Assessment of Bridge Remaining Fatigue Life through Field Strain Measurement

With the aging of existing steel bridges and the accumulated stress cycles under traffic loads, assessment of remaining fatigue life for continuing services has become more important than ever, especially for decisions on structure replacement, deck replacement or other major retrofits. Experiences from engineering practices have indicated that fatigue analysis based on specification loads and distribution factors usually underestimates the remaining fatigue life of existing bridges by overestimating the live load stress ranges. Fatigue evaluation based on field measured stress range histograms under actual traffic load proves to be a more accurate and efficient method for existing bridges. This paper describes the application of such method in assessing the remaining fatigue life of bridge structures. Current AASHTO specifications for fatigue evaluation of existing bridges are reviewed and compared. Case studies of three major highway bridges are discussed. Finally a procedure for evaluating the fatigue life of existing bridges based on case-specific field strain measurement is proposed for practicing engineers and bridge owners.