Fatigue Analysis of the I-75 Bridge over the Kentucky River at Clays Ferry

Fatigue analysis was performed on AASHTO category E welded connections on the southbound I 75 bridge over the Kentucky River at Clays Ferry, Kentucky. That analysis was based on the stress-range histogram data provided by Law Engineering of Louisville, Kentucky. That data were obtained from strain gages installed at 6 test locations on the downstream truss. The fatigue analyses consisted of safe-life and fatigue-crack growth analyses. Safe-life predictions were based on AASHTO fatigue design (SN) curves. To use those curves, equivalent constant-amplitude stresses were derived from the stress histograms. Those stresses and loading frequencies were modified to reflect anticipated increases in traffic volume and loading over the life of the structure by appropriate multiplicative adjustment factors. Four different methods of load prediction were used with combinations of the stress summing methods, total traffic, and truck traffic. In the majority of cases, the safe-life estimates exceeded 50 years. One overly conservative load-prediction method provided safe-life estimates as low as 15 years. Fatigue-crack growth analyses were performed at each test location using iterative crack-growth calculations by computer. The software program employed for that purpose required assumptions regarding initial and final size of the crack, crack geometry, and material properties. Analyses of hypothetical cracks at each test location was performed assuming a 1-inch initial size and a 6-inch final size. The fatigue-crack growth analyses predicted very slow growth of the cracks over that crack-length interval. For the present loading rates, those crack-length interval growth rates exceeded 350 years at all test locations. The fatigue analyses indicated that the six test locations (gusset connections) were in a reasonably reliable condition from a fatigue standpoint to allow their continued use in the bridge over the next 50 years. Supplemental inspections, analyses and, possibly, gusset retrofits are warranted if the truss is to be retained in the new bridge. Existing cracks in the gusset connections should be repaired to preclude further crack growth