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Re: Bolting issues

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Majid:

Here's what I know.

>> Does <the RCSC Spec. > says how many turns? or how much extra tension
>> is okay?

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
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.

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.

>> excessive pretensioning load must affect shear capcity of the bolt at
>> "factored loads" not "service loads", specially when shear plane
>> passing through threads

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."

>> I have sound theoretical reasons for why shear-tension interaction
>> exist, and therefore, it must be applicable to bolts as well...One
>> figure <in Kulak's book> (4.17) supports this idea and one does not
>> (4.15)

You have to distinguish between externally applied tension and
pretension when you talk about shear tension interaction. 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 talk too much. (-:

Charlie