Fisher Presentation

8/3/2019 Fisher Presentation

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Fatigue Design: Its past, what it is today andFatigue Design: Its past, what it is today andits futureits future

John W. Fisher Professor Emeritus &John W. Fisher Professor Emeritus &Director Emeritus, ATLSS Center LehighDirector Emeritus, ATLSS Center Lehigh

UniversityUniversity

Engineering Research Center



Failures due to Fatigue and/or Fracture• – ’

– Mine hoists – Pressure vessels & storage tanks

–

– Railroad car axles

(Surprising ) Failures in welded structures before and after WW II –

– Bridge girders

• Earliest welded bridge fractures in Germany, Belgium and

~ • Led to the rule of not welding transverse to tension flange (abig mistake!)





Early Research - AASHO Road Test steel

beams with cover- lates:1958-1960



Cracks developed at ends of cover plate weld toes



Early AWS & AASHTO Specification

– Prior to 1965 only “token” design provisions

– Relied on Goodman Dia rams AWS ; Data from steel industr ;

University research (few beam specimens and many small

tension specimens)

f K F ro1

• Allowable stresses determined from equations which accounted for:

2

– – Stress Ratio ‘R ’

– Detail category LL DL

DL

S

S R



Smax

static strengthlimit

2

Smin = Smaxf ro

k1f ro

1 – k2R

Smax =

Smin

Smin

Smax

R =

basis for Maximum stress design at

specific cycles



Specifications Provided by AWS 1950’s &

AASHO 1965 used Smax for desi n

• Higher strength steels had higher allowable maximumstress in 1965

girder due to changes in ‘R ’

• Different equations were to be used when maximums ress was n compress on

• Detail categories were used (different than used today)

– AISC ado ted stress ran e in 1969

– AASHTO changed the coefficients k1 and k2 in 1971to make all steels the same and indirectly use stress



HS20 Design Truck used until 1995:

A D



Table 1.7.2B-Stress Cycles

Main (longitudinal) Load carrying members

Type of Road Case ADTT Truck loading Lane loading

Freeways, 2500 or more 2,000,000

Expressways,

major

highways and

I over 2,000,000

for single truck

500,000

Freeways,

Expressways,

ma or II < 2500 500,000 100,000

highways andstreets

Other

Highways and

streetsIII 100,000 100,000



1960’s fatigue tests on large scale welded details

made use of statistical experiment design



Internal Flawe ange e



Weld Toe Defect



Slag Inclusion Discontinuity

100m

A

A

Slaginclusionsatfusionboundary



Effect of Minimum stress, R-ratio



Effect of steel grade (250 t0 700MPa



Residual Stress from welding





Statistical distribution at all stress levels



Residual Stress at Weld Toe



Fatigue Design Curve - Basis• at gue es gn urve represents on t e

95% Probability of Survival Life based on experimentaldata

– Also called the lower bound curve

– For large data set this curve is 2SD shifted from the

2SDforalarge

setofdata

• The design curve

corresponds to 2.3%

)N( LBnP f

large data set

NLΒΝ

LowerBound

DesignCurve MeanCurve



1974 AASHTO Interim Specifications

Major RevisionMajor Revision• Fatigue Provisions

– LL

– Material Strength is Immaterial

– Significantly revised detail categories (6 categories,-

– Figures were added to help identify details

– Established foundation for all subsequent design

requ remen s• Fracture Provisions

– Charpy V-Notch (CVN ) requirements adopted innc u e n mater a spec cat on





Detail Categories1974 – 2009 AASHTO & 1969 - 1999 AISC

B'



Bridge loading & representative vehicle



Compare with Actual Load Spectrum

Notemanytrucksareheavierthan



Typical response of floor beam and deck

plate to 5-axle Truck

CH27 Diaph cutout

M P a

CH89 on

Deck

Seconds



Typical S r histogram for girder with cover plate25000

20000

2000

2500

3000

3500

15000

0

500

1000

1500

10000

1 1 1 1 1 1

5000

0

2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5



Stress range histogram at orthotropic deck rib wall

from wheel loads1000000

100000

R27W

0.01% freq. of

1000

10000

o f

C y c l e s (8.15 ksi)

100 N u m b e r

1

10

0 . 0 - 0 . 5

0 . 5 -

1 . 0

1 . 0 -

1 . 5

1 . 5 - 2 . 0

2 . 0 - 2 . 5

2 . 5 - 3 . 0

3 . 0 - 3 . 5

3 . 5 -

4 . 0

4 . 0 -

4 . 5

4 . 5 - 5 . 0

5 . 0 - 5 . 5

5 . 5 - 6 . 0

6 . 0 - 6 . 5

6 . 5 - 7 . 0

7 . 0 - 7 . 5

7 . 5 - 8 . 0

8 . 0 - 8 . 5

8 . 5 - 9 . 0

9 . 0 - 9 . 5

Stress Range (ksi)



Miner’s Rule – Effective stress range

S

12

2

1

1

N

n

N

n D

N

n D

i

i

i

S r, max

S2

S re

S1

n N N2 N2 N11

N



Effective stress range, S re

ree

e

i

i

i i

i

i

i

S N

N

n

N n D

atfailuretocyclesthe:

