Threshold Fatigue Stress Range, Factor of Safety

[jmggks]

The AISC code is pretty clear about what factors of safety are included for strength checks. Does anyone know if the fatigue threshold stress range in Appendix 3 includes any safety factor? If not, how much safety factor is appropriate if your objective is unlimited fatigue life?

I am working on a structure that supports an automated crane system for plating. I plan to use a fatigue design load that should occur infrequently but is possible. A load equal to 80% of my design load is likely every 4.5 minutes, and a load equal to about 55% of my design load is likely every minute.

Thanks for your comments.

 

[jmggks]

Just found this in AISC Design Guide 7:

Fatigue provisions have a 95 percent reliability factor (two standard deviations below mean curve of test data) for a given stress range, and expected life condition. Thus, it is
reasonable to expect that 5 percent of similar details can experience fatigue failure before the expected fatigue life is expired.

 

[dhengr]

Jmggks:
So, now, the trick is to pick the perfect detail, which exactly fits the testing; never to load it outside of the testing regime, and to do perfect fabricating, so as not to introduce any unwanted defects. Then, cross your fingers that the plating environment doesn’t eat it up in three years. We should always be cognizant of the potential for fatigue problems in our design. But, a little more attention to conservative design and stress values, good clean load paths, details without any stress raisers, details which are easy to fab., good clean welding, etc. usually buy you much more than a bunch of fracture mechanics calcs.