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1996 National Steel Const.ruction Conference CRANE GIRDER DESIGN

An examination of design and fatigue considerations

Julius P. Van De Pas, P.E.,(top) is a project engineer andJames M. Fisher, P.E., Ph.D., isvice president with ComputerizedStructural Design (CSD), aMilwaukee-based consultingengineering firm. Fisher is amember of the AISC Committeeson Specifications and on Design,Fabrication and Erection ofStructural Steel Buildings. Thisarticle is based on a paper theypresented at the 1996 NationalSteel Construction Conference.

PROPER FUNCTIONING OFOVERHEAD BRIDGE CRANES ISDEPENDENT upon propercrane runway girder design anddetailing. The runway designmust account for the fatigueeffects caused by the repeatedpassing of the crane, and thedetails must not createrestraints that limit the girdersability to deflect under theapplied crane loads. The runwaygirders should be thought of as apart of a system comprised of thecrane rails, rail attachments,electrification support, cranestop, crane column attachment,tie back and the girder itself. Allof these items should be incorpo-rated into the design and detail-ing of the crane runway girdersystem.Stiffer elements of a structur-al assembly tend to attract load.This holds true for crane girders.In a statically loaded member,the tendency for an attachmentto "draw" load can often beneglected. However, with therepeated application ofloads thiscondition can lead to fatiguedamage. Relative deflectionsbetween adjacent members mayoften be neglected in staticallyloaded structures. In dynamical-ly loaded structures these rela-tive movements can result insome form of fatigue damage. Ithas been estimated that 90% ofcrane girder problems are associ-ated with fatigue cracking. Toaddress these conditions, thispaper will briefly discuss thephenomena of fatigue damage,then the nature of crane loadswill be discussed followedwith adiscussion of typical connectionsand details, lastly a designexample will be provided.The basic phenomena offatigue damage has been under-stood for may years. Engineershave designed crane runwaygirders that have performed with

minimal problems while beingsubjected to millions of cycles ofloading. The girders that areperforming successfully havebeen properly designed anddetailed to: limit the applied stressrange to acceptable levels. avoid unexpected restraintsat the attachments andsupports avoid stress concentrationsat critical locations avoid eccentricities due torail misalignment or cranetravel minimize residual stressesRunway systems that haveperformed well have been prop-erly maintained by keeping therails and girders aligned andlevel.

FATIGUE DAMAGEFatigue damage can be char-acterized as progressive crackgrowth due to fluctuating stresson the member. Fatigue cracksinitiate at small defects orimperfections in the base materi-al or weld metal. The imperfec-tions act as stress risers thatmagnify the applied elasticstresses into small regions ofplastic stress. As load cycles areapplied, the plastic strain in the

small plastic region advancesuntil the material separates andthe crack advances. At thatpoint, the plastic stress regionmoves to the new tip of the crackand the process repeats itself.Eventually, the crack sizebecomes large enough that thecombined effect of the crack sizeand the applied stress exceed thetoughness of the material and afinal fracture occurs. Commongrades of structural steel andcommon sizes of members usedin interior applications are notprone to brittle fracture. Thetypical situation occurs whencracks reach a noticeable size

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~ ~nd are repaired before cata-trophic failure occurs. A dam-ged girder can be evaluated fortness for purpose using varioustigue life prediction techniquesnd fracture mechanics. Theseethods are outside the scope ofs discussion.The phenomena of fatigueamage or crack growth is con-idered to occur in three stages:itiation; propagation; and finalacture. The crack initiations affected by the initial flawize, the amount of residualtress, the presence of corrosionnd the applied stress range.ost of the fatigue life of annwelded or unnotched members taken up in the initiation ofhe crack. Fabricated memberspically will have small defectsrom the welding process thatan be considered as initiatedracks. In this case, the entireseful life of the section is takenp in crack propagation. Theseful life of the element is usu-ally met when the crack reachesan objectionable size.Crack propagation occurshen the applied loads fluctuaten tension or in reversal fromension to compression.luctuating compressive stressill not cause cracks to propa-ate. However, fluctuating com-ressive stresses in a region ofesidual tensile stress will causeracks to propagate. In this case,he cracks will stop growingafter the residual stress iseleased or the crack extends outfthe tensile region.The general design solutionso ensure adequate service life ofmembers subject to repeatedoads are to limit the buildup ofresidual stress, limit the size ofinitial imperfections, and to limitthe magnitude of the appliedstress range. The AISCSpecification limits the allowablestress range for a given servicelife based on the anticipatedseverity of the stress riser for agiven fabricated condition. Inaddition, it requires conformancewith Chapter 9 of the AWS Dl.lStructural Welding Code("Dynamicallv Loaded Struc-

