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RE: UBC 1633.2.4

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Peter,

I would be careful about just ignoring the participation of the rigid
exterior walls altogether.  You could find a situation where the walls
suck up so much load that your moment frame really doesn't do anything
(not too likely but somewhat possible depending on how the walls are
configured).  Keep in mind that just because your walls are not designed
as shear walls, that does not mean that they cannot behave that way.  If
you have rebar in the CMU walls for out-of-plane wind loads, then that
same rebar can cause your CMU walls to act a reinforced shear walls.  Not
to mention that even if they do "fail", they can still effect the
behaviour of the moment frames.  For example, you could find a situation
depending on wall configuration where some walls are weaker that others,
"fail" quickly, but still cause an unexpected distibution of the lateral
loads to the moment frames that create an unexpectly higher lateral load
on some of your moment frames.

The point is that just believing that failure of the CMU walls won't
effect the system may not be the most prudent move.  If it were me, I
would still need to prove to myself that the attraching of load by the CMU
walls and hte subsequent failure would not adversely affect the response
of the moment frames.  And that means a potentially significant effort on
my part.  Thus, to me, it seems more efficient to provide for relative
movement between the wall and the structural frame.

Around here, most CMU backup walls are anchored back by use of channel
slots and dove tail anchors.  The dove tail anchors are usually placed in
the bed joints, so they are better "equipped" to accomidate vertical
movement.  Even in this configuration, they can accomidate horizontal
lateral movement both by some rotation and by potentially slipping some in
the joint.  The end result is that this type of system is typically fine
for use in low seismic zones like the Detroit area.  Now, use of such a
system in a higher seismic zone might be more interesting, but since I am
not in a high seismic zone, I have yet to really deal with that
interesting problem.

HTH,

Scott
Ypsilanti, MI


On Sat, 2 Nov 2002, Peter Griem wrote:

> Scott,
>
> I was actually hoping to eliminate a lot of work by ignoring the
> participation of the rigid exterior walls altogether.  There would be no
> seismic analysis done on the exterior walls whatsoever.  Plus I would not
> have to detail horizontally slotted connections at each floor level.  The
> theory being - if the walls help out, great, if they fail since they were
> never designed for the shear, there's a whole steel moment frame system
> capable of taking the design seismic load.
>
> I don't feel too comfortable about this idea, but I'm trying to understand
> why I shouldn't implement it.  The more I read 1633.2.4.1, the more I think
> that it intends to refering to adjacent elements, not "adjoining" in the
> sense of attached.
>
> Detroit area is in a similar seismic zone - are masons and steel fabricators
> used to seeing large horizontal slots in attachment clips?
>
> How about other low-siesmic areas?  More input from thelist would be greatly
> appreciated.
>
> Peter
>
> -----Original Message-----
> From: Scott Maxwell
> To: 'seaint(--nospam--at)seaint.org'
> Sent: 11/2/2002 9:22 AM
> Subject: Re: UBC 1633.2.4
>
> That seems to make sense to me.  You should keep in mind, however, that
> if
> you do not provide the slotted hole (or someother mechanism for relative
> movement) then your exterior wall WILL take some of the lateral load up
> to
> some point when it "fails" and loses its stiffness (due to the fact that
> your exterior CMU wall is likely much stiffer than your moment frame).
> Thus, to me at least, you seem to be creating more work for yourself by
> having to analyze how the CMU walls attracting lateral load affects the
> behaviour of your structure.  It seems much easier to supply some type
> of
> connection that will allow relative movement between the moment frame
> and
> the CMU walls (i.e. 1633.2.4.2), especially if you are talking about a
> low
> rise building with CMU walls that are stacked from the foundation.
>
> HTH,
>
> Scott
> Ypsilanti, MI
>
>
> On Fri, 1 Nov 2002, Peter Griem wrote:
>
> > I have a moment frame building in Seismic Zone 1 that has non-load
> bearing
> > exterior CMU walls (supported entirely on the foundation).
> >
> > Can I consider the exterior wall to be an adjoining rigid element
> > (1633.2.4.1) or does it have to be considered and exterior element
> > (1633.2.4.2)?
> >
> > It seems that it makes a BIG difference in how you attach the CMU wall
> back
> > to the steel structure.  It looks like you can rationalize that if the
> > failure of an adjoining rigid element doesn't impair an independently
> > designed steel moment frame, then you can IGNORE the requirements for
> > slotting a connection horizontally.  Is that right?  What am I
> missing?
> >
> > Thanks,
> >
> > Peter Griem, P.E.
> > Glastonbury, CT
> >
> >
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