Fatigue and Fracture Critical Members

National Steel Bridge AllianceA Division of the American Institute of Steel Construction, Inc.

nsbansba

FATIGUEand

FRACTURECRITICALMEMBERS

An Overview

Informational:

The National Steel Bridge Alliance has publishedthis document in its continuing effort to enhance

the state-of-the-art of steel bridge design andconstruction in the United States.

DisclaimerAll data, specifications, suggested practices and drawings presented herein, are based on the best available information and delin-eated in accordance with recognized professional engineering principles and practices, and are published for general informa-tion only. Procedures and products, suggested or discussed, should not be used without first securing competent advice respect-ing their suitability for any given application.

Publication of the material herein is not to be construed as a warranty on the part of the National Steel Bridge Alliance - orthat of any person named herein - that these data and suggested practices are suitable for any general or particular use, or offreedom from infringement on any patent or patents. Further, any use of these data or suggested practices can only be madewith the understanding that the National Steel Bridge Alliance makes no warranty of any kind respecting such use and the userassumes all liability arising therefrom.

FATIGUE is when cracking occurs due to cyclic loading. Cyclicloading, primarily in tension, can lead to fatigue cracking even if the yield stress of thematerial, weld metal or fastener is never exceeded. The structural failure occurs whenthe fatigue crack grows to a sufficient size to cause an unstable fracture. Such fatigue-induced fracture can develop from a minute crack or flaw existing at some mechanicalor metallurgical discontinuity or a location of stress concentration. With successiveload repetitions, the crack or flaw may grow and propagate through the material untilthe affected member fractures.

The fatigue strength of a connection or detail on a member is governedby three variables:

(1) Number of cycles of loading causing tension(2) Range of service load stress

(i.e. difference between maximum and minimum stress) (3) Initial size of a flaw or discontinuity.

A great deal of research has been carried out regarding fatigue over the past fortyyears. That research led to bridge specifications categorizing the various joint detailswhich allows engineers to address potential fatigue in the design process. These andother fatigue related AASHTO specifications, when carefully followed, assure a bridgedesigner that potential adverse impact of fatigue on the safety of the bridge is eithereliminated or greatly reduced.

FATIGUEand

FRACTURECRITICALMEMBERS

An Overview

Informational:

Charles D.Gorman, P.E.

Senior Structural Consultant

Construction Marketing

Bethlehem Lukens Plate

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FATIGUE and FRACTURE CRITICAL MEMBERS - An OverviewInformational:

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n December 15,1967, the SilverBridge over the

Ohio River at Point Pleasant,West Virginia collapsed with-out warning and forty-sixpeople lost their lives. Thebridge was an eyebar chainsuspension bridge with a 700-foot center span and 380-footend spans. The investigationfollowing the failure estab-lished that stresses had notexceeded the stress levels forwhich the bridge wasdesigned. The main cause ofcollapse was determined tobe the brittle fracture of twocorrosion cracks at a pinholein one of the eyebars. Thearrangement of the eyebars

was such that fracture of anyone eyebar would cause com-plete collapse of the struc-ture. This failure led to aFederal law mandating bienni-al inspections of the nation’sbridges.

Over the next ten years,FHWA inspections revealedthat a number of bridges hadexperienced major cracks, for-tunately without loss of thestructures. As an example, in1969 the Bryte Bend Bridgenear Sacramento experiencedlarge cracks. In May of 1975the two-girder LafayetteStreet Bridge in Minneapolis-St. Paul experienced cracksthat initiated at a lateral con-

nection plate and propagatedthe full depth of the web.Another example was theQuinnipiac River Bridge on I-91 near New Haven,Connecticut. In 1973 a crackwas discovered that had orig-inated in an unfused butt weldin the interface between alongitudinal stiffener and theweb. The crack propagatedthrough a large portion of theweb and into the bottomflange. Another fractureoccurred in the I-79 Bridgenear Pittsburgh at NevilleIsland in 1977. This crack initi-ated at a groove weld detailthat resulted in the fracture ofboth the web and the flange.

These and a number of

Historical Backgroundother bridge anomalies thatoccurred during the sixtiesand seventies led the FederalHighway Administration,AASHTO, American WeldingSociety, the steel industry andresearchers to review thedesign, material and fabrica-tion provisions being used atthe time. Based on thisreview, changes or additionsto the specifications havebeen made over the years. Asa result, there are now exten-sive provisions to addressfatigue, material toughnessrequirements, a bridge weld-ing code and the requirementof a Fracture Control Plan.

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FRACTURE CRITICAL MEMBERS (FCM) are defined as tension members ortension components of bending members (including those subject to reversal of stress), the failure of which would be expected to result incollapse of the bridge. In general terms, a member or component may qualify as fracture critical if all three of the following conditions exist:

(1) Member or component is in tension or subjected to tension

(2) Loads or forces have no alternate path to travel but through the member

(3) Failure of the member or component would result in collapse of the bridge.

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he AASHTO bridgespecification assignsa fatigue category to

various types of fabricationdetails as well as plain materi-al. The categories are general-ly related to the stress con-centrations associated withthe detail and the initial flawsizes related to the fabricationprocess.

