Civil Engineering Reference
In-Depth Information
Redundancy analysis
15.1 Basics of BRidge Redundancy
Redundancy is the quality of a bridge to perform as designed in a damaged
state because of the presence of multiple load paths. Conversely, nonredun-
dancy 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 struc-
ture. Three types of redundancy, load path, structural, and internal redun-
dancies, 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 com-
ponents 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 langes and webs in two-girder bridges
• Tension langes and webs in single or double box girder bridges
• Tension langes and webs in loor 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
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