Numerous steel highway bridges were built during the construction boom between the late 1950s and late1970s and are still in use today. Fatigue cracking can be considered as a main source of deterioration forthese bridges. It is reported that the largest category of observed fatigue cracks are those caused by out-ofplanedistortion. Locations where transverse structural components are framed into longitudinal girdersthrough web stiffeners not attached to the flanges (such as diaphragms or cross-frames) are mostsusceptible. In the current study, a web stiffener detail is fatigue tested under different cyclic loadingconditions. As-welded specimens are tested, along with specimens retrofitted by grinding and rewelding,post-weld treatment (i.e. needle peening), or using adhesively bonded fiber reinforced polymer (FRP)attachments. Direct strain and deflection measurements are compared with finite element (FE) analysispredictions, and local (hot-spot) stresses are compared with hot-spot stress design curves. A time-seriesbased damage detection method is also explored for fatigue crack depth prediction using strain data. Themethod is validated using small- and large-scale specimen strain data. It is found that damage measuresbased on strain measurements in the vicinity of the critical hot-spot are closely correlated with the truecrack depth.
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