2 Session Outline Definition Three stages of the results of fatigue Premise of fatigue designHistoryFatigue evaluationRetrofit
3 Fatigue DefinitionReduced 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)
5 Three Stages of Fatigue Failure Crack InitiationInitiation from a point of high stress concentration.Stress concentration can result from weld flaws, out-of-plane distortion, fabrication details or fatigue prone detailsRESULT OF FATIGUE
7 Fabrication Flaws Plug and tack weld Incomplete fusion Slag inclusions and porositiesBlowholes and undercutsStart and stop positionsCraters and arc strikesBack-up barsIntersecting weldsNicks, Notches, and Indentations – Beam handling devices such as liftingtongs develop intense pressure at the point of contact and can causemeasurable indentations and gouges.Chain marks – When transporting steel beams, chains are commonly usedto secure the beam to the truck.Out-of-Plane Bending Forces – Beams must be securely blocked to resistcyclic sidesway movement during truck or rail transport. There have beenextreme cases where cracks have initiated in beams before they have beenerected.
9 Three Stages of Fatigue Failure Stable Crack PropagationCrack continues to grow under cyclic loading until it reaches critical size.Once a fatigue crack has initiated, applied cyclic stresses may cause rapid propagation of the crack across the member section until it reaches a critical size, at which time the member may fracture.
10 Three Stages of Fatigue Failure FractureWhen the crack propagates to the critical size fracture will occur.Fracture is the separation of the member into two parts
11 Fatigue Design Parameters Number of Cycles (75 year life) – 4 considerations100,000500,0002,000,000Over 2,000,000Number of cycles depends on road classification (AASHTO)Case I more than 2500 ADTT (average daily truck traffic)Case II less than 2500 ADTTStress Range (thru analysis)Differences in maximum and minimum live load stressesAllowable stress range based on number of cycles and detail typeNote lane loading is less severe than truck loading (single truck load is the load that causes the worst fatigue condition)
13 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 componentNo test specimen lost their load-carrying capacity as a result of compression cracks
17 Fatigue Design Parameters Fatigue Detail Types (AASHTO)Assignment of Stress Categories for various detailsRedundancyDifferent allowable for non-redundant and redundant membersTest were made to determine allowable stress range for various details and plotted against number of cyclesAllowable Stress v. number of cycles S-N CurvesIf computed stress range is less than the allowable stress range = infinite life for the detail
28 Moment Cover Plate Related Issues Large number of ODOT’s interstate was constructed with using steel rolled beams with coverplates located over the piersField splice locationProvide extra capacity over piers in higher moment area.Allowed similar beam size along bridge. Economical design.Bridge Standard Drawings 1950’s and 1960’s
29 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 stressesLater discovered that these coverplated details where all E or E’ category fatigue prone detailsLevel of low allowable stress range
33 Crack Propagation at Cover Plate Ends POSSIBLE CRACK LOCATIONSCrack Propagation at Cover Plate EndsOf significance to the inspector is that cracks are only readily detectable visuallyas a through crack after most of the fatigue life of the detail is gone. Therefore, abridge engineer must be notified immediately whenever cracks are found in aflange.
34 Fatigue Evaluation BDM Method A (evaluation of remaining life)Based on NCHRP Report 299AASHTO 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 rangeA 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.
35 FATIGUE ANALYSISMethods 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.
37 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 lifeRemaining safe life of the detail, 98 (for redundant members) percent probability that the actual life remaining will exceed the remaining safe life
38 Method A - Guide Specifications for Fatigue Evaluation of Existing Steel Bridges STEP 1 - Design assumptions and input valuesFatigue 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%
39 Method A - Guide Specifications for Fatigue Evaluation of Existing Steel Bridges STEP 2 - Run the structural anaylsisDesign MomentsStress rangeCheck against the limiting stress range if less than infinite life then finishedSTEP 3 - Compute remaining life of detailParametersADTT , "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 availableCalculated reliability factor "Rs", basic reliability factor "RS0", "FS1", "FS2", "FS3"
40 Fatigue Life Calculation Y calculated fatigue lifeA current ageK, C, f detail factorsR reliability factorSr Stress rangeY= [f x K (10^6)/T x C x (R x Sr)^3] - AVariation in Stress range very sensitive to the calculationsEx. 30 year old structureSr = 2 ksi yields 50 yearsSr = 2.5 ksi yields 10 yearsSr = 3 ksi yields negative life
41 Method A - Guide Specifications for Fatigue Evaluation of Existing Steel Bridges The following should be submitted1. 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"
43 Fatigue Strength Analysis Method 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.
44 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).
46 Report ContentsThe Fatigue Analysis should provide the information as requested in table form which is easy to followSeries of computer output is not necessary, actual not wantedAny background information to better understand assumptions should be provided, for example existing plan details
47 Fatigue Retrofits Composite design - Do nothing option End bolted cover plate retrofitsWorthy of mentionFatigue of sign supports a source of fatigue problems due to cyclical loading of truck traffic passing or wind. Western states. No inspection program
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