T-18 NCHRP 12-81 Fatigue on the Serviceability

8/18/2019 T-18 NCHRP 12-81 Fatigue on the Serviceability

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Update on NCHRP Project 12-81

AASHTO T-18 Meeting

Austin, TX

July 10, 2012

Evaluation of Fatigue on the

Serviceability of Highway Bridges


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Project Team Members

Mark D. Bowman, Prof. of Civil Engr., Purdue Univ., West

Lafayette, IN Gongkang Fu, Prof. of Civil Engr., Illinois Institute of Technology,

Chicago, IL

Ed Zhou, URS Corp., Program Manager, Bridge Evaluation,

Testing & Retrofit, Hunt Valley, MD Robert J. Connor, Assoc. Prof of Civil Engr., Purdue Univ., West

Lafayette, IN

Amol Godbole, Graduate Research Assistant, Purdue Univ., West

Lafayette, IN


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NCHRP 12-81 Panel Members

Mr. Barton J. Newton (Chair), California DOT

Dr. Sreenivas Alampalli, New York DOT

Ms. Laura M. Amundson, Parsons Brinkerhoff

Dr. Lian Duan, California DOT

Mr. Hussam Z. Fallaha, Florida DOT Mr. Thomas K. Koch, North Carolina DOT

Mr. Keith L. Ramsey, Texas DOT

Dr. James A. Swanson, University of Cincinnati

Dr. William Wright, Virginia Tech University

Mr. David B. Beal, NCHRP

Dr. Waseem Dekelbab, NCHRP


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Primary Objective

Revise and update Section 7 “Fatigue

Evaluation of Steel Bridges” of the AASHTO

MBE


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Section 7 Items to Improve

Improved methods utilizing reliability-based approach to assessthe fatigue behavior and aid bridge owners in making

appropriate operational decisions

Guidance on evaluation of retrofit and repair details used to

address fatigue cracks Guidance for evaluation of distortion induced fatigue cracks

Guidance for the evaluation of tack weld induced fatigue cracks

Guidance for field evaluation procedures

Adjustment of truck loading factors to account for multiple laneloading


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Project Status

Draft Final Report – September 2011

Panel Member Comments – December 2011

Revise & Submit Final Report – January 2012

Final Editorial Stage – Ongoing


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Riveted Members (7.2.1)

Riveted members currently allowed to be Category

C per MBE (versus Category D in LRFD)

Concerns expressed about Category C rating for

riveted members in poor condition

Exception added in Sect. 7.2.1 to account for poor

condition of riveted connections:

“The exception is for riveted members of poor physical

condition, such as with missing rivets or indications

of punched holes, in which case Category D shall be

used.”


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Tack Weld and Riveted Members

MBE does not provide directguidance on the evaluation of

tack welded members.

Tack Welds are widely used in

bridges with built upsections.

Classified as Category E in

LRFD

Considerable cost to remove by

grinding


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Tack Weld Tests

Tack Weld Specimen Tack Weld Test Setup Tack Weld Specimen Cracks


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Test Results Comparison

1

10

100

1.00E+05 1.00E+06 1.00E+07 1.00E+08

S t r e s s R a n g e ( k s

i )

Number of Loading Cycles

Comparison of Test Results with AASHTO Mean Fatigue Curves(For Net Section Stress)

Category B Mean Curve Category C Mean Curve Category D Mean Curve

Test Results Test Results (Runouts)


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Tack Weld Summary

All tests lie above category D mean curve, and fit

category C curve

All tests exceed category C design curveDetails with tack welds may be classified as Category

C detail

Current category E rating for tack welds is too

stringent


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Tack Weld Evaluation (7.2.1)

Tack weld evaluation spelled out in Sect. 7.2.1:

“Tack welds may be evaluated based upon the requirementsof Category C, given in LRFD Design Table 6.6.1.2.3-1.”


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Estimating Stress Ranges (7.2.2)

Equation for Effective Stress Range Modified

Delete existing equation and add:

( Δf)eff = RpRsΔf

“Rp = The multiple presence factor, calculated as described in

Article 7.2.2.1 for calculated stress ranges, or 1.0 formeasured stress ranges”


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Multiple Presence Factor for Fatigue

• Important to consider for trusses and two girder bridgeswith multiple lanes.

