# A Fatigue Question?



## jascia1919 (Sep 22, 2007)

If I know for sure a steel member and its connection is under compression. Eg. Steel Arch splice full pen weld.

Live load will only change the magnitude of compression but not the sign.

Do I still need to check the fatigue as per AASHTO or other codes.

It is a real question I have from my job. I am thinking theoretically I should check. But actually it won't control.

Thanks.


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## MA_PE (Sep 23, 2007)

jascia1919 said:


> If I know for sure a steel member and its connection is under compression. Eg. Steel Arch splice full pen weld.
> Live load will only change the magnitude of compression but not the sign.
> 
> Do I still need to check the fatigue as per AASHTO or other codes.
> ...


yes. I need to revisit the code to get the numbers but I recall there that fatigue limits the stress range due to live load regardless if it reverses sign or not.


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## awesomepogi (Oct 28, 2008)

Fatigue involves stress range &amp; not just stress reversal per se. Just get the absolute value of the difference between max &amp; min stresses &amp; compare that range to the allowable.

If you're using SN curves, the allowable stress range is specified for corresponding fatigue stress application vs. no. of cycles.

Appendix 3 of AISC 13th ed. deals with fatigue design


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