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Suggested Citation:"Chapter 1." National Academies of Sciences, Engineering, and Medicine. 2019. Proposed Modifications to AASHTO Culvert Load Rating Specifications. Washington, DC: The National Academies Press. doi: 10.17226/25673.
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Suggested Citation:"Chapter 1." National Academies of Sciences, Engineering, and Medicine. 2019. Proposed Modifications to AASHTO Culvert Load Rating Specifications. Washington, DC: The National Academies Press. doi: 10.17226/25673.
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1 C H A P T E R 1 Background Load rating bridges and culverts is a key step in assessing the condition of structures that go under and over our highways and roads. The Manual for Bridge Evaluation, 3rd Edition (MBE) provides detailed guidance for the rating process for bridges but has only limited guidance for the treatment of culverts. This is a significant issue as culverts constitute about 130,000 of the structures listed in the National Bridge Inventory which includes spans of 20 ft and greater, but also includes millions more culverts with spans less than 20ft. In recent years, the Federal Highway Administration (FHWA) and the American Association of Highway and Transportation Officials (AASHTO) has begun to focus on the issue of rating culverts. This effort has shown significant issues in rating concrete box sections which have been providing good service but are being rated as deficient when analyzed by current codes. Further, there is little guidance provided to rate concrete culverts of other shapes or of other materials. Corrugated structural plate structures have spans of 25 feet and larger, but little guidance is offered for rating. Deep corrugated metal structures, which are relatively new to the AASHTO specifications, also have long spans and are not addressed in the MBE at all. Thermoplastic and fiberglass culverts have smaller diameters and are not considered in the NBI, but rating is required in some states, thus these structures too should be addressed in the MBE. Surveying / Soliciting Data The objective of the survey for this project was to determine the overall impressions of the states regarding the specifications related to culverts and to solicit data for building a suite of culvert bridges for use in later regression testing of the specification recommendations. To begin the review, a survey related to culvert rating was compiled using the web software SurveyMonkey® and distributed. The survey content is listed in Appendix A. The survey distribution was compiled from a list provided by AASHTO and included the AASHTO SCOBS (Subcommittee on Bridges and Structures) email list along with the RADBUG (Rating and Design Bridge User Group-AASHTOWare BrDR) email list. In all, 42 respondents provided complete survey responses; incomplete survey responses (those that only answered the first one or two questions) were discarded. In all, 37 states completed the survey, plus the District of Columbia, and the U.S. Army Corp of Engineers. Three states had two responses from different individuals. Of those 42 respondents, 19 requested follow-up interviews. The additional interviews were conducted by phone, with the exception of 1, which was conducted via email. Four states that requested follow-up interviews did not respond in time to set up interviews for this report. The detailed results of the survey are provided in Appendix B. A summary discussion of the survey and follow-up interviews are provided in the following sections. A cursory review of the survey results and the follow-up interviews indicate the following:  Concrete culverts are used extensively, followed by steel corrugated culverts. Only one state indicated that they rated thermoplastic culverts and they are not reporting rating issues for those types of structures. Reinforced concrete culverts are not rating well but don’t show physical signs of distress.  Seven of the states surveyed are using Ohio DOT’s metal corrugated spreadsheet in some form for the rating of metal culverts. (Ohio, Iowa, Idaho, Louisiana, California, Oregon, Michigan).  There seems to be a severe rating penalty for metal corrugated culverts when moving from the just above the 12” fill height to just below that height.  Large fill heights often present a problem, causing the rating for culvert structures to fail under dead load conditions.

2  AASHTOWare BrDR™ is used by many states for reinforced concrete box culvert analysis. Several states use BRASS.  A variety of research efforts were described in the survey results. The survey and the literature have provided a number of studies that were pertinent to this project including the following: Caltrans Culvert Study – Caltrans instituted a study of reinforced concrete box sections (RCBs) to investigate how they might be rated to meet current FHWA requirements. As part of this study, Caltrans created BrDR input files for RCBs designed under both current and historical standards. Caltrans provided us with about 140 such files representing a variety of sizes (single and double boxes), depths of fill, and date of construction (1922-2010). Many of these input files were used in subsequent project phases to evaluate (regression testing) current and proposed design and rating practices. Ohio DOT and Michigan DOT Metal Culvert Spreadsheets and Supporting Research - Ohio DOT has developed a series of three spreadsheets for rating metal culverts:  Standard metal culverts  Low cover metal culverts  Metal box culverts The spreadsheets are based on Design Data Sheet No. 19 which was produced by the National Corrugated Steel Pipe Association in 1995. Despite their age, the data sheet and the Ohio DOT spreadsheets provide good guidance for rating metal culverts. At least seven states use the spreadsheets and Michigan has modified it for their own purposes. PennDOT Box Culvert Live Load Distribution Study – PennDOT initiated a study of live load distribution on RCBs to investigate the differences between the distributions in the AASHTO Standard and the initial edition of the AASHTO LRFD Specifications. The study was completed by Dr. McGrath. The study provided a recommendation that the AASHTO Standard Specification distribution is more appropriate for RCBs and AASHTO subsequently adopted this recommendation. The study also developed insights into load distribution that are pertinent to this project.

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Over the past decade, significant state and federal resources have been expended to develop a state‐of-the-art set of reliability‐based bridge design and load rating specifications, including Load and Resistance Factor Design (LRFD) and Load and Resistance Factor Rating (LRFR). However, these design and rating methods were developed for larger bridge structures, and may result in overly conservative ratings when applied to buried culverts. Of the more than 600,000 records in the National Bridge Inventory, over 130,000 represent culverts, thus constituting a significant proportion of the nation’s bridge infrastructure.

The TRB National Cooperative Highway Research Program's Web-Only Document 268: Proposed Modifications to AASHTO Culvert Load Rating Specifications proposes modifications to the culvert load rating specifications in the Manual for Bridge Evaluation and revises the AASHTO LRFD Bridge Design Specifications accordingly.

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