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Re: Bolting issues

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Dear Charlie,

I enjoy your talk whether is short or long, and I greatly appreciate your input.

>Here's what I know.
>When you install a bolt, you are talking about a range of from 1/3 turn
>to 2/3 turn from snug to induce the pretension for the turn-of-nut
>method. Other methods use different indicators, but the required turn
>will likely be very similar. Rotational capacity tests demonstrate that
>high-strength bolts can withstand several turns before failure in
>torque-induced tension, particularly A325's, which are quite ductile. I

Is this regardless of grip length and bolt size? If we are thic confident,
Where can we set a practical limit for the maximum no. of turns?

>think any extra tension is okay, as long as the bolt doesn't break
>during installation. All you're really doing is over-rotating and
>stretching the bolt a little more. But by that time, you're in the
>plastic portion of the bolt elongation curve, so even a large
>over-rotation only makes a small difference in the installed pretension.

Over-rotating can make a 10% to 30% increases in pretensioning load beyond
70% specified preload. I agree, this is not much of concern of this is only
during the instalation, or under service loads where shear is resisted by
fricion. However this additional tension will significantly reduces shear
resistance simply to almost zero under factored loads, IF WE BELIEVE THERE
IS STILL THE SAME PRETENTIONING LOAD AT THIS STAGE, AND IF WE BELIEVE THERE IS A
SHEAR-TENSION INTERACTION THOUGH.

Also, if in tightening a bolt only an additional 1/10 of a turn would cause
it to fail, is this bolt still accpetable. What happens to its margin of safety?

>Also, when you let go of the wrench, the stress state in the bolt
>changes from combined tension/torque to tension only, which is not
>>nearly as demanding. Add all this up and the worst stresses the bolt
>>will ever see (if designed to meet the AISC and RCSC Specifications) are
>>induced during installation.

Please, correct me if I am wrong. When I am turning the nut, the main torque
is to overcome frictional torque developed between nut and the faying
surface. Only a smaller protion (depending on the friction coeff. bolt
thread and nut)of the torque is transferred to the bolt shank which may be
in interaction with
pretension during the tightening stage. This is portion equals the small
frictional torque developed between the head and the other side of faying
surface, or the torque applied by another spod wrench used to hold the bolt
from rotational slip during installation. My point is: "The magitude of the
external torque applied is not the same as torsion induced in the bolt shank".

Moreover, even if this torque produces any considerable shear stress will be
maximum only at the primeter of the bolt shank or threads and zero at the
bolt centre. Whearas for ultimate shear resulted from bearing action the
whole cross section uniformly reaches its ultimate shear strength, of 0.6fu.
So maximum stresses during instulations are not quite comparable and can not
be considered worst than those resulted from ultimate loads.

>Shear strength is independent of pretension because any pretension that
>existed prior to loading will be released by shear deformations that
>occur before failure. That's why the tabulated bolt shear and tensile
>strengths are good for any installation condition (i.e., snug-tight or
>pretensioned). The shear plane passing through the threads does affect
>the shear strength, however. The question of service loads vs. factored
>loads is really not applicable. Bolts don't fail at service loads. Slip
>occurs just above the service range, but that is an awefully benign
>"failure."

This is where I have problem understanding how this could happen, and this
is exactly where considering interaction or no can be justified. 

Why shear deformations in the transfevrse direction could affect
longitudinal deformation in a bolt? In fact even if there is the sligtest
effect, assuming it to be due to large deformations, it must be in direction
of increasing that tension not decreasing it. I would agree with you if you
say AFTER removing the shear, pretension may be lost or reduced, but NOT
WHEN MAXIMUM SHEAR FORCE IS STILL IN EFFECT.

>You have to distinguish between externally applied tension and
>pretension when you talk about shear tension interaction. 

But external or internal, it changes the clamping force while the tension in
the bolt remains constant. Unless external tension exceeds the tensile
capacity of the bolt causing, which will not only bring the clamping force
down to zero but also causes the bolt to fail.

>The interaction of externally applied shear and tensile forces must be
>considered. Interaction between externally applied shear and the
>installed pretension is not a design consideration, however, for the
>same reason as stated previously (i.e., you'll lose all pretension due
>to shear deformations of the bolt shank prior to failure). Kulak Figure
>4.17 is for interaction of externally applied shear and tensile forces
>only. Figure 4.15 shows clearly that shear strength is independent of
>installed pretension. In fact, it shows that a bolt still has the same
>shear strength after 1.5 turns as it had at snug and 1/2 turn!

I would believe it if you give me a reason why pretensioning force could be
all lost due to shear deformations. I found too few data supporting Fig.
4.15. Either there must be a logical reason to support that old experimental
data or it was just matter of luck with a few bolts having long grip which
behaved in very ductile manner.

>>I talk too much. (-:

Me too.

I thank you so much for your input.

Regards,


Majid Sarraf
www.uottawa.ca/~msarraf


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                   \               Majid Sarraf             /
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