Pma

where N is the number of stress cycles, S is the nominal stress range, and A is a constant particular to the detail category, as given in Table 34.1.

AISC and AWS also have a Category F with a slope greater than 3 for checking the shear stress in the throat of welds. AASHTO and CSA S16 require checking the shear stress in the throat of welds according to Category E rather than Category F. However, the minimum weld size requirements and the weld strength requirements generally will ensure sufficient weld throat to avoid fatigue. There are few, if any, documented fatigue cracking cases associated with shear stress through the throat of fillet welds, therefore it is unnecessary to check the shear stress range in the throat of welds for fatigue and this will not be discussed further.

Eurocode 3 (EC3)' [20] and the British Standard 7608 [21] also use a nominal stress approach, but they each have unique sets of S-N curves with different category labels. However, the end result of checking the stress range will be approximately the same regardless of which of these sets of S-N curves is used, since all of these design specifications are empirical and are calibrated with the same database of full-scale test results. None of these sets of S-N curves has any inherent advantages or will estimate the fatigue life of a detail more accurately than any other set of S-N curves. Seven of the fourteen Eurocode curves are the same as the seven AASHTO curves, as indicated in Table 34.1. The American Bureau of Shipping (ABS) guidelines [22], the U.K. Health and Safety Executive [23], and other groups in the marine industry use S-N curves from the British Standards (BS 7608) [21]. The BS 7608 S-N curves can be associated approximately with the AASHTO S-N curves so that knowledge of the fatigue strength of details in this specification can be translated to an equivalent AASHTO category. Table 34.1 cross-references the BS 7608 S-N curves with the nearest AASHTO S-N curves.

In the nominal stress range approach, each detail category has a constant-amplitude fatigue limit (CAFL), which is also given in Table 34.1. The CAFL is a stress range below which no fatigue cracks occurred in tests conducted with constant-amplitude loading. These limits are shown as the horizontal lines on the right side of the S-N curves in Figure 34.1. The Eurocode and BS S-N curves define the fatigue limits differently.

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