AISC Fatigue Design - Which Category?

[bootlegend]

I am looking at a bulk material storage bin and thinking about the many cycles of loading and unloading. Suppose I have simple span floor beams with a "floor" plate bolted to the top flange of the beams. What case would be appropriate for the fatigue checks on the beam at the bolt location? Table A-3.1 (15th edition) category 1.5 is for members with drilled or reamed holes but only lists two options: pretensioned bolts, with a threshold stress of 10 ksi, or empty holes with a threshold stress of 7 ksi. I'm planning on non-pretensioned bolts. I can't imagine a snug-tight bolt would be a worse condition than the open hole. Thoughts?

 

[r13]

IMO, you don't have fatigue problem, as your loading is rather static and non-cyclic.

 

[phamENG]

There's no option for non-pretensioned bolts because, in fatigue applications, pretension is really the only good bolting option. If you really have fatigue loading on the joint, then the constant stress fluctuation/reversals are going to loosen a bolt that isn't pretensioned (in most situations, anyway).

Do you really have a fatigue situation, though? How often will this thing be filled and emptied? What's the design service life?

 

[bootlegend]

The bin could see 1,000,000 cycles over 30 years. Probably wouldn't completely empty every cycle so the stress range might be less than I'm using. I was more interested in thoughts on why no snug-tight option was listed. I think phamENG has it. I'm not using the bolts to carry the cyclical load in this case (except to brace the beam flange which I don't want the bolts to loosen for that either).

Thanks!

 

[Agent666]

Is this scenario really a stress reversal from tension to compressive stresses though. Sounds more like a stress range possibly wholly in either the tension or compression range if emptying/filling over time. My understanding of fatigue is you predominantly need a stress reversal over and above the minimum values for consideration and a minimum number of cycles over some limit.

Edit - By stress reversal I mean cycles into the tension and compression stress ranges.

 

[phamENG]

bootlegend - be careful. If this bin is going to vibrate a lot, pretension may still be a good idea as the snug tight nuts could still back off under high frequency vibrations.

Agent - as I understand it, cycles across the T/C line are the worst case, but even oscillations within tension or compression can still induce a fatigue failure if the magnitude of the stress variation is high enough.