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1 Fatigue Evaluation of Steel Bridges The following report summarizes the results of the research effort undertaken as part of NCHRP Project 12-81. This research project has a focus on Section 7 âFatigue Evaluation of Steel Bridgesâ in AASHTOâs The Manual for Bridge Evaluation (MBE) first issued in 2008. The MBE combines the Manual for Condition Evaluation of Bridges, Second Edition (2000) and its 2001 and 2003 Interim Revisions with the Guide Manual for Condition Evaluation and Load and Resistance Factor Rating (LRFR) of Highway Bridges, First Edition in 2003 and its 2005 Interim Revisions. The objective of this research is to develop a revised and updated Section 7 for the MBE, to meet the needs of the user. A view exists among some fatigue evaluation engineers that the MBE is overly conservative, because some bridges with satisfactory service history are accordingly determined to have nega- tive remaining fatigue lives. A number of factors may have contributed to this conservatism: overestimated load distribution factors, unintended composite action ignored, the S-N curveâs lower bound being used, etc. However, not all cases of fatigue evaluation are believed to be overly conservative. For example, truss or two-girder bridges carrying more than one lane of traffic may have un-conservative fatigue life estimates because of the single lane loading prescribed in the MBE. When multiple lanes are carried by the two trusses or girders, the fatigue life may be significantly overestimated because possible simultaneous loads on other lanes are ignored. On the other hand, conventional analysis methods generally overestimate the live load stress ranges in truss bridges because unintended composite actions are often ignored. In general, a larger amount of uncertainty is involved in fatigue evaluations compared with bridge strength evaluations or load ratings. Furthermore, the demand for a realistic fatigue eval- uation is much higher than that for a fatigue design, because an over-conservative evaluation result could cost considerably more than an over-conservative design. An un-conservative result is, of course, not desired either. Besides the uncertainty factors mentioned above, there are also other sources of uncertainty in the fatigue evaluation process. They include the scatter nature of the S-N curves, variable truck loads including significant site-to-site variations, approximations in structural analysis or load effect estimation, etc. The inherent uncertainties, however, can be reduced using more refined analyses or field measurements to better define the stress range at the details in question. The research program of this project aims toward the revision of Section 7 of the MBE to advance the state of the art and the practice. Items specifically identified as in need of improve- ment include: 1. Improved methods utilizing a reliability-based approach to assess the fatigue behavior and aid bridge owners in making appropriate operational decisions. 2. Guidance on the evaluation of retrofit and repair details used to assess fatigue cracks. 3. Guidance for the evaluation of distortion-induced fatigue cracks. S u m m a r y
2To address these needs a number of analytical and experimental studies were performed. The analytical studies were used to examine various aspects that influence the fatigue behavior. These topics ranged from truck loading effects on bridge structures to fatigue resistance related factors that affect the predicted fatigue life. Both analytical and experimental studies were used to fur- ther develop an understanding of distortion-induced deformations and the structural behavior of various retrofit details used to repair a bridge structure with distortion-induced fatigue crack- ing. Moreover, early in the study it was decided that it would be beneficial to perform a series of experimental tests to study the influence of tack welds on riveted joints. A summary of some of the key findings from the study is provided below. Finite fatigue life predictions based upon use of an approximate curve to estimate the lifetime average ADTT (average daily truck traffic) based upon the present single lane ADTT were found to lead to inconsistencies and errors in the prediction of the remaining fatigue life. The use of a closed form solution for the effect of traffic growth developed in NCHRP 12-51 is recommended for inclusion in the updated version of Section 7. The resistance factor for the finite fatigue life was found to be well correlated with the 95th percentile for the minimum life. The values for RR were correspondingly recalculated for the evaluation fatigue life and the mean fatigue life levels and are suggested for inclusion in the revised Section 7 provisions. Multiple presence of trucks was found to have some influence on the loading used for fatigue evaluation. The primary factors involved were the ADTT level, the number of lanes available, and the bridge span length. WIM (weigh-in-motion) data with a high-resolution time stamp of 0.01 seconds from four different states and for different bridge configurations were used to study the effect of multiple trucks on various bridge structures. It was found that an equation involving the three predominant variables could be used to reasonably model multiple truck presence. Remaining fatigue life was found to have an undesirable connotation and it was believed that a new methodology to evaluate fatigue serviceability would be useful. Hence, a non- dimensional parameter, named the fatigue serviceability index, was developed to evaluate the condition and the assessment outcome with respect to fatigue. The method uses bridge age, predicted fatigue life, structural configuration, and bridge importance to determine the fatigue evaluation. When the bridge age exceeds the predicted fatigue life of a given bridge detail then the remain- ing life predicted in the current MBE Section 7 provisions gives a negative fatigue life. In the current Section 7 requirements, the user has the option to either reassess the life using new information, to assume a greater risk in the fatigue life estimation, or to retrofit the detail that has developed the problem. Using ânewâ information involves some additional effort and cost since better information such as WIM data or strain measurements are needed. Consequently, an additional option was developed to recalculate the cumulative frequency distribution based upon satisfactory performance with no observed fatigue cracking so that a positive remaining life would be produced at all times. The new option utilizes a modified frequency distribution with the same reliability factor as the original estimate. An experimental study was conducted to evaluate the fatigue strength of members with tack welds. A number of existing bridge structures, especially older riveted structures, have tack welds that were used for fit-up during construction and which were simply left in place. The tack welds are currently classified as Category E details in the LRFD Bridge Design Specifica- tion. This fatigue category provides a correspondingly low fatigue life prediction, which may require a costly retrofit or removal by grinding the tack welds off the primary structure when the bridge evaluation is performed. Consequently, cyclic tests were conducted to determine if the fatigue strength is indeed higher than Category E, since few data on tack welded members are available. A higher fatigue strength classification may remove the need for unnecessary
3 retrofits or repairs. The following observations were made based on the tack weld analysis and testing results: ⢠Finite element analysis indicates that the weld toe of the first line of tack welds experiences the maximum stress. Hence, it was expected that the weld toe of the first tack weld would be the critical location for fatigue due to the stress concentration at that location. This was confirmed through the fatigue testing, as all of the fatigue cracks that formed were observed to initiate at the weld toe of the first tack weld. ⢠Variations in the number of tack welds, length of tack welds, position of the tack weld relative to the fastener hole, and orientation of the tack welds relative to the load were all studied. The number, length, and orientation of the tack welds were not observed to significantly affect the fatigue strength of the tack welds. ⢠Based upon the results of seventeen cyclic tests, it was found that the cyclic strength of the tack welds all exceeded the mean value of the Category D curve and was closest to the mean fatigue strength for the Category C curve. Moreover, all of the fatigue test results exceeded the Category C fatigue design curve. Hence, it was concluded that the fatigue strength can be adequately modeled using a Category C design life as given per the LRFD Bridge Design Specification. Distortion-induced fatigue cracking of steel bridge web gap details was studied. A survey of state transportation officials was conducted to evaluate current fatigue inspection and evalua- tion procedures. The results of the survey revealed that distortion-induced fatigue cracking is the most frequently encountered type of fatigue distress observed by various state transportation agencies. Both softening and stiffening, in addition to hole drilling, were reported as methods being used to retrofit distortion-induced cracking. Both analytical modeling and experimental testing were used to evaluate the behavior of retrofits used to stiffen the connection and mitigate distortion-induced fatigue cracking. The following observations were made: ⢠Finite element analysis was used to study the stiffness and response of WT sections used for retrofit elements to mitigate distortion-induced cracking. The WT section is typically installed to bridge the gap between the vertical connection plate and the girder flange, with the WT flange attached to the girder flange and the WT web stem attached to the vertical connection plate or stiffener. It was found that increasing the thickness of the flange of the WT section is significantly more effective in controlling out-of-plane distortions than increasing the web thickness. ⢠Finite element analysis was conducted to analytically study forces developed in cross frame angle members of representative bridges since they frequently provide the out-of-plane forces that cause distortion of the girder web. It was found that out-of-plane deformations decreased significantly after a retrofit was installed, but the force in the brace was found to increase notably, often twofold or more for a given differential girder displacement. Predicting the out-of-plane force in the retrofit is difficult because it was found to be influenced by the dif- ferential deflection of adjacent girders, the size and length of the cross-brace members, the length of the girder web gap, the thickness of the girder web, and the geometry of the retrofit detail. These factors must be accounted for through refined analysis or field measurement if the retrofit forces are to be accurately assessed. Otherwise, a sufficiently stiff retrofit must be installed to minimize the distortion and transfer the out-of-plane force to the girder flange. ⢠Experimental testing was used to evaluate the cyclic performance of three retrofit details bolted to the girder flange and the vertical connection plate: WT sections, double angles, and single angles. Thirteen test specimens were used with variations in the retrofit thicknesses and web gap dimensions. It was observed that fatigue cracking initiated very quickly in the
4subcomponent girder web at the web gap for all girder sections tested when subjected to out-of-plane distortions. Although the cracks initiated quickly, growth slowed considerably as it propagated away from the web gap region due to softening of the connection. (After the retrofit, the fatigue cracks were not removed by hole drilling to permit evaluation while still in this most critical condition.) No subsequent fatigue cracks were observed to occur for any of the WT or double-angle retrofit details which were installed. The fatigue cracks left intact were observed to not grow further or to only grow by a small amount. A number of single- angle retrofits were observed to develop active fatigue cracks, but none of them failed after at least 5,000,000 additional loading cycles after retrofit. The cracking is believed to be due to a lack of symmetry and greater flexibility of the single-angle retrofit. ⢠If a stiffening retrofit is used, it is recommended that either a WT section or a pair of angles should be used if possible. Thicknesses greater than ½-in should be utilized to provide suf- ficient connection stiffness with at least four bolts used to connect the retrofit to the web and flanges. If single angles are used for the retrofit, then a relatively thick angle should be used. Also, retrofit holes should always be drilled to remove the crack tip of the distortion-induced fatigue cracks that have been detected.
