FATIGUE BEHAVIOR OF VARIABLE LOADED BRIDGE DETAILS NEAR THE FATIGUE LIMIT

Abstract

The findings of the current NCHRP Project 12-15(5), Behavior of Variable Loaded Details Near the Fatigue Limit, are highlighted. The main focus of the research is the examination of welded bridge details in the high-cycle, long-life regime. Large-scale plate girders with coverplate, web attachment, and web stiffener details are subjected to fatigue loading that simulates actual truck traffic. A Rayleigh type stress spectrum is used with the inclusion of an occasional overload exceeding the constant-amplitude fatigue limit. The frequencies of occurrence being considered for the overloads are 0.1, 0.05, and 0.01 percent. Prior research indicated that fatigue crack propagation occurred even when the effective stress range was below the constant-amplitude fatigue limit and the exceedance rate of the limit was as low as 0.1 percent. The current test specimens also allow for a detailed study of distortion-induced fatigue cracking at a connection plate web gap detail. Results indicate that the retrofit method of drilled holes at the crack tip is inadequate at high levels of distortioon. In addition to the experimental work, a review of fatigue test data generated around the world since the AASHTO fatigue provisions were adopted in 1974 was completed. This study has allowed for a reassessment of the provisions. A summary of the proposed revisions to the specifications is given.