AISC LRFD 1.pdf

276Comm. A-KAPPENDIX KCONCENTRATED FORCES, PONDING, AND FATIGUEK3. DESIGN FOR CYCLIC LOADING (FATIGUE)Because most members in building frames are not subject to a large enough numberof cycles of full design stress application to require design for fatigue, the provisionscovering such designs have been placed in Appendix K3.When fatigue is a design consideration, its severity is most significantly affected bythe number of load applications, the magnitude of the stress range, and the severityof the stress concentrations associated with the particular details. These factors arenot encountered in normal building designs; however, when encountered and whenfatigue is of concern, all provisions of Appendix K3 must be satisfied.In general, members or connections subject to less than a few thousand cycles ofloading will not constitute a fatigue condition except possibly for cases involvingfull reversal of loading and particularly sensitive categories of details. This isbecause the admissible static design stress range will be limited by the admissiblestatic design stress. At low levels of cyclic tensile stress, a point is reached wherethe stress range is so low that fatigue cracking will not initiate regardless of thenumber of cycles of loading. This level of stress is defined as the fatigue threshold,F TH . .Fluctuation in stress which does not involve tensile stress does not cause crackpropagation and is not considered to be a fatigue situation. On the other hand, in elementsof members subject solely to calculated compressive stress, fatigue cracksmay initiate in regions of high tensile residual stress. In such situations, the cracksgenerally do not propagate beyond the region of the residual tensile stress, becausethe residual stress is relieved by the crack. For this reason, stress ranges that arecompletely in compression need not be investigated for fatigue. For cases involvingcyclic reversal of stress the calculated stress range must be taken as the sum of thecompressive stress and the tensile stress caused by different direction or pattern ofapplied live load.When fabrication details involving more than one category occur at the same locationin a member, the stress range at that location must be limited to that of the mostrestrictive category. By locating notch-producing fabrication details in regions subjectto a small range of stress, the need for a member larger than required by staticloading will often be eliminated.Extensive test programs (Fisher, Frank, Hirt, and McNamee, 1970, and Fisher,Albrecht, Yen, and Klingerman, 1974) using full size specimens, substantiated bytheoretical stress analysis, have confirmed the following general conclusions:LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION