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From page 67... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 2 E-2 SECTION 7 7.1 -- LOAD-INDUCED VERSUS DISTORTIONINDUCED FATIGUE C7.1 Fatigue damage has been traditionally categorized as either due to load-induced or distortion-induced fatigue damage. Load-induced fatigue is that due to the in-plane stresses in the steel plates that comprise bridge member cross-sections. Read the entire page →
From page 68... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 3 E-3 Tack welds may be evaluated based upon the requirements of Category C, given in LRFD Design Table 6.6.1.2.3-1. As uncertainty is removed reduced from the evaluation by more refined analysis or site-specific data, the increased certainty is reflected in lower partial load factors, summarized in Table 7.2.2.1-1 and described in Articles 7.2.2.1 and 7.2.2.2. Read the entire page →
From page 69... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 4 E-4 as: Rp = 0.988 + 6.87x10-5 (L) Read the entire page →
From page 70... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 5 E-5 Table 7.2.2.1-1 -- Partial Load Factors: Rsa, Rst, and Rs Fatigue-Life Evaluation Methods Analysis Partial Load Factor, Rsa Truck-Weight Partial Load Factor, Rst Stress-Range Estimate Partial Load Factor, Rsa For Evaluation or Minimum Fatigue Life Stress range by simplified analysis, and truck weight per LRFD Design Article 3.6.1.4 1.0 1.0 1.0 Stress range by simplified analysis, and truck weight estimated through weigh-inmotion study 1.0 0.95 0.95 Stress range by refined analysis, and truck weight per LRFD Design Article 3.6.1.4 0.95 1.0 0.95 Stress range by refined analysis, and truck weight by weigh-inmotion study 0.95 0.95 0.90 Stress range by field-measured strains N/A N/A 0.85 For Mean Fatigue Life All methods N/A N/A 1.00 a In general, s sa stR R R= 7.2.2.1.1 -- For the Determination of Evaluation or Minimum Fatigue Life In the calculation of effective stress range for the determination of evaluation or minimum fatigue life, the stress-range estimate partial load factor shall be taken as the product of the analysis partial load factor and the truck-weight partial load factor: s sa stR R R= (7.2.2.1.1-1) If the effective stress range is calculated through refined methods of analysis, as defined in LRFD Design Article 4.6.3: 0.95saR = (7.2.2.1.1-2) Read the entire page →
From page 71... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 6 E-6 7.2.2.1.2 -- For the Determination of Mean Fatigue Life In the calculation of effective stress range for the determination of mean fatigue life, the stress-range estimate partial load factor shall be taken as 1.0. 7.2.2.2 -- Measuring Estimated Stress Ranges C7.2.2.2 The effective stress range may be estimated through field measurements of strains at the fatigue-prone detail under consideration under typical traffic conditions. Read the entire page →
From page 72... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 7 E-7 ( ) 2 s dead load compressiontensionR f f∆ > ( ) Read the entire page →
From page 73... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 8 E-8 Otherwise, the total fatigue life shall be estimated as specified in Article 7.2.5. 7.2.5 -- Estimating Finite Fatigue Life 7.2.5.1 -- General C7.2.5.1 Three Four levels of finite fatigue life may be estimated: • The minimum expected fatigue life (which equals the conservative design fatigue life) Read the entire page →
From page 74... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 9 E-9 where: RR = Resistance factor specified for evaluation, minimum, or mean fatigue life as given in Table 7.2.5.21-1 A = Detail-category constant given in LRFD Design Table 6.6.1.2.5-1 n = Number of stress-range cycles per truck passage estimated according to Article 7.2.5.2 g = Estimated annual traffic-volume growth rate in percentage a = Present age of the detail in years [(ADTT) Read the entire page →
From page 75... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 10 E-10 Figure C7.2.5.1-1 -- Lifetime Average Truck Volume for an Existing Bridge Read the entire page →
From page 76... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 11 E-11 Table 7.2.5.1-1 Resistance Factor for Evaluation, Minimum or Mean Fatigue Life, RR Detail Category (from Table 6.6.1.2.5-1 of the LRFD Specifications) RR Minimum Life Evaluation 1 Life Evaluation 2 Life Mean Life A 1.0 1.5 2.2 2.9 B 1.0 1.3 1.7 2.0 B' 1.0 1.3 1.6 1.9 C 1.0 1.3 1.7 2.1 C' 1.0 1.3 1.7 2.1 D 1.0 1.3 1.7 2.0 E 1.0 1.2 1.4 1.6 E' 1.0 1.3 1.6 1.9 7.2.5.2 -- Estimating the Number of Cycles per Truck Passage The number of stress-range cycles per truck passage may be estimated (in order of increasing apparent accuracy and complexity) Read the entire page →
From page 77... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 12 E-12 • Through the use of LRFD Design Table 6.6.1.2.5-2, • Through the use of influence lines, or • By field measurements. 7.2.6 -- Acceptable Remaining Fatigue Life The remaining fatigue life of a fatigue-prone detail is the total fatigue life, as determined through Article 7.2.5, minus the present age of the bridge. Read the entire page →
From page 78... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 13 E-13 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 7.2.6.2 Recommended Actions Based on Fatigue Serviceability Index The fatigue ratings and assessment outcomes as given in Table 7.2.6.2-1 are recommended as a guideline for actions that may be undertaken based on the obtained value for the fatigue serviceability index. A better fatigue rating may be assumed for Q values at the boundary of two ranges. Read the entire page →
From page 79... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 14 E-14 7.2.7.2 -- Recalculate the Fatigue Life Serviceability Index 7.2.7.2.1 -- Through Accepting Greater Risk In general, the evaluation 1 life of Article 7.2.5 is used in determining the remaining fatigue serviceability index life of a bridge detail according to Article 7.2.6. If the evaluator is willing to accept greater risk of fatigue cracking due to: • Long satisfactory cyclic performancefatigue life of the detail to date, • A high degree of redundancy, and/or • Increased inspection effort, e.g., decreased inspection interval, or • Some combination of the above the remaining fatigue serviceability indexlife may be determined using a fatigue life approaching the mean fatigue life of Article 7.2.5. Read the entire page →
From page 81... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 16 E-16 Table 7.2.7.2-1 Cumulative Distribution Function (x) for Standard Normal Variable x x 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.0 0.5000 0.5040 0.5080 0.5120 0.5160 0.5199 0.5239 0.5279 0.5319 0.5359 0.1 0.5398 0.5438 0.5478 0.5517 0.5557 0.5596 0.5636 0.5675 0.5714 0.5753 0.2 0.5793 0.5832 0.5871 0.5910 0.5948 0.5987 0.6026 0.6064 0.6103 0.6141 0.3 0.6179 0.6217 0.6255 0.6293 0.6331 0.6368 0.6406 0.6443 0.6480 0.6517 0.4 0.6554 0.6591 0.6628 0.6664 0.6700 0.6736 0.6772 0.6808 0.6844 0.6879 0.5 0.6915 0.6950 0.6985 0.7019 0.7054 0.7088 0.7123 0.7157 0.7190 0.7224 0.6 0.7257 0.7291 0.7324 0.7357 0.7389 0.7422 0.7454 0.7486 0.7517 0.7549 0.7 0.7580 0.7611 0.7642 0.7673 0.7704 0.7734 0.7764 0.7794 0.7823 0.7852 0.8 0.7881 0.7910 0.7939 0.7967 0.7995 0.8023 0.8051 0.8078 0.8106 0.8133 0.9 0.8159 0.8186 0.8212 0.8238 0.8264 0.8289 0.8315 0.8340 0.8365 0.8389 1.0 0.8413 0.8438 0.8461 0.8485 0.8508 0.8531 0.8554 0.8577 0.8599 0.8621 1.1 0.8643 0.8665 0.8686 0.8708 0.8729 0.8749 0.8770 0.8790 0.8810 0.8830 1.2 0.8849 0.8869 0.8888 0.8907 0.8925 0.8944 0.8962 0.8980 0.8997 0.9015 1.3 0.9032 0.9049 0.9066 0.9082 0.9099 0.9115 0.9131 0.9147 0.9162 0.9177 1.4 0.9192 0.9207 0.9222 0.9236 0.9251 0.9265 0.9279 0.9292 0.9306 0.9319 1.5 0.9332 0.9345 0.9357 0.9370 0.9382 0.9394 0.9406 0.9418 0.9429 0.9441 1.6 0.9452 0.9463 0.9474 0.9484 0.9495 0.9505 0.9515 0.9525 0.9535 0.9545 1.7 0.9554 0.9564 0.9573 0.9582 0.9591 0.9599 0.9608 0.9616 0.9625 0.9633 1.8 0.9641 0.9649 0.9656 0.9664 0.9671 0.9678 0.9686 0.9693 0.9699 0.9706 1.9 0.9713 0.9719 0.9726 0.9732 0.9738 0.9744 0.9750 0.9756 0.9761 0.9767 2.0 0.9772 0.9778 0.9783 0.9788 0.9793 0.9798 0.9803 0.9808 0.9812 0.9817 2.1 0.9821 0.9826 0.9830 0.9834 0.9838 0.9842 0.9846 0.9850 0.9854 0.9857 7.2.7.3 -- Retrofit The Bridge C7.2.7.3 If the recalculated fatigue life serviceability index is not ultimately acceptable, the actual fatigue life serviceability index may be increased by retrofitting the critical details to change improve the detail category and thus increase the lifefatigue serviceability index. Read the entire page →
