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Re: Steel: Moment Connections

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Interesting question by Jan and more interesting comments by Stan.

I have a few points which may make you ponder even more, and question the
whole idea of using slip critical bolts for moment connections.

>The slip critical connection uses design values
>for which the connection is not expected to slip
>at allowable load levels.  Such a connection can
>be used in those cases where you do not want the
>connection to slip.  For the standard hole size
>you would not expect slippage to exceed 1/8 inch
>if it were not for the fact that the allowable
>bearing values for bearing in the connection
>assumes that some inelastic elongation of the
>holes is acceptable.  In other words, when your
>joint sees allowable force level loads, you could
>get quite a bit of reltive movement between
>parts.  Slip critical connection are useful where
>relative positions of the parts connected needs to
>be held fairly fixed, or the relative rigidities
>of various parts of the system need to stay within
>limited range.  In moment connections, the stress
>reversals one is designing for may make the slip
>very pronounced.

In a moment connection there is two type of bolt slip which may cause
problem, 1) Horizontal 2) Vertical

Horizontal: This one is due to the realtive rotation beam web with respect
to the column flange. When this happens, we end up having only the top and
bottom plate resisting moment, and contribution of web is lost until beam
end rotates large enough so that bolts start work in bearing action.
Interestingly this rotation could be larger than rotational ductility of
joint, i.e. web contribution will not exist until connection fails (most
likely due to non-ductile weld failure). If we simply ignore the
contribution of web, then we have to rely on overstrength and ductility of
top ,or bottom plate (depending on which one is in tension) with their
typically brittle welds!


Vertical: This is due to shear caused by gravity, plus shear due to 
conteraction end moment caused by earthquake load, i.e (left end moment +
right end moment )/ beam length.

If we want to totally eliminate slip of bolts for a moment connection in a
eartquake resistant frame, we have to design for a resultant factored force,
horizontal factored force to provide moment resistance for the web, and
vertical component, factored shear due to gravity and factored shear due to
end moments  combined (contarary to routine practice of using service loads
to design SC bolts). Lets for simplicity forget a bout horizontal component.
Now,
As for loads, I will give it, at least, 30% extra shear compared to shear
due to specified loads. As for bolts shear resistance (considering no
special surface preperation) you have roughly half shear resitance of bolts
in bearing if they are to be used as slip critical. That means you need 2.6
(1.3 x 2= 2.6)times no. of bolts you would normally design for shear to
control slip! Increasing no. bolts means, more holes. This causes serious
reductiuon in the web cross section, as well as tab plates. Tab plate
thickness easy to increase, but beam web not as easy!.    
  

>If your bolts are not designed as slip critical in
>a moment connection, then at allowable load level,
>they will likely be prone to slip some.  The
>flange welds then become the stiffest part of the
>system and end up taking the shear as well as the
>force that you designed for.  So, use slipcrticial
>bolts, right?  Unfortunately, Northridge post
>mortems on moment joints hinted that, due to a
>sort of tension field action in the web, the
>flange welds might try to take much of the shear
>anyways.

>
>Order of completion may matter here.
>
>For bolted web (slip critical or not) and full
>pen. flanges:
>First weld flanges.  Tension the bolts later.

But even doing this does not help much. Because most of the shear due to
gravity is applied after placement of the beam (Dead loads, live loads and
earthquake). unless we almost complete the construction and let the live
load on and then tension the bolts! Or, still a high percentage of shear can
be taken by top and bottom plates.

I think we should let the web plate bolt slip and think of providing more
flexiblity for the plates to reduce their share in resisting vertical shear,
and providing more ductility for them. I think this is done in most of
proposed detailing for moment connections, such as dogbone, etc.. 

Regards,


Majid Sarraf

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                   \               Majid Sarraf             /
                    \             Ph.D Candidate           /
                     \   Department of Civil Engineering  /
                      |        University of Ottawa      |
                      |           Ottawa, Ontario        |
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