AISC LRFD 1.pdf

App. K3.] DESIGN FOR CYCLIC LOADING (FATIGUE) 121Since the shear rigidity of the web system of steel joists and trusses is less than thatof a solid plate, their moment of inertia shall be taken as 85 percent of their chords.K3. DESIGN FOR CYCLIC LOADING (FATIGUE)This appendix applies to members and connections subject to high cycle loadingwithin the elastic range of stresses of frequency and magnitude sufficient to initiatecracking and progressive failure (fatigue).1. GeneralThe provisions of this section apply to stresses calculated on the basis of unfactoredloads. The maximum permitted stress due to unfactored loads is 0.66F y .Stress range is defined as the magnitude of the change in stress due to theapplication or removal of the unfactored live load. In the case of a stress reversal, thestress range shall be computed as the numerical sum of maximum repeated tensileand compressive stresses or the numerical sum of maximum shearing stresses ofopposite direction at the point of probable crack initiation.In the case of complete-joint-penetration butt welds, the maximum design stressrange calculated by Equation A-K3.1 applies only to welds with internal soundnessmeeting the acceptance requirements of Section 6.12.2 or 6.13.2 of AWS D1.1.No evaluation of fatigue resistance is required if the live load stress range is lessthan the threshold stress range, F TH . See Table A-K3.1.No evaluation of fatigue resistance is required if the number of cycles of applicationof live load is less than 2 10 4 .The cyclic load resistance determined by the provisions of this appendix is applicableto structures with suitable corrosion protection or subject only to mildly corrosiveatmospheres, such as normal atmospheric conditions.The cyclic load resistance determined by the provisions of this appendix is applicableonly to structures subject to temperatures not exceeding 300°F (150°C).The Engineer of Record shall provide either complete details including weld sizesor shall specify the planned cycle life and the maximum range of moments, shearsand reactions for the connections.2. Calculation of Maximum Stresses and Stress RangesCalculated stresses shall be based upon elastic analysis. Stresses shall not be amplifiedby stress concentration factors for geometrical discontinuities.For bolts and threaded rods subject to axial tension, the calculated stresses shallinclude the effects of prying action, if any.In the case of axial stress combined with bending, the maximum stresses, of eachkind, shall be those determined for concurrent arrangements of the applied load.For members having symmetric cross sections, the fasteners and welds shall bearranged symmetrically about the axis of the member, or the total stresses includingthose due to eccentricity shall be included in the calculation of the stress range.For axially loaded angle members where the center of gravity of the connectingwelds lies between the line of the center of gravity of the angle cross section and theLRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION