FATIGUE CRACKING IN WELDED STEEL BRIDGES

A number of localized failures have developed in welded steel bridge components because of fatigue crack propagation, which in some instances has led to brittle fracture. Increases in the number of trucks and allowable weights has led to a rapid accumulation of loading cycles. A large number of structures experiencing cracking have welded details that have been identified as susceptible to fatigue crack propagation only after they were built. Oversimplification of member interactions and connection behavior allowed by design codes has resulted in a large number of cases of distortion-induced fatigue cracking. Many of the structurally deficient bridges are over 50 years old. Environmental corrosion has built up over time and has caused increasing amounts of damage. As the infrastructure ages and the costs for new construction escalate, maintenance and rehabilitation are becoming increasingly important to the continued operation of the transportation system. Existing bridge structures have been fabricated under a wide variety of practices and operate in a full spectrum of service conditions. As such, no single set of guidelines can adequately ensure the safety and reliability of all the existing structures. Periodic inspection for accumulated damage and deterioration is critical to acceptable operation of the nation's bridges. Continued technology transfer and education of engineers and qualified inspectors will increase the effectiveness of the inspection and maintenance process. Typical types of fatigue damage found in welded steel bridges are reviewed, specific examples are cited, common retrofit procedures are examined, and proper investigation practices are outlined.