• Use WIM data to study truck patterns in several states.

• Critical factors include number of lanes, ADTT, and span

length.

• Equation with these factors ranges from 1 to 1.10 for various

conditions. Increases stress range accordingly.


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Measuring Stress Ranges (7.2.2.2)

Definition of Δf i in Eq. 7.2.2.2-1 slightly modified toavoid underestimation of effective stress range caused by

truncation effects when measured stress range histograms are

used

Δf i > ( Δf TH )/2


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Determining Fatigue-Prone Details (7.2.3)

Rs eliminated from Equation 7.2.3-1

Refer to effective stress range as defined in 7.2.2 for

appropriate factored loading


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Infinite-Life Check (7.2.4)

Modify the three possible combinations to account for the

correct load factor for (Δf)max used in Equation 7.2.4-1

Use Rp times Fatigue I Load Combination (infinite life) for use

of fatigue truck

Use 2.0 (Δf)eff ;for calculated stress ranges with fatigue truck

determined by WIM along with Rs = 1

Use larger of maximum (Δf i ), 2.0(Δf)eff , or other suitable

value; for measured stress ranges use Rs=1


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Estimating Finite Fatigue Life (7.2.5)

Introduce an additional fatigue life levelMinimum, Evaluation 1, Evaluation 2, Mean

Modify the finite life Equation 7.2.5.1-1where:

Y = Total fatigue life of the detail in yearsg = estimated annual traffic-volume growth rate

a = present age of detail

[ ]1

3log (1 ) 1

365 ( ) (( ) )

log(1 )

a R

SL eff PRESENT

R A g gn ADTT f

Y g

− + + ∆ =

+


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Estimating Finite Fatigue Life (7.2.5)

Modify (ADDT)SL to use present number of trucks per day

rather than average over fatigue life

Eliminate Figure C7.2.5.1-1

Provide new Table 7.2.5.1-1 to add Evaluation 2 Life and

modify the RR values


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Fatigue Serviceability Index (7.2.6)

Method for providing a relative evaluation of the fatigue serviceability of astructural detail – eliminate remaining life to measure serviceability

Dimensionless qualitative measure

Recommended Actions Based on the Fatigue Serviceability Index

Introduce Equation 7.2.6.1-1

Y aQ GRI

N

− =


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Y aQ GRI

N − =

where:

a = Bridge age in yearsY = Total fatigue life of the detail in years

N = Greater of Y or 100 years

G = Load Path Factor

R = Redundancy Factor

I = Importance Factor


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The load path factor, G:

The redundancy factor, R:

Importance Factor, I:

Table 7.2.6.1-1 Load Path Factor G

Number of Load Path Members G

1 or 2 members 0.8

3 members 0.9

4 or more members 1

Table 7.2.6.1-2 Redundancy Factor R

Type of Span R

Simple 0.9

Continuous 1

Table 7.2.6.1-3 Importance Factor I

Structure or Location Importance Factor, I

Interstate HighwayMain Arterial State

Route

Other Critical Route

0.90

Secondary Arterial

Urban Areas

0.95

Rural RoadsLow ADTT routes

1.00


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Table 7.2.6.2-1 added for recommended actions based on theFatigue Serviceability Index, Q:

Fatigue Rating and Assessment Outcomes

Fatigue Serviceability

Index, Q Fatigue Rating Assessment Outcome

1.00 to 0.50 Excellent Continue Regular Inspections

0.50 to 0.35 Good Continue Regular Inspections

0.35 to 0.20 Moderate Continue Regular Inspections0.20 to 0.10 Fair Increase Inspection Frequency

0.10 to 0.00 Poor Assess Frequently

< 0.00 CriticalConsider Retrofit,

Replacement or Reassessment


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Negative Life Options

• Current bridge age exceeds predicted life with no apparent

fatigue damage (Unrealistic and inaccurate assessment?)

