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ODOT Structure Project Manager Seminar Fatigue Analysis PowerPoint Presentation
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ODOT Structure Project Manager Seminar Fatigue Analysis

ODOT Structure Project Manager Seminar Fatigue Analysis

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ODOT Structure Project Manager Seminar Fatigue Analysis

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  1. ODOT Structure Project Manager SeminarFatigue Analysis

  2. Session Outline • Definition • Three stages of the results of fatigue • Premise of fatigue design • History • Fatigue evaluation • Retrofit

  3. Fatigue Definition • Reduced material resistance under fluctuating stresses or reversals, which may culminate in cracks or failure after a number of cycles. • Fatigue is the tendency of a member to fail at stress levels below yield stress when subject to cyclical loading (Truck loading)

  4. Stress – Strain Curve • Add stress strain curve

  5. Three Stages of Fatigue Failure • Crack Initiation • Initiation from a point of high stress concentration. • Stress concentration can result from weld flaws, out-of-plane distortion, fabrication details or fatigue prone details

  6. Fabrication Flaws • Plug and tack weld • Incomplete fusion • Slag inclusions and porosities • Blowholes and undercuts • Start and stop positions • Craters and arc strikes • Back-up bars • Intersecting welds

  7. Porosity

  8. Three Stages of Fatigue Failure • Stable Crack Propagation • Crack continues to grow under cyclic loading until it reaches critical size.

  9. Three Stages of Fatigue Failure • Fracture • When the crack propagates to the critical size fracture will occur.

  10. Fatigue Design Parameters Number of Cycles (75 year life) – 4 considerations • 100,000 • 500,000 • 2,000,000 • Over 2,000,000 Number of cycles depends on road classification (AASHTO) • Case I more than 2500 ADTT (average daily truck traffic) • Case II less than 2500 ADTT Stress Range (thru analysis) • Differences in maximum and minimum live load stresses • Allowable stress range based on number of cycles and detail type • Note lane loading is less severe than truck loading (single truck load is the load that causes the worst fatigue condition)

  11. Stress Range • Live Load Stress Range • Fatigue is only considered for tension or stress reversal situation. • Tensile portion of stress cycle drives or propagates the fatigue crack, no matter how small the tension component • No test specimen lost their load-carrying capacity as a result of compression cracks

  12. Fatigue Design Parameters Fatigue Detail Types (AASHTO) • Assignment of Stress Categories for various details Redundancy • Different allowable for non-redundant and redundant members Test were made to determine allowable stress range for various details and plotted against number of cycles • Allowable Stress v. number of cycles S-N Curves • If computed stress range is less than the allowable stress range = infinite life for the detail

  13. Redundancy Load Path • Having three or more main load carrying members Non- Redundant Redundant

  14. Session Outline • Definition • Three stages of the results of fatigue • Premise of fatigue design • History • Fatigue evaluation • Retrofit

  15. Fatigue History Tidbits • 1930’s railroad bridges • Riveted steel highway bridge construction 1940’s & 1950’s • Early 1960’s cracks formed at the AASHO Road Test program (by John Fisher) • 1967 Silver Bridge (stress corrosion initialized fatigue of eyebar)) Initiate Bridge Inspection program • 1968 NCHRP fatigue research (Lehigh) begins • 1970 I-95 Yellow Mill Pond Bridge (cover plated beam) • AASHTO specs – 1973 & 1989

  16. Moment Cover Plate Related Issues Large number of ODOT’s interstate was constructed with using steel rolled beams with coverplates located over the piers • Field splice location • Provide extra capacity over piers in higher moment area. • Allowed similar beam size along bridge. Economical design. • Bridge Standard Drawings 1950’s and 1960’s

  17. Moment Cover Plate Problem • Significant stress concentration at coverplate end due to abrupt change of cross-section. • Weld at ends of coverplate caused weld termination transverse to flow of stresses • Later discovered that these coverplated details where all E or E’ category fatigue prone details • Level of low allowable stress range

  18. Rolled beam Bottom cover plate Bottom cover plate

  19. A A POSSIBLE CRACK LOCATIONS Crack Propagation at Cover Plate Ends

  20. Fatigue Evaluation BDM Method A (evaluation of remaining life) Based on NCHRP Report 299 • AASHTO Guide Specification for Evaluation of Existing Steel Bridges (prediction of remaining life of fatigue prone detail) Method B (allowable stress range) • AASHTO Standard Specification for Highway Bridges (new design) allowable stress range A fatigue analysis of all existing steel members to be re-used or rehabilitated shall be included as part of the Structure Type Study, PDP Step 7.