;

mi

i

m

re

i

m

riim

ree

i

i

i

m

rii

i

m

r

nSSn

S A N

n

S A N

n

m AS A N constantsareandwhere,Recall,

riii

i

i

i

m

rii

i

i

i

iim

re

S f n

n

n

Sn

n

r S

)of occurenceof (frequency Let,

33

rangestresscubemeanRoot

i

riire S f S

m

i

m

riire

i

m

rii

m

re

i

S f SS f S

/ 1



Random variable fatigue tests on large girders



Stress-Range spectrum - Fatigue Resistance -

0.01% Exceedence CAFL

Sre



Applicability of Miner’s Rule

• Shortcomings – does not consider the sequence effects in loading

– does not consider the mean stress effects in loading

• However Miner rule rovides ood correlation for random loading which is what most systems see

•

– non-linear damage models may be more appropriate

• ner s u e on y app ca e n e n e e

– not in the Infinite Life



Subsequent Specification Revisions-

• Revisions for fatigue design truck loading = HS15 or 75% HS20• Revisions & enhancements to fatigue resistance provisions for:

• Orthotropic decks

• Modular expansion joints

• 2009 (Interim)

– Revision in the approach of infinite life check

• Introduction of two fatigue limit states

–

– Elimination of provisions related to orthotropic decks

– Introduction of section to address Constraint Induced Fracture



AASHTO Fatigue Truck – HS15: LRFD



Correlated to Live Load Spectrum on actual

bridges in service over 50 years• AASHTO fatigue limit-state load range for primary members:

– 2 x HS15 or HS30 (108 kips)

•• Based on tests on hundreds of brid esBased on tests on hundreds of brid es

S re S r,maxr,max

uee

0.01% prob. of

exceedence

(LRFD)

% Ou

GVW



Metamorphosis



Examples of Cat. E or E' cover

late details desi ned in 1960’s



Example of Longitudinal

Welded Attachment after 5 years of service

I-84 Bridge…Actually worse

an a egory



Lafayette Street Bridge: St. Paul Minnesota(May 1975)

n ersec ng roove

Welds and Defect



oanBrid e

2000



Crack-like Condition at Shelf PlateGirder F FB-5



Girder E P.P. 28Crack Origin

Cleavage Fracture Cleavage Fracture



Constraint Induced Fracture (CIF)• a ure o oan r ge s owe t e poss ty o

constraint induced fracture•

with no evidence of fatigue crack growth

– Pop-in Initiation at intersecting welds• Lack of direct connection of the shelf plate to the

transverse connection plate created a large geometric

• Forensic investigation showed that the crack like

geometry and the high triaxiality resulted in fracture

• Small gaps prevents yielding and a triaxial stress state

from weld shrinkage



Primary and Secondary Stresses•

subjected to in-plane stresses called primary stresses

– P/A stress or Mc/I stress from mechanics

– Stresses arising due to out-of-plane deformation of elements are negligible

• econ ary s resses genera e ue o e orma on oelement either from secondary out of plane deformationand/or second order load-deformation interaction

– Secondary deformation arises due to incompatibilityin displacement between connecting elements

– deformation to cause second order stresses



Small Web Gap Cracking• oor eam connect on p ate not connecte to anges

(concerns about welding to tension flange) – Small deformable web a at connection late co e



Web Gap Cracking in Plate Girders & Box’sWeb Gap Cracking in Plate Girders & Box’s



Web Gap Cracking at Top FlangesWeb Gap Cracking at Top Flanges

Flange

Trans..

Plate



Web Gap Cracking –Top Flange



Web Gap Cracking atWeb Gap Cracking at



Crack Development is on Both Sides



How to Prevent Web Gap Cracking

• Positive attachments between components – Eliminates relative dis lacements between

components

• Adequate flexibility of connections to accommodate

• Examples:

– Weld or bolt transverse connection plates to flanges

– Weld gusset plates to transverse connection platesand web

•



Retrofit at Bottom Flanges



Bolted retrofit at top flange



Softening with Large Web Gaps - -

displacementtooccuroverlongerlengthorwebgap

(L)

Mustbesufficientlyflexible

tobesuccessful



Connection SofteningConnection Softening

ua y con ro s cr ca w ereconnection plate is removed



Hot Spot Stress Approach - DNV• Tubular Structures :DNVHotSpotStress

LocalStressPulledby

.

s s

NominalStress

S t r

Out-of-plane



Hot-spot Stress Approach – IIW, ABS• Tubular and Non-tubular Structures

S H.S. St/2

s s

e s s S0.4tSH.S.

3t/2

L o c a l S t r

L o c a l S t St

0.1√(r*t)Weld

Toe

t/2 3t/2Weld

Toe t/2 3t/2 Weld

Toe

Weld

Toe 0.4t t

ABSHot-SpotStress IIWHot-SpotStress



Design Curves





Questions

• Why is a fatigue truck (HS15) used todesi n for fati ue resistance in the

AASHTO specifications rather than the

HS20 truck used for stren th desi n?• What would you consider the most

resistant structure?

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Fisher Presentation