• Eliminate use of “negative life” or “remaining life” terms and

instead refer to fatigue serviceability index:

Re-label Section 7.2.7 to “Strategies to Increase Fatigue

Serviceability Index”


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Increase Fatigue Serviceability Index (7.2.7)

• Art. 7.2.7.2.1 Select alternate (increased) risk level.Current Spec (Minimum, Evaluation, Mean) life option.

New spec will add one more level (Min, Eval 1, Eval 2,

Mean).

•Art. 7.2.7.2.2 More accurate data. Retain options toassess bridge through modification of Rs by refined

analysis, site truck WIM data, or field evaluation.

• Art. 7.2.7.2.3 Truncated Fatigue Life Distribution. Modifyprobability distribution using present age to re-compute

fatigue life if satisfactory performance is demonstrated.


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Modify Probability Distribution (7.2.7.2.3)

• Use present age to modify probability distribution and re-

compute fatigue life

• Virtually eliminates negative life prediction

• Method only works based upon no cracking from field

data & information


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Truncated Distribution (7.2.7.2.3)

Equations added for each of the fatigue life levels for a modifiedestimate. All based upon mean life estimate.

Example for minimum fatigue life:

Y’minimum = 2.19Ymean exp{0.73φ-1[0.039(1-P)+P]-0.27}

where

Y’minimum = updated minimum life in years

Ymean = mean life in years without updating

φ-1 = inverse of standard normal variable cumulative probability

functionP = probability of fatigue life being shorter than current age before

updating


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Distortion-Induced Fatigue Evaluation (7.3)

Additional language added to denote cause of distortion

induced cracking

Art 7.3.1 Language added to discuss need to assess

distortion-induced cracking

Art 7.3.2 Retrofit Options noted

Softening retrofit

Stiffening retrofit


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Fracture-Control for Older Bridges (7.4)

Re-wording of existing requirements

Commentary notes re-worded. Constraint-induced fracture

mentioned.


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Alternate Analysis Methods (7.5)

Option added for use of fracture mechanics or hot-spot stress

analysis to determine finite life in lieu of Y in Article 7.2.6

Commentary language added to note when such case may be

considered.


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Questions?


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Multiple Presence Factor (7.2.2.1)

Equation for Multiple Presence of vehicles provided:

Rp = 0.988 + 6.87(10)-5

(L) + 4.01(10)-6

(ADTT) +0.0107/(nL) >= 1.0

Where L = span in feet

ADTT = avg. trucks/day for all directions and all lanes

nL = number of lanes


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Fatigue Serviceability

Example


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Bridge DetailsWelded plate girder with partial-length welded cover plate

Evaluation 1 Life used to assess the bridge fatigue serviceabilityBridge Age = a = 45 years

[(ADTT)SL]PRESENT = 2,350

Δf eff = 3.75 ksi for 70-ft simple span girders

Traffic growth = 2% = 0.02; n = 1

RR = 1.2 due to Category E detail; A = 11.0(10)8 ksi3

Y = 44 years

[ ]

1

3log (1 ) 1

365 ( ) (( ) )log(1 )

a R

SL eff PRESENT

R Ag g

n ADTT f Y g

−

+ +

∆ =+


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Fatigue Serviceability IndexNo. load paths (girders) = 4; so G = 1

No. spans =1 (simple span); so R = 0.9

N = larger of (100 or Y) = 100

Interstate Bridge; so I =0.9

Q = [(Y-a)/N]GRI = [(44-45)/100]*1*0.9*0.9 = -0.01From Table 7.2.6.2-1 the fatigue rating is “Critical”. In this case

there is no cracking noticed after a thorough inspection, so it

is decided to reassess the fatigue serviceability.

The procedure outlined in 7.2.7.2.3 will be used.


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Updated Fatigue Life Evaluation

Welded plate girder with partial-length welded cover plate

Equations added for each of the fatigue life levels for a modifiedestimate. All based upon mean life estimate.