The fatigue categories areidentified with letters ‘A’through ‘F’. Category ‘A’ rep-resents plain material and hasthe best fatigue resistance. Asthe categories go from ‘A’ to‘F’, their resistance to fatiguediminishes. For example, theend of a welded cover plate isa Category ‘E” detail. TheStandard Specification pro-vides two tables, one forredundant structures and onefor non-redundant structures.The LRFD Specification hasjust one table of allowablestress ranges irrespective ofredundancy. Each table liststhe allowable stress range forvarious numbers of cycles ofapplied load for each of thefatigue categories. It is the

responsibility of the engineerto evaluate the design toensure that, for the expectednumber of cycles of appliedloads, the stress range at vari-ous joint details does notexceed the allowable for thatparticular detail. At times,designers chose to use detailsthat permit a higher allowablerange than is needed.Generally, these details addextra cost to the structureand provide no increase inreliability. For example, aCategory B detail provides noadditional value, but requiresunnecessary additional cost,when a Category C detail fora crossframe-stiffener connec-tion plate detail is sufficient forthe stress-range.

According to theAASHTO Bridge Designspecification, cross frame con-nection plates are to be rigid-ly connected at both the topand bottom flanges to pre-vent distortion inducedfatigue. Two common meth-ods are used: (a) the connec-tion stiffener is fillet weldeddirectly to the flanges, a

Category ‘C’ detail and (b) thestiffener is welded to a plateor tee (commonly called a tabplate) that is bolted to thetension flange, a Category ‘B’detail. The Category ‘C’ detailis the most economical detailfor plate girder bridges.Girder designs that satisfystrength and deflection crite-ria usually satisfy Category ‘C’

stress ranges at cross framelocations. When necessary,careful placement ofdiaphragm locations awayfrom the points of maximumstress can often eliminate theneed for the tab plate detail.Researchers and AASHTOhave endorsed the Category‘C’ detail of welding stiffenersto either the compression ortension flange. However, anumber of designers use a

Category ‘B’ detail because ofhistoric reluctance to weld toany tension flange.

When Category ‘C’ stressrange limits are met in thedesign process, the use of aCategory ‘B’ detail adds costbut no increased reliability tothe design. If a Category ‘B’cross frame connection is

used at the stiffener to flangelocation, that requires only aCategory C detail, the engi-neer still needs to evaluatethe Category ‘C’ conditionthat would also exist at thestiffener to web weld.Likewise the Category ‘C’condition at the stiffener totab plate weld also needs tobe evaluated.

Fatigue CategoriesT

Fatigue cracks do not propagate in bridges thatare designed, detailed and fabricated by today’sstandards. For all practical purposes, if thestress range at a detail is less than the allowablerange, unstable crack growth does not occur.

n order for a member to be classified as fracture critical, the member must be in tension or subject to tensile stress. Members orcomponents that are not subject to tensile stress are not fracture critical. Further, for a bridge member to be fracture critical, itmust also be the only load path available. A fracture critical member must be a primary member, the fracture of which would

cause collapse of the bridge. This is defined by AASHTO as non-redundant.Tension flange and web plates in one or two-girder bridges maybe examples of fracture critical members. However, it is unlikely that a two-girder bridge will collapse despite the failure of one of the flanges.

Redundancy is often provided by lateral distribution of loads through the concrete slab,the cross bracing/diaphragms and participation of non-structural elements

such as curbs and railing.

he vast majority of bridges do not havefracture critical members. However, it isimportant to recognize when they exist.

AASHTO specifications require the bridgedesigner to identify and designate FractureCritical Members (FCM) on the contractdrawings. Any member or component thatqualifies and is designated as fracture criti-cal must meet a separate set of require-ments and special fabrication procedures.These special fabrication procedures needto be included in a Fracture Control Plan(FCP) which is required as a part of the

ANSI/AASHTO/AWS Bridge WeldingCode.

Fracture CriticalI

T

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FATIGUE and FRACTURE CRITICAL MEMBERS - An Overview

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Fracture Critical or Not

Fracture Control PlanThe Fracture Control Plan (FCP) is defined in Section

12 of the ANSI/AASHTO/AWS Bridge Welding Code,which details provisions regarding the fabrication ofbridge members designated as fracture critical.

Implementation of the Fracture Control Plan will helpto ensure that steel bridges with critical tension compo-nents serve a useful serviceable life over the periodintended in the original design.

• A plate girder bridge with three or more girders is a redundant structure and does not containmembers that should be considered fracture critical.

• For one or two girder bridges, while considered non-redundant, fracture critical requirementsare limited to the tension components such as the web and tension flanges. For straight simplespan bridges, only the web and the bottom flange would be fracture critical.

• For trusses, certain tension members may be FCM.

• The tie of a tied arch is generally considered fracture critical.

• Any hanger-type members, such as a pin and hanger on a girder bridge or the hanger trussmembers on a suspended truss, may be fracture critical.

