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Re: Column design

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Ashraf,

If you are "governed" by ACI 318 (i.e. it is either required as code/law
or recommended for use), then you can point the others to section 8.8 of
ACI 318 (I am looking at ACI 318-02, but is largely the same for older
versions back to ACI 318-89).  That section states:

"8.8.1 - Columns shall be designed to resist the axial forces from
factored loads on all floors or roof and the maximum moment from the
factored loads on a single adjacent span of the floor or roof under
consideration.  Loading condition giving the maximum ratio of moment to
axial load shall also be considered.

8.8.2 - In frames or continuous construction, consideration shall be given
to the effect of unbalanced floor or roof loads on both exterior and
interior columns and of eccentric loading due to other causes.

8.8.3 - In computing gravity load moments in columns, it shall be
permitted to assume far ends of columns built integrally with the
structure to be fixed.

8.8.4 - Resistance to moments at any floor or roof level shall be provided
by distributing the moment between columns immediately above and below the
given floor in proportion to the relative columns stiffness and conditions
of restraint."

The point is that this section clearly indicated that typical CIP R/C
frames must have their columns designed to resist moments due to gravity
loads.  This makes sense as most concrete frames will detailed such that
beam-to-column connections have significant rigidity such that significant
moment is transfer from beam to column.  In other words, in "typical" CIP
concrete frames beam-to-column connections are considered fixed _NOT_
pinned.  Thus, there will be moments in the columns (especially exterior
columns) due to gravity loads.  This is certainly true is you use the ACI
method (section 8.3.3) to determine moments in beam members instead of
doing a frame analysis, but should also be true if you do a frame analysis
of a typical R/C CIP frame.

The only way one could realistically argue that no monents would be
present due to gravity loads would be to make "discontinuous" members,
which is rather difficult to due in CIP concrete.  It can be done with
precast concrete, but even in such situations you will typically still
have moments in the columns due to gravity loads (i.e. typical precast
system is to have simply supported beam sitting on a haunch/corbel at the
column, which means now your gravity load is now applied eccentrically to
the column).

HTH,

Scott
Adrian, MI


On Fri, 16 Dec 2005, Ashraf Manjappara wrote:

> Hi everybody,
> Good to see your comments and it shows how the structural engineering
> community view  such erronous assumptions.
> I would like to thank everybody for your  moral support for me.
> In Kuwait wind velocity is taken as 160 kmph and not considered as
> seismically active area. But Kuwait Oil company says structure should be
> designed for EQ also using UBC.
> Robert:
> Here ACI and BS code is using widely. I dont think they have point from
> codes.
> Eli:
> They argue that upto 6 stories no need to think about lateral force
> resistance system using columns since lift walls will take care of lateral
> force and no need to design  columns for moments from the vertical loads.
> Wind speed is very high. What you said is absolutely  right regarding EQ
> since Iran and Saudi's north western area which is considered as seismically
> active area.And nobody is bothered about the life of people if an EQ happens
> with all these trash designs.May God save the people here.
> Suresh:
> The issue is not with regard to lateral force system. My opponents argue
> that no need to consider moment due to vertical loads in columns.
> Kevin:
> You are right and my morale to fight with them is now boosted.
> Padmanabhan:
> Really what you said is right.
> regards to all
> Ashraf
>
>
> On 16/12/05, Ashraf Manjappara <ashraf.manjappara(--nospam--at)gmail.com> wrote:
> >
> >
> >
> > ---------- Forwarded message ----------
> > From: Kevin Below <kevinbelow(--nospam--at)videotron.ca>
> > Date: 16-Dec-2005 04:27
> > Subject: RE: Column design
> > To: seaint(--nospam--at)seaint.org
> >
> > Wow.  That's a tough situation Ashraf.  I support you 100 %, but it sounds
> > like you will have to take on a whole country.
> >
> > You may want to check the fire rating of a 20 cm column.  I think that the
> > Canadian code for example, will require a reduction of capacity if you need
> > a fire rating.
> >
> > Probably the code (depends on which code you are using) will not require
> > you to consider biaxial bending on corner columns, but certainly uniaxial
> > bending on edge columns must be considered.
> >
> >
> >
> >
> > génio structure
> >
> > *Kevin Below, ing., Ph.D. en Structures*
> >
> > *290, rue Seigneuriale***
> >
> > *Beauport, (Québec) G1C 3P8***
> >
> > *Tél. : (418) 660-6969 poste 272***
> >
> > *Fax : (418) 660-6463***
> >
> >  -----Original Message-----
> > *From:* Ashraf Manjappara [mailto: ashraf.manjappara(--nospam--at)gmail.com]
> > *Sent:* 15-Dec-05 16:54
> > *To:* seaint(--nospam--at)seaint.org
> > *Subject: *Column design
> >
> > I have to design a RCC framed structure to support a LL of 22kN/m2 with a
> > span of 6m,bay length of 4.2m. Architect is insisting on 20cm X 40cm wide
> > column, 20cm is the width parallel to span.
> > I am not able to design that column for moment since width is just
> > 20cm.Could you suggest a way to reduce the column moment or else an
> > alternate solution.
> > As far as I know most of the designers in Kuwait does not  design end or
> > corner columns of RCC frames for moment. They simply assume that it's simply
> > supported and design column for axial load only and they design the beam for
> > simply supported  moment. For eg. for udl they design the beams of frames
> > for wl2/8 in one span frames.
> > Its really shocking for me. I started working in Kuwait last month only.
> > So many designers here are saying that no need to design column for moment
> > if building is  below 6 stories. I am not able to understand their logic and
> > they are not accepting my view that every column should be designed for the
> > moment it attracts.  They are forcing me to design columns only for axial
> > load alone.so please make your comments.
> > regards
> > Ashraf
> >
> > --
> > No virus found in this incoming message.
> > Checked by AVG Free Edition.
> > Version: 7.1.371 / Virus Database: 267.14.0/203 - Release Date: 2005-12-15
> >
> >
> > --
> > No virus found in this outgoing message.
> > Checked by AVG Free Edition.
> > Version: 7.1.371 / Virus Database: 267.14.0/203 - Release Date: 2005-12-15
> >
> >
> > --
> > S.M.Ashraf
> > Structural Engineer
> > Al Habshi Consultants Office
> > P.O.Box 27154
> > Safat 13132, Kuwait
> > Tel. 965-2430103
> >
>
>
>
> --
> S.M.Ashraf
> Structural Engineer
> Al Habshi Consultants Office
> P.O.Box 27154
> Safat 13132, Kuwait
> Tel. 965-2430103
>

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