Review of Steel Bridges with Fracture-Critical Elements

This paper presents findings of NCHRP Synthesis Project 35-08 that gathered available information on bridges with fracture-critical members (FCM) from the literature, from a survey of bridge owners and consultant inspectors, and from targeted interviews. In the 1970s, material, design, fabrication, shop inspection, and in-service inspection requirements were improved for steel bridges in general. Special provisions for FCM were then implemented, mainly in reaction to bridge collapses. These requirements transformed the industry and the design of modern bridges so that fatigue and fracture are rare in bridges built in the past 20 years. There is a hidden initial cost in some cases because more expensive superstructure designs are being used than necessary to maintain an acceptable reliability level because of restrictions or more subtle prejudice against bridges with FCM. The major impact on life-cycle costs is the additional mandate for hands-on in-service inspection of FCM. There are also varying definitions of “fracture critical,” and consequently there is wide disagreement in classifying different types of superstructures as fracture critical. Numerous bridges have had a full-depth fracture of a fracture-critical girder and did not collapse, usually because of the alternative load-carrying mechanism of catenary action of the deck under large rotations at the fracture. The capacity of damaged superstructures (with FCM removed from the analysis) may be predicted with refined three-dimensional analysis. However, there is a strong need to clarify the assumptions, load cases and factors, and dynamic effects in these analyses. This paper reports on results of this study.