Page 85 - Bridge and Highway Structure Rehabilitation and Repair

60 SECTION 1 ADMINISTRATIVE ISSUES

2.4 THE ROLE OF REDUNDANCY AND FRACTURE CRITICAL MEMBERS
2.4.1 Defi nitions
Redundancy is a desirable structural quality to have in any structure. It reduces the risk of
failure and increases safety. It means that in an assembly of members prior to the collapse of an
overstressed member, the load carried by that member will be redistributed to adjacent members
or elements, which do have the capacity to temporarily carry additional load. This redistribution
of peak stress to members with lower stress would prevent the collapse of the structure and is

referred to as redundancy. This is a kind of bonus offered by the configuration of members act-
ing together as an assembly.
There are three types of redundancies which may be described as:

1. Structural redundancy: Structural redundancy is defined as that redundancy which exists as

a result of the continuity within the load path. Any statically indeterminate structure may
be said to be redundant. For example, a single span is statically determinate and cannot dis-
tribute load or stress to another span. It is therefore non-redundant. A continuous two-span
bridge has structural redundancy.
However, AASHTO conservatively classifies exterior spans as non-redundant where the

development of a fracture would cause two hinges which might be unstable.
2. Load path redundancy: Load path redundancy refers to the number of supporting elements,
usually parallel, such as girders or trusses. For a structure to be non-redundant, it must have
two or less load paths (i.e., load carrying members), like the ones which only have two beams
or girders. The failure of one girder will usually result in the collapse of the span, hence
these girders are considered to be non-redundant and fracture critical.
3. Internal redundancy: With internal redundancy, the failure of one element will not result in
the failure of the other elements of the member. The key difference between members which
have internal redundancy and those which do not is the potential for movement between the
elements. Plate girders, which are fabricated by riveting or bolting, have internal redundancy
because the plates and shapes are independent elements. Cracks which develop in one ele-
ment do not spread to other elements.

Conversely, plate girders fabricated by rolling or welding are not internally redundant,
and once a crack starts to propagate, it may pass from piece to piece with no distinction un-
less steel has sufficient toughness to arrest the crack. Internal redundancy is not ordinarily

considered in determining whether a member is fracture critical but as affecting the degree
of criticality.
2.4.2 Fracture Critical Members (FCMs) Linked to Redundancy

Inspection and maintenance of FCMs are important in avoiding a collapse. Some load car-
rying bridge members are more critical to the overall safety of the bridge and, thus, are more
important from a maintenance standpoint. Although their inspection is more critical than other
members, the actual inspection procedures for FCMs are no different.
The AASHTO manual “Inspection of Fracture Critical Bridge Members” states that “Mem-
bers or member components (FCM’s) are tension members or tension components of members
whose failure would be expected to result in collapse.” To qualify as an FCM, the member or
components of the member must be in tension and there must not be any other member or system
of members which will serve the functions of the member in question should it fail. The alternate
systems or members represent redundancy. Once an FCM is identified in a given structure, the

information should become a part of the permanent record file on that structure. Its condition

should be noted and documented on every subsequent inspection. The criticality of the FCM
should also be determined to fully understand the degree of inspection required for the member
and should be based upon the following criteria: