Fatigue Evaluation of Steel Bridges (2012)

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61 The conclusions and suggestions in this chapter are based upon an analysis and evaluation of data gathered in experimen- tal laboratory studies and of truck data gathered previously by various state transportation agencies. Also, an evaluation of existing fatigue analysis procedures was conducted and rec- ommendations for modifying those procedures were devel- oped through the incorporation of recent research results by others and the observations developed in this study. • Finite fatigue life predictions based upon the use of a curve to approximate the lifetime average ADTT based upon the present single-lane ADTT were found to lead to incon- sistencies 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 Project 12-51 by Fu et al. (2003) 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 mini- mum life. The values for RR were correspondingly recalcu- lated for the Evaluation Life and the mean life levels and are suggested for inclusion in the revised Section 7 provisions. • Presence of multiple trucks was found to have some influ- ence on the loading used for fatigue evaluation. The pri- mary factors involved were the ADTT level, the number of lanes available, and the bridge span length. WIM 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 reason- ably 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 service- ability index (FSI), was developed to evaluate the condi- tion and the assessment outcome with respect to fatigue. The method uses bridge age, predicted fatigue life, struc- tural 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 remaining life predicted in the current MBE Section 7 provisions gives a negative fatigue life. A method was developed to recalculate the cumula- tive frequency distribution based upon satisfactory perfor- mance with no observed fatigue cracking so that a positive remaining life would be produced at all times. The new method 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. – Finite element analysis indicated 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 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 orienta- tion of the tack welds were not observed to significantly affect the fatigue strength of the tack welds. – Based upon the results of 17 cyclic tests, it was found that the cyclic strength of the tack welds all exceeded the mean value of the Category D curve, were closest to the mean fatigue strength for the Category C curve, and exceeded the design fatigue strength for Category C. C h a p t e r 4 Conclusions

62 the fatigue cracks were not removed by hole drilling to permit evaluation while still in this most critical con- dition.) 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 ret- rofit. The cracking is believed to be due to a lack of sym- metry 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 uti- lized to provide sufficient 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, ret- rofit holes should always be drilled to remove the crack tip of the distortion-induced fatigue cracks that have been detected. Proposed Revisions to MBE Section 7 A number of revisions have been proposed to the provi- sions in Section 7 of the Manual of Bridge Evaluation. The suggested revisions are shown with a strike out through dele- tions to existing provisions and new underlining shown for new provisions. The new proposed provisions for Section 7 of the MBE are shown in Appendix E of this document. Moreover, examples to illustrate use of the proposed Section 7 provisions are pro- vided in Appendix F. Options for Design Consideration Based upon the results of this study for the evaluation of fatigue serviceability, a number of options for inclusion in the LRFD Bridge Design Specification may wish to be con- sidered. These are noted below. • Presence of multiple trucks, either side-by-side in adja- cent lanes or back-to-back in the same lane, may cause increased stresses that are not considered when a single lane of loading only is used for fatigue design. For most common bridge applications this factor is not very domi- nant and the single truck for a Fatigue II loading is quite adequate. However, for certain structures, such as two- girder bridges or trusses that support multiple lanes and with long span lengths and a high ADTT volume, then a multiple lane loading stress range amplification factor can 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. Both analytical modeling and experi- mental testing were used to evaluate the behavior of retrofits used to stiffen the connection and mitigate distortion- induced fatigue cracking. – Results of a survey conducted to evaluate current fatigue inspection and evaluation procedures revealed that distortion-induced fatigue cracking is the most fre- quently encountered type of fatigue distress observed among various state transportation agencies. Both soft- ening and stiffening, in addition to hole drilling, were reported as methods being used to retrofit distortion- induced cracking. – Finite element analysis was used to study the stiffness and response of WT sections used for retrofit elements to mitigate distortion-induced cracking. It was found that increasing the thickness of the flange of the WT section is more effective in controlling out-of-plane distortions than increasing the web thickness. – Finite element analysis was conducted to analytical study forces developed in cross frame angle members of repre- sentative 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 two- fold or more for a given differential girder displacement. Predicting the out-of-plane force in the retrofit is quite difficult because it was found to be influenced by several different factors, including the differential deflection of adjacent girders, the size and length of the cross-brace members, the size 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 crack- ing initiated very quickly for all girder sections tested when subjected to out-of-plane distortions. Although the cracks initiated quickly, growth slowed consider- ably as it propagated away from the web gap region due to softening of the connection. (After the retrofit,

63 become quite important. A multiple presence factor was developed for inclusion in MBE Section 7 to model this loading situation. • Tack welds are classified as Category E in the LRFD Bridge Design Specification. A number of fatigue tests were con- ducted in conjunction with this study and it was found that the cyclic lives were most closely aligned with the Category C mean fatigue strength. Although the Category E classifica- tion may be partially to discourage the practice of leaving tack welds in place after fabrication, it appears that the clas- sification can be improved to be no worse than Category D. • The finite fatigue life is based on an approximation when accounting for the growth of traffic volume by estimat- ing the ADTT in a single lane of traffic averaged over the design life. The LRFD commentary indicates that it may be best to consult with a traffic engineer. However, a closed form solution for incorporating traffic growth into the finite fatigue life equation was developed in conjunction with NCHRP Study 12-51 for existing bridges, and per- haps a procedure similar in approach should be developed for inclusion in the finite-life equation directly in Chapter 6 or the single lane ADTT expression in Chapter 3.