IO

current AASHTO fatigue-design curves should beequally applicable to predict the fatigue behaviorof v¡eathered as well as unweathered structural-steelcornponents.

REFERENCES

1. Standard Specifications for High\ray Bridges.AASHTO, Washington, D.C., 1977.

2. Surface Texture. Report ANSI B 46.1-1978.Àmerican National Standards fnstitute, Nerr'York, 1978.

3. J.W. Fisher, K.H. Frank, M.A. Hirt, ând B.M.McNamee. Effect of Weldments of the FatigueStrength of Steel Beans. NCHRP Report I02.TRB, National Research Council, washington,D.C.,1970,114 pp.

4. J.W. Fisher, P.A. A1brecht, B.T. Yen, D.J.Klingerman, and B.M. McNanee. Fatigue Strengthof Steel Beams vríth Welded Stiffeners and At-tachments. NCHRP Report L47. TRB, NationalResearch Council, Washington, D.C., 1974, 85 pp.

5. F.C. Lea and J.G. Whitman. The Failure ofGirders Under Repeated Stresses. Welding Re-search Supplement, Vol. 18, No. I, Jan. 1939.

6. J.D. Nee. Fatigue Strength of USS "T-1' Con-structíonal Alloy Steel Beans with and withoutStiffeners. Applied Research Laboratory, UnitedStates Steel Corporation, l,lonroeville, pê. rFeb. L966.

7. D.R. Sherman and J.E. Stallrneyer. Fatigue of"T-ln Beams. Status Report of Fatigue Corn¡nit-têe. Welding Research CounciI, Universíty ofIIlinois, Urbana, May 1963.

8. lù.M. Wilson. Flexural Fãtigue Strength ofSteel Beams. Engineering Experinent StationBul-l-. 377. University of Illinois, Urbana, Vol.45, No.33, Jan.1948.

9. E.G. Signes et al. tr'actors Affecting the Ea-tigue Strength of Welde¿l High Strength Steels.British lilelding Journal, Vol. 14, No. 3, 1967.

10. F. Watkinson et a1. The Fatigue Strength oflfelded .foints in High Strength Steels andIqethods for Its fnprovenìent. In proc., Con-ference on Fatigue of Wel-ded Structures, TheWelding Institute, Brighton, England, July 1970.

11. T. Kunihíro, K. fnouè, and T. Fukusa. Atmos-pheric Exposure Study of Weathering Steel.Research Laboratory Report 729. Ministry ofConstruction, Tokyo, Japan, 1972.

]-2. P. Albrecht. Fatigue Behavior of 4-year Weath-ered 4588 Steel Specimens with Stiffeners andAttachments. Report EÍ1WA/MD-8]-/02. Departmentof Civil Engineering, University of Maryland,College Park, July 1978.

13. P.R. Simnon. Arc Welding of ¡¡eathering Steels.welding Journal, Dec. 1968.

14. K. Yamada. Japanese Experience on WeatheringSteel Bridges. Department of Civil Engineering,Nagoya Uníversity, Furo-Cho, Chikusa-Ku,Nagoya, Japan,1983.

15. P. Albrecht and J.c. Cheng. Fatigue of 8-yearWeathered AutÕmatically welded A588 Steel Stif-feners. Departnent of Civil Engineering, Uni-versity of Mary1and, College park, June 1982.

ntblicdtion of thís paper sponsored by Committee on Steel Bridges.

Notice: The material in this pøper is itúended for general information only.Any use oÍ this materiøl in relation to any specific application should be bøsedoìt independent examînation and verification ol its unrcstricted avdilabiliry forsuch use, dnd a determination of suitability lor the ãpplication by professiornþquøffied personnel. No license under any United States Steel Corporøtionpatents ot other propr¡etory interest ¡s implied by the publication of thß paper.Those making use of or relying on the mnterial assume all rísks and liabilityarising from surh use or reliance.

