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Re: Concrete Load Brg Wall with Conc Beams

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James,

A lot of this depends on how you are choosing to detail the various
connections.  I was mainly pointing out that you do have the option of
treating the exterior connection as having no restraint.  Obviously, the
reality would have to mirror the design intent.  In your case, it would
appear that you could achieve a "no restraint" connection for both the
beam and slab by using a haunch that would take the gravity loads but not
impose too much rotational restraint (i.e. moment connection) on the beam
or slab.  This may not be an option, however.


As far as crack issues in dealing with both the beam and/or slab in a
no-restraint condition at the one end, this again would largely be an
issue of how the connections are detailed.  Assuming that you could create
a connection of the beam to the wall that would effectively transfer the
gravity loads (shear) in some sort of monolithic connection, you could
still provide tooled joint at the top of the beam that could "force" the
create to develop at the tooled joint.  The problem that I can see with
this would be that then the shear would likely have to be transmitted to
the wall by way of shear-friction, if the crack propigated far enough down
through the beam section from the top.

If you do detail it to get a pinned-type end, then development of the
positive steel into the wall can be a tricky issue.  On the one hand, the
development is not necessary since in theory there will be no moment at
that location, but obviously you still need rebar to bridge the connection
for shear purposes (unless you do use a haunch or something similar).  I
believe that the bars would still need full development lengths for use as
reinforcement in shear friction, but don't know for sure if that is within
the intent of the ACI 318 code.

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.

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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