NCHRP Research Report 883  presents a proposed AASHTO guide specification for the analysis and identification of fracture-critical members and system-redundant members. The report describes the analysis methodology and provides application examples. The analysis methodology is based on comprehensive 3-D finite element analyses (FEA) and case studies to evaluate the redundancy of new and existing steel bridges with members traditionally designated as fracture-critical members (FCMs) including simple- and continuous-span I-girder and tub-girder, through-girder, truss, and tied-arch steel bridges. The material in this report will be of immediate interest to steel bridge engineers.

Advances in new materials have brought about renewed discussion of the traditional definition of what constitutes a fracture-critical (FC) steel bridge. For example, modern steel bridges are often built of high-performance steel and fabricated using higher quality welding procedures with a composite deck slab and, therefore, are inherently more capable of carrying redistributed loads through alternate paths. Currently, the stringent fabrication and maintenance inspection requirements for FC bridges steer owners and designers away from building new FC bridges although allowed by the AASHTO code with the appropriate design, construction, and inspection procedures. The increased maintenance inspection costs are also a concern of existing FC bridges.   These additional costs, and any structural concerns, could be eliminated or minimized by re-examining the FC designation and reclassifying bridges where justified.  But, designers and owners often have deferring opinions on the definition of a FC bridge. The AASHTO code does not give guidance on the appropriate methods for fracture analysis. Although individual designers have developed methods for this issue, there is no consensus on the loadings, the approach, and what constitutes failure.