Update on NCHRP Project 12-81 AASHTO T-18 Meeting

Austin, TX July 10, 2012

Evaluation of Fatigue on the Serviceability of Highway Bridges

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

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

Primary Objective

Revise and update Section 7 “Fatigue Evaluation of Steel Bridges” of the AASHTO MBE

Section 7 Items to Improve Improved methods utilizing reliability-based approach to assess

the 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 lane

loading

Project Status Draft Final Report – September 2011 Panel Member Comments – December 2011 Revise & Submit Final Report – January 2012 Final Editorial Stage – Ongoing

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.”

Tack Weld and Riveted Members MBE does not provide direct

guidance on the evaluation of tack welded members.

Tack Welds are widely used in bridges with built up sections. Classified as Category E in

LRFD Considerable cost to remove by

grinding

Tack Weld Tests

Tack Weld Specimen Tack Weld Test Setup Tack Weld Specimen Cracks

Test Results Comparison

1

10

100

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

Stre

ss R

ange

(ksi)

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)

Tack Weld Summary All tests lie above category D mean curve, and fit

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

C detail Current category E rating for tack welds is too

stringent

Tack Weld Evaluation (7.2.1)

Tack weld evaluation spelled out in Sect. 7.2.1: “Tack welds may be evaluated based upon the requirements

of Category C, given in LRFD Design Table 6.6.1.2.3-1.”

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 for measured stress ranges”

Multiple Presence Factor for Fatigue • Important to consider for trusses and two girder bridges with 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.

Measuring Stress Ranges (7.2.2.2)

Definition of Δfi in Eq. 7.2.2.2-1 slightly modified to avoid underestimation of effective stress range caused by truncation effects when measured stress range histograms are used

Δfi > (ΔfTH )/2

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

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 (Δfi ), 2.0(Δf)eff , or other suitable

value; for measured stress ranges use Rs=1

Estimating Finite Fatigue Life (7.2.5) Introduce an additional fatigue life level Minimum, Evaluation 1, Evaluation 2, Mean

Modify the finite life Equation 7.2.5.1-1 where: Y = Total fatigue life of the detail in years g = estimated annual traffic-volume growth rate a = present age of detail [ ]

13log (1 ) 1

365 ( ) (( ) )log(1 )

aR

SL effPRESENT

R A g gn ADTT f

Yg

−

+ + ∆ =

+

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

Fatigue Serviceability Index (7.2.6) Method for providing a relative evaluation of the fatigue serviceability of a

structural 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 GRIN− =

Fatigue Serviceability Index (7.2.6) Y aQ GRI

N− =

where: a = Bridge age in years Y = 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

Fatigue Serviceability Index (7.2.6) 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 Highway Main Arterial State Route Other Critical Route

0.90

Secondary Arterial Urban Areas

0.95

Rural Roads Low ADTT routes

1.00

Fatigue Serviceability Index (7.2.6) Table 7.2.6.2-1 added for recommended actions based on the

Fatigue 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 Inspections 0.20 to 0.10 Fair Increase Inspection Frequency 0.10 to 0.00 Poor Assess Frequently

< 0.00 Critical Consider Retrofit,

Replacement or Reassessment

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”

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 to assess 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. Modify probability distribution using present age to re-compute fatigue life if satisfactory performance is demonstrated.

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

Truncated Distribution (7.2.7.2.3) Equations added for each of the fatigue life levels for a modified

estimate. 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

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

updating

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

Fracture-Control for Older Bridges (7.4) Re-wording of existing requirements Commentary notes re-worded. Constraint-induced fracture

mentioned.

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.

Questions?

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

Fatigue Serviceability Example

Bridge Details Welded plate girder with partial-length welded cover plate Evaluation 1 Life used to assess the bridge fatigue serviceability Bridge Age = a = 45 years [(ADTT)SL]PRESENT = 2,350 Δfeff = 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 ) 1365 ( ) (( ) )

log(1 )

aR

SL effPRESENT

R A g gn ADTT f

Yg

−

+ + ∆ =

+

Fatigue Serviceability Index No. 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 = -0.01 From 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.

