Civil Engineering Association

Probabilistic Fatigue Life Estimation of Steel Bridges by Using a Bilinear S-N Approach

This paper focuses on estimating the fatigue life below the constant amplitude fatigue threshold (CAFT) of steel bridges by using a
probabilistic approach on the basis of a bilinear stress life (i.e., the S-N approach). The current AASHTO S-N approach uses a single S-N line
for predicting the fatigue life. However, because of the variation of actual applied live-load stress cycles, this approach very often results in a
severe underestimation of the useful life of structures. It implies that fatigue damage in respective structural steel details may be overestimated.
To improve fatigue life estimation, a bilinear S-N approach is integrated into a probabilistic framework that can model the uncertainties
associated with the fatigue deterioration process. In this approach, the equivalent stress range is computed by using two S-N slopes and
several probability density functions associated with stress ranges. These probabilistic functions are determined on the basis of stress-range
bin histograms from long-term monitoring. An existing bridge that is expected to experience finite fatigue life is used to illustrate the application
of the proposed approach.