Members

Fatigue Strength of Weathered and Deteriorated Riveted

JOHANNES M.M. OUT, JOHN W. FISHER? and BEN T. YEN

ÀBSIRACr

A study has been performed on the fatigueresistance of corroded and deterioratedriveteil ¡ne¡nbers. The need for this studyarose from the concern with the large numberof riveted structures functioning today thathave various degrees of corrosion and poten-tial fatigue dånage. The validity of AASHTOand A¡nerican Railway Engineering Associationcategory D that is generally used for rivetêdconnections is uncertain, particularly nearthe fatigue 1imit. A series of fatiguetests was carried out on 80-year-old steelbridge stringers with a riveted built-up

cross section. The stringers erere signifi-cantly corroded along the cornpression flangeand 1ocaI1y at the tension flange. Thestress rânges that were applied vrereselecÈed between the fatigue timits of cle-sign categories C and D. The corroded re-gion of the tensíon fIånge proved to be thenost severe conclition, varying between cate-gories C and E. The category D fatiguelinit appears to be applicable to the rivetdetail studied. The reduction of the com-pression flanges had no effect on the per-fornance of the netnber. A strong frictionalbond between section components was found tohave a beneficial effect on fatigue life. A

out et aI.

series of reduced-tenperature tests on acracked stringer tlíd not induce fracture ofthe cracked component and confírmed the re-dundancy of riveted built-up sections fabri-cated fron mild steel.

Major concerns of bridge engineers today are thesafèty of old riveted structures and the potentialfatigue damâge that has accumulated. Many of thesestructures were fabricated and placed into serviceat the beginning of the century. The question ofsafety is of increasing irnportance as ever-intensi-fying traffic, deteriorating cornponent.s, and accumu-Iation of large numbers of cycles are a reality forhighwayr railroad, and ¡nass transit bridges.

The erítería adopted for control of fatigue andfracture of new brÍdge structures åre based on stud-ies of modern welded construction and ongoing labo-râtory resèarch on welded members. Most olderbridges were cônstructed of riveted built-up ¡ne¡n-bers. Research is needed to êstabtish better êsti-mates of the fatigue resistance of rÍveted built-upsections.

Most of the previous laboratory work has beencarried ouÈ on simple butt splices. A further liní-tation is that none of the previous testíng has beenperforned \dith stress ranges less than 97 MPa. Boththe AASHTO and the American Railway Engineering As-sociation (AREA) specifications use a lorder-boundestinate based on these limited data to classify thefatigue strength of riveted built-up mernbers lI,2l.This lower bound corrêsponds to categÕry D in thejoint classification system. A description and sum-nary of the data base used are given in the cornmen-tary to the AREA specifications (?).

A recent literature survey by the authors con-firned the validity of category D as a lower-boundestinate for fatigue strength. l'igure I shords thecollected test results for normal clamping force andbearing ratios as permitted by the specifications.The test results for bearing conditions that exceedthe specification limíts are shown in Figure 2,whích shows that some results fall below category D.

A pilot test program on the high-cycle fatiguêbehavior and fracture resistance of rivetêd built-up

11

members in their weathered and deterlorated state isdescribed. Stringers fron an actual britlge wereused in this study. The need for this investigationstens from the desirability to ascertaín erhether ornot a dÍfference exists between splices, built-upsections, cover-plate ter¡ninations, and other de-tails of rívete¿l built-up members. Category D fa-tigue restrictions impose an enorrnous penalty on fa-tigue resistance. It is of particular interest toinvestigate the applicability of this category nearthe fatigue or endurance limÍt because no experinen-tal data are available at values less than 97 MPa.

FATTGUE TEST PROGRAII,I

Fatigue tests were conducted on four rivetedbuilt-up stringers taken fro¡n a railroad bridge.The purpose of the test program was to

1. Establish fatigue life data on rivet details,with particular focus on the high-cycle regíon; and

2. Establish the fatigue behavior of the deteri-orated built-up rnenber.

The second objective deals with the effect 'offorce redistribution to the other section conponentsas a result of crack extenslon at a rivet detail orcorroded flange angIe. this redistribution raisesstresses and produces progressive crack initiationantl propagation in those components. Note thatriveted built-up ¡nenbers possess a ilegree of .redun-õlancy, which allows for an important increnent oflife after cracking develops in one conponent. Thefactor of redundancy did not appear ín the najorityof the previous experínents, which had been con-ducted on simple splices.

The project concentrated on high-cycIe fatiguebehaviori that is, near the constant cycle fatigueIinit of ÀÀSHTO and AREA categories C and D of 69and 48 MPa, respectively. This generally neant con-tinuation of cycling beyond 107 cycles.

Test Specinens

The fatigue tests were conducted on ¡nenbers that hadbeen re¡noved frorn a railroail bridge. It v¡as athree-span riveted truss briclge that supported a

o(L

Il¡l(,z.lØall¡l(rFu,

lo' lo5 106 lo7 106

NUMBER OF CYCLES

FIGURE I Fatigue resistance of riveted steel connections with normal clampingforce and low bearing ratio (= 1.5).

