Fatigue behavior of welded connections enhanced with UIT and bolting

Abstract

A common problem in bridges employing welded steel girders is development of fatigue cracks at the ends of girder coverplates. Fatigue cracks tend to form at the toes of the transverse welds connecting a coverplate to a girder flange since this detail has a region of very high stress concentration. Because many aging bridges employ these fatigue-prone, AASHTO fatigue Category E or E' details, a means to effectively enhance the fatigue lives of these details is being sought. A research project funded by the Kansas Department of Transportation was undertaken at the University of Kansas to investigate the fatigue life enhancement afforded by two retrofit methods. One retrofit method was similar to a method described in the AASTHO Bridge Design Specification (AASHTO 2004) and involved pretensioned bolts being added to the ends of coverplates near the transverse welds. Unlike the AASHTO bolting procedure, the modified bolting procedure studied during this project utilized coverplates having transverse fillet welds that were left in the as-fabricated state. The other retrofit method was the use of a proprietary needle peening procedure called Ultrasonic Impact Treatment (UIT). Results of the research project showed that UIT was highly effective at enhancing the fatigue lives of coverplate end details while the bolting procedure was ineffective. Weld treatment with UIT resulted in an improvement in fatigue life over control specimens by a factor of 25. This translated in an improvement from an AASTHO fatigue Category E detail rating to and AASHTO fatigue Category A detail rating. The modified coverplate bolting procedure tested during this project had either no effect on fatigue life or, in some cases, had a detrimental effect on fatigue life. The coverplate bolting procedure included in the AASHTO specification allows a coverplate end detail to achieve a fatigue Category B resistance when bolted rather than transversely welded. Therefore, the modified bolting procedure tested during this project was much less effective at enhancing fatigue life than either the AASHTO bolting procedure or UIT.