3421 Fatigue Resistance

The approach to designing and assessing structures for fatigue is empirical and is based on tests of full-scale members with welded or bolted details. Such tests indicate that

• The strength and type of steel have only a negligible effect on the fatigue resistance expected for a particular detail [6-10].

• The welding process also does not typically have an effect on the fatigue resistance [11,12].

• The primary effect of constant-amplitude fatigue loading can be accounted for in the live-load stress range [6-9], that is, the mean stress is not significant.

The independence of fatigue resistance from the type of steel greatly simplifies the development of design rules for fatigue since it eliminates the need to generate fatigue data for every type of structural steel. Testing that has been performed on welded details in stainless steel show that the fatigue strength of stainless steel details is comparable to that of ferritic structural steel details [13].

The reason that the dead load has little effect is that, locally, there are very high residual stresses. In details that are not welded, such as anchor rods, there is a mean stress effect [14]. A worst-case conservative assumption, that is, a high tensile mean stress, is made in the testing and in the design of these nonwelded details.

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