FCM PDF
FCM PDF
FCM PDF
Welding Code (AWS) and are applied to tension members whose bridges.
fracture could lead to bridge collapse. (Another bridge inci-
dentthe failure of a pin-and-hanger assembly, which trig-
gered the collapse of one span of the Mianus River Bridge in It is essential to understand that the FCP was specifically
1983served as the impetus for enhanced field inspection re- developed in response to failures (i.e., brittle fractures) in non-
quirements for these same members.) redundant tension members that occurred in the 1970s. Such
members, which may be either entirely (e.g., a truss member)
The Three-Legged Stool or partially (e.g., a flexural member) in tension became known
Today, a total fracture control plan (FCP) is often illustrated as fracture critical members (FCMs). An FCM is defined by the
as a three-legged stool, where each leg is made up of a part of Code of Federal Regulations (23CFR650 Bridges, Structures and
the plan, as illustrated in Figure 1. (Since the introduction of Hydraulics) as a steel member in tension, or with a tension
the FCP, the authors are not aware of any failures in fracture element, whose failure would probably cause a portion of or the
critical members fabricated to the FCP. Hence, the FCP con- entire bridge to collapse.
cept appears to be serving its intended purpose.) Prior to the FCP, the design of tension members was based
Robert Connor (rconnor@purdue.edu) is an associate professor of civil engineering and director of the S-BRITE Center at Purdue University.
Karl Frank (karl.frank@hirschfeld.com) is chief engineer with Hirschfeld Industries. Bill McEleney (mceleney@aisc.org) is managing director
of NSBA. John Yadlosky (john.yadlosky@hdrinc.com) is HDRs bridges and structures operations director.
AASHTO/AWS D1.5 Bridge Welding Code, Article C12.2.2 lapse of the bridge. As an example, fatigue cracks were found in
Commentary on Definitions late 1970 at cover plate terminations on the Yellow Mill Pond
Tension members or member components whose failure bridge, which carried I-95 in Connecticut. The girders had nu-
would not cause collapse of the bridge are not fracture criti- merous small cracks and although one girder almost completely
cal. Compression members and portions of bending members fractured, the bridge continued to carry traffic.
in compression may be important to the structural integrity of While a portion of these members is subjected to tension
the bridge, but do not come under the provisions of this plan. due to bending, failure of a single stringer or girder would not
Compression components do not fail by fatigue crack initiation result in collapse of the bridge or even a part of the roadway.
and extension, but rather by yielding or buckling. Multiple stringers supported by transverse floor beams are also
inherently redundant.
From the American Railway Engineering and Floor beams. Some engineers have chosen to classify floor
Maintenance-of-Way Association (AREMA): beams fracture critical, perhaps in consideration of the support
AREMA Manual for Railway Engineering, Chapter 15, Article of the roadway. Floor beams should be assessed for FCM sta-
9.1.14.2a tus in the same manner as any other bridge memberi.e., is
Fracture critical members (FCM) are defined as those fracturing of a floor beam likely to result in the collapse of the
tension members or tension components of members whose bridge? Regarding roadway support, consider the following:
failure would be expected to result in collapse of the bridge 1. Is the bridge deck composite with the stringers and floor
or inability of the bridge to perform its design function. The beams? If so, in order for the riding surface to collapse,
identification of such components must, of necessity, be the the entire floor system must suffer a fracture.
responsibility of the bridge designer since virtually all bridges 2. Are there continuous stringers over the floor beams? Con-
are inherently complex and the categorization of every bridge tinuous stringers offer an alternate load path for the ve-
and every bridge member is impossible. However, to fall within hicle load.
the fracture critical category, the component must be in tension. 3. How are the floor beams framed into the main longitudi-
Further, a fracture critical member may be either a complete nal elements? Can a failed floor beam in conjunction with
bridge member or it may be a part of a bridge member. the bridge deck carry load via an arching action spanning
AREMA Manual for Railway Engineering, across the fracture?
Chapter 15, Article 9.1.14.2b 4. Assuming the tension side of the floor beam fails, is it rea-
Members or member components whose failure would not sonable to assume the entire floor beam would suffer a
cause the bridge to be unserviceable are not considered frac- full-depth fracture?
ture critical. Compression members and member components In most cases, floor beams in conjunction with continuous
in compression may, in themselves, be critical but do not come stringers and the continuity of the deck will provide a redundant
under the provisions of this Plan. system capable of carrying the vehicle load without a collapse.
