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RE: Shear-off bolted connection

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Mark,

>It dosen't make any difference wheather the bolt is stressed to 100% or
>200% of yield, as long as the bolt dosen't fail during erection, it will
>perform adequately.


1- The proof load, or specified pretensioning force in the bolt must be
about 70% of tensile resistance of the bolt (I think this is more or less
the same for both CISC and LRFD). There is usually no distinct yield
strength in bolt material, so your 200% of yield may be around ultimate
capacity of bolt.


2- an exessively pretensioned bolt does not fail during construction but
could probably reach about 95% of it tensile resistance in tension in
service state. This translates to very small shear resistance left in the
bolt for ultimate state, where the bolt is resisting shear by bearing
action. So it may appear
an O.K. practice for service loads, but it is seariously on a the unsafe
side for ultimate resistance deisgn.

Majid Sarraf
www.uottawa.ca/~msarraf



>  This is true irrespective of the way in which a bolt
>is tensioned.  The reason for this is that it is the frictional resistance
>provided by the clamping force that transfers the shear in the connection. 
>The force is the bolt can't exceed the initial clamping force unless you
>subsequently pull on the bolt in excess of the original clamping force, and
>if the subsequent tensile force exceeds the capacity of the bolt it will
>fail irrespective of the method of tensioning.
>
>I find it interesting that you have had problems with bolts breaking, while
>I have not had this problem.   The fact that the problem is not universal
>suggests that the problem cannot be laid totally at the steps of the DTI's.
> Were any tests undertaken to verify the mill cert's?  Was the manufacturer
>of the DTI's asked to comment on the problem?  
>
>Given the polarized nature of the discussion regarding twist-off vs DTI's I
>would be reluctant to jump to a conclusion until both sides have had a
>chance to explain the problem.  They are both guilty of providing selective
>information.
>

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