tures"), which provides criteriafor limiting the severity of stressrisers found in weld metal andthe adjacent base metal.It should be noted that higherstrength steel does not have alonger fatigue service life thanA36 steel. That is, the rate ofcrack growth is independent ofthe yield strength of the material.Similarly, the rate of crackgrowth is not effected by thetoughness of the material. A

given cross section of highertoughness will be able to resistthe effect of a larger crack without fracture. However, at thisstage of the service life of themember, only a few additionalcycleswould be gained by havinga material of greater toughness.Thus, the AISC Specificationprovisions regarding fatigue con-ditions are independent of mate-rial strength and toughness. Thematerial design requirements forstrength and toughness are thesame for crane runway girdersas for statically loaded girders.

CRANE LOADSEach runway is designed tosupport a specific crane or groupof cranes. The weight of thecrane bridge and trolley and thewheel spacing for the specificcrane should be obtained fromthe crane manufacturer. Thecrane weight can vary signifi-cantly depending on the manu-facturer and the classification ofthe crane. Based on the manu-

facturer's data, forces are: deter-mined to account for impact, lat-eral loads, and longitudinalloads. The AISC Specification,and most model building codesaddress crane loads and set min-imum standards for these loads.The AISE Technical Report No.13 Guide for the Design andConstruction of Mill Buildingsalso sets minimum requirementsfor impact, lateral and longitudi-nal crane loads. The AISErequirements are used when theengineer and owner determinethat the level of quality set bythe AISE Guide is appropriatefor a given project. It should benoted that the latest edition ofthe BOCA National BuildingCode has adopted the AISEGuide for the purpose of deter-mining crane loads.Vertical crane loads aretermed as wheel loads. The mag-nitude of the wheel load is at itsmaximum when the crane is lift-ing its rated capacity load, andthe trolley is located at the endof the bridge directly adjacent tothe girder. In addition to shearand bending stresses in the gird-er cross section, the wheel loadsresult in localized stresses underthe wheel. AISE TechnicalReport No. 13 provides an equa-tion for calculating this localizedstress. The method is based onconsidering the top flange andrail as beams on an idealizedelastic foundation. The axialstiffness of the web determines

Modern Steel Construction I March 1996/49

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the stiffness of the elastic foun-dation. The compressive stress isoriented parallel to the axis ofthe member and can be added tothe flexural compressive stress.Localized bending stresses at thetop flange to web juncture canalso occur when the rail is notaligned directly over the girderweb. To minimize fatigue crack-ing at the junction of the web tothe top flange the AISETechnical Report No. 13 requiresa full penetration weld plus con-toured fillet welds between theweb and top flange. It should benoted that the localized wheelloads will occur with each pas-sage of the wheel. A girder sup-porting a four wheel end truckwill experience four stress fluc-tuations for each passage of thecrane.The vertical wheel loads aretypically factored using the sameimpact factor. Itaccounts for theeffect of acceleration in hoistingthe loads and impact caused bythe wheels jumping over irregu-larities in the rail. Bolted railsplices tend to cause greaterimpact than welded splices. Inthe U.S., most codes require a25%increase in loads for cab andradio operated cranes and a 10%increase for pendant operatedcranes. .Lateral crane loads are orient-ed perpendicular to the cranerunway and are applied at thetop of the rails. Lateral loads arecaused by: acceleration and decelera-tion of the trolley and loads non vertical lifting unbalanced drive mecha-nisms oblique or skewed travel ofthe bridgeExcept for the case of the trol-ley running into the end stops,the magnitude of lateral loaddue to trolley movement andnonvertical lifting is limited bythe coefficient of friction betweenthe end truck wheels and rails.Drive mechanisms are eitherequal on each side of the crane orthey are balanced to align thecenter of the tractive force withthe center of gravity of the crane

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........and lifted load. If the drivemechanism is not balanced,acceleration and deceleration ofthe bridge crane results in skew-ing of the bridge relative to therunways. The skewing impartslateral loads onto the crane gird-er. Oblique travel refers to thefact that bridge cranes can nottravel in a perfectly straight linedown the center of runway. Itmay be thought of as similar tothe motion of an automobile withone tire under inflated. The ten-dency of the crane to wander canbe minimized by properly main-taining the end trucks and therails. The wheels should be par-allel and the should be in similarcondition. The rails should bekept aligned and the surfacesshould be smooth and level. Apoorly aligned and maintainedrunway can result in larger lat-eral loads. The larger lateralloads will in turn reduce the ser-vice life of the crane girder.The AISC Specification andmost model building codes setthe magnitude of lateral loads at20%of the sum of the weights ofthe trolley and the lifted load.The AISE Technical Reportvaries the magnitude of the lat-eralload based on the function ofthe crane (see Table 1).

Longitudinal crane forces aredue to either acceleration anddeceleration of the bridge craneor the crane impacting thebumper. The tractive forces are

Table 1: AISE Crane Side Thrusts

Crane TypeTotal Side thrust% of lifted load

Mill crane .40Ladle cranes .40Clamshell bucket& magnet cranes 100(including slab &billet yard cranes)Soaking pit cranes 100Stripping cranes 100Motor roommaintenance cranes, ect. 30Stacker cranes 200(cab-operated)

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limited by the coefficient of fric-tion of the steel wheel on therails. The force imparted byimpact with hydraulic or springtype bumpers is a function of thelength of stroke of the bumperand the velocity of the craneupon impact with the crane stop.The longitudinal forces should beobtained from the crane manu-facturer. If this information isnot available, the AISETechnical Report provides equa-tions that can be used for deter-mining the bumper force.Consideration of fatiguerequires that the designer deter-mine the anticipated number ofload cycles. It is a common prac-tice for the crane girder to bedesigned for a service life that isconsistent with the crane classi-fication. The correlation betweenthe CMAA crane designationsand the AISC loading conditionscan be seen in Table 2. TheMBMA Low-Rise BuildingSystems Manual provides a

Table 2: Crane LoadingConditionsCMAA Crane AISC LoadingClassification Condition

A, B IC,D 2

E 3F 4

design method for reducing theAISC loading condition for thegirder. This method accounts forthe fact that for many cranes theloading that is applied on a regu-lar basis is less than the maxi-mum wheel load.For cranes with scheduledproduction tasks, the number ofcycles can be directly calculatedbased on anticipated use.

DETAILING & FABRICATIONCONSIDERATIONS

WeldingThe vast majority of stress ris-ers that lead to crack propaga-tion are weld defects. Common

weld defects are: lack of fusion orpenetration, slag inclusions,undercut, and porosity. Lack offusion and penetration or cracksare severe stress risers. Slaginclusions and undercut are sig-nificant defects in areas of rela-tively high stress. It should benoted that surface defects are farmore harmful than burieddefects. Also, the orientation ofthe defects is important. Planerdefects normal to the line ofapplied stress are more criticalthan defects parallel to the lineof stress.Visual inspection during fabri-cation is the most useful methodof ensuring adequate qualitycontrol of the fabricated ele-ments. Itshould be noted thatvisual inspection is mandatory(per AWS, for the contractor) forboth statically and dynamicallyloaded structures.The fabrication sequenceshould be controlled to limitrestraint during welding so as to

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~educe the residual stresses cre-ted by the welding process. Forxample, when fabricating alate girder, the splices of thelange and web plates should beade before the flanges and webates are welded together.

Tiebacks are provided at thend of the crane runway girderso transfer lateral forces fromhe girder top flange into therane column and to laterallyestrain the top flange of therane girder against buckling.he tiebacks must have ade-uate strength to transfer theteral crane loads. However, theiebacks must also be flexibleough to allow for longitudinalovement of the top of the gird-r caused by girder end rotation.he amount of longitudinalovement due to the end rota-ion of the girder can be signifi-ant. The end rotation of a 40oot girder that has undergone a

deflection of span over 600 isabout .005 radians. For a 36 inchdeep girder this results in .2" ofhorizontal movement at the topflange. The tieback must alsoallow for vertical movement dueto axial shortening of the cranecolumn. This vertical movementcan be in the range of 1/4 in. Ingeneral, the tie back should beattached directly to the topflange of the girder. Attachmentto the web of the girder with adiaphragm plate should beavoided. The lateral load pathfor this detail causes bendingstresses in the girder web per-pendicular to the girder crosssection. The diaphragm platealso tends to resist movementdue to the axial shortening of thecrane column.Bearing StiffenersBearing stiffeners should beprovided at the ends of the gird-ers as required by the A'lS'CSpecification paragraphs K1.3

