AISC Appedix 3

[toluun]

I had a question about equation A-3-1 in AISC Appendix 3. The formula uses a value of nSR, which is defined as number of stress range fluctuations in design life. Is this the total number of cycles that occur on the bridge over all stress ranges and its design life? Or is it the number of cycles that occur at the stress range Fsr. I am just a little confused.


I can provide the data I am looking at if need be.

Thanks for your time!!

 

[ishvaaag]

In my opinion, given this as the sole number for "stress range fluctuations in design life" you in some way need to get to this number. Maybe you can ascertain (by whatever the methods) not only one nSR, but several, nSRi, referring to stress rantes SRi. That is you may technically have come out to that the structure along its life will support about 500 times at the support 50 MPa stress range, but you also see that it likely will have to stand 30 MPa 20000 times. These numbers are easier to get if trains or trucks passing, somewhat more difficult if snow loadings, and even more if wind actions. Hence see where we are. But in my view AISC, at least in the words, is not asking for cumulative damage effects of daily fluctuations to get an input number for nSR, it is enough that you use a number justifiable in whatever the way, that could include that or no. If you are able to join the data in a comprehensive check for just a nSR at a given SR through any kind of cumulative damage analysis (or even just accepting it from the logic of the problem or reputable source) then you are done for checks starting with just such nSR and SR.

So, in my view, not ALL numbers of cycles corresponding to ALL ranges of stress enter nSR. As for checks only concomitant actions need be checked, in fatigue, only the number of times a given level of stress is ranged is counted (or one equivalent number if you integrate the effects for that).

 

[ishvaaag]

erratas

rantes in 2nd line is ranges
3d line suppress after 500 times "at the support"

 

[JAE]

The way I interpret it - if you are designing a portion of a structure that undergoes tension when the structure is loaded, you then have to limit the stress in that member, under any of the future load applications, to less than F_SR. The number n_SR is the number of load applications, whether those loads get you all the way to F_SR or not.

Example:
You check tension on a flange of a bridge beam. Say the beam experiences 2000 loadings per day of vehicles....i.e. 2000 times per day the flange goes into tension above its static tension.
The life span of the bridge is assumed at 50 years.
The value of n_SR would be 2000 x 365 x 50 = 36,500,000.

Now not all of those loadings will be the same - some small cars, heavier trucks, etc. But each load application that causes tension would be included in the number.

You then determine F_SR from the equation and then design a beam flange to have stress lower than that prescribed amount.

 

[ishvaaag]

I agree with JAE in that in such way you would be in agreement with AISC. I only pointed as well that you could reduce all the checks to just one at the worst stress range per item for a number higher that resumes the cumulative damage of all other cycles at lower stress range. So in my quote, for example you only would check once at the 50 MPa stress range but the number of cycles might convert from 500 to 580 after considering the cumulative damage in fatigue that the lower cycles cause; it is a matter of searching for such procedure in fatigue related books. In doubt, the simple way of JAE works and keeps tight to the code.

 

[toluun]

Hey guys

Thanks so much for your help!

I was hoping you guys could help me with another aspect of fatigue....the fatigue categories specified by AISC. I trying to find the detail shown by the attached picture I guess I am torn between category D and E. This is mainly because I am not 100% certain on what defines a pin plate.

Also it should be noted that the connections in the picture are riveted.