Recently, fatigue cracking has been observed in the field at a number of different structural details. In one instance, complete fracture of a tension flange followed fatigue crack growth at the toe of a transversely welded cover plate in a 13-year-old bridge. Subsequent inspection of 15 other cover-plate ends in the same bridge revealed that the two beams adjacent to the cracked member were also cracked through about one-half the flange thickness. Smaller fatigue cracks were detected at several other cover-plate ends. This report combines the results of two studies. NCHRP Project 12-15, "Detection and Repair of Fatigue Cracking in Highway Bridges," demonstrated that either peening the weld toe or applying a gas tungsten arc remelt process can be successful in improving fatigue strength in the laboratory. Two state-of-the-art reports completed as part of this study are included in Part II of this report. Existing methods of nondestructive inspection are described in Appendix A and their reliability and adaptability for detection of fatigue cracks in welded highway bridges are evaluated. Typical existing and currently designed welded bridge details are reviewed in Appendix B and those most susceptible to fatigue crack growth are classified. NCHRP Project 12-15(2), "Retrofitting Procedures for Fatigue-Damaged Full-Scale Welded Bridge Beams", included further work on peening and the gas tungsten arc remelt process; this study also (1) demonstrated the applicability of these methods in the field, (2) provided new test data on the low stress-range behaviour of full-size bridge beams, and (3) examined the fatigue strength of beams with cracks at the ends of transverse stiffeners. Recommendations based on the tests of full-size beams were used to modify the fatigue provisions of the AASHTO Standard Specifications for Highway Bridges during 1978. (Author/publisher)
Abstract