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Re: Steel: Moment Connections

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LibertyEng(--nospam--at) wrote:
> I'm designing connections for a steel fabricator for a building in Maryland.
> I have a question that comes up regularly in designing moment connections.
> Why do folks specify that the beam web connections for shear use slip critical
> bolts?  The beam flanges are to be welded to the column flanges.  I haven't
> seen an AISC design example that uses slip critical bolts for the shear load.
> Many of the examples in Volumes 1 & 2 of the AISC Manual use bearing bolts
> (type X or N connections).  Under what conditions are type SC shear
> connections required in beam to column moment connections?  When are type X or
> N shear connections OK in beam to column moment connections?  Thanks in
> advance for any input.
> Jan Harris, PE
> Liberty Engineering, PC

My understanding is as follows:

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.

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

Order of completion may matter here.

For bolted web (slip critical or not) and full
pen. flanges:
First weld flanges.  Tension the bolts later.

For slip critical bolts in web in concert with
welded off shear tab and full pen. flanges:
First weld flanges.  Tension the bolts later.  Do
the shear tab weld to web last.

Stan Johnson, PE, add eggs, oil, and
water and stir thoroughly...   :)