T2

single railroad track. The bridge was owned by theSouthern Railroåd Cornpany and was located near Mar-shaI1, North Carolina, spanning the French Broad IvyRiver. ft rdas built ín 1903 and demolished in 1982for being obsolete rather than for nalfunctioníng.It lras in service until denolition. Strain Íreasure-ments made while in service indicate that about Ipercent of the stress cycles exceealed the category D

fatigue Iimitt thus the cumulative fatigue da¡nagefro¡n service was negligible (l). The bridge rdasconstructed by the Phoenix Steel Company. The con-struction material used was ¡neilium steel, an earlyalloy steel. In 1903 alloy steels had just replaced

Transportation Research Record 950

wrought iron as thè principal material for netalstructures.

Fritz Engineering Laboratory acquired six of thestringers. They were built-up l-shapes that were Irn deep and 0.32 n wide, and consisted of a web plateand four angles that r.rere connected to the web bytwo rows of rivets (Figurè 3). fheir originallength e¡as 7.32 m. As cut frorn the bridge, theyvrere about 6.10 m long.

The conalition of the stringers appeared to besatisfactory. Their surface had been protected by aheavy tar coating ancl was eventually cleaned by sandblasting. Inspection indicated that the tension

oo-

l¡,(tzGlng,l¡,Gl-ø,

lorlolloet03þ.I{UTIBER OF CYCLES

FIGURE 2 Fatigue resistarice of riveted steel connections with normal clampingforce and high bearing ratio (t 1.5).

FIGURE 3 Stringers of the French Broad hy River Bridge.

Br'¿¡o.3s +.' r 9z'

L 6'¡6'r lz'

Out et aI.

flange was relativety undanaged except at sectionswhere the cross frame hacl been connected to the web(Figure 4). The botton inside flange angle is seento be severely corroded, so that a retluced thicknessresulted as well as a partly eliminated rivet headat that sêctÍon. The regions of reduced thicknesscontained a set of notches and associated stressconcentrations, which ploved to be rather severe.

Signíficant deterioration was generally presental.ong the compression flanges' as íllustrated inFigure 5, which shows a thickness reduction of theflange where the cross tíes had rested on thestringers. Several long cracks were founcl ín thecompression flange at thê location of the lateralbracing connection plates' as illustrateil in Figure6. It appears Èhat the bracing connection had pro-vided restraint to the top flange adjacent to a tie-bearing poínt.

Test Setup

The stringers were tested in a four-point bendingsetup. The repeated loads were applÍed by tldoAnsler 245-kN hydraulic jacks and one pulsâtorr asiltustrated in Fígure 7. The distance between thejacks was 1.53 m. The loading signal ha¿l a constantamplitude, and the freguency of the cycle $tas 520cycles per ninute.

Lateral braces were attached adjâcent to eachjack to prevent lateral movêÍìent caused by theslight ¿listort.ion and lack of syrûnetry. of the dete-riorated section. The high freguency of the vari-

able loads requíre¿l special care totíon of the setup and the jacks.

13

prevent vibra-

FIGURE 5 View of compression flange showing significantreduction in outstanding legs.

FIGURE 6 Preexisting crack in compression flange next to bracingconnections.

FIGURE 4 Flange angle corrosion at cross'frame connections. FIGURE 7 Fatigue testing setuP.

2 62.1

3 62.1

4 55.2

14

Results of Fatigue Tests

General Rernarks

Four of the stringers have been tested. The atten-tion focusecl on the following details: (a) the cor-roded region of the flange angles, and (b) theriveted connection betr¡een the web and angles. Thedata in Table I sun¡narize the test results for the

TABLE I Observed Cracking, Corroded Area

Ao Grossa Ao Netb No._of Cycles(MPa) (MPa) (106) Comment

1 73.4 75.24.404.99

0.851.45

39 .7lc

t.t95.377.6f

Note: Ratio R= omin/omax æ 0.1.uStr"sr r"ng" at fu¡l section.oslaaaa rung" at sect¡on reduced by corrosion.cTest discontinued.dTest stopped because of change in condition.

fatigue strength of the corroded region. For eachtest bea¡n, the gross section stress range (at theful-I section) and the net section stress range (atthe section reduced by corrosion) are tabulated.Results are tabulated for the number of cycles untilfirst detected cracking, until fail-ure of thè cor-roded ang1e, as well ãs the number of cycles untíIfailure of the section, if applicable.