Example for Evaluation 1 fatigue life:

Y’eval1 = 2.19Ymean exp{0.73φ-1[0.074(1-P)+P]-0.27} where

Y’eval1 = updated minimum life in years

Ymean = mean life in years without updating

φ-1 = inverse of standard normal variable cumulative probabilityfunction

P = probability of fatigue life being shorter than current age beforeupdating


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Updated Fatigue Life Evaluation

For Category E mean life RR = 1.6, and Ymean = 53.1 years

P = probability of fatigue life being shorter than current age before updating

P = probability per Eq. 7.2.7.2.3-5 gives P = 0.176

Example for Evaluation 1 fatigue life:

Y’eval1 = 2.19Ymean exp{0.73φ-1

[0.074(1-P)+P]-0.27}

where

Y’eval1 = 2.19(53.1)exp{0.73φ-1[0.074(1-0.176)+0.176]-0.27}

Y’eval1 = 53 years

updated FSI value

Q = (53-45/100)(1)(0.9)(0.9) = 0.06The detail now has a rating of “poor” with an assessment outcome of “assessfrequently”


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Section 7 of MBE

Considered as being overly conservative Negative remaining lives obtained for some bridges with

satisfactory service history.

Factors contributing to this conservatism include over-

estimated load distribution factors, ignoring unintendedcomposite action, and use of design (min) S-N curves

Un-conservative factors include assumption of single lane

loading

Users of specification may not have resources to performdetailed analysis or measurements.


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Research Goals

Assess the fatigue performance of details with fit-up tack weldsleft intact to determine the need and methods to retrofit suchdetails.

Develop effective methods to assess and design effective retrofit orrepair procedures for details susceptible to distortion-induced

cracking.

Evaluate current use of S-N curves with a linear extension belowCAFL for long life behavior with multiple slope S-N curves used

by foreign countries.

Develop the “Fatigue Serviceability Index” concept to assess theserviceability of the fatigue limit state

Revise Section 7 of the Manual with Commentary and Examples.


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Finite Element Analysis

Finite Element Model

of the tack weld specimen Sections along whichstresses are measured


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Finite Element Analysis

Stress Distribution in Specimen for 3, 2 and 0 Tack Welds respectively


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Tack Weld Tests

17 Specimens in Test Matrix

No. of Tack

Welds

Tack Weld

Position

Tack Weld

Length

No. of Specimens Tested at Sr Value

20 ksi 12 ksi 12 ksi

2 L


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Tack Weld Tests

S.No. Specimen Elapsed Cycles

(* denotes runouts)Stress (ksi) Specimen Condition

1 TW-3LN-12-1 5,253,000* 12 No cracks

2 TW-3LN-12-2 5,103,000* 12 No cracks

3 TW-3LN-12-3 6,316,000 12 Crack at weld toe spreading into bolt hole

4 TW-3LN-20-1 1,066,000 20 Crack at weld toe spreading into bolt hole

5 TW-3LN-20-2 843,000 20 Crack at weld toe spreading into bolt hole

6 TW-3LN-20-3 1,294,000 20 Crack at weld toe just starting to spread into plate thickness

7 TW-3LL-12-1 6,223,000* 12 No cracks

8 TW-3LL-12-2 6,243,000 12 Crack at weld toe spreading into bolt hole

9 TW-2LN-12-1 8,324,000 12 Crack at weld toe, crack length 21/32 inch

10 TW-2LN-12-2 8,259,000 12 Two cracks; one remaining at weld toe and the other spreading ¼

inch into plate thickness

11 TW-2LM-12-1 7,061,000 12 Crack at weld toe spreading into bolt hole

12 TW-2LM-12-2 6,507,000 12

Three cracks; Two cracks spreading into bolt hole and the other

spreading ¼ inch across plate width.

13 TW-2LM-12-3 7,400,000 12 Two cracks; one spreading into bolt hole and the other spreading

1/8 inch across plate width.

14 TW-3LF-12-1 7,667,000* 12 No cracks

15 TW-3LF-12-2 7,546,000* 12 No cracks

16 TW-2TN-12-1 5,513,000 12 Crack at weld toe spreading into bolt hole

17 TW-2TN-12-2 7,570,000 * 12 No cracks

Summary of Test Results (With Net Section Stresses)


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Tack Weld Tests

Specimen TW-3LL-12-2 Weld Cracks

( 3 Long Welds)

Specimen TW-2LM-12-1

Weld Cracks

(2 Modified Position Welds)

Specimen TW-3LN-12-3 Weld Cracks

( 3 Normal Welds)

Specimen TW-3LN-20-2

Weld Cracks

(3 Normal Welds at 20 ksi)


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Tack Weld Tests

Simple lap connection involving apair of plate members attached to

a test central plate.

