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FATIGUE CRACKING IN LONGER SPAN BRIDGES

Long-span bridges, with the exception of the Silver Bridge failure at Point Pleasant, West Virginia have not exhibited serious problems with fatigue cracking and fracture. On occasion a stringer or floor beam has experienced cracking, which is a maintenance problem. When a main girder has cracked, the redundancy of the structure prevents collapse. Obviously, any cracked member is costly to repair and often restricts traffic until the repair has been completed. In general, the fatigue-crack problems that have developed in longer span structures are not significantly different from those experienced with shorter spans. Cracking has occurred at poor details, at large weld flaws, and from secondary and displacement-induced stresses. In this paper we present three case studies to examine the actual behavior of several large structures where fatigue cracking developed. One case examined is the cracking in stringer webs of a suspension bridge from out-of-plane displacement at floor beam connections. Strain measurements were obtained in order to assist with determining the cause of the cracking and to model the appropriate response. Further investigations were carried out on differently modified test bays in order to determine if the structural response could be altered so that further crack growth could be prevented or other courses or remedial action undertaken. The final two case studies examine the cracking at details where large lack-of-fusion regions have resulted and permitted fatigue-crack growth to occur. In one case, lack-of-fusion conditions at groove-welded splices in longitudinal stiffeners were investigated. In the second case, similar undesirable conditions existed at lateral bracing connection plates that were welded to girder webs and transverse stiffeners. These conditions permitted fatigue-crack growth to occur in the transverse connection of the lateral plate and stiffener and into the main girder web. The actual live-load stress range in longer span structures is generally of lesser magnitude and the frequency of occurrence is not as great as observed in shorter span bridges. Hence, it often takes longer for crack growth to develop in longer spans. Nevertheless, performance and behavior will eventually be impaired if fatigue-crack growth develops. Attention to details is shown to be of paramount importance in all bridge structures if fatigue and fracture are to be prevented.