AISC LRFD 1.pdf

Comm. K3.] DESIGN FOR CYCLIC LOADING (FATIGUE) 277(1) Stress range and notch severity are the dominant stress variables for weldeddetails and beams.(2) Other variables such as minimum stress, mean stress, and maximum stress arenot significant for design purposes.(3) Structural steels with yield points of 36 (250) to 100 ksi (690 MPa) do notexhibit significantly different fatigue strength for given welded details fabricatedin the same manner.Because the design stress range may be readily calculated from the equation for themean curve minus two standard deviations of the actual test data using modern handcalculators or computers, the past method which relied upon multiple tables ofcycles of loading, stress categories, design stress ranges, and illustrative exampleshas been replaced by a single table (Table A-K3.1). In the new format, the situationdescription, the stress category, the ingredients for the applicable equation, instructiveinformation and pertinent illustrative examples are presented in separate cellsacross individual table rows. The sites of concern for potential crack initiation arepresent in text and in example sketches. Similar format and consistent criteria isbeing developed for the AWS Code and other Specifications.A detail not covered by earlier editions of the Specification has been added (Frankand Fisher, 1979) to cover tension-loaded plate elements connected at their end bytransverse groove or fillet welds in which there is more than a single site for the initiationof fatigue cracking, one of which will be more critical than the othersdepending upon welded joint type and size and material thickness. Regardless ofthe site within the joint at which potential crack initiation is considered, the designstress range provided is applicable to connected material at the toe of the weld.The fatigue resistance of bolts subject to tension is predictable in the absence of pretensionand prying action; and in this edition of the specification criteria are providedfor such non-pretensioned details as hanger rods and anchor rods. In the caseof pretensioned bolts, deformation of the connected parts through which pretensionis applied introduces prying action, the magnitude of which is not completely predictable(Kulak et al., 1987). The effects of prying are not limited to a change in theaverage axial tension on the bolt but includes bending in the threaded area under thenut. Because of the uncertainties, definitive criteria for calculating prying effectsand definitive criteria for design stress range are not included in the specification.To limit the uncertainties regarding prying action on the fatigue of pretensionedbolts in details which introduce prying, the design stress range provided in TableA-K3.1 is appropriate for extended cyclic loading only if the prying included in theapplied load is small.Non-pretensioned fasteners are not permitted under the Specification for jointssubject to cyclic shear forces. Bolts installed in joints meeting all the requirementsfor slip-critical connections survive unharmed when subject to cyclic shear stressessufficient to fracture the connected parts, for which criteria are provided in Section2 of Table A-K3.1.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION