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AISC Fatigue

AISC Fatigue

AISC Fatigue

Looking in the 13th & 14th edition of AISC, for the most part, it doesn't really matter what your yield strength is under fatigue loads. IOW, going from 36 ksi to 50 doesn't really help under most circumstances. But how about going to something like Grade 105? If you (for example) were in a situation like Case 8.5(a) (see table A-3.1, p.16.1-177 in the 13th edition)......I would think you would have a somewhat higher threshold stress than 7 ksi.

Does anyone know of a way to deal with it.....or are you stuck trying to hunt down S-N curves outside the code.


RE: AISC Fatigue

For higher grade bolts you can refer to the RCSC for more specific fatigue data. Some bolt manufacturers like Unbrako will have better more specific data also. The AISC category for 8.5 is very generic as it is applied to a wide range of threaded fasteners. Incidentally, if you pretension bolts and account for the pretensioning reduction effect on the calculated fatigue stress range, you will often have a low stress range and will not need the higher threshold.

In general using the AISC nominal stress approach, the material ultimate strength will not come into play in calculating fatigue resistance. Ultimate is used for fatigue rather than yield. You can use more detailed methods for endurance (fatigue) limits such as those found in a mechanical engineering text like Shigley. These apply to non-welded material.

Ultimate strength does not come into play at all for welded connections for the weld throat and HAZ material. Using AISC/AWS fatigue details is good in this case. If you get into more advanced methods of calculating fatigue resistance such as Effective Notch Stress or fracture mechanics then you may find the resistance is affected by the ultimate strength or other material properties. These two approaches are seldom conducive for practical engineering business/project needs.

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