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RE: Two-way flat plate moment frame question

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Scott,

You have made a very important observation hidden away in your last paragraph which I think must be emphasized.

Even if the slab system is not used in the calculation the lateral force resistance, the slab must still be designed and detailed for the amount of drift that it will experience or failure/collapse of the slab could occur, in either flexure or shear.

Regarding the amount of slab to be used in the stiffness calculations, I would assume no more than the column strip width for a flat slab or the effective flange width for and beam/slab system with relevant reductions for cracking.


At 01:14 PM 22/10/2003 -0400, you wrote:
Mark:

To my understanding, while your comments are largely true in practice
(i.e. what is done for "real life" buildings in high seismic zones), there
are beginnings of making use of such frames for lateral loads.  Two ACI
committees have been working with this issue...ACI 421 and ACI 352.  ACI
352.1R-89 (Reapproved 1997) starts to get into it to a rather small
degree, if I recall correctly.  ACI 421.1R-99 deals with shear
reinforcement for slabs in general.  But, the most relevent document is
not available yet.  Committee 421 is working on finishing up (they are
supposedly responding to TAC comments) "Seismic Design of Slabs for
Punching Shear".  They also co-sponsered sesssion with 352 on "Flat Slab
Design for Lateral Loads".

You are correct the shear becomes a major issue when using flat slab
systems as the frames to take lateral loads, especially punching shear.
Thus, they are looking at ways to provide shear reinforcing.  They are
looking at things like stud rails, shear heads, and just "plain old"
stirrups (rather tough to do in relatively thin slabs).

But you are correct, to my knowledge, that generally speaking some "other"
sort of lateral system is typically used in the "real world" when dealing
with flat slabs systems, such as beams on some of the column lines to act
as the frames or shear walls.  But, even some such systems have had
problems in the past in line with your comments about shear.  There were
several flat slab systems, if I recall, in the Northridge quake that
suffered some punching shear problems.  They were low rise flat plate
systems with capitals (I believe...memory is rusty).  The lateral system
was then a moment frame comprised of the exterior bays which had beams
along the exterior walls.  The problem arose in that the interior flat
plate slab-column connections were not really detailed to handle the drift
that occurred, which "created" some signicant punching shear moments that
the slab was not really able to handle.

Regards,

Scott
Ypsilanti, MI


On Wed, 22 Oct 2003, Mark Gilligan wrote:

> Using a slab to act as part of a  frame is generally not advised when
> dealing with seismic loads.  The problem is that under the high drifts that
> will likely occur, cracks will occur at the interface to the column which
> will reduce the shear capacity of the slab.  In fact you may find that the
> code dose not allow such systems in some locals.
>
> Mark Gilligan
>
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