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1  Load rating of bridges and culverts with missing or incomplete as-built information (BCMI) presents a unique challenge for bridge owners. The AASHTO Manual for Bridge Evaluation (MBE) offers general guidance on how to handle these bridges, but the states must decide exactly how the rating is completed. Consequently, states have developed pro- cedures that vary widely in approach and level of detail. The objectives of this project are to gather and synthesize information on the methods that states currently use to perform load ratings of bridges and culverts with missing or incomplete as-built information and, in doing so, to establish the current state of practice. This synthesis report includes a literature review, a survey of state agencies, follow-up interviews, and a presentation of case examples provided by the states. A comprehensive search of major online databases was conducted to identify relevant literature on the subject. Diagnostic and proof load testing of bridges was the focus of many of the sources identified. The results of a diagnostic load test are used to calibrate a model of the bridge or to adjust parameters used in the load rating. Proof load tests lead directly to the load rating for a specific vehicle (test vehicle) at the operating level. A handful of recent studies assessed nondestructive testing (NDT) technologies that are particularly useful in dealing with bridges with partial information; American Concrete Institute (ACI) 228.2R is a useful source on NDT of concrete bridges. A survey of state agencies generated 46 responses, for a response rate of 92%. The survey indicated the following: ⢠Nearly all state inventories have some bridges and culverts with missing or incomplete as-built information in their inventories. ⢠Many states reported that their inventories include nonâstate-owned bridges that they are nonetheless responsible for load rating. ⢠The most common types of bridges with partial as-built information are reinforced con- crete slab, culvert, and girder bridges. ⢠Some of our estimates suggest that between 9.5% and 18% of the bridges in the National Bridge Inventory (NBI) are lacking information. ⢠Of the 46 responding states, 61% of the states reported having a formal procedure for load rating BCMI; of those, 78% say their procedure is documented. The level of detail in the documented state procedures varies, from not more than a restatement of the guidelines provided in the AASHTO MBE to a breakdown by the type of bridge and step-by-step guidance on how to complete the rating. Chain drag, ground-penetrating radar, dye penetrant, infrared thermography, ultra- sound, eddy current induction rebar locator, and cover meter are the most commonly used NDT technologies. Just under half of the survey respondents reported using some type of S U M M A R Y Load Rating of Bridges and Culverts with Missing or Incomplete As-Built Information
2 Load Rating of Bridges and Culverts with Missing or Incomplete As-Built Information destructive material testing to assist with load rating BCMI. Of the 46 states, 23 reported having conducted a diagnostic load test while eight reported having performed a proof load test to assist in the load rating of a bridge or culvert with missing or incomplete as-built information. The procedures for load rating BCMI were divided into 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 foundation, all states refer to the AASHTO MBE. An EB rating is based merely on knowledge and experience and does not involve analysis or calculations. To assist in the load rating, many states use field measurements, historical information, and comparisons to bridges of a similar design that do have plans: this is designated as an FH rating. Procedures that fall into the RE category use an assigned rating approach that depends on the year (or assumed year) that the bridge was built and its NBI condition ratings. For the DL category, the known or assumed design load and an assumed rating are employed to estimate the capacity of the bridge or culvert with insufficient as-built information; then, all other ratings are calculated as proportions of a ratio of live load effects. States applying the FT procedure typically make use of available testing technologies and have completed one or more proof tests to assist in load rating their BCMI. Note that the boundaries between these categories are not strict, and states sometimes adopt aspects of more than one approach. To illustrate the diverse approaches, seven case examples are presented from seven dif- ferent states. The case examples and the associated results are highlighted in Table S-1. Case Example Location (State) Procedure Category Bridge Type Rating Approach 1 CO EB RC Culvert Combine visual field evaluation with a long history of the structure carrying all traffic. 2 CA DL Prestressed Concrete Slab Confirm no signs of deterioration with a field visit and consider the bridgeâs history of carrying traffic. Assume a design load and set the inventory rating for this load to 1.0. Establish rating factors for all other vehicles by calculating the ratio of the moment they create divided by the moment created by the assumed design truck. 3 OR DL Concrete Channel Beam Confirm condition rating of the bridge. If the bridge has a service history of more than 20 years, assume a capacity equal to the highest load effect caused by legal vehicles (up to an SU4). Then, calculate rating factors for all other vehicles based on a ratio of the moment they produce and the moment of the worst legal vehicle. If the condition rating is low, reduce all rating factors. 4 ID RE Reclaimed T-Beam Perform field evaluation, with particular focus on the condition rating of the superstructure from the last inspection, which is used to set inventory and operating ratings for an HS-20 truck, applying a table of established values. Then, compare these ratings to ratings of similar bridges that are about the same age, producing adjustment factors for calculating the operating ratings for legal loads. 5 RI FH Masonry Arch Combine measurements from a field evaluation with material properties taken from the AASHTO MBE. Create a finite element model to calculate all dead load effects and then use influence lines to run all legal and design vehicles across the bridge to receive a rating per the allowable stress method. 6 MA FT RC Culvert Use detailed field measurements and nondestructive test methods to essentially create a set of plans for analyzing the bridge. Enter the information gathered into the AASHTO Culvert LFR, producing load ratings for the design vehicle and all legal vehicles. 7 FL FT Prestressed Slab Unit Perform proof test with two incrementally loaded test trucks and with strain and deflection gauges monitored in real time. Compute load ratings for all Florida legal loads by using the field testing procedure outlined in the AASHTO MBE, generating load rating factors that all exceeded 1.0. Table S-1. Case example summary.