The FCP addresses the following items in detail:• Base metal requirements• Welding process • Consumable requirements• Welding procedure specification• Contractor requirements• Thermal cutting• Repair of base metal• Straightening, curving and cambering• Tack welds and temporary welds• Weld inspection• Repair welding

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s discussed earlier,FCM are membersor components that

are “critical” to the overall sur-vival of the bridge and are,therefore, non-redundant.

Per AASHTO, an exampleof a redundant structurewould be a multi-girder bridgewith three or more girders. Ifone girder cracks, the loadscarried by that girder can betransferred to adjacent girders

versusA through the deck and cross

frames. Although damaged,the bridge can remain in limit-ed service. The defining of amember as redundant ornon-redundant is somewhatambiguous. Present specifica-tions do not definitively defineredundancy. In addition to thequestion of an alternate loadpath, internal redundancy isoften believed to exist. Anexample of internal redun-

dancy would be a built-upmember with bolts, similar tothe riveted members of thepast. The justification or basisfor built-up member redun-dancy is that if a crack occursin one component it is unlike-ly to propagate into adjacentcomponents. This redundancydefinition was used for the tiegirder of the Blue WaterBridge tied arch betweenMichigan and Canada. The tie

was designed as a box built upfrom angles and plates boltedtogether.

The redundancy questionis the subject of a number ofresearch studies that shouldresult in definitive specificationprovisions. The use of three-dimensional finite elementanalysis may resolve the alter-nate load path issue in thefuture.

RedundancyNon-Redundancy


Designer and Fabricator rolesregarding fracture critical

esigners and fabricators who review design plans need to investigate membersthat are designated as fracture-critical. The reviewer should determine if the

fracture critical designations are properly limited to tension components. Compressioncomponents such as the top flange in a simple span bridge should not be required tomeet fracture critical criteria. In many cases it is incorrect for entire members to bedesignated fracture-critical or for plate girder structures of more than two girders toinclude fracture-critical material. Even two-girder structures with fracture-critical mem-bers may warrant re-evaluation based on past research.

pecifying fracture-critical where it is required is the responsibility of the engineer.Specifying fracture-critical cannot be a substitute for good engineering judge-

ment since it adds cost but not reliability to the bridge.

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n many cases, a two-girder, non-redundant bridge can be fabricat-ed and constructed more eco-

nomically than a three girder, redundantstructure. Applying the Fracture Critical

Plan to the twogirder schemeresults in a structurethat can be expect-ed to provide alevel of safety andreliability equivalentto the three girderstructure. A majorconcern is whenFracture Criticalprovisions are spec-ified and not justi-fied. As theCommentary forthe FractureControl Plan states,“The FractureControl Plan mustnot be used indis-

criminately by designers as a crutch to besafe and to circumvent good engineeringpractice.” Specifying a member as fracturecritical requires stricter fatigue require-

andAdvantagesDisadvantages

I ments for every fatigue category. Thesestricter requirements include; increasedmaterial toughness, increased mill testing,more restrictive fabrication procedures,

and more rigorous inspections.Indiscriminately specifying FCM mayincrease the cost of a bridge withoutincreasing the reliability.

D

Applying the

Fracture

Critical Plan to

the two girder

scheme results

in a structure

that can be

expected to

provide a level

of safety and

reliability equiv-

alent to the

three girder

structure. S

For additional fatigue and fracture related information, please refer to:

• American Association of State Highway and Transportation Officials, Standard Specificationsfor Highway Bridges, Sixteenth Edition,Washington D.C., 1996.

• American Institute of Steel Construction, Steel Bridges:The Best of Current Practice, AISC,1985.

• ANSI/AASHTO/AWS Bridge Welding Code, 1996.

• American Railway Engineering and Maintenance-of-Way Association, Manual for RailwayEngineering, Chapter 15; Steel Structures, AREMA.

• Barsom, J.M.,The Development of AASHTO Fracture-Toughness Requirements for Bridge Steels, American Iron and Steel Institute, February 1975.

• Bethlehem Steel Corporation, Economical Details for Bridges: Cross Frames and Cross FrameConnections – Technical Bulletin,TB-315A.

• Fisher, J.W., Bridge Fatigue Guide, American Institute of Steel Construction, 1977.

• Fisher, J.W., Fatigue and Fracture in Steel Bridges, John Wiley & Sons, 1984.

• Fisher, John W., Geoffrey L. Kulak and Ian F.C. Smith, "A Fatigue Primer for Structural Engineers",National Steel Bridge Alliance, May 1998.

• Miller, D.K., Design Fire: Consider Overall Structural Performance When Specifying Fatigue Details,The Welding Innovation Quarterly,Volume X, No. 2., 1993.

• National Steel Bridge Alliance,Volume I, Chapter 3, Highway Structures Design Handbook.

• Rolfe, S.T., J.M. Barsom, Fracture and Fatigue Control in Structures, Prentice Hall, 1977.

• Salmon, C.G., J.E. Johnson, Steel Structures, Harper Collins, 1990.

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Fatigue and Fracture Critical Members