allowable compressive stress under bolt head 1

[StevePAA]

when doing an FEA that includes clamping forces for fasteners in a bolted joint, what is the allowable stress for the area under the bolt head that's subject to the clamping force? When you have a joint with large diameter fasteners, and allow for a ~20000 lbf clamp load you can see higher than typically allowable stresses (>.6Sy) under the bolt heads just due to the clamping force, w/o any external loading.

So, assuming these stresses are away from the edge of the part, and there's no fatigue loading, what's an "industry" acceptable value for these stresses under the bolt heads? By this I mean referenced to some sort of code, from a reliable source, etc- something that I can quote.

I have an analysis report someone else did that uses a value of .9Sy for this, but I'm not sure where this came from. I've tried googling, but bearing stress is typically referenced to shear so maybe I'm not using the correct terminology?

Thanks In Advance!

 

[corus]

Check in the UK structural standard BS 5950, referred to here

http://www.scribd.com/doc/8163901/P325-Introduction-to-Steelwork-Design-to-BS-595012000

and search for bearing stress or table 11.2


corus

 

[CoryPad]

You are in an area that isn't widely standardized. VDI 2230 has the best information on the subject. Synonymous names to bearing stress are surface pressure and contact stress.

The idea is to limit plastic deformation during external loading of the joint because that can lead to lost clamping force. Thus, a stress just less than the yield stress is a good estimate. Note that surface pressure is non-uniform and that it is not equal to a von Mises-style equivalent stress. Usually the allowable surface pressure is greater than the material allowable stress (von Mises, other).

 

[StevePAA]

Thanks for the answers.

Corus- the structural codes don't deal with this from what I've seen. Their definition of bearing stress is the material outboard of the bolt hole tearing away due to transverse loading (wrt to the bolt axis). What I am talking about is different- the contact pressure directly under the bolt head from just tightening the bolt.

CoryPad. Yes, I agree with what you've said and this has been my approach. In my case, the only additional loading on the bolts that will occur is in the event of an "accident". So, if that happens, and there are a few thousandths of an inch of additional yield under the bolt heads, that will be ok as the machine will be unoperable (accident= stopped machine) and thus no bolts will unexpectedly back out over time. The idea is to just keep everything contained in the event of an accident and it will.

For the analysis, when I apply a conservative estimate of the clamping force for a 5/8" gr 8 bolt, with no other loading, I am at a peak Von Mises of ~25ksi on the part under the bolt heads; this is ~.70Sy with no other loading; so, if you assume .60Sy allowable per ASD, then it's obviously exceeded. From what I've seen, this isn't directly taken into account in structural, although I suppose it is indirectly dealt with by having standards for bolt holes from the edge and from each other.

Thanks again, and if anyone has more info, keep it coming.

 

[rb1957]

isn't this a bearing problem (rather than a compressive stress) ?