Fracture-critical members are treated in the AASHTO LRFD Specifications and in the AASHTO Guide Specifications for Fracture Critical Non-Redundant Steel Bridge Members.
A fracture-critical member (FCM) or member component is a tension member or component whose failure is expected to result in collapse of the bridge or the inability of the bridge to perform its function. Although the definition is limited to tension members, failure of any member or component due to any type of stress or strain can also result in catastrophic failure. This concept applies to members of any material.
The AASHTO Standard Specifications for Highway Bridges contains provisions for structural integrity. These recommend that, for new bridges, designers specify designs and details that employ continuity and redundancy to provide one or more alternate load paths.
Also, external systems should be provided to minimize effects of probable severe loads.
The AASHTO LRFD specification, in particular, requires that multi-load-path structures be used unless there are compelling reasons to the contrary. Also, main tension members and components whose failure may cause collapse of the bridge must be designated as FCM and the structural system must be designated nonredundant. Furthermore, the LRFD specification includes fracture control in the fatigue and fracture limit state.
Design of structures can be modified to eliminate the need for special measures to prevent catastrophe from a fracture, and when this is cost-effective, it should be done. Where use of an FCM is unavoidable, for example, the tie of a tied arch, as much redundancy as possible should be provided via continuity, internal redundancy through use of multiple plates, and similar measures.
Steels used in FCM must have supplemental impact properties as listed in Table 1.2. FCM should be so designated on the plans with the appropriate temperature zone (Table 1.2) based on the anticipated minimum service temperature. Fabrication requirements for FCM are outlined in ANSI/AASHTO/AWS D1.5.
High Performance Steels (HPS), as discussed in Art. 1.5 provide an opportunity to significantly increase reliability of steel bridges. With impact properties for this steel usually exceeding 100 ft-lb at 10F, it easily meets the requirements for fracture critical material.
For example, the HPS70W material requirement for welded, 4-in thick plates, in FCMs in a temperature zone 3 application is 35 ft-lb at 30F (see Table 1.2).
