Bolt Shear Strength in Pretension Condition 2

[jiamin]

We have a sharp argument in our office regarding a bearing type bolt connection with a pretension value smaller than the Minimum Bolt Pretension specified in AISC-13th, Table J3.1.

One of our engineer says, with any value of pretension which is over 20% of corresponding available tensile stress, the bolt shear capacity will be reduced and has to be calculated base on section J3.7---Combined Tension and Shear in Bearing-Type Connection.

My oppinion is that the pretension is not considered in combined tension and shear working condition. The combined tension and shear refer to working load in both tension and shear direction. As is shown in the attached diagram, with working load only in shear direction, even though there is pretension in bolts, the bolt shear capacity still can be calculated base on shear only condition. Am I right?

Thanks very much for your precious oppinion.

 

[nutte]

I agree with you. The bolt pretension is almost the full bolt tensile allowable. That means a pretenioned bolt, if you apply the pretension as a tension in the J3.7 equations, will have very little shear capacity.

Ask your colleague for some documentation supporting his claim. The user note in the specification section J3.7 tells you this section won't control for stresses less than 20% allowable. His confusion stems from the difference between applied tension and the internal bolt pretension.

 

[slickdeals]

I have found this link to be invaluable in understanding something that is counter-intuitive to most engineers.

http://www.boltscience.com/pages/basics1.htm

It has good analogy based on sprints for bolt pretensioning.

 

[rb1957]

the way i read nutte's reply is that he's disagreeing with you. FWIW, i agree with nutte, that the pretension reduces the available shear. i'd use Rt^2+Rs^2 = 1, 'cause it's easy and conservative; there are all sorts of interaction formula.

 

[nutte]

rb1597, you misunderstand my post. I do not believe the pretension reduces the allowable shear.

I was satying that if you apply the pretension as an applied tension, you'd wind up with little shear reserve left. That makes no sense, and it doesn't follow the literature. Therefore, it cannot be correct.

 

[slickdeals]

Here is AISC's reponse:

http://www.modernsteel.com/steelinterchange_details.php?id=646

Pretensioned Bearing-Type Joints

Please let me know whether a bearing-type connection can be pretensioned.

If yes, should the pretension (70% of the proof load of the bolt) be taken into account and checked for combined shear and tension when using N- or X-type bolts?
What is the purpose behind pretensioning bearing type joints? Is the pretension to be in accordance with RCSC 2004? Will prying occur in a pretensioned joint?
Answer

A bearing-type connection may be snug-tightened or pretensioned. Hence, a bearing connection can be pretensioned.

The pretension force should not be considered part of the tensile load on the fastener. These are not additive. The tension load in a pretensioned bolt does not change when an external tension force is applied until the parts separate, which would require a tension force in excess of the pretension. Additionally, shear deformations occur prior to bolt failure. These deformations also relieve the pretension prior to bolt failure.

The purpose behind pretensioning is generally to minimize the variation of force in the bolt. This should not be confused with slip-critical connections, which are designed to prevent slip. Refer to Section J1.10 of the 2005 AISC specification (

http://www.aisc.org/2005spec)

and Section 4.2 of the 2004 RCSC specification (

http://www.boltcouncil.org)

for cases when pretensioned joints are mandated.

Sergio Zoruba, Ph.D., P.E.

 

[jiamin]

Thanks Guys for your precious information. That means bolt shear capacity will not be reduced by pretension in the bolt. That matches a test result I remember way back when we preloaded the bolt and get a higher shear capacity (even 15% higher than the AISC code value) than let the bolt loose.

 

[rb1957]

did you get a higher shear result because the mateial was above spec (like 90% is) or because of friction ?

 

[jiamin]

I think the higher shear result mostly comes from the friction.

 

[slickdeals]

I contacted Dr. Geoff Kulak, please see his attached response

"The pretension in a bolt is the result of relatively small elongations in the axial direction that are produced as the nut is turned on. As the bolt is loaded in shear, these small axial deformations are dissipated as a zone of yielding develops in the bolt in the vicinity of the shear plane(s). So, in both this intuitive description and by measurements taken during tests, the pretension in the bolt is zero, or at least approaching zero, at the time the ultimate shear strength is reached. There is plenty of physical testing that supports the hypothesis. ( I should also remind you that whether the designer is using allowable stress design (ASD) or ultimate strength design (LRFD), the basis for the rules in all North American specifications is ultimate strength, i.e., LRFD)."