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RE: Steel: Moments in joist seats.[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Steel: Moments in joist seats.
- From: "Horning, Dick/CVO" <dhorning(--nospam--at)CH2M.com>
- Date: Wed, 16 Sep 1998 12:34:42 -0600
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)aol.com[SMTP:Eskelsev(--nospam--at)aol.com] > Sent: Wednesday, September 16, 1998 9:53 AM > To: seaint(--nospam--at)seaint.org > 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 > SJI > 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 > > >
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