and KI.4. The AISE Guiderequires that full penetrationwelds be used to connect the topof the bearing stiffeners to thetop flange of the girder. Filletwelds are considered to be inade-quate to transfer the concentrat-ed wheel load stresses into thebearing stiffener. The bottom ofthe bearing stiffeners may be fit-ted (preferred) or fillet welded tothe bottom flange. All stiffener togirder welds should be continu-ous. Horizontal cracks have beenobserved in the webs of cranegirders with partial height bear-ing stiffeners. The cracks startbetween the bearing stiffenerand the top flange and run longi-tudinally along the web of thegirder. There are many possiblecauses for the propagation ofthese cracks. One possible expla-nation is that eccentricity in theplacement of the rail on the gird-er causes distortion of the girdercross section and rotation of thegirder cross section. At the sup-

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port, the girder can not rotate,and the cross sectional distortionis concentrated into web bendingabove the stiffener. Crackingmight also occur if the tie backdetail holds up one edge of thecrane girder restricting themovement caused by axial short-ening ofthe crane column.Intermediate StiffenersIf intermediate stiffeners areused, the AISE Guide alsorequires that the intermediatestiffeners be welded to the topflange with full penetrationwelds, the stiffeners should bestopped short of the tensionflange in accordance with theAISC Specification provisionscontained in Chapter G. TheAlSE Guide also requires contin-uous stiffener to web welds forintermediate stiffeners.

Cap ChannelsChannel caps or cap plates arefrequently used to provide ade-

quate top flange capacity totransfer lateral loads to thecrane columns. The commonheuristic is that a wide flangereinforced with a cap channelwill be economical if it is 20pounds a foot lighter than aunreinforced wide flange mem-ber. It should be noted that thecap channel or plate does not fitperfectly with 100% bearing onthe top of the wide flange. Thetolerances given in ASTM A6allow the wide flange member tohave some flange tilt along itslength, or the plate may becupped or slightly warped, or thechannel may have some twistalong its length. These condi-tions will leave small gapsbetween the top flange of thegirder and the top plate orcharmed The passage of thecrane wheel over these gaps willtend to distress the channel orplate to top flange welds.Because of this phenomena, capplates or channels should not be

used with class E or F cranes.The Channel or plate welds tothe top flange can be continuousor intermittent. However, theAISC Allowable stress for thebase metal is reduced fromCategory B for continuous weldsto Category E for intermittentwelds.Column Cap PlateThe crane column cap plateshould be detailed so as to notrestrain the end rotation of thegirder. If the cap plate girderbolts are placed between the col-umn flanges, the girder end rota-tion is resisted by a force couplebetween the column flange andthe bolts. This detail has beenknown to cause bolt failures.Preferably, the girder should bebolted to the cap plate outside ofthe column flanges. The columncap plate should be extendedoutside ofthe column flange withthe bolts to the girder placed out-side of the column flanges. The

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......................lumn cap plate should not beade overly thick as this detailquires the cap plate to distortallow for the end rotation ofe girder. The girder to capate bolts should be adequate tonsfer the tractive or bumperrces to the longitudinal craneacing. The engineer shouldnsider using slotted holes per-ndicular to the runway or'ersize holes to allow tolerancer aligning the girders atop thene columns.

A horizontal truss can be usedresist the crane lateral forces,e truss is designed to spantween the crane columns.pically, the top flange of therder acts as one chord of thess while a back up beam actsthe other chord. The diagonalembers are typically angles.eferably, the angles should belted rather than welded. Theane girder will deflect down-

ward when the crane passes, theback up beam will not. Thedesign of the diagonal membersshould account for the fixed endmoments that will be generatedby this relative movement.Walkways can be designedand detailed as a beam to trans-fer lateral loads to the cranecolumns. The lacing design mayneed to be incorporated into thewalkway design. Similar to thecrane lacing, the walkway con-nection to the crane girder needsto account for the vertical deflec-tion of the crane girder. If thewalkway is not intended to act abeam, then the designer mustisolate the walkway from thecrane girder.The AISE Guide requires thatcrane runway girders with spansof 36 feet and over for buildingclassifications A, B, and C orrunway girder spans 40 feet andover in class D buildings shallhave bottom flange bracing. Thislacing is to be designed for 21J2%

of the maximum bottom flangeforce, and is not to.be welded tothe bottom flange. Cross bracesor diaphragms should not beadded to this bracing so as toallow for the deflection of thecrane beam relative to the back-up beam.Sidesway WebBucklingCrane runway girders shouldbe checked to ensure adequatecapacity to resist sidesway webbuckling. Equation Kl-7 con-tained in the AISC Specificationshould be used to perform thischeck. This criteria is likely tocontrol the member size forcrane runway girders with capplates, welded girders with larg-er top flanges and girders withbraced compression flanges. Itseems likely that the foregoingAISE limitations on the length ofunbraced tension flanges werecreated to address the sideswayweb buckling phenomena. Thesidesway web buckling criteria

