Fatigue and Remaining Life Assessment of Steel Bridges More than 50 Years Old

The present bridge network in Quebec includes approximately 9000 bridges of different types and represents a 9 billion dollars asset. Many bridges show signs of deterioration well before the end of the design life, supposedly between 50 to 100 years. Truck loading has increased by more than 40% in the past 40 years, and bridges have not been reinforced in order to comply with the new codes. The accumulated fatigue and remaining life of highway bridges will be reviewed in this paper. Railroad bridges are excluded because their loading analysis uses a different approach. The condition of steel bridges in full service for more than 50 years, which is the average age of most of bridges in operation in Canada, must be evaluated in a realistic way. Fatigue, severe climatic conditions such as temperature difference between hot and cold seasons, de-icing salt and corrosion are deterioration factors. At low temperatures, steel looses its ductility, thus increasing the risk of brittle fracture. Steel used 50 years ago was poorly deoxidized during pouring, and consequently fatigue cracks can propagate more easily through the impurities.