As clearly stated in these specifications, compression mem- The authors have observed cases where engineers have clas-
bers or components of members in compression are not to sified floor beams as FCMs on bridges where the floor beams
be considered FCM. Both AREMA and AWS use essentially are spaced very closely, such as three feet or less. It is difficult to
the same definitions and state that compression members do imagine that failure of a floor beam spanning from main girder
not come under the provisions of the FCP. Further, redun- to main girder spaced so closely could result in collapse of the
dant members do not come under the provisions of the FCP. bridge or roadway. If one were to idealize the main girders as
The use of the phrase do not also leaves no interpretation supports between which the floor beams span, the cross section
and differs from other typical specification type verbiage, such that carries the load would be comprised of multiple girders
as should or may. (i.e., floor beams). Hence, by definition, the floor beams could
not be classified as FCMs at such close spacing.
FCM or not? If a floor beam is judged to be fracture critical, only the por-
In the interest of providing guidance, a few typical members tion subjected to tensile stresses should be subjected to the FCP.
found in steel bridges are listed along with basic rationale for If the floor beam is a rolled beam, while the entire beam would
either classifying or not classifying the member as an FCM. be required to meet the more stringent CVN material require-
Multi-girder bridges and stringers. Bridges with mul- ments, only the portion in tension is subjected to the FCP fabri-
tiple longitudinal members, such as girder bridges with three cation and inspection requirements. Hence, welds made to the
or more girders or stringer beams of long-span bridges, are ex- compression flange would not be subjected to the FCP even
amples of members with alternate load paths in the event of a though the rolled beam is a single piece of steel. If the floor
fracture. Their criticality is similar to the bridge deck, where beam is a fabricated plate girder, the tension flange and the web
fracture would result in local failure of the deck but not col- must meet the more stringent CVN material requirements of
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bridge
crossings
the FCP. However, only the portion of the web that is in ten- visions of the FCP. For example, see this excerpt from AWS
sion needs to meet the FCP fabrication requirements. The top Article 12.2.2.2 Attachments:
flange, which is only in compression, would not be considered Any attachment welded to a tension zone of an FCM mem-
fracture critical. Also, if the floor beam is designed as a simply ber shall be considered an FCM when any dimension of the at-
supported member, small negative moments that may be pro- tachment exceeds 100 mm [4 in.] in the direction parallel to the
duced due to a shear connection at the ends would not justify calculated tensile stress in the FCM. Attachments designated
classifying the top flange as FC material. FCM shall meet all requirements of this FCP.
Primary longitudinal girders. While the FCP applies to The FCP clearly states the attachment must be located on
various elements, it was failure in elements such as primary lon- the portion of the member subjected to tensile stresses. Hence,
gitudinal girders that led to the development of the plan. The a longitudinal stiffener that is welded to a girder in the tension
classic main girders of a two-girder bridge can reasonably be zone of the web plate must meet the FCP, while a longitudinal
classified as FCMs since failure of one of the beams may be ex- stiffener in the compression zone of a web plate does not need
pected to lead to collapse of the bridge. In the absence of any to meet the FCP, as shown in Figure 2. Note that even though
rigorous system analysis, the portions of the girders subjected the attachment is welded to a web platewhich is designated as
to tension (flange and web) would be classified as FCMs and be FCM in terms of the material selection (see AWS C12.2.2.2)
required to meet the FCP, while the portion of the girder that is due to the fact that a portion of the web is in tension (since
only subjected to compression does not, as illustrated in Figure 2. the welding of the longitudinal stiffener is on the compression
Tension chords or diagonals in trusses. Generally speak- portion of the web) there is no need to invoke the FCP. Note
ing, most tension diagonals and chords in trusses would be clas- also that short attachments, such as a transverse stiffener, which
sified as FCMs. is always less than 4 in. long in the direction of primary stress,
Tie girders. Generally speaking, tension ties would be clas- need not be classified as FCM.
sified as FCMs.
Miscellaneous attachments to FCMs. In addition to pri- Ongoing Research
mary members, certain attachments must also be classified as There are currently several research projects under way fo-
FCMs and be fabricated to the requirements of the FCP. The cusing on bridges and bridge members traditionally classified as
reason for this is to ensure that components such as longitudi- fracture critical. Individual projects are studying the following
nal stiffeners meet the same requirements as the base metal of areas:
the primary member. Further, the welds used to attach these Member-level redundancy. This research effort is examin-
components to the primary member must also meet the pro- ing the strength and fatigue performance of both riveted and
Figure 2 Example of classification of FCM components on a plate girder (created by Robert Connor).
JANUARY 2015