Updated Fatigue Life Evaluation Welded plate girder with partial-length welded cover plate Equations added for each of the fatigue life levels for a modified

estimate. 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 probability

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

updating

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) = 0.06 The detail now has a rating of “poor” with an assessment outcome of “assess

frequently”

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 unintended composite 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 perform detailed analysis or measurements.

Research Goals Assess the fatigue performance of details with fit-up tack welds

left intact to determine the need and methods to retrofit such details.

Develop effective methods to assess and design effective retrofit or repair procedures for details susceptible to distortion-induced cracking.

Evaluate current use of S-N curves with a linear extension below CAFL for long life behavior with multiple slope S-N curves used by foreign countries.

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

Revise Section 7 of the Manual with Commentary and Examples.

Finite Element Analysis

Finite Element Model of the tack weld specimen

Sections along which stresses are measured

Finite Element Analysis

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

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 <1-in 2

3 L <1-in 3 3 2 (FT)

2 L <1-in 3 (MP)

2 T <1-in 2

3 L >1-in 2

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)

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)

Tack Weld Tests Simple lap connection involving a

pair of plate members attached to a test central plate.

Outer splice plates tack welded in place and “lightly” bolted to the central plate.

Bolts tightened to approximate the clamping force of a comparatively sized rivet.

1

10

100

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

Stre

ss R

ange

(ksi)

Number of Loading Cycles

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)

1

10

100

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

Stre

ss R

ange

(ksi)

Number of Loading Cycles

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

Test Results Test Results (Runouts)

1

10

100

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

Stre

ss R

ange

(ksi)

Number of Loading Cycles

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

Test Results Test Results (Runouts)

Tack Weld Summary The number and length of the tack welds not does not seem

to 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

Fatigue Serviceability Index

00.05

0.10.15

0.20.25

0.30.35

0.40.45

0.50.55

0.60.65

0.70.75

0.80.85

0.90.95

1

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

Fatigue Serviceability Index

00.05

0.10.15

0.20.25

0.30.35

0.40.45

0.50.55

0.60.65

0.70.75

0.80.85

0.90.95

1

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

FSI

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

3 Girder, 2 Span, Interstate Bridge

2 Girder, 2 Span, Interstate Bridge

4 Girder, 1 Span, Interstate Bridge

3 Girder, 1 Span, Interstate Bridge

2 Girder, 1 Span, Interstate Bridge

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)

Background Subsequent study by Fisher et al (1990) found that a positive

attachment 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

Distortion Tests Retrofit Finite Element Analysis

Flange 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

Distortion Tests

0

2

4

6

8

10

12

14

16

0 0.05 0.1 0.15 0.2 0.25 0.3

Load

(kip

)

Deformation (in)

Load vs Deformation

F 0.500 W 0.375 F 0.500 W 0.500F 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

Load

(kip

) 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

Distortion Tests To study the effectiveness of various retrofit attachments

after 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

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)

Distortion Tests

Web-to-Flange weld crack in 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

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

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

DA t2 = 3/4

X X X X

t3 = 7/8 X X

SA t3 = 7/8

X X X X

t4 = 1 X X

Pseudo-Displacement Control

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.00E+00 2.00E+05 4.00E+05 6.00E+05 8.00E+05 1.00E+06 1.20E+06

Dist

ortio

n (in

ch)

Cycles

Web Gap Distortion vs Cycles

Distortion Tests Specimen Result

DT-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-WT075 Cracks Increased in Length After Retrofit

DT-D2-WG1-WTRH075 No Increase in Crack Length After Retrofit DT-D1-WG2-WT075 Cracks Increased in 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

Forc

e (k

ip)

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

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 plotted

using both the AASHTO and Eurocode S-N Curves specified for this detail Both the Eurocode S-N curve and the AASHTO S-N Curve seem to perform

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

S-N Curve Development

1

10

100

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

Stre

ss R

ange

(ksi

)

Cycles to crack

Test Results Eurocode Category 80 AASHTO Category C