Specifications and Design Relations

During the last 35 yr (1930's) extensive laboratory fatigue data have been accumulated for numerous types of structural members, connections and materials. These data have provided the basis on which fatigue design provisions have been developed for the principal design specifications for bridges. In the development of these provisions: (1) The expected life of the structure (number of cycles of loading); (2) the stress ratio (ratio of minimum stress to maximum stress); (3) the type of members, details and fabrication; and (4) the type of material in the structure have been taken into account. Although these fatigue provisions provide for improved designs, much still remains to be done to better relate bridge designs to loading conditions in the structures and to the behavior of the structures under such loading conditions.