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Bridge FatigueMythsBy Dennis R. Mertz, P.E., Ph.D.

Categorizing Details: What if a detailexperiences no applied tension?

M a n y t i m e s , e n g i n e e r s l o o k a t w e l d e d s t e e lbridge details , either on contract drawings or onactual bridge members, and categorize the detailsint o one of th e AAS H TO-specified det a il cat egories,A through E, based merely on the details geome-try . However , such categor iza t ion is premature .The detail must be expected to experience appliedtensile stresses due to the specif ied design loadsbefore they should ever be considered as a fatigue-sensitive detail and labeled as one of the ASSHTOdeta il ca tegories.

This fact is inherent to all AASHTO specifica-t ions, yet even learned steel experts can become

confused by information beyond that contained int h e s p e c i f i c a t i o n s . F o r e x a m p l e , k n o w i n g t h a tresidual stresses play an importa nt role in the per-formance and design of fat igue-sensit ive details ,even experts have suggested restricting the use ofwha t t hey deem t o be f a t igue- sens i t ive de t a i l s ,even if the d eta il does not experience tensile str ess-es due to the design loads. This misinterpretationof th e specifica tions results beca use they know t ha tresidual st ress due to welding ar e tensile near thedetail .

R e s i d u a l s t r e s s e s a r e v e r y l oc a l i z e d . Th e s elocked- in s t resses may cause cracking in a very

localized region near a weld, but these cracks willnot grow if the applied stresses do not include atensile component.

With the 1974 I nter i m AASH TO Speci f i cat ions,t he AASH TO Stand ard Speci f i cat ions for H ighwayBr idgesno longer considered details that experi-enced only fluctuating comprehensive stresses forfa t igue des ign . In N ati onal Cooper at i ve H ighw ayResear ch Progr am( N C H R P) Repor t 1 47, wh ichreported on one of th e research efforts tha t formedthe basis of the interim specif icat ions, ProfessorJ ohn W. Fisher of Lehigh University observed:

Failures occurred due to destruction of the pri-

mary tension f lange of all beams with details sub-jected to tension- tension and par t ia l reversa l o fstress. Crack growth also was observed in the com-p r e s s i o n f l a n g e . H o w e v e r , t h e g r o w t h a r r e s t e daf ter the cracks grew out o f the tensi le res idualstress region unless there was a reversal of stress.There were no failures when the f lange was sub-jected to compression-compr ession st ress cycle.

I n t h e c u r r e n t e d i t i o n o f t h e S t a n d a r d Speci f i ca t ion f or H ighway Br idges, 16th Edit ion,

details to be considered for fatigue are tabularizedi n T a b l e 1 0 . 3 . 1 B . I n t h i s t a b l e , o n e c o l u m n i sdef ined as Kind o f St ress . Examinat ion o f th iscolumn reveals that no entry for details subjectedto compressive stresses alone, only those experi-encing a range o f tens i le s t resses or reversa l o fstresses involving both tension and compressionduring t he stress cycle, a re considered for fa tigue.

Th e L RF D Br i d ge Des i gn Spec i f i c a t i on, 1s tEdition, is more explicit in their description of theappl ica t ion o f the fa t igue provis ions . In Art ic le6.6.1.2.1, the provision states: These provisions

shal l be appl ied only to de ta i ls subjected to netapplied tensile stress. In other words, only if dur-ing the passa ge of a truck the detail is ant icipatedto cycle into tension due to the net applied stress-es-both due to dead load and live load-is the detailconsidered for fa tigue.

Design for Fatigue: How many cyclesare enough, or is a bridges fatiguelife gone after 2 million cycles?

A typical fa t igue-res is tance curve , in log- log

space, is shown in Figure 1. The sloping portion ofthe curve represents the f inite-life fat igue resis-t ance . Alo ng t h i s par t o f t he curve , fo r a g ivenstress range, a corresponding finite life defined byt he curve i s a n t i c ipa t ed . The da shed ho r iz on t a lpor t ion o f the curve represents the in f in i te- l i fefa tigue resista nce. If all of th e stress ran ges experi-enced by a de t a i l a re le ss t han t he s t re ss r angedefined by the horizontal line, it is anticipated thatthe de ta i l wi l l no t crack. The dashed hor izonta lport ion of the curve is called the constant-ampli-tude fa t igue threshold.

Ignoring, for the moment, the constant-ampli-

tude fatigue threshold, the curve can be thought to

BRIDGECROSSINGSPractical Information For The Bridge Industry

No. 4, February1997

Reprinted from Modern Steel Construction

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represent the locus of points of equal fatigue dam-age, as shown in Figure 2. Anywhere in the regionto the left of the curve, the steel detail is consid-e r e d s a f e (t h e t e r m u n c r a c k e d w o u l d n o t b eappro pr ia t e , a s a l l ma t e r i a l s co n t a in ve ry smal lflaws). Anywhere in the region to the right of thecurve, the steel detail is considered cracked ( thet e r m u n s a f e w o u l d n o t b e a p p r o p r i a t e a s t h ec r a c k s m a y b e s m a l l e r t h a n t h e c r i t i c a l s i z e ) .Anywhere along the curve, the details would expe-rience equal fatigue damage (simplistically thoughtof as having a crack size equal to the size used tode f ine c r ack ing) . Th is equa l amo unt o f f a t iguedamage accumula t e s f a s t e r ( in le ss numbers o fcycles) a t h igher s t ress ranges , and s lower end,however t he dam age is considered equal a nyw herea long th e curve despite the ma gnitude of the stressrange.

