Session Outline Definition Three stages of the results of fatigue Premise of fatigue design History Fatigue evaluation Retrofit
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)
Three Stages of Fatigue Failure 1.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
Three Stages of Fatigue Failure 2.Stable Crack Propagation Crack continues to grow under cyclic loading until it reaches critical size.
Three Stages of Fatigue Failure 3.Fracture When the crack propagates to the critical size fracture will occur.
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)
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
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
Moment Cover Plate Related Issues Large number of ODOTs 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 1950s and 1960s
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
A A POSSIBLE CRACK LOCATIONS Crack Propagation at Cover Plate Ends
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.
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.
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
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%
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"
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
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"
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.
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).
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
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