The test results on the rivet details are sum-marized in Table 2. Only the test alata for those

TABLE 2 Summary of Test Resr¡lts at the Rivet Details

No. of Cyclesb (106)Âo Netâ

Test (MPa)

Transportation Research Record 950

sectional elements. lhis reclistribution often gen-erated rapid cracking in those elenents. Thiseffect is shown in Figures I and 9. Soon after

FIGURE B Crack at corroded area of testbeam l

::llDr:s

FIGURE 9 Surface of crack in corroded a¡ea of test beam l.

severing of the corroded angle, the opposite flangeangle developeil several cracks within a short periodof tirne.

Fatigue Resistance of Corroilecl Region

The four stringers tested all had the region of re-duced thickness fron corrosion. The degrees of re-ductionr however, were quite clifferent and variedfron 5 to 40 percent of the angle 1eg area. Figure8 shows the cracked-reduced section of beam l. Thecrack surface at this section is shown in Figure10. Cracking was observed to initiate at multiplesites in the region neâr the edge of the angle wherethe thickness is minimal.

The corroded section of bea¡n 2 after fracture isshown in Figure 11. Before Èhe test a small exist-ing crack had been detected in the corro¿le¿l section,which was removed by grinding. Thís segment isshown in Figure 12.

The degree of corrosion of the third stringer Ísshown in Figure 13. No cracking was observed at

64.8

63.4

57.2

Crack found > 104 mmA¡gle severedSection failed

AngÌe severedSection fâiled

No failu¡e

CYack found >7.6 mmFirst hole drilledSection spliced

Nc Nr Comment

65.9

5 9.3

56.2

46.8

45.8

6.527.348.70

t2.98

36.50

t8.26

25.94

25.94

Argle, bottom hole, top of holeAngle, bottom hole, top of holeArgle, bottom hole, top of holeAngle, bottom hole, top of hole

18.25 Test stoppedWeb plate, bottom hole, bottomof hole

36.50 Test stopped38.69 Argle, bottom hole, fillet, top of

hole39.72c Angle, bottom hole, fillet, top of

hole39.72c Angle, bottom hole, fillet, top of

holeA¡gle, bottom hole, top of hole,

shear spanAngle, bottom hole, top of hole,

shear span

Notet Only cracks at rivet holes not affected by a cracked section are included." Slress rangc al scct¡on rcduced by corros¡on.O*"

= t,r" until first cracking, and Nt = l¡tc unlil fâilure of component.c'Iest

stopped.

fatigue cracks that were not affecte¿l by cracking ofthe corroded section or other adjacent sections areliste¿l. It was observeil that progressing crackingof a section caused force redistribution from thecracked sectíôn components to the other cross-

'I+

Out et 41.

FIGURE 10 Set of cracks developed because of force redistributionin section.

FIGURE 11 Cracked section at corroded area of testbeam 2.

this section Èhroughout the test. The thicknessreductíon was less than half the original thickness.

Figure 14 shows the crack in the reduce¿l sectionof bea¡n 4. This crack was arrested by drillingholes centered on the crack tip after it had grownto a length of about 64 nm. The crack subsequentlyreinitiated aî.ter 2.2 ¡nillion additional stress cy-c1es, and the section was finalJ.y spficed to pernitcontinued testing of the stringer.

The fatigue test data of the corroded region de-tail of the flange angle are shown in Figure 15.The stress range is defined on the net section.

The corro¿led areas of stringers 2 and 4 (see Fig-ures 11 and t4) had a fatigue life at the first ob-served cracking that !'¡as lower than that provided bycategory E. The corro¿led detaÍl of stringer 1 (seeFigure 8) had a fatigue resistance slightly lowerthan category c. Stringer 3 (see Figure 13) did notcrack at this location.

The deterioration of the flange angle of bean 3

FIGURE 12 Initial crack in corroded area of test beam 2.

FIGURE l3 Corroded area of test beam 3.

FIGURE 14 Corroded area of test beam 4 with crack, reinitiatedafter having holes d¡illed.

15

l6

À l{o Dolocloòþ Croclhg h Gorroded Areo

I I To¡l l{o.

o c?ocllng

o An¡[cFollür.a Srcllon

NUHEER OF CYCLES

FIGURE 15 Fatigue resistance of corroded area details.

Transportation Research Record 950

FIGURE 16 Fatigue cracked riveted section in test beam 3, afterseveral reduced temperature tests.

FIGURE 17 Cracked sr¡rface of crack at rivet hole rururing intobottom flange.

oÀ

l¡¡(9zGlnu,¡¡¡Ê,Fv,

tot

was not severe enough to have a crack iniÈiate. Theseverity of the corroded regions of stringers 1, 2,and 4 r¡ere greater than that predícted by consitler-ing the loss of cross-sectionâI area only, as theyall plot well belos¡ category A. Apparently, theroughness of the surface and the associated stressconcentrations are the major factors in making thisdetail more severe than anticipatecl.

The ¡nechanism of crack for¡natíon in the corrodedarea is as follo¡rs. First, several small cracksform on the rough surface at the deeper notchesclose to the angle tip. These cracks coalesce andforn a long, shallow surface crack. This surfacecrack then propagates through the flange thicknessat the tip and beco¡nes an eilge crack, whereas smallcracks continue to form in the corro¿led surface andto coalesce with lhe eclge crack. The crack length¡neasured at the bottom surface significantly lagsbehind the length rneasured at the corroded topsurface.

À final note should be nade on the crack lengthat discovery in èhe various cases. For beam 4, thecrack was s¡nall when detected--7.6 nn. This condi-tion is identified in Figure 15 as cracking. Thecondition where the crack length has increased toapproximately the length at discovery for bearn I isrepresented by point Cr. Hence the fatigue strengthof the corrodetl sections of beams I anil 4 are corn-parable.

All three stringers that developed cracks in thecorroded section sustained a substantiat number ofadditional stress cycles before the section faitecl.The numbers of cycles at failure of the angte of thecross section are inclicated by angle failure andsection failure, respectively.

Fatigue Resistance of Rivet Detaí1s

Fatigue cracks forned at rivet holes, as illustratedin Figure 16. The crack surface of one of these ¿le-talLs ls shown in Fígure 17. The fatígue strengthsof the rívet details that cracked independently ofeach other are summarizecl in Table 2 anil plotted inFígure 18 as a function of the net section stressrange. In Figure I8 cracking denotes first observedcracking. Failure of the flange angle developed forstringer 3 only. The cracked angles of the remain-ing stringers were retrofitte¿l before failure coulcl

out et al.

oo.

t¡Jc,2GU'tf,l¡lGFat

t06t05to'NUMBER OF CYCLES

FIGURE lB Fatigue resistance of rivet details'

occur. No tests were continued until failure of theriveted section because of cracking at rivet holes.

Fatigue cracking was observed to occur belon thefatígue limit for category c (69 MPa). Tv¡o cracksdeveloped below the fatigue linit of category D

(48.3 lrfPa). Both of these cracks were located ín ashear span. The stress condition that corresPondsto bendÍng and shear is slightly nore severe thanbending alone if the rivets are in bearing.

The literature review tlemonstratecl that clanpingforce and bearing ratío were the principal variablesinfluencing the fatigue behavior of riveted joints(4). Most of the cracke¿l-rivet details are locatedin a constanÈ monent region, so that the rivets clo

not transrnit a bearing force. ghis is â favorablecontlition. At the sane time, the rivets appear tobe tight, which ls favorable as well.

The avaiLable shop drawings ¿lo not indicate thenethod of hole preparation. The apparent distortionof the holes may be a result of punching or drivingof the rivets.

There \ras no clear evidence of cracks existing atthe beginning of the tests. fn one or two instancesa dark oxiile was found on the crack surface, butsometines this oxide was located away from the hole.In at leåst one instance it appeareal this had oc-curred when the anglês were f1a¡ne cut from the sec-tion in or¿ler to expose the crack surfaces. None ofthese cracks corresponded to an appreci-ab1y ¿lif-ferent fatigue life than others without any indica-tion of oxide.

The majority of the rivet detaíls vlere observe¿lto develop first cracking at the top of the hole'even though the norninal stress range is higher atits bottom. Þ{ost cracks initiated at the inner sur-face between web plate and angle at the edge of thehole. It is possible that fretting has aicled cracki nitiation.

observations during the test íntlicated that therewas a significant influence of a frictional bond be-tvreen the web plate and angles. This bond was froma paint and corrosion procluct. The significance ofthis ínfluence lies in the effect it had on thepropagation velocity of the crack. The fríctionalresistance betvreen web plate and ang1e, v¡hich wasadjacent to the crack as well as ahead of the crack

front, reduced the compliance of the cracked plateand the crack opening displacement. This decreasedthe crack growth rate and extended the fatiguelife. crack growth neasurements indicate that thecrack grorvth rate was fairty constant with increas-íng crack length, as long as the crack propagated inthe vertical leg of the angle. This phenonenon istreate¿l in nore detail elselrhere (5).

The fatigue resistance of the riveted stringersis compared with the category c and D resistancelínes, which are shown to represent riveted rnernbers(?) in Figure 19. AIso Plotte¿l are the results ofother tests on rivetecl tnembers. Fatigue life is de-fined as first observe¿l cracking in all cåses. Thetruss joints tested by Reernsnyder (6) and the hangermenbers tested by Baker antt Kulak Q) were subjecteclto nuch higher stress range levels than the string-ers and therefore yieldeit ¡nuch shorter fatigue1ives. All three sets of test alata confirm thatcategory D is a lower bound for the fatigue lifer asdefíned by the developnent of a snall crack.

cracking of Deteriorated conpression Flänges

The sígnificant loss of sectíon of the conpressionflange apparent in Figure 5 yfas typical of everytest beam. In extreme cases this loss was suffÍ-cient to result in the development of fatigue crack-ing in the outstanding legs of the ansle. Fígure 20

shows one such fatigue crack. The crack surfacesare shoe¿n in Figure 21.

The cracks always started at the section that hadthe largest thickness reduction. The explanationfor this phenornenon is Èhat the compressional stresscycle resulted in yieltling on the net ligãments ofthe reduced section as a result of stress concentra-tion and reducedl area. During the unloading part ofthe stress cycle a residual tension stress fieldforrned, which pernitted crack initiation. rn thesecircunstances the crack propagated towaral the heelof the angle ancl arrested after having grovtn out ofthe region of yielding. None of the cracks propa-gated past the heel. Hence the cracks in the co¡n-pression flange did not ailversely affect the load-carrying capacity of any of the test beams.

L7

( I Tesl No.

o Crock¡ng

o Angle Foilure

18

NUMBER OF CYCLES

FIGURE 19 Fatigue resistance of riveted members.

Transportation Research Record 950

FIGURE 2t Fatigue crack su¡face in deteriorated compressionllange of test beam 1.

nitrogen, the tenperåture of the enclosed section ofthe stringer was decreased to approximately -40"c.The tenperature vras monitored by three temperaturegauges attached to the web plate at mid-depth and tothe top and bottom angles. After reaching the re-quired tenperature, the temperaturê was maintainedby regulating the nitrogen flow.

v¡ith the cracked section being at this low tem-peraturer a static l-oad that corresponded to themaxi¡nu¡n load of the repeated load cycle was ap-pIÍeil. The cyclic loading was then resumed at a

frequency of. 260 cycles per minute. The crack frontadvanced in a stable, fatigue rnode. The cyclicIoading was continued for a period of approximately0.5 hr at the reduced tenperature. Then the crackwas propagated at roon tenperature for an additíona112.5 to 25 mn. This procedure was then repeated.

l.laterial Fracture Character istics

An indication of the fracture characterístics of thematerial was obtained by perforrning a series ofcharpy V-notch inpact tests on J-8 specimens takenfro¡n a tension flange angle of one of the string-ers. Temperatures varied from -18o to 66oC. Theresults are shown in Eígure 22.

À l-arge variation in absorbed inpact energy canbe seen at test temperatures between 21" and 44oC.

ot

¡¡J(9z.ts

anvlt¡l.l)-3t

l06l05to.

H "'t;*fu#l,t$ì,,,,,

FIGURE 20 Fatigue crack in deteriorated compression flange oftest beam l.

REDUCED TEMPERATURE TEST

objective and Procedure

A reduced temperature test was carried out on thethird stringer. Thê objêctive was to evaluaÈe thebehavior of the rnemberr given a fatigue crack, underlow temperatures. More specifically, the objectivewas to deterrnine if brittle fracture of a componentwould occur and how that would affect the behaviorof the cross section. At the start of the test afatigue crack extended fron lhe top of the hole tothe edge of the angle and fron the botto¡n of thehole to the angle fillet, as shown in Figure 16. Nocracking was observed in the other flange angle orin the web plate at that cross section.

The test procedure r.ras as follo!¡s. The cyclictest was interrupted so that an insulating box ofStyrofoam could be built around the sectionr whichcontained a grid of copper tubes with openings atregular distance through which liquid nitrogen couldbe injected into the closed space. By injecting the

rti',,]

-(9El¡JzUJ

c)l¡Jlnfroqd)

Out et al.

-20 0 roTEMPERATURE,OC

FIGURE 22 Charpy V-notch characteristics of flange angle.

The estinated 20.3 J transition temperature wasabout 20oC. Hence the naterial woulil satisfy theinpact energy requirement for zone 2 of the AASHTOand AREA specif ications.

Results of Reduced Temperature Test

The reduced temperature test was performed for anumber of different crack lengths. These lengthsand the number of stress cycles experienced underreduced ternperature are given in Table 3.

TABLE 3 Reduced TemperatureTest Results

Test a¡ (mm) AN (cycles)

I2

3

45

67

Note: T È -40'C. (omax)s - 6t MPa. ¿ =2.6 x I 0-3 fS-l¡. u¡ = "ìîãtit"ngrh

at s¡årtof test measured ât bottom of flange fromangle co¡4er to crack tip.aCrack iû fillet.bsingle static test.c Test ? discontinùedl no fracture.

No unstable crack extension occurred in thecracke¿l angle at any stage. During the process ofcrack extension the gross section stress increasedby about 30 pêrcent. It is apparent fron the datâin Table 3 that significant numbers of stress cycleswere applied at each crâck length increnent.

The rnaxi¡nun estinated stres,ç intensity factorduring the test v¡as 45 MPa(m¡1/2 if an edge crackwâs assurned fro¡n the heel of the angle. The testresults suggest that a significant temperature shiftis applicable because the alynanic fracture toughnessat 21oC was thís oraler of nagnitude (8).

rn addition, the results indicated that the fric-tional restraínt between the cracked angle and the

19

uncracked web, which was noted ln the section onFatigue Test Program, was beneficial because thecrack opening was Enall. The associated stress in-tensity factor vras probably snaller than predicatedby assumÍng an edge crack.

No ailverse behavior was experienced at the sec-tion when the flange angle crâcked in two under thereduced ternperature. The renaining resLstance ofthe cross section was sufficient to carry the ap-plied loa¿|.

SI'I.{MÀRY AND CONCI,USIONS

The experirnental studies carried out on fourereathered and deteriorated riveted members providedinfor¡nation on the extreme Iife behavior of suchrnembers. In adldition, they have also provided in-formation on the behavior of severely corroded re-gions and their susceptibility to fatigue crackgrowth. The principal findlngs are as follows.

l. The extrene Iife fatigue resistance of theweb flange riveted connection appears to be close tothe category D fatigue linit. Several fatiguecracks were found to ¿levelop in the rivet detalls âtstress ranges betneen 46 and 66 UPa after 8 to 30million cycles.

2. The fâtigue resistance of the corro¿led sec-tíon was observed to vary between category E and C,depending on the severity of the corrosion and theloss of cross-sectional area. This degree of sever-ity cannot be accounte¿l for by considering only theloss of area. The four test results suggest thatwhen the thickness of the outstanding flange angleis reduced until less than half remains, the proxirn-ity of the corrocled area to the opposite surface re-duces the fatigue resistance. Those sÈringers thathad more than half of their flange thickness renìove¿lby corrosion initiated fatigue cracks near the cate-gory E resistance curve. When about half the thick-ness was avaíIable, lnltiation was observed to occurnear the category C resistance curve. A lesserlevel of thickness did not result in crack initi-ation.

3. Severing of a component of the built-up sec-tion ilid not im¡nediately lnpair the cyclic loadingcapacity of the nernbers. BetÌreen 0.5 to 1 millloncycles of a stress range of 62 to 69 MPa on thegross section Ìrere reguired before the load-carryingcapacity was conpletely destroyed. Cracks for¡neclslow1y in the other angle and in the web plate. Al1four test beans exhibitetl redundant behavlor oncecracks developed that severed a flange angle.

4. Significant bond was observedl to exist be-tween the angles and web plate in their painted andcorroded condltion. This reduced the opening of thecrack and extended the fatigue life.

5. Fatigue cracks that forned in the deterio-rated legs of the conpression flange were observedto arrest near the heel of the angle. None of thesecracks affected the load-cârrying capacity 'and thefatlgue resistance of the stringers.

6. Reduced tenperature tests at periodic inter-vals of extension of a crack grown fron a rivet holeinto the outstanding leg of an angle did not resultin unstable crack growth. Even wÍth 95 percent ofthe angle section cracked, the crack extension ¡nodewas stable.

ACKNOWLEDGMENT

This paper is based on a part of Fri.tz nngineeringLaboratory Project 483, iThe Fatigue Strength ofWeathered and Deteriorated Riveted Members." Theproject was carried out under the sponsorshíp of theUnivêrsity Research Program, U.S. Department of

50

_a

27.939.46l .088.9

t 14.3142.2| 52.4'

I 9.000'0b22,00012,0009,000

I 5,00017,000

:

20

Transportation. The experiments were con¿lucted atFritz Engineering Laboratory, Lehigh Uníversity,Bethlehern, Pennsylvania.

Appreciation is due to the technical staff:Robert Dales, Charles Hittinger, Kermit Eberts,David Kurtz. Raymond Kroner, Peter de Car1o, andRussell I-ongenbacht to Suth Grirnes and GiovanniVecchioi to Richard Sopkot to George rrwín for valu-able advicei an¿l to the sources that contríbutealtest ¿lata.

RXFERENCES

1. Interim Specifications: Bridges. AASHTo, wash-ington, D.C., 1979.

2. Irlanual for Railroad Engineering. A¡nerican Rail-way Engineering Assoclatlon, Wâshlngton, D.C.,1980, Chapter 15: Steel Briilges.

3. .T.W. Fisher, B.T. Yen, W.J. Frank, and P.B.Keatíng. An Assessment of Fatigue Damage in theNorfolt and western Railway Bridge 651 at Han-nibal, !4issouri. Tech. Report 484-1(83). FritzEngineering taboratoryr LehÍgh Universityr Beth-lehern, Pa., Dec. 1983.

4. J.M.M. Out, J.W. Fisher, and 8.1. Yen. The Fa-tígue Behavior of lveathered and DeterioratedRiveted trlembers--Phase I. Tech. Report 483-I(83). Frítz Engineering Laboratory, LehighUniversity' Bethlehemr Pa., Sept. 1983.

5. J.M.!.{. OuÈ: The Fatigue Behavíor of a Weatheredand Deteriorated Riveted Menber. llasterrs the-sis. tehigh University, Bethlehern, Pa. r May1984.

6. H.S. Reensnyder. Fatigue Life Extension ofRiveted Connections. ASCEr Journal of thestructural Division, vol. l01r No. sT12, Dec.1975, pp. 259I-2608.

7. K.A. Baker and c.L. KuLak. Fatigue Strength ofTwo Steel Details. Structural Engineering Re-port 105. Departnent of Cívil Engineering' Uni-versity of Alberta, E¿lrnonton, Albertar Canadla,Oct.1982.

8. S.T. Rolfe and J.M. Barsom. Fracture and Fa-tigue ControL in structures! Applications ofFracture l{echanics. Prentice-Ilallr EnglewooilCliffs' N.J.' I977.

Publication of this paper sponsored by Committee on Steel Bridges.

Stresses in Hanger Plates of Suspended Bridge GirdersJAMES R. BDLLENOIT, BEN T. YEN, and JOHN W. FISHER

ABSTRACT

Hanger plates in suspended span bridges arebriefly exanined for in-pIane bending. Thehanger plates are designecl as tension men-

, bersi however, measurements indicate thatbending occurs as well. This bending isproducecl by the reLative rotation at theencls of the pJ.ate and is caused by the fric-tional bond between the hanger plate, pinrand girder web assembly. A finite-element¡¡odel of the hanger plates in¿licates that anonunifor¡n stress distríbution exists acrossthe width of the plate at the pinhole. The¡naximu¡n stress concentration factors werecalculated to be 4.6 ancl 1.8 for 6.9 MPa (lksi) axial and bending stressr respectively.A fatigue strength analysis was conducted tocletermine the 1ífe of the hanger plates.

À frequêntly a¿lopted arrangenent for short- an¿lnedium-span steel bridges is the three-span struc-ture with a suspeniled portion in the middle (seeFigure 1). The suspendled steel girders are usuallyconnected to the overhanging girders by hinges andhanger plates. This arrangènent renders the sus-pended girders in a simply supported condition.

The hanger plates, which are attacheil to thegirders by pins (as illustrated in Figure 21, areprimary bridge conponents. The plates are designetlto undertake the reactlons of the rocker supports of

FIGURE I Three-span bridge with suspended girders.

the suspen¿led span. Because the pins of hangerplates are assumed to rotate freely, the hangers areassu¡ned to sustâin tension forces.

In actual cases hanger plates nay be subjectecl toin-plane bending because of friction at the pins,and to out-of-plane bendíng because of skewness ofthe bridge or other reasons. Broken hanger plateshave been founcl in bridges (!). Some results of abrlef study on in-plane bending of hanger plates arepresented here.

MEASURED STRESSES

Because hanger ptates are assumed to take tensionforces, Iive-load stresses ín hanger plates areexpecteal to be tensile in nature. Measurements byelectrical resistancê strâin gauges on hanger platesof two highway brídge girders índicated that strainsvaried from tension to compression, or vice-versa,as vehicles traversêd the bri¿lqes (j¿).

Exa{nples of recorcled strain-tine traces are sho¡tnin Figure 3. These results irnply that the hangerplates were subjecte¿l to more than sinple tensíon.