Fatigue damage may occur to members supporting machinery, cranes, vehicles, and other mobile equipment. Such damage is not likely in members subject to few load changes or small stress fluctuations. For example, full design wind or seismic loads occur too infrequently to justify stress reductions for fatigue. Fatigue as a design consideration is affected by the magnitude of the stress range, the number of load cycles, and the severity of the stress concentration associated with a particular structural detail.
Stress range is defined as the magnitude of the change in stress (ignoring sign) due to the application or removal of the live load. Unfactored live loads and determination of stresses by elastic analysis is used for fatigue design in both LRFD and ASD specifications.
The stress range is calculated as the algebraic difference between minimum and maximum stress, or the numerical sum of maximum shearing stresses of opposite direction at the point of probable crack initiation.
The AISC LRFD Specification has a different approach to fatigue design compared to that of the ASD Specification. The LRFD specification stipulates that, if the stress range is less than the fatigue threshold stress, FTH, fatigue is not a design consideration. Also, fatigue need not be considered when the number of cycles of application of live load is less than 20,000.
Structural details are grouped into several stress categories, A through F, that represent increasing severity of stress concentration. Table 6.26 defines the stress categories, the fatigue threshold stress (maximum stress range for indefinite design life), FTH (ksi), and the fatigue constant, CÆ’ , used to calculate a design stress range for each category. See the AISC LRFD Specification for fatigue detail diagrams. They are generally similar to those in Table 6.28.
The range of stress at service loads must not exceed the design stress range (ksi) computed using Eqs. 6.72a through 6.72d, as applicable. For all stress categories except category F and category C’
The AISC ASD specification simply limits the stress range depending on the stress category and the number of loading cycles (Table 6.27). The stress category descriptions (Table 6.28) differ somewhat from those in Table 6.26. For increasing repetitions of load, the allowable stress ranges decrease.
Design of members to resist fatigue cannot be executed with the certainty with which members can be designed to resist static loading. However, it is often possible to reduce the magnitude of a stress concentration below the minimum value that will cause fatigue failure.
In general, avoid design details that cause severe stress concentrations or poor stress distribution. Provide gradual changes in section. Eliminate sharp corners and notches. Do not use details that create high localized constraint. Locate unavoidable stress raisers at points where fatigue conditions are the least severe. Place connections at points where stress is low and fatigue conditions are not severe. Provide structures with multiple load paths or redundant members, so that a fatigue crack in any one of the several primary members is not likely to cause collapse of the entire structure.