It is unlikely that all portions of a steel bridge plate will have the same CVN value reported by the mill supplying the steel. This is due to several factors, including natural variation in the material and differences in the processing of various plates. It is important, however, that the bridge community have a specification that ensures, with reasonable certainty, that the toughness at any location in a plate is sufficient for an acceptable level of performance. The specification must consider, along with other factors, the spatial variations in toughness for normalized plates versus plates supplied in the as-rolled condition. The AASHTO "Guide Specifications for Fracture Critical Non-Redundant Steel Bridge Members", published in 1978, cover the toughness requirements for bridge steel for fracture-critical members. Based on the failure investigation of a fracture-critical member, the specification later was tightened through a requirement for increased sampling and testing at a lower temperature. Subsequently, additional research was performed in an AISI-sponsored study, and as a result, the requirements for materials to be used in regions with service temperatures in Zones 1 and 2 were restored to their original form. However, the research that supported that change did not include material processed for fracture-critical-member applications in Zone 3. NCHRP Project 12-31, "Notch Toughness Variability in Bridge Steel Plates," was initiated with the objective of developing needed improvements in bridge steel plate CVN impact-toughness testing and acceptance criteria for applications in regions with service temperatures in Zone 3. The researchers evaluated existing literature and data, and performed analytical studies and laboratory testing to develop new data. This report documents the work performed under Project 12-31 and discusses the testing procedure used and the statistical analyses performed in the preparation of the proposed specifications. The specification calls for testing both ends of as-rolled plate, versus one end for normalized plate, because of the greater variability in toughness in as-rolled plate. Additionally, an increase in minimum allowable test values, as a percentage of required average toughness, is proposed for both plate types. The changes are recommended for all temperature zones. The specification recommended in this report should provide a mechanism for better assurance of the suitability of steel bridge plate with respect to toughness. The AASHTO Highway Subcommittee on Bridges and Structures is expected to act on the proposed specification at its annual meeting in 1994. The proposed specification is based on the premise that average toughness of steel plate, computed from CVN test results, is a sufficient predictor of fracture performance. Additional research to correlate CVN values to the fracture behaviour of full-thickness plate also is recommended in the report.
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