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63  The objectives of this synthesis report were to gather and synthesize information on the methods that states currently use to load rate bridges and culverts with missing or incomplete as-built information and, in so doing, to establish the current state of practice for load rating bridges of this type. The synthesis included a literature review, a survey of state agencies, follow-up interviews, and a presentation of case examples supplied by the states. Summary Literature Review A comprehensive search of major online databases was conducted to identify relevant literature on the subject. Key findings included the following: ⢠Diagnostic and proof load testing of bridges is the focus of many of the sources identified. This emphasis is understandable because bridge load testing has been a topic of consider- able interest in the research community in recent decades as advances in sensors and data acquisition technologies simplified bridge load testing. A diagnostic load test can be used to calibrate a bridge model or to adjust parameters for the load rating. Proof load tests offer the benefit of loading the bridge to levels above a target load and lead directly to the load rating for a specific vehicle (test vehicle) at the operating level. The Transportation Research Board [23] provides a primer on bridge load testing and serves as a recent source for the application of diagnostic and proof load testing for bridge load rating. ⢠AASHTOâs MBE [2] outlines commercially available proven NDT technologies that can be used in the load rating of a bridge. A few recent studies specifically assessed NDT technologies that are particularly useful in addressing BCMI. For example, ACI [24] is a recent and useful source for NDT for concrete bridges. ⢠Researchers have recently brought to bear machine learning to study the problem of load rating bridges with missing information. Although these approaches may not help in load rating individual BCMI, they could be used to assess these bridges at a system level. ⢠Several states document their procedures for load rating BCMI in manuals, guidelines, or memoranda. These documents were cited in the report and were included in the state of practice. ⢠The search identified a few other studies that conducted surveys of bridge owners and reported on methods used to load rate BCMI. However, no scholarly literature addresses exactly what âengineering judgmentâ involves and how it should be applied to load rating BCMI. C H A P T E R 5 Summary and Conclusions
64 Load Rating of Bridges and Culverts with Missing or Incomplete As-Built Information Survey A survey of state agencies and follow-up interviews were performed to identify and better understand current practices. The survey was sent to the members of the AASHTO Committee on Bridges and Structures, with 46 responses, for a response rate of 92%. The survey reached the following conclusions: ⢠Nearly all states have some BCMI in their inventories. ⢠Many states reported nonâstate-owned bridges in their inventories that they nonetheless are responsible for load rating. ⢠The most common types of bridges with missing information are reinforced concrete slab, culvert, and girder bridges. ⢠A very rough estimate of the total number of BCMI in the country ranges from 58,600 to 111,700. This estimate displays considerable uncertainty, but it does provide some sense of the scope of the problem nationwide. ⢠Follow-up interviews suggested that locally owned bridges more commonly lack information compared to state-owned bridges. ⢠A slight majority of states report some formal procedure for load rating BCMI; of those, most document the procedure. The level of detail in the documented state procedures varies, from not more than a restate- ment of the guidelines in the AASHTO MBE [2] to a breakdown by type of bridge and a step- by-step guideline on how to complete the rating. Just over half of the responding states reported using some type of NDT technologies to assist in load rating BCMI. Among the most commonly cited technologies were chain drag, GPR, dye penetrant, infrared thermography, ultrasound, and profometer rebar locator and cover meter. Just under half of the respondents noted the use of some type of destructive material testing (e.g., concrete cores) to assist in load rating BCMI. Slightly more than half of the responding states reported conducting a diagnostic load test to assist in load rating BCMI. However, proof load testing is much more infrequent: less than 20% of the states conducted a proof load test to support load rating BCMI. Procedures for Load Rating BCMI The procedures for load rating BCMI were broken down into the following five categories: ⢠Experience-based rating (EB) ⢠Field measurement and historical data (FH) ⢠Rational evaluation rating (RE) ⢠Design load rating (DL) ⢠Field testing (FT) As a starting point, all states refer to the AASHTO MBE [2] for load rating BCMI. An EB rating is based simply on the knowledge and experience of the rater and does not involve analysis or calculations. Many states employ field measurements, historical information, and comparisons to bridges of a similar design that have plans, which is denoted as an FH load rating. Procedures that fall into the RE category take an assigned rating approach that depends on the year (or the presumed year) that bridge was built and the NBI condition ratings of the bridge. For the DL category, the known or assumed design load and an assumed rating are applied to estimate the capacity of the BCMI, and all other ratings are then proportioned based on a ratio of live load effects. The states in the FT category utilize testing technologies and have conducted one or more proof tests when load rating their BCMI.
Summary and Conclusions 65  Case Examples Seven case examples were presented from seven different states, illustrating the various approaches. Six of the bridges were reinforced or prestressed concrete, and one was a masonry arch. The seven case studies can be summarized as follows: ⢠In the Colorado case example, a reinforced concrete box culvert was rated based strictly on field inspection and engineering judgment (i.e., the EB procedure). The bridge received an HS-20 inventory rating of 36 tons, an operating rating of 40 tons, and a permit rating of 96 tons. ⢠In the California case example, a reinforced concrete slab bridge was rated using the DL procedure. The inventory rating for the HS-20 vehicle was set equal to 1.0, and ratings for the other vehicles were derived from it. ⢠The Oregon case example was a reinforced concrete channel bridge, rated under the DL approach. The bridge received a load rating of 0.75 for the HL-93 vehicle. ⢠The Idaho case example was a concrete T-beam bridge built in 1960, rated with the RE approach. The bridge had an NBI condition rating of 4 for its superstructure, an inventory rating of 0.33, and an operating rating of 0.56 for the HS-20 vehicle. ⢠The Rhode Island case example was a masonry arch culvert built in 1901 that was rated with the FH procedure. The state conducted a detailed inspection of the three-barrel arch to assess the condition of the culvert and the fill depth. The state modeled the bridge in GT Strudl and applied ASR to rate the bridge. The final inventory ratings all exceeded 1. ⢠The Massachusetts case example was a concrete frame bridge with NBI condition ratings of 6. The bridge was rated by using the FT approach. The state used impact-echo and GPR testing to estimate reinforcing details. ⢠The Florida case example was a prestressed concrete slab bridge built in the 1960s that was rated based on proof load testing (i.e., the FT approach). The bridge safely carried the target proof load, as specified in the AASHTO MBE, and exhibited linear behavior. The ratings for the bridge were derived directly from the test results. Conclusions The AASHTO MBE [2] offers general guidance on load rating of bridges and culverts with missing or incomplete as-built information. Beyond that, states have assumed responsibility for developing and implementing more extensive guidelines, and many of those are documented in state policies and procedures. Field measurements, comparisons with similar bridges from the same era, and use of historical information serve as the foundation for most if not all of the state procedures. The more in-depth procedures base the load rating on the condition of the bridge, ratings for the known or assumed design loads of the bridges, and results from field testing tech- nologies. Some elements are common to the many procedures in use throughout the country, suggesting some sharing of ideas among the states over the years. Although between 58,600 to 111,700 bridges in the country likely suffer from missing or incomplete information, the states generally seem comfortable with their procedures and the resulting load ratings for BCMI. Possible future research includes the following topics: ⢠NDT can be used to determine missing geometric and material property information and assess bridge condition; however, no standard procedures are in place that would promote effective application of NDT in load rating BCMI. Research to quantify the uncertainties asso- ciated with various NDT measurements of interest and to develop standard NDT workflows would be beneficial in load rating BCMI.
66 Load Rating of Bridges and Culverts with Missing or Incomplete As-Built Information ⢠No NDT methods are currently available to support the reliable estimation of effective pre- stress or the condition assessment of strands in prestressed concrete members. This topic warrants further research. ⢠Research has not addressed how the load ratings obtained with the various procedures would compare when applied to the same bridge and whether one method is more conservative than another. Research would be beneficial if it systematically compared the load ratings (based on the different approaches) for a suite of different types of bridges. The research could also consider the cost to complete the load rating and the economic cost if the result is a load restriction. ⢠With the introduction of the AASHTO SHV legal loads, a question remains about whether the approaches used to rate BCMI are still applicable to these SHV loads. Alternatively, some bridges may have experienced enough of these loads in service to be rated without a detailed analysis or testing. This is an area worthy of additional research.