  21. FATIGUE ANALYSIS • Methods are useful as indicators of the relative severity of the fatigue detail. So they should both be evaluated along with any other pertinent information that could help in reaching a conclusion. • ODOT will review the analysis for final determination as to whether the members require fatigue related upgrading.

  22. Method A - Guide Specifications for Fatigue Evaluation of Existing Steel Bridges • Remaining mean life of the detail, 50 percent probability that the actual life remaining will exceed the remaining mean life • Remaining safe life of the detail, 98 (for redundant members) percent probability that the actual life remaining will exceed the remaining safe life

  23. Method A - Guide Specifications for Fatigue Evaluation of Existing Steel Bridges STEP 1 - Design assumptions and input values • Fatigue truck - HS15 truck with back axle spacing fixed at 30 ft. • Live load distribution factor more realistic based on field data less conservative than Std. Spec. • Allowance for alternate methods, finite element, instrumentation, this would require special consideration by the department. • Section properties - For non-composite decks if the deck shows no signs of detachment, may increase section properties 30% in positive and 15% in negative regions. • Impact 10%

  24. Method A - Guide Specifications for Fatigue Evaluation of Existing Steel Bridges STEP 2 - Run the structural anaylsis • Design Moments • Stress range • Check against the limiting stress range if less than infinite life then finished STEP 3 - Compute remaining life of detail Parameters • ADTT , "Ta", growth rate "g" and present age of structure in years. • Typical growth rate of between 2% and 4% • Back calculate using actual traffic counts if available • Calculated reliability factor "Rs", basic reliability factor "RS0", "FS1", "FS2", "FS3"

  25. Fatigue Life Calculation • Y calculated fatigue life • A current age • K, C, f detail factors • R reliability factor • Sr Stress range Y= [f x K (10^6)/T x C x (R x Sr)^3] - A Variation in Stress range very sensitive to the calculations Ex. 30 year old structure Sr = 2 ksi yields 50 years Sr = 2.5 ksi yields 10 years Sr = 3 ksi yields negative life

  26. Method A - Guide Specifications for Fatigue Evaluation of Existing Steel Bridges The following should be submitted 1. A table showing: - Remaining safe and mean fatigue life - Moments and stress ranges at each detail and location being evaluated. 2. A list of assumptions and input values used for each detail and location being evaluated including: - Live load distribution factor - Wheel and axle spacing of the fatigue truck used as defined in the guide specification. 3. Location and section properties of the detail and a narrative stating whether those section properties are composite or non-composite. 4. ADTT , "Ta", growth rate "g" and present age of structure in years. 5. Impact percentage (10%) 6. Calculated reliability factor "Rs", basic reliability factor "RS0", "FS1", "FS2", "FS3"

  27. Fatigue Strength AnalysisMethod B • In applying loads for fatigue stresses, a single lane of traffic shall be used. (live load distribution factor of S/7) • The design loading shall be HS20.

  28. Method B - Current Standard AASHTO Specifications for Fatigue The following should be submitted: 1. A table showing moments and stress ranges at each detail and location being evaluated. (strength analysis also required) 2. A list of assumptions and input values used for each detail and location being evaluated including: Live load distribution factor (S/7) & Fatigue vehicle used (HS20-44) 3. Location and section properties of the detail and a narrative stating whether those section properties are composite or non-composite. (If you do both then the benefit of going to composite can be determined).

  29. Report Contents • The Fatigue Analysis should provide the information as requested in table form which is easy to follow • Series of computer output is not necessary, actual not wanted • Any background information to better understand assumptions should be provided, for example existing plan details

  30. Fatigue Retrofits • Composite design - Do nothing option • End bolted cover plate retrofits Worthy of mention Fatigue of sign supports a source of fatigue problems due to cyclical loading of truck traffic passing or wind. Western states. No inspection program