Table 10.3.1A of the Standar d Speci f i ca t i onsrepresents fatigue-resistance curves for all of thefa t igue categor ies , A through E . The a l lowablestress ranges for more than two million cycles arethe const a nt-a mplitude fa tigue th resholds. The dif-ference in the values for redunda nt a nd non-redun-dant bridges represent the different consequenceso f c r ack ing in redundan t ve rsus no n- redundan tbridges. The codewriters at tempted to arbitrarilyincrease s a fe t y aga ins t f a t igue in o n- redundan t

b r i d g e s . T h e a l l o w a b l e s t r e s s e s f o r r e d u n d a n tbridges ar e the laborat ory-derived values.

The L RFD Speci f i cat i onincludes and equat ionto define fat igue resista nce of ea ch fat igue ca tegory(Equation 6.6.1.2.5-1). When 100,000, 500,000 andt wo mi l l io n cyc le s a re p lugged in t o t he genera lequat ion in the LRFD Specifi cati on, the allowables t r e s s r a n g e s f o r r e d u n d a n t b r i d g e s i n t h eS t a n d a r d S p eci f i ca t i on s r e s u lt . F u r t h e r , t h eco ns t an t - ampl i t ude f a t igue t h resho lds g iven inTable 6.6.1.2.5-1 of the L RFD Speci f i ca t i ona r eequal t o the allowa ble stress ra nges for more tha nt w o m i l l i o n c y c l e s i n t h e t a b l e f o r r e d u n d a n t

bridges in the Stand ar d Specifi cati ons.

Thus, the specified resistance of the StandardSpecificationsa n d L RFD Speci f i cat ionare identi-cal , with the exception t ha t t he LRFD Specifi cati ont r e a t s redundan t ve rsus no n- redundan t b r idgesdifferently.

The true differences between the two specifica-tions lie on t he loa d side of the equa tion. Since thecurve shown in Figure 2 represents equal fat iguedamage, the two specif icat ions are comparable intheir respect ive magni tudes o f s t ress range andcycled yield equal fat igue damage on the curves,w hich a re common t o ea ch specifica tion.

The codewriters who developed the fatigue pro-visions of the Stand ar d Specifi cati onsdid not w antto require that designers deal w ith a loa ding specif-ic to fatigue. They used multiple HS20 truck andlane lo ads fo r t he f a t igue checks , jus t a s t hesel o a d s a r e u s e d f o r s t r e n g t h c o n s i d e r a t i o n s .Knowing that these are f ict i t iously high loads forfatigue, the codewriters specified that a fictitiouslylow number of cycles be considered for fatigue. Thehigher resultant stress range in conjunction withthe lower that actual number of cycles results infa t igue damage comparable to the ac tual br idge .This f ict i t iously low number of cycles has led toconfusion.

The codewriters who developed the fatigue pro-

visions of the LRFD Specifi cati onwa nted the num-ber of cycles for the fatigue check to be realistic sobr idge eva lua t o r s co u ld be t t e r co mprehend t heactual remaining like of the bridge in comparisonto the number of cycles used for design. Instead ofdesigning for, say, two million cycles, the designerwill consider tens of millions of cycles when design-ing to the L RFD Speci f i cat i on. Thus, a new loadw a s requ ired for fa t igue : 75% of a s ing le HS 20truck (or an HS15 t ruck) wi th a f ixed rear-ax lespacing of 30. This load is representative of all thet rucks that wi l l cross the br idge dur ing i t s l i fe .Theoretically, if every tr uck tha t crossed the bridge

during its life-both those heavier and those lighterthan the fat igue truck-was replaced by the fat igue

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t ruck, fa t igue damage equal to the ac tual fa t iguedamage would result . The stress ranges result ingfrom the new fatigue load, in conjunction with thehigher, more realistic number of cycles, yields com-p a r a b l e f a t i g u e d a m a g e a s t h e S t a n d a r d Spec i f i c a t i ons, y e t w i l l n o t l e a d e v a l u a t o r s t obelieve th a t t he fat igue life is over a fter tw o million

tru cks have crossed th e bridge.Denni s R. M er tz , P.E. , Ph.D . , teaches br id ge

design at t he U ni ver sit y of D elaw are. H e is the pri -

ma r y a u t h or o f t h e f a t i g u e p r ov i s i o n s o f t h e

AASH TO L RFD B r i d ge Desi g n Speci f i ca t i o n s.

M er tz cur r ent ly is stud ying th e sim pli f icati on of th e

desi gn pr ovisions for steel I -gir der br id ges.

NATIONALSTEELBRIDGEALLIANCE1405 Lilac Drive North, Suite 212Golden Valley, MN 55422-4528ph: 612/591/9099 fax: 612/591/9499

The mission of The National Steel Bridge Alliance (NSBA),

which was formed in 1995, is to enhance the art and

science of the design and construction of steel bridges. Its

activities include organizing meetings, conferences and

national symposia, conducting the Prize Bridge Awards

competition, supporting research, developing design aids,

and providing assistance to bridge owners and designers.

The NSBA membership includes representatives from all

aspects of the steel bridge industry.