AISC bolt pretension 1

[andsolhe]

Hi

I'am reading the AISC 'specification for steel structural buildings 2010' (ASD), and find chapter J.3 a bit confusing when it comes to bolt pretension and tensile strength.

So my question is:
Why is the minimum bolt pretension set to 0.7*Fu according to table J3.1 and allowable tensile strength set to Fu/omega = 0.5*Fu as in equation J3-1? It seems like the bolt pretension force is higher than the allowable tensile force. But that doesn't make sense, so I hope that someone at this forum can help me understand.

Thanks

 

[WillisV]

http://msc.aisc.org/globalassets/modern-steel/steelwise/30753_steelwise_bolting_web.pdf

 

[JAE]

Thanks, WillisV - I've copied out the relevant Q&A from that link:

[blue]Must the installed pretension be added to the tension or shear load when designing a bolted joint?[/blue]
It is important to remember that pretension (i.e. installation tension) is always ignored when determining the design strength of a bolted joint. The initial pretension is reduced to zero (or near zero) as a bolt is loaded to failure in shear and/or tension. This effect is due to the deformations that occur prior to bolt failure. As mentioned in commentary Section 5.1 of the RCSC specification “measurements taken in laboratory tests confirm that the pretension that would be sustained if the applied load were removed is essentially zero before the bolts fails in shear (Kulak et al., 1987). Thus, the shear and tensile strengths of a bolt are not affected by the presence of an initial pretension in the bolt.” In the case of a tension connection, the initial pretension and the external tension add up to the point of separation of the connected parts. However, this increase is relatively small and is customarily ignored, as it occurs prior to bolt failure.

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[andsolhe]

Thanks WillisV.

Some of the bolted connections I'am working with is analysed in ANSYS, which include bolt pretension. So my bolt force will never get below the initial pretension.
But I guess the tensile strength for the bolts should be checked without pretension included.