1474Load Induced Fatigue

According to a parametric study reported by Horton et al. (2000, 2003), the design of welded plate girders made entirely from HPS70W or hybrid HPS70W and conventional (Grade 50W) steel is more likely to be controlled by load-induced fatigue when compared to those made entirely of conventional steel (Grade 50W). Using detail category C' and C (AASHTO 2000b), and assuming full composite action in both positive and negative bending regions as well as a lower bound (conservative) fatigue limit equal to one-half the constant amplitude fatigue threshold value for all designs, Horton et al. concluded that load-induced fatigue often governed the design of HPS welded girders that were less than 150 ft (45.8 m) in length. For span length equal to or exceeding 200 ft (61 m), the effect of load-induced fatigue becomes negligible. The reason load-induced fatigue governs the design of shorter span but not longer span girders is because dead-load to total stress ratio increases with span length. As the dead-load to total stress ratio increases, the corresponding live-load to total stress ratio will decrease, thus rendering a lower stress range for live-load stress and less pronounced fatigue effects. It should be pointed out that the study reported by Horton et al. was conducted using a rather conservative fatigue limit that ignored the effect of average daily truck traffic on the value of the fatigue limit. The actual volume of truck traffic used for the design of a particular bridge may result in an increase in the allowable fatigue limit and thus reduce or even eliminate the effect of load-induced fatigue on the design of the bridge.

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