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From page 24... ... 24 As described in the previous chapter, individual items in Section 7 that were identified for revision were examined. This chapter presents the results of the examination of the items. Read the entire page →
From page 25... ... 25 Given this observation, there have not been obvious reasons and motivation to recommend a new model to better describe the fatigue load effect than the current AASHTO fatigue truck, although the latter's lack of realistic modeling is obvious. At this point, it is recommended or further emphasized, as done in Section 7 of the AASHTO MBE, that WIM data gathered at or near a particular site with well maintained and calibrated equipment is clearly the most reliable data to be used for fatigue load effect estimation. Read the entire page →
From page 26... ... 26 Concept of Multiple Presence Factor For strength limit states, MPF is intended to facilitate estimation of the load effect in a structural component due to all truck loads on the span, with reference to the load effect in the same component due to only one lane of truck load. Therefore, MPF for evaluation is formulated similarly as follows: MPF N-Lane Load Effect in Component One-Lane = Load Effect in Component LE LE total onelane = ( ) Read the entire page →
From page 27... ... 27 for two-girder-, two-truss-, and two-arch-systems, where each primary member needs to carry all the lanes (as opposed to multiple beams carrying several lanes so that some beams do not participate in carrying certain lanes at all) Read the entire page →
From page 28... ... 28 0 0.25 0.5 0.75 1 0 5 000 10000 15000 M PF /N ADTT Shear: Span = 220ft 2-lane dat a 3-lane dat a 4-lane dat a 2-lane regression 3-lane regression 4-lane regression Figure 23. Comparison of proposed MPF and computed MPF using WIM data divided by number of lanes (shear in 220ft span) Read the entire page →
From page 29... ... 29 As illustrated, the real fatigue life of the detail is a random variable, expressed using the curve of probability distribution with little triangle symbols. The total life estimated according to the AASHTO MBE (2011) Read the entire page →
From page 30... ... 30 Therefore, the truncated probability P at Y = a = current age is P = probability of fatigue life being shorter than current age before updating based on no crack found =   +     Φ Ln a 2.19Ymean 0 27 0 73 12 . Read the entire page →
From page 31... ... 31 tistical distribution for a given N For design purposes, allowable nominal stress ranges are usually defined two standard deviations below the mean stress ranges. Read the entire page →
From page 32... ... 32 not on a two standard deviation shift which would give lower stress range values. Based on this, using the 95th percentile line as minimum life, the resistance factor RR was recalculated for mean life. Read the entire page →
From page 33... ... 33 Fatigue Category A B B' C D E E' Probability of Failure 5% 1.0 1.0 1.0 1.0 1.0 1.0 1.0 10% 1.3 1.2 1.2 1.2 1.2 1.1 1.2 15% 1.5 1.3 1.3 1.3 1.3 1.2 1.3 20% 1.7 1.4 1.4 1.4 1.4 1.3 1.4 25% 1.9 1.5 1.5 1.6 1.5 1.3 1.5 30% 2.1 1.6 1.6 1.7 1.6 1.4 1.6 35% 2.2 1.7 1.6 1.8 1.7 1.4 1.6 40% 2.4 1.8 1.7 1.9 1.8 1.5 1.7 45% 2.7 1.9 1.8 2.0 1.9 1.6 1.8 50% 2.9 2.0 1.9 2.1 2.0 1.6 1.9 Table 7. Variation of RR with probability of failure. Read the entire page →
From page 34... ... 34 base value (i.e., based on fatigue resistance alone) to a reduced value that reflects greater consequences from the lack of ability to redistribute the load (load path factor) Read the entire page →
From page 35... ... 35 Fatigue Serviceability Index, Q Fatigue Rating Assessment Outcome 1.00 to 0.50 Excellent Continue Regular Inspection 0.50 to 0.35 Good Continue Regular Inspection 0.35 to 0.20 Moderate Continue Regular Inspection 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 Table 13. Fatigue rating and assessment outcomes. Read the entire page →
From page 36... ... 36 an approximate remaining life of 20 years gets reduced to a life of approximately 13 years for the worst possible bridge condition. The FSI can also become negative if negative remaining lives are obtained. Read the entire page →
From page 37... ... 37 a (yr) Minimum Remaining Life Using Manual Recommended Approach (yr) Read the entire page →
From page 38... ... 38 a (yr) Minimum Remaining Life Using Manual Recommended Approach (yr) Read the entire page →
From page 39... ... 39 a (yr) Minimum Remaining Life Using Manual Recommended Approach (yr) Read the entire page →
From page 40... ... 40 involves determining how much the bolts need to be tightened in order to have the same clamping effect as a rivet. Bolt Tightening Procedure Need for Developing Procedures Tack welds have been used frequently in bridge structures to temporarily hold members in place before riveting. Read the entire page →
From page 41... ... 41 0 10 0 20 0 30 0 40 0 50 0 60 0 0 1 0 2 0 3 0 4 0 5 0 6 0 To rq ue (l bft) Tension (kip) Read the entire page →
From page 42... ... 42 Finite Element Analysis of Tack Weld Specimen Motivation for Analysis It is important to know how the stress will flow in the tack weld specimen. Knowledge of the stress distribution is useful for finding the locations where stress concentration occurs during the cyclic loading. Read the entire page →
From page 43... ... 43 0 5 10 15 20 25 30 St re ss R an ge (k si) 0 6 5 4 3 2 1 Distance from plate edge (inch) Read the entire page →
From page 44... ... 44 area of the net section. The reduced 9 ksi stress range in the net section indicates that some of the stress range has been transferred into the base plate through the tack weld toes which lie ahead of the net section. Read the entire page →
From page 45... ... 45 Most of the specimens which experienced fatigue cracks had only one weld which cracked. Some specimens experienced crack initiation at multiple tack welds and simultaneous crack growth. Read the entire page →
From page 46... ... 46 Category B Design Curve Category C Design Curve Test Results Test Results (Runouts) Read the entire page →
From page 47... ... 47 earlier, Connor and Fisher (2006) describe a situation where a retrofit detail with a small thickness was not fully effective in preventing further crack growth at a detail with distortion-induced fatigue cracking, while a thicker detail used elsewhere on the same bridge was effective in halting further crack growth. Read the entire page →
From page 48... ... 48 F 0.500 W 0.375 F 0.500 W 0.500 F 0.500 W 0.625 F 0.500 W 0.750 0 2 4 6 8 10 12 14 16 0 0.1 0.15 0.05 0.2 0.25 0.3 Lo ad (k ip) Deformation (in) Read the entire page →
From page 49... ... 49 F 0.750 W 0.375 F 0.750 W 0.500 F 0.750 W 0.625 F 0.750 W 0.750 0 5 10 15 20 25 30 35 0 0.1 0.15 0.05 0.2 0.25 0.3 Lo ad (k ip) Deformation (in) Read the entire page →
From page 50... ... 50 F 0.375 W 0.625 F 0.500 W 0.625 F 0.625 W 0.625 F 0.750 W 0.625 F 1.000 W 0.625 0 5 10 15 20 25 30 35 40 0 0.1 0.15 0.05 0.2 0.25 0.3 Lo ad (k ip) Deformation (in) Read the entire page →
From page 51... ... 51 It can be observed from these plots that in general, as both web and flange thicknesses are increased, the maximum load capacity of the retrofit before failure increases. From the plots where the flange thickness of the retrofit is kept constant, it can be seen that changing the web thickness has comparatively little effect on the maximum load capacity of the retrofit as well as the stiffness of the retrofit which is given by the elastic slope of the load vs. Read the entire page →
From page 52... ... 52 Clearly from the previous analysis, the flange thickness is a governing variable influencing the stiffness as well as the maximum load carrying capacity for the WT retrofit. Hence for the WT retrofits, the primary variable that will be changed for observing the behavior of the retrofit will be the flange thickness. Read the entire page →
From page 53... ... 53 For the Bridge B and Bridge A models shown in Figures 59 and 60, the upper flanges of the exterior girders were fixed in place while the middle girder was displaced downward. The web gap distortion was measured. Read the entire page →
From page 54... ... 54 Figure 62. Bridge A -- stress distribution before and after retrofit. Read the entire page →
From page 55... ... Web Gap - 0.5" Angles - 0.5" Web Gap - 0.5" Angles - 0.75" Displacement 0.5 Distortion (inch) Before/After Ratio Displacement 0.5 Distortion (inch) Read the entire page →
From page 56... ... 56 Web Gap - 0.5" Angles - 0.5" Web Gap - 0.5" Angles - 0.75" Displacement 0.5 Force (kip) After/Before Ratio Displacement 0.5 Force (kip) Read the entire page →
From page 57... ... 57 and 1.75 in. were used, and two different cross bracing thicknesses of 0.5 in. Read the entire page →
From page 58... ... 58 Web Gap - 0.5" Bracings - 0.5" Web Gap - 0.5" Bracings - 0.75" Displacement 0.5 Force (kip) After/Before Ratio Displacement 0.5 Force (kip) Read the entire page →
From page 59... ... 59 Specimen Web Gap Length (inch) Distortion (inch) Read the entire page →
From page 60... ... 60 Figure 66. Typical double-angle and single-angle retrofits. Read the entire page →

From page 24...

... 24 As described in the previous chapter, individual items in Section 7 that were identified for revision were examined. This chapter presents the results of the examination of the items.