allowable fatigue stress 2

[dgkhan]

Suppose I have a beam in which actual bending stresses are 33 ksi. It is grade 50, compact, braced @ < Lc, hence it is OK. What will be allowable fatigue stress in it. Let load cycle be > 2,000,000

 

[JAE]

Fatigue max. allowable stress depends on the type of stress you are looking at. For basic flexural stress in a beam, per AISC (ASD 9th Edition) you'd use the example diagrams and find stress example 2 applicable for stress in plain material.

Per Table A-K4.2 you'd find you have stress category A.

For 2,000,000 cycles you have Loading Condition 4 found in table A-K4.1.

For Loading Condition 4 you have a max. stress range = 24 ksi per Table A-K4.3.

 

[dgkhan]

Thanks JAE, I hope it was more easy i.e. simply reduce more allowable stress to cater for fatigue.

 

[nutte]

dgkhan, it is slightly more complex than you had hoped. Also keep in mind that the value that JAE calculated is the stress range, not the actual stress. If your beam is always loaded at 33 ksi (unlikely, if it has any live load), your stress range would be zero. If its maximum stress is 33 ksi tension, minimum 10 ksi tension, your stress range would be 23 ksi. And finally, if it went into load reversal, say 33 ksi tension max, 10 ksi compression min, your stress range is 43 ksi.

I recently performed some fatigue design checks. The connection material checks were by far the most time consuming. You need to figure your actual stress ranges and then check each component for each applicable portion of the table in the fatigue section of the AISC specification.

 

[dgkhan]

Thanks nutte. Your explanation was very clear. Looks like it's altogether a different angle to look on each component of frame unless it is similar and figure out fatigue stresses, and to me connections becomes controlling. So normally in building design I do not design connections. But if fatigue is a concern, than it is my baby too.