From page 82... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 17 E-17 relatively few stress-range cycles at fatigue-prone details. However, depending upon the magnitude of the out-ofplane distortion and the geometry of the web gap detail, the crack growth may be slow and a significant period of time may be required before they become large enough to be detected visually.As such, existing bridges which have experienced many truck passages, if uncracked, may be deemed insensitive to distortion-induced cracking, even under heavier permit loads. Read the entire page →
From page 83... ... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 18 E-18 should be considered. One critical issue for this approach is to size the retrofit connection of sufficient thickness and strength for the loading forces to be generated at the new connection. Read the entire page →
From page 85... ... E-20 E.2 Section 7 (Incorporating Recommended Changes) Read the entire page →
From page 86... ... E-21 SECTION 7 FATIGUE EVALUATION OF STEEL BRIDGES 7.1 LOAD-INDUCED VERSUS DISTORTIONINDUCED FATIGUE Fatigue damage has been traditionally categorized as either load-induced or distortioninduced. Load-induced fatigue is that due to the inplane stresses in the steel plates that comprise bridge member cross-sections. Read the entire page →
From page 87... ... E-22 The base metal at net sections of riveted connections shall be evaluated based upon the requirements of Category C, given in LRFD Design Table 6.6.1.2.3-1, instead of Category D as specified for new designs. 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. Read the entire page →
From page 88... ... E-23 7.2.2.1 Calculating Estimated Stress Ranges The multiple presence factor Rp shall be calculated as: Rp = 0.988 + 6.87x10-5 (L) Read the entire page →
From page 89... ... E-24 7.2.2.1.1 For Determination of Evaluation or Minimum Fatigue Life In the calculation of effective stress range for the determination of evaluation or minimum fatigue life, the stress-range estimate partial load factor shall be taken as the product of the analysis partial load factor and the truck-weight partial load factor: Rs = RsaRst (7.2.2.1.1-1) If the effective stress range is calculated through refined methods of analysis, as defined in LRFD Design Article 4.6.3, Rsa = 0.95 (7.2.2.1.1-2) Read the entire page →
From page 90... ... E-25 7.2.2.2.1 For Determination of Evaluation or Minimum Fatigue Life Where field-measured strains are used to generate an effective stress range, Rs, for the determination of evaluation or minimum fatigue life, the stress-range estimate partial load factor, shall be taken as 0.85 7.2.2.2.2 For Determination of Mean Fatigue Life Where field-measured strains are used to generate an effective stress range, Rs, for the determination of mean fatigue life, the stress-range estimate partial load factor, shall be taken as 1.0. 7.2.3 Determining Fatigue-Prone Details Bridge details are only considered prone to load-induced fatigue damage if they experience a net tensile stress. Read the entire page →
From page 91... ... E-26 2.0( f) eff ; for calculated stress range due to a fatigue truck determined by a truck survey or weigh-in-motion study with Rs=1.0 Larger of maximum ( if ) Read the entire page →
From page 92... ... E-27 ( ) efff = The effective stress range as specified in Article 7.2.2 Table 7.2.5.1-1 Resistance Factor for Evaluation, Minimum or Mean Fatigue Life, RR Detail Category (from Table 6.6.1.2.5-1 of the LRFD Specifications) Read the entire page →
From page 93... ... E-28 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 The number of members that carry load when a fatigue truck is placed on the bridge is used to select the load path factor; e.g., two members for a two-girder bridge and for a typical truss structure; four or more members for a multi-beam or multigirder bridge; etc. For diaphragms and secondary members, use G = 1. Read the entire page →
