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# Infinite and Finite Fatigue Checks

## Infinite and Finite Fatigue Checks

(OP)
My friends,

When should one consider Fatigue 1 and when Fatigue 2?. It is just not clear to me. Textbooks adopt different approaches and can be quite confusing. For the same ADDT(SL) of 6800 (200000x0.2x2x0.85 based on a 2 lane rural intersates simple span stel beam-slab), some texts will go ahead with Fatigue 2 and some will try Fatigue 1.

IJR

### RE: Infinite and Finite Fatigue Checks

Fatigue I relates to infinite life and maximum stress. It's independent of AADT. In theory, if you meet Fatigue I you're good for Fatigue II.

Fatigue II is based on AADT and finite life.

### RE: Infinite and Finite Fatigue Checks

(OP)
bridgebuster,

could you be more precise?. I understand from your statement and from what I have read that Fatigue II is the design criteria. It must always be checked. But if fatigue I, which is not a criteria, is satisfied, then Fatigue II is satisfied by default. Is that what you are saying?

IJR

### RE: Infinite and Finite Fatigue Checks

Too bad AASHTO can't be more precise. Here's my understanding: You have to design for Fatigue I and in theory your detail has 99.9999999999% chance of infinite life. Fatigue II is when you're designing for something other than HL-93, e.g., permit load or some other vehicle. In theory, the alternate load creates a situation where the detail may now have a finite life. Of course it's possible that the number of cycles which would create the finite life condition may never be reached.

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