Fatigue of Welded Steel Bridge Members Under Variable-Amplitude Loadings

A project is reported the objective of which is to acquire fatigue data on welded bridge members under variable-amplitude random-sequence stress spectrums, such as occur in actual bridges, and to develop an analytical method of predicting variable-amplitude fatigue behavior from constant-amplitude fatigue data. Pertinent past work was studied with particular emphasis on field mearurements of stresses in bridges under traffic. A theoretical study was made to predict, from various hypothese, the fatigue behavior of the small specimens and beams being tested during the program. A program of variable-and constant- amplitude fatigue tests of small specimens was performed to obtain crack growth data and to determine the effects of various stress- spectrum parameters on fatigue life. A program of variable- and constant-amplitude fatigue tests of relatively large beams of various steels was also performed. The results were evaluated, and methods of utilizing the results for design and specification purposes were developed. Field stress measurements of bridges under traffic were collected and evaluated. Distributions were then selected for the variable amplitude stress spectra to be used in the experimentation. An experimental program was also planned on small plate specimens and large beam specimens. Details of the experimental program are outlined. The project findings are discussed and the test results are reviewed. The results of the study provide a link between the type of variable-amplitude random-sequence fatigue loadings that actually occur on bridges and the constant-amplitude fatigue data and allowable stress values that are commonly used in designing such bridges. Applications of the findings are reviewed.