×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Bolt Shear Strength in Pretension Condition

Bolt Shear Strength in Pretension Condition

Bolt Shear Strength in Pretension Condition

(OP)
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.

 

RE: Bolt Shear Strength in Pretension Condition

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.

RE: Bolt Shear Strength in Pretension Condition

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.

RE: Bolt Shear Strength in Pretension Condition

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.

RE: Bolt Shear Strength in Pretension Condition

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.

RE: Bolt Shear Strength in Pretension Condition

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 (www.aisc.org/2005spec) and Section 4.2 of the 2004 RCSC specification (www.boltcouncil.org) for cases when pretensioned joints are mandated.

Sergio Zoruba, Ph.D., P.E.
 

RE: Bolt Shear Strength in Pretension Condition

(OP)
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.  

RE: Bolt Shear Strength in Pretension Condition

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

RE: Bolt Shear Strength in Pretension Condition

(OP)
I think the higher shear result mostly comes from the friction.

RE: Bolt Shear Strength in Pretension Condition

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)."

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources