Variable-Amplitude Fatigue Strength of Structural Steel Bridge Details: Review and Simplified Model

This paper reviews eight previous studies on the variable-amplitude (VA) fatigue strength of structural steel details: (1) four studies in the finite-life regime in which the number of cycles to failure of the specimens are equal to or shorter than the number of cycles at the intersection between the sloped S-N line and the VA fatigue limit, hereafter called the transition life; and (2) four studies in the infinite-life regime that is, at numbers of cycles greater than the transition life. The VA data correlate well with the constant-amplitude data when the former are plotted in terms of an equivalent root-mean-cube stress range. The effects of the following variables on the fatigue strength of structural details are discussed: block sequence in a stress range spectrum, spectrum size, type of spectrum, minimum stress, and type of steel. A so-called long-life factor quantifies how far a type of detail was cycled into the finite-life regime. Based on the results of the literature review, the writers recommend that the current AASHTO log-log bilinear equations for calculating the fatigue life be replaced with a single equation similar to the equation for predicting the fatigue crack growth rate in metals. This simplified model more accurately predicts the fatigue life near the VA fatigue limit.