Outer splice plates tack welded in

place and “lightly” bolted to thecentral plate.

Bolts tightened to approximate

the clamping force of a

comparatively sized rivet.


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1

10

100

1.00E+05 1.00E+06 1.00E+07 1.00E+08

S t r e s s R a n g e ( k s i )


Comparison of Test Results with AASHTO Design Fatigue Curves

(For Net Section Stress)

Category B Design Curve Category C Design Curve Test Results Test Results (Runouts)


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1

10

100

1.00E+05 1.00E+06 1.00E+07 1.00E+08

S t r e s s R

a n g e ( k s i )


Comparison of Test Results with AASHTO Mean Fatigue Curves

(For Gross Section Stress)

Category C Mean Curve Category D Mean Curve Category E Mean Curve



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1

10

100

1.00E+05 1.00E+06 1.00E+07 1.00E+08

S t r e s s R

a n g e ( k s i )


Comparison of Test Results with AASHTO Design Fatigue Curves

(For Gross Section Stress)

Category C Design Curve Category D Design Curve Category E Design Curve



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Tack Weld Summary

The number and length of the tack welds not does not seemto have a very significant effect on the fatigue life.

Modified position tack weld tests cracked a little sooner than

normal position, but same general behavior

No cracking for fully tightened bolt specimens - bolts draw

away stress from the weld toe.

Tack weld test results:

All lie above category D mean curve

Current category E rating for tack welds is too stringent

Details with tack welds may be classified as Category C detail


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Fatigue Serviceability Index

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Remaining Life (Years)

Variation of (Y-a)/N with Remaining Life

Bridge Age 0

Bridge Age 10

Bridge Age 20

Bridge Age 30

Bridge Age 40

Bridge Age 50

Bridge Age 60

Bridge Age 70

Bridge Age 80

Bridge Age 90

Bridge Age 100


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Fatigue Serviceability Index

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

F S I

Remaining Life (Years)

Influence of G, R, I factors on the FSI (Bridge Age 35 years)

4 Girder, 2 Span, Rural Bridge

4 Girder, 2 Span, Interstate Bridge







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Distortion-Induced Cracking

Widespread phenomenon About 90% of all fatigue cracking

is the result of out-of-plane

distortion at fatigue sensitive

details. (Connor and Fisher(2006))

Retrofit Options

Hole drilling (softeneing)

Attachments (stiffening)


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Background

Subsequent study by Fisher et al (1990) found that a positiveattachment was necessary for retrofit for higher stresses.

Connor and Fisher (2006) presented general guidelines for

evaluation and retrofit of details susceptible to out-of-plane

distortion fatigue. The study recommended use of heavy back-to-back angles or comparable WT sections with four bolts in

each leg


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Distortion Tests

Retrofit Finite Element AnalysisFlange Thickness

(inch)

Web Thickness

(inch)

3/8 3/8

1/2 1/2

5/8 5/8

3/4 3/4

1

Retrofit Model

Retrofit Shear Failure Retrofit Flexure Failure


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Distortion Tests

0

2

4

6

8

10

12

14

16

0 0.05 0.1 0.15 0.2 0.25 0.3

L o a d

( k i p )

Deformation (in)

Load vs Deformation

F 0.500 W 0.375 F 0.500 W 0.500

F 0.500 W 0.625 F 0.500 W 0.750

Load vs. Deformation

for Constant 1/2” Flange Thickness

0

5

10

15

20

25

30

35

0 0.05 0.1 0.15 0.2 0.25 0.3

L o a d ( k

i p )

Deformation (in)

Load vs Deformation

F 0.375 W 0.500 F 0.500 W 0.500 F 0.625 W 0.500

F 0.750 W 0.500 F 1.000 W 0.500

Load vs. Deformation

for Constant 1/2” Web Thickness

Flange Thickness more influential than Web Thickness on the Retrofit Stiffness


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Distortion Tests

To study the effectiveness of various retrofit attachmentsafter distortion–induced cracking has occurred in the web

gap

Total of 13 test specimens:

7 specimens having 1.5 inch web gap with WT retrofit

3 specimens having 0.75 inch web gap with double angle

retrofit

3 specimens having 0.75 inch web gap with single angle retrofit


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Distortion Tests

Test Matrix of WT Specimens

Distortion Test Setup

WT Retrofit

Flange

Thickness (in)

Distortion 0.01 in Distortion 0.02 in

Web Gap

0.75 inch

Web Gap

1.5 inch

Web Gap

0.75 inch

Web Gap

1.5 inch

0.5 X X X X

0.75

X X X

X X X

X (RH)

X (RH)

X (B)

X (B)


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Distortion Tests

Web-to-Flange weld crackin Specimen DT-D2-WG1-WT050

Specimen DT-D1-WG1-WTRH075

Stiffener-to-Web Weld Cracks

0.5 inch thick WT

Retrofit for Specimen

DT-D2-WG1-WT050

0.75 inch thick WT Retrofit with

Drilled Retrofit Hole for Specimen

DT-D1-WG1-WTRH075


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Distortion Test Summary

All WT, DA, and SA retrofit tests completed All WT and DA retrofit details examined effective in

arresting or slowing crack growth

Retrofit holes did not experience crack re-initaition

WT flange thickness most effective parameter for controlling

displacement; greater than 5/8-in flange thickness best

SA thickness needs to be greater to control displacement and

control retrofit cracking


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Distortion Tests

Test Matrix for Distortion Tests(Completed Tests shown in yellow)Connection

Type Detail Thickness Differential Distortion, 0.01 in Differential Distortion, 0.02 in

Web Gap, g1 =

3/4

Web Gap, g2 =

1 1/2

Web Gap, g1 =

3/4

Web Gap, g2 =

1 1/2

WT

t1 = ½

(Flange Thickness)

X X

X X

t2 = 3/4

(Flange Thickness)

X X X

X X X

X (RH)

X (RH)

X (B)

X (B)

Differential Distortion, ∆1 Differential Distortion, ∆2

DAt2 = 3/4

X X

X X

t3 = 7/8X

X

SA

t3 = 7/8X X

X X

t4 = 1X

X


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Distortion Tests

Specimen ResultDT-D1-WG1-WT050 No Increase in Crack Length After Retrofit

DT-D1-WG1-WT075 No Increase in Crack Length After Retrofit

DT-D2-WG1-WT050 Cracks Increased in Length After Retrofit


DT-D2-WG1-WTRH075 No Increase in Crack Length After Retrofit


DT-D1-WG1-WTB075 No Increase in Crack Length After Retrofit

Distortion Test Results

WT Retrofit

0

5

10

15

20

25

30

35

40

45

0 2 4 6 8 10

F o r c e

( k i p )

Displacement (milli inches)

Force Vs Web Gap Distortion

LVDT1 (South Side)

LVDT2 (North Side)

Force Vs Web Gap Distortion Plot for Specimen DT-D2-WG1-WT075


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S N C D l t


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S-N Curve Development

Foundation underlying fatigue evaluation of steel bridges in Manual. Developed primarily based on constant amplitude cyclic loading fatigue test

data.

Advancement in understanding long-life fatigue behavior under low-magnitude and variable-amplitude cyclic loading

NCHRP Report 354: partial length cover plates, web attachments,

and transverse web stiffeners Supports the conservative straight-line extension of the fatigue resistance

curves

NCHRP Report 336: transverse connection plate test results plottedusing both the AASHTO and Eurocode S-N Curves specified for thisdetail

Both the Eurocode S-N curve and the AASHTO S-N Curve seem to performreasonably well

AASHTO S-N curve is simpler to use with a linear slope


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S-N Curve Development

1

10

100

1.00E+05 1.00E+06 1.00E+07 1.00E+08

S t r e s s R a n g e ( k s i )

Cycles to crack

Test Results Eurocode Category 80 AASHTO Category C


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T-18 NCHRP 12-81 Fatigue on the Serviceability

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