From page 94... ... E-29 7.2.7 Strategies to Increase Fatigue Serviceability Index 7.2.7.1 General If the fatigue serviceability index is deemed unacceptable, the strategies of Articles 7.2.7.2 and 7.2.7.3 may be applied to enhance the fatigue serviceability index. 7.2.7.2 Recalculate Fatigue Serviceability Index 7.2.7.2.1 Through Accepting Greater Risk In general, evaluation 1 life of Article 7.2.5 is used in determining the fatigue serviceability index of a bridge detail according to Article 7.2.6. Read the entire page →
From page 95... ... E-30 This strategy is based upon achieving a better estimate of the fatigue life. 7.2.7.2.3 Through Truncated Fatigue Life Distribution When a negative fatigue serviceability index is obtained according to Article 7.2.6, the detail's fatigue serviceability index may be updated using equations below for mean, evaluation and minimum lives, provided a field inspection finds no evidence of fatigue cracking at the detail. Read the entire page →
From page 96... ... E-31 Table 7.2.7.2-1 Cumulative Distribution Function (x) for Standard Normal Variable x x 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.0 0.5000 0.5040 0.5080 0.5120 0.5160 0.5199 0.5239 0.5279 0.5319 0.5359 0.1 0.5398 0.5438 0.5478 0.5517 0.5557 0.5596 0.5636 0.5675 0.5714 0.5753 0.2 0.5793 0.5832 0.5871 0.5910 0.5948 0.5987 0.6026 0.6064 0.6103 0.6141 0.3 0.6179 0.6217 0.6255 0.6293 0.6331 0.6368 0.6406 0.6443 0.6480 0.6517 0.4 0.6554 0.6591 0.6628 0.6664 0.6700 0.6736 0.6772 0.6808 0.6844 0.6879 0.5 0.6915 0.6950 0.6985 0.7019 0.7054 0.7088 0.7123 0.7157 0.7190 0.7224 0.6 0.7257 0.7291 0.7324 0.7357 0.7389 0.7422 0.7454 0.7486 0.7517 0.7549 0.7 0.7580 0.7611 0.7642 0.7673 0.7704 0.7734 0.7764 0.7794 0.7823 0.7852 0.8 0.7881 0.7910 0.7939 0.7967 0.7995 0.8023 0.8051 0.8078 0.8106 0.8133 0.9 0.8159 0.8186 0.8212 0.8238 0.8264 0.8289 0.8315 0.8340 0.8365 0.8389 1.0 0.8413 0.8438 0.8461 0.8485 0.8508 0.8531 0.8554 0.8577 0.8599 0.8621 1.1 0.8643 0.8665 0.8686 0.8708 0.8729 0.8749 0.8770 0.8790 0.8810 0.8830 1.2 0.8849 0.8869 0.8888 0.8907 0.8925 0.8944 0.8962 0.8980 0.8997 0.9015 1.3 0.9032 0.9049 0.9066 0.9082 0.9099 0.9115 0.9131 0.9147 0.9162 0.9177 1.4 0.9192 0.9207 0.9222 0.9236 0.9251 0.9265 0.9279 0.9292 0.9306 0.9319 1.5 0.9332 0.9345 0.9357 0.9370 0.9382 0.9394 0.9406 0.9418 0.9429 0.9441 1.6 0.9452 0.9463 0.9474 0.9484 0.9495 0.9505 0.9515 0.9525 0.9535 0.9545 1.7 0.9554 0.9564 0.9573 0.9582 0.9591 0.9599 0.9608 0.9616 0.9625 0.9633 1.8 0.9641 0.9649 0.9656 0.9664 0.9671 0.9678 0.9686 0.9693 0.9699 0.9706 1.9 0.9713 0.9719 0.9726 0.9732 0.9738 0.9744 0.9750 0.9756 0.9761 0.9767 2.0 0.9772 0.9778 0.9783 0.9788 0.9793 0.9798 0.9803 0.9808 0.9812 0.9817 2.1 0.9821 0.9826 0.9830 0.9834 0.9838 0.9842 0.9846 0.9850 0.9854 0.9857 7.2.7.3 Retrofit If the recalculated fatigue serviceability index is not ultimately acceptable, the actual fatigue serviceability index may be increased by retrofitting the critical details to improve the detail category and thus increase the fatigue serviceability index. Read the entire page →
From page 97... ... E-32 fatigue-prone details. However, depending upon the magnitude of the out-of-plane distortion and the geometry of the web gap detail, the crack growth may be slow and a significant period of time may be required before they become large enough to be detected visually. Read the entire page →
From page 98... ... E-33 7.4 FRACTURE-CONTROL FOR OLDER BRIDGES Bridges fabricated prior to the adoption of AASHTO's Guide Specifications for FractureCritical Non-Redundant Steel Bridge Members (1978) may have lower fracture toughness levels than are currently deemed acceptable. Read the entire page →

From page 67...

... SECTION 7: FATIGUE EVALUATION OF STEEL BRIDGES 2 E-2 SECTION 7 7.1 -- LOAD-INDUCED VERSUS DISTORTIONINDUCED FATIGUE C7.1 Fatigue damage has been traditionally categorized as either due to load-induced or distortion-induced fatigue damage. Load-induced fatigue is that due to the in-plane stresses in the steel plates that comprise bridge member cross-sections.