Does bolt pre-tension come into play with AISC Eq J3-3a? 1

[JAE]

The specification J3.7 in AISC (I'm looking at the thirteenth edition) has a forumla for available tensile strength in a bolt when it is also subjected to shear.

The commentary uses this phrase [blue]"Tests have shown that the strength of bearing fasteners subject to combined shear and tension resulting from externally applied forces..."[/blue]

We have a bolted connection where there is only shear. However, these bolts are to be pre-tensioned.
There is no other applied tensile force in the bolts or connection.

Does the pre-tensioning require use of section J3.7? Or is pre-tensioning not considered an externally applied force?

 

[CELinOttawa]

I think we have to take this one back to the fundamentals: What is the state of stress in the bolt? We also need to look at what STATE you are considering. At Ultimate Limit State the pre-tension is down to zero due to elongation and slip. At Serviceability Limit State, pre-tensioning to any degree would need to be included, in my opinion, but you could also reduce the total shear in your calculation by any amount of friction developed by your plates.

Thus, I'd make it:
SLS = Total tension including pre-tensioning, plus shear (reducing applied shear by that transfered through friction)
ULS = Applied tension, applied shear (make no reduction for transfer of load through friction unless specifically designed as a slip-critical connection).

I don't know much of the AISC code, but it cannot get around basic mechanics.

 

[desertfox]

Hi Jae

I am not familiar with AISC but I am looking at some bolts on a product for the steel industry which have failed in service.
The failure appears to be from principle tensile stresses on the bolts, characterised by the crack in them growing at 45 degrees to the bolt axis.
Studying the loadings, it appears that we have only external shear loads on the bolts, so the only tensile stress the bolts can possibly see is that of the pretension loading.
I am attempting to show that the preloading coupled with the external shear loads result in the combined stress exceeding the yield stress of the fastener and thus causing them to fail.

 

[spats]

"Externally applied forces" is the key. Pre-tension is not an externally applied force. Bolt pre-tensioning is achieved as a result of small axial elongations as the nut is turned. These elongations are relieved as shear deformations and shear yielding take place. This has been confirmed by testing. Bolt shear capacity is not dependent on, or affected by, bolt pre-tensioning.

In a similar fashion, you don't have to reduce bolt tension capacity because of pre-tensioning. This is because pre-tensioning results in a clamping force (compression in the plates) that must be overcome before you put any significant, additional tension in the bolts.

 

[JAE]

I think we found a previous thread that confirmed that the bolt pre-tension does not kick you into the combined shear + tension checks for bolts.

I don't have the thread link handy but it talked about the shear deformation essentially releasing the pre-tension stress which apparently nullified the tension.

If I can find the link I'll post it.

 

[desertfox]

Hi JAE

I found an article on the subject and it clearly states that preload does not effect the shear capacity.

See clause 2.4 in this link

http://preserve.lehigh.edu/cgi/view...xt=engr-civil-environmental-fritz-lab-reports