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was introduced into the AISCSpecification in the NinthEdition. Runway girdersdesigned prior to this time wouldnot have been checked for thiscriteria.At present, the AISC methoddoes not address the condition ofmultiple wheel loads on a singlespan.

are typically resisted by verticalX-bracing in the plane of thecrane girder. The use of kneebraces to create a rigid frame toresist longitudinal crane forcesshould be avoided. The kneebrace picks up the vertical wheelload each time the wheel passesover the brace. K braces are sub-ject to the same behavior. If alacing system is used to resistlateral loads, this same systemcouldbe used to transfer longitu-Knee Braces or KBracesThe longitudinal crane forces

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dinal forces to the plane of thebuilding columns. Then thecrane vertical bracing could beincorporated into the buildingbracing at the building column.Rail Attachments

The rail to girder attachmentsmust perform the followingfunc-tions: transfer the lateral loadsfrom the top of the rail tothe top ofthe girder. allow the rail to float longi-tudinally relative to the topflange ofthe girder hold the rail in place later-ally. allow for lateral adjustment

or alignment of the rail.The relative longitudinalmovement of the crane rail to thetop flange of crane girder iscaused by longitudinal expansionand contraction of the rail inresponse to changes in tempera-ture and shortening of the girdercompression flange due to theapplied vertical load of thecrane.There are four commonlyaccepted methods of attachingcrane rails to crane girders.These are: hook bolts, rail clips,rail clamps, and patented railclips. To varying degrees thesefour methods perform the func-tions previously mentioned. Theauthors are aware of installa-tions that have the rails weldeddirectly to the top flanges of thegirders. This method is not rec-ommended. The rails may lackthe controlled chemistry thatwould ensure goodquality welds,and there is no provision for lon-gitudinal movement or lateraladjustment ofthe crane rails.Hook bolts are only appropri-ate for attaching light rails sup-porting relatively small and lightduty cranes. Hook bolts shouldbe limited to CMAAClass A, B.and C cranes with a maximumcapacity of approximately 20tons. Hook bolts work well forsmaller crane girders that do nothave adequate space on the topflange for rail clips or clamps.Longitudinal motion ofthe cranerail relative to the runway girder

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' 1may cause the hook bolts toloosen or elongate Therefore,crane runways with hook bolts.should be regularly inspectedand maintained. AISC recom-mends that hook bolts beinstalled in pairs at a maximumspacing of 24 in. on center. Theuse of hook bolts eliminates theneed to drill the top flange of thegirder. However, these savingsare offset by the need to drill therails.Rail clips are one piece cast-ings or forgings that are usuallybolted to the top of the girderflange. Many clips are held inplace with a single bolt. The sin-glebolt type ofclip is susceptibleto twisting due to longitudinalmovement of the rai1.This twist-ing of the clip causes a cammingaction that will tend to push therail out ofalignment.There are two types of railclamps, tight and floating. Railclamps are two part forgings orpressed steel assemblies that arebolted to the top flange of thegirder. The AISE TechnicalReport No. 13 requires that railclips allow for longitudinal floatof the rail and that the clipsrestrict the lateral movement to0..25 in. inward or outward.When crane rails are installedwith resilient pads between therail and the girder, the amountof lateral movement should berestricted to 1/32-in. to reduce thetendency of the pad to work outfromunder the rail.Patented rail clips are typical-ly two part castings or forgingsthat are bolted or welded to thetop flange of the crane girder.The patented rail clipshave beenengineered to address the com-plex requirements of successfullyattaching the crane rail to thecrane girder. Compared to tradi-tional clips, the patented clipsprovide greater ease in installa-tion and adjustment and providethe needed performance withregard to allowing longitudinalmovement and restraining later-al movement The appropriatesize and spacing of the patentedclips can be determined from themanufacturer's literature.

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