Redundancy is the quality of a bridge to perform as designed in a damaged state because of the presence of multiple load paths. Conversely, nonredundancy is the lack of alternate load paths, meaning the failure of a single primary load-carrying member would result in the failure of the entire structure. Three types of redundancy, load path, structural, and internal redundancies, have been identified much earlier. Recently, the FHWA provides a new definition for these three types of redundancy in the FHWA Bridge Design Handbook (FHWA/NSBA/HDR 2012), and they are summarized in Table 15.1. In general, redundancy issue should exist for all types of bridges. However, of all bridge construction materials, only steel bridge members may have such designation as fracture critical, and with regard to the topic of structural redundancy, the nonredundant steel members are the fracture critical members (FCMs). FCMs are those in axial tension or tension components of bending members whose failure would result in the failure of the structure. These elements are labeled as such on the contract drawings and are subjected to more stringent design, testing, and inspection criteria than those that are part of a redundant system (Fu and Schelling 1989, 1994; Fu 2000). Caltrans (2004) made a list of members or components, including but not limited to the following, identified as FCMs:
• Tension ties in arch bridges • Tension members in truss bridges • Tension flanges and webs in two-girder bridges • Tension flanges and webs in single or double box girder bridges • Tension flanges and webs in floor beams or cross girders • Tension braces in the cross frame of horizontally curved girder bridges • Attachments welded to an FCM when their dimension exceeds 100 mm
(4″) in the direction parallel to the calculated tensile stress in the FCM • Tension components of bent caps • Splice plates of an FCM
Moreover, Caltrans made a comprehensive flowchart for identifying FCMs of complex steel bridges in Figure 15.1.