs. Review fatigue-truck models and algorithms for predicting remaining fatigue life. This information shall be assembled from technical literature and from unpublished experiences of engineers and bridge owners. Review literature and practice on predicting changes in truck-weight histograms following changes in truck weight. Task 2. Describe the types and degrees of damage to bridge components (e.g., prestressed beams, steel girders, bridge decks) caused by increases in truck weight. Identify the data required to estimate the network cost of these damages. Prepare a recommendation on the priority for developing methodologies to estimate the cost of these damages. Develop an estimate of the cost and time to prepare a methodology for each significant type of damage to bridge components. Task 3. Based on the information obtained in Task 1, pro- pose an algorithm that predicts changes in truck-weight his- tograms and in fatigue-truck models caused by changes in legal and permit truck weight. Illustrate the application of the algorithm with specific examples. Task 4. Outline a methodology to determine network maintenance, repair, and replacement costs resulting from fatigue damage to steel girder bridges subjected to increased truck weights. The outline shall include data requirements. Provide a discussion of how network costs will be estimated when data are missing or inadequate. Task 5. Submit an interim report, within 6 months, to doc- ument Tasks 1 through 4 for review by the NCHRP. The report should contain a detailed proposed work plan for the comple-

tion of the project and specifically identify the methodolo- gies that can be developed with the available funds. The contractor will be expected to meet with the NCHRP to review the report. Project panel approval of the proposed work plan must be received before work on the remaining tasks is started. Task 6a. Based on the approved work plan, develop the fatigue-damage network-cost estimating methodology out- lined for steel girder bridges in Task 4. Task 6b. For other types of damage included in the approved work plan, develop methodologies for estimating the increased bridge network maintenance, repair, and replace- ment costs associated with proposed increases in truck weights. Provide a discussion of how network costs will be estimated when data are missing or inadequate. Task 7. Prepare illustrative application examples. Task 8. Prepare a detailed functional plan for developing a software module that can be integrated with AASHTOWare Bridge Ware to implement the methodologies developed. Task 9. Submit a final report that documents the entire research effort and includes the methodologies as a stand- alone document. In addition, provide a companion executive summary that outlines the research results. A second phase of this project was also conducted after fulfilling the above tasks. That phase had a main objective of developing a software module to implement the proposed methodology for estimating bridge network costs due to changes in truck weight limits. As a result, an Excel program was developed to facilitate such analyses to be performed by highway agencies in the country. A users manual was also developed along with the software to assist with application. The software module and its users manual are contained in the accompanying CRP-CD-37. 1.3 ORGANIZATION OF REPORT Chapter 2 presents a comprehensive review on relevant subjects, with an emphasis on state of the art and state of the practice. In that chapter, Section 2.1 discusses approaches used in previous studies involving estimating bridge network costs. Section 2.2 summarizes the efforts and the results of a survey of state and other transportation agencies inside and outside the United States. These results helped prioritize cost impact categories to be included in the recommended methodology. Section 2.3 presents current understanding on the mechanisms of fatigue or deficiency of bridge compo- 2 nents caused by heavy trucks. Section 2.4 reviews previously proposed methods for predicting changes in truck weight his- tograms as a result of truck weight limit changes. Section 2.5 offers a summary for that chapter. Chapter 3 presents the concept of the recommended methodology developed in this project. Section 3.1 discusses the general structure and the principles for the estimation methodology. The requirement for data of the methodology is addressed as one of the major factors considered in the process of design and development. Section 3.2 is used to introduce the concept of a recommended method for predicting changes in truck load spectra. This is a fundamental step for all the cost- impact categories covered here, because truck load spectra are the driving force for the relevant bridge network costs. The fol- lowing sections then present the estimation methodology for each of the four cost-impact categories, as follows: (1) Fatigue of existing steel bridges, (2) Fatigue of existing reinforced con- crete (RC) decks, (3) Deficiency due to overstress for existing bridges, and (4) Deficiency due to overstress for new bridges. Section 3.7 discusses the principles for summing the costs for individual cost-impact categories. Chapter 4 summarizes the conclusions of this study. Appendix A presents the procedure of the recommended methodology for estimating bridge network costs as a result of truck-weight limit changes. This methodology is for U.S. highway agencies at various levels to predict such costs for planning purposes. It covers four cost impact categories men- tioned above. This appendix has separate sections respectively for each of the four categories prioritized in this study. The concept of this methodology is presented in Chapter 3, includ- ing the supporting theory and background information. Appen- dix A also contains several data sets to be used as the default data for application when more detailed site-specific or jurisdiction-specific data are not available. They are intended to meet the minimum requirement for input data to facilitate implementation of the recommended methodology. The recommended methodology has been applied to two bridge networks as presented in Appendix B. The first exam- ple is for the bridges of two routes in the state of Idaho for an increase in permit truck weight limits. In that example, the upper limit was increased from 467 kN (105 kips) to 574 kN (129 kips) for gross vehicle weight (GVW). Three scenarios were investigated: (1) The truck weight limit change is effec- tive only for the two specific routes; (2) The change is imple- mented in the entire state; and (3) The change is legalized in the entire state (i.e., no permit would be needed to carry the weight of 129 kips if the Bridge Formula is satisfied). For these three scenarios, only the bridges on the two specified routes were covered. The second example applies the rec- ommended methodology to the state of Michigan for legal- izing the 3S3 truck configuration in the entire state. The first example covers a smaller number of bridges within the state network. This small size of network permitted a relatively

more detailed analysis. In contrast, the second example esti- mates the impact costs for a much more extensive network for the entire state of Michigan. These examples illustrate the application of the recommended methodology. The attachments to this report include the developed soft- ware module named “Carris” and its users manual. The soft- ware is written using the Microsoft Excel for interactive 3 application. Two application examples have been prepared for using the software. One is the Idaho Example’s Scenario 2 and the other is the Michigan Example. The software for the examples and the users manual are contained in CRP-CD-37. Note that application of software requires the user change either of the examples in the CD to the case of interest, as explained in the user’s manual.