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Re: Concrete Load Brg Wall with Conc Beams

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I ran the model and deflections with a pinned end doubled the deflection based on gross section. Did not run a cracked anaylsis as the load bearing wall has become a non-issue. I have persuaed our client to stay with the 2 feet square exterior columns for this seven story structure that I had in the original design.
Thanks for all your comments
James


From: Gil Brock <gil(--nospam--at)raptsoftware.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Re: Concrete Load Brg Wall with Conc Beams
Date: Fri, 26 Jul 2002 11:20:11 +1000

At 08:45 PM 25/07/02 -0400, you wrote:
James,
To me, deflection should not be a real issue, since even with a pinned
connection on one end, the deflection will likely not increase to
dramatically.  You should, however, check the deflection if you have any
concerns.

Actually, deflection could increase significantly depending on the previous degree of fixity.

For example, the increase in deflection from full fixity to a pin at one end is approximately 2.5 times the original deflection plus the effect of the increase in cracking at mid-span in the bottom and in the top at the interior support.

So even with partial fixity previously, the deflections could easily increase by 30 to 100% when a pin is introduced depending on the original degree of fixity.

The end result is that if you choose your connection details to allow some
rotation (note that it doesn't have to be much...you can verify this by
doing a slope-deflection calculation to find out the potential rotation at
the pinned end), then most of you issues that you raise become much
easier.  I can see using haunchs, as mentioned before.  I can also
potentially see building up the wall, stopping it just below the floor
level, building the beam and slab to sit on the wall but have some sort of
construcion joint so that it is not monolithic, then continue the wall
above the slab level without it being monolithic to the slab and beam.
The difficult part with this scenario would be to continue the vertical
steel up through the connection area to still connect the upper and lower
wall sections, yet not have that steel unduely retrain the beam and slab
rotation.

HTH,

Scott
Ypsilanti, MI

On Thu, 25 Jul 2002, James Lane, P.E. wrote:

> Scott wrote the beam end could be designed as a pinned end.
>
> Yes, a designed pinned end would increase the positive steel in the beam. > The negative moment still would want to occur until the concrete cracks. You > could provide some small negative steel bars to control cracking yet not > exceed the stiffness or strength of wall steel ensuring the wall was not the > weaker element. What are your thoughts about after the beam cracks, shear > capacity of the beam, bottom reinforcing that is not developed, increased > rotation/deflection, and a cracked floor slab? Also the flat plate adjacent
> to the beam is fixed and the steel is developed in the wall.
> James
>
>
> >From: Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>
> >Reply-To: <seaint(--nospam--at)seaint.org>
> >To: seaint(--nospam--at)seaint.org
> >Subject: Re: Concrete Load Brg Wall with Conc Beams
> >Date: Fri, 19 Jul 2002 21:22:53 -0400 (EDT)
> >
> >James,
> >
> >One simple solution is to allow the end of the beam at the wall to be an
> >"unrestrained" end (i.e. "pin" connection).  This would mean that you
> >would not need to develop any negaitve moment steel since there would be > >no negative moment. The down side is that the positive moment in the beam
> >will increase.  Under section 8.3.3 of ACI 318-99, the positive moment
> >would become wu*ln^2/11 instead of wu*ln^2/14 (this assumes that you meet > >the general requirements of 8.3.3...two or more spans; spans approximately
> >equal, etc).
> >
> >This solution is primarily there for your type of situation...that is when > >the support condition really does not support a restrained connection that
> >would require a significant negative moment.
> >
> >HTH,
> >
> >Scott
> >Ypsilanti, MI
> >
> >
> >On Fri, 19 Jul 2002, James Lane, P.E. wrote:
> >
> > >
> > >
> > > A 12 inch thick load bearing wall multiple stories. Heavily reinforced > > > concrete beams on grid coming into bearing wall. Is it ok not to develop
> >the
> > > beam bars (not enough wall thickness. Wall must satisfy ACI 14.4.
> > >
> > > I say negative steel should be developed by a haunch or something
> >similar
> > > and design wall for additional haunch moment. Looking for other
> >engineers
> > > comments regarding development of beam steel in exterior walls.
> > >
> > > I have heard one response that does not develop the beam steel and
> >treats it
> > > as a partial fixity letting the negative steel yield and redistributing
> >beam
> > > moments. If you do not develop the beam steel how can you be sure that
> >the
> > > end moment does not pull out the partial developed bars causing failure?
> > >
> > >
> > >
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Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. (ABN 99003351504)
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022              Fax +61 7 3807 8422
email:          gil(--nospam--at)raptsoftware.com
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