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RE: Steel: Moments in joist seats.

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I recall a warehouse-type building in the Portland area which collapsed in
the '70's after a heavy snowfall.  The roof framing was open-web joists
supported on really skinny little pipe columns, which couldn't accommodate
the joist deflections under the snow load.  As a minimum, IMHO you should
get all the dead load on before connecting the chord extension, and then do
it with bolts in longitudinally slotted holes.  I would also size the column
to be able to handle some nominal moment cranked in at the top.  It's hard
to picture the chord forming a plastic hinge before the cap plate yields in
bending about its thickness.  The problem  would be less severe if you have
a joist framing in from each side, of course.

> ----------
> From: 	Eskelsev(--nospam--at)[SMTP:Eskelsev(--nospam--at)]
> Sent: 	Wednesday, September 16, 1998 9:53 AM
> To: 	seaint(--nospam--at)
> Subject: 	Steel: Moments in joist seats.
> In the case of a joist-girder resting on top of a tube column, there is a
> common detail which extends the top plate of the column a couple of inches
> along the seat of the joist.  The joist then has a top chord extension
> (TCX)
> to match the coverplate length.
> The question was raised by the EOR, of whether the offset of the
> centerline of
> the column and the bearing point of the column by 5 inches would cause a
> moment in the column and in the top chord of the joist-girder.  I can see
> how
> this might occur, but this is the only time it has been questioned.  The
> specifications don't address the possibility of a moment being induced by
> deflection of the bearing plate.
> (The SJI specifications call for a design based on simple span and that is
> 98%
> of the time what the engineer is happy with.  The other 2% of the time,
> the
> engineer normally specifies what moment will be applied to the top and
> bottom
> chords and what details are required for attachment.)
> My intuition (never to be totally trusted) tells me that the top chord
> would
> plastic hinge at the attachment to the much stiffer seat creating the
> idealized pin condition anyway.  This may cause problems of its own.
> Has anyone else run across this detail and thought about the potential
> moment?
> Thanks for your time.
> Valerie Eskelsen