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Re: Column design

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Dear Ashraf,

I have not been able to read all the mails on the subject, as I have caught
the thread midway. As such, I am not sure if I fully understand your
problem, but in as much as I have been able to understand the problem, you
are in the process of designing a building in Kuwait, which you say is a
seismically active area and also that the wind speed is significant. You
have a peripheral RCC column that has a structural depth, h, of 20 cm (about
8 inches), across the building edge and a structural width of 40 cm (about
16 inches), along the building edge. The column is supposed to receive a CIP
beam spanning at right angles to the building edge. The question you have
raised is whether you can design the column for axial force only, thereby
neglecting the moments. You have also alluded to the fact that the building
has other (I suppose, fairly rigid) elements that would be expected to carry
the lateral loads, in its entirety. I am assuming that the column is an edge
column (not a corner column), that you have a CIP slab rigidly connected to
the system, that the supported beam belongs to an intermediate floor (not
the roof) and lastly that you are located in seismic zone 1. Correct me if I
am wrong.

Your question is: Is it necessary to design this column for moments? Let me
share with you how I see your question. Let us look at the gravity moments,
first. I suppose there is no doubt in any one's mind that there would be a
gravity moment, in the column, on the basis of an elastic analysis. This is
to say when the analysis is based on realistic sectional properties and
lengths of the beam and the column. If you wish to pick up your shears and
moments in the beam, on the basis of such an analysis, you do not have the
choice of neglecting the moment in the column. On the other hand, if you
provide a hinge in the beam at the beam-column junction, in your analysis
model -- thus departing from the assumption of a rigid moment-carrying
connection -- the shear and moment diagrams of the beam would be accordingly
modified and you would have no gravity moment on the column, on the basis of
this model. In this case, you can design the beam with a simply supported
end and must make sure that the top rebars in the beam, at this end, can not
develop any moment. If, for instance, you provide rebars at the beam top
that can develop a moment at the beam-column junction, your assumption of a
hinge would break down. You must also remember that even when you depart
from the elastic analysis but still provide a clear load path that can
transmit the gravity loads, that load path would become available before
failure and thus the strength aspect would be adequately addressed. But you
still need to estimate the minimum eccentricity that may nevertheless be
present, in a real-life structure and design your column at least for this.
Serviceability (cracking and deformations, in this case) is, however, to be
considered separately. In general, departure from elastic analysis does
result in serviceability issues, which may be negligible in some cases --
for instance when the departure is not severe -- and significant in others.

Let us consider the lateral loads next. If you have fairly rigid elements,
more or less symmetrically located within the building, and also a rigid
diaphragm present, the seismic load share of the columns may be greatly
reduced, even on the basis of an elastic analysis. But still there may be
some moments, proportionate to the stiffness share of the column. You would
thus not be departing from the elastic analysis -- not significantly at
least, when you choose to practically design the columns for a certain
nominal moment capacity. As yet another approach, it may still be
permissible to neglect even this lateral-load share of the columns and
designate certain specified members to transmit the seismic loads, to the
exclusion of all other vertical members. But when you make such an
assumption, you must make sure that your column -- now supposedly relieved
of the lateral-load carrying function -- does not have to transmit any local
lateral-load carrying function. For instance, you have to ensure that the
wind load acting on the cladding, can be transmitted to the slab-system,
independent of the column, possibly in the vertical direction.

To sum it up, if there actually are columns in the system and you were to
assume that there are none -- at least that they are not available for
moment transmission -- you must make sure that no moments are actually
caused by the rebar detailing and also that the lateral loads can be
transferred independent of the columns that are being counted out of the
system. In such a case I would not even be inclined to accept the
six-storied limit, that you mention.

However, frankly I am still not sure if I fully understand the advantages of
choosing to neglect the gravity moments in the columns. If I were to design
a building like yours, I would not even consider neglecting the moments, as
I would find no good reason to do this and woud be reluctant to forego many
advantages that an integral system provides!


I hope it helps.


With kind regards,


Sincerely,


Rizwan Mirza
Rizwan Mirza, Consulting Engineers
Lahore, Pakistan
-----------------------------------------------
----- Original Message ----- From: "Scott Maxwell" <smaxwell(--nospam--at)engin.umich.edu>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, December 20, 2005 5:27 AM
Subject: Re: Column design


Yeah...but for a column (end/exterior column or interior) "just
supporting a pre-cast beam" will still have the load from the beam applied
eccentrically to the column (unless if is a roof beam-to-column
connection, in which case it could be a concentric load).  And that
eccentric load will result in the moment in the column (not matter what
"load pattern" used for an exterior column but only for "unbalanced" loads
for an interior column).

You could in theory have the beam (exterior) or beams (interior) sit on
the column and then have the next stories column sit on the top of the
beams.  But you would still have some eccentric load cases for the
interior columns.  And detailing such a beast so that the beams can just
sit in bearing, but the column can not translate laterally relative to
beam, is rather tough thing to do.

The real point is that typical reinforced cast in place construction you
design the concrete system as a frame.  This includes exterior columns.
This because typical CIP concrete construction has all the beam to column
joints as continual, integral concrete...and thus, such joints cannot
really behave anywhere close to a pinned connection.

Regards,

Scott
Adrian, MI


On Mon, 19 Dec 2005 ASQENGG2(--nospam--at)aol.com wrote:


As I said you have to detail it properly, make sure negligible moment is
transferred to the column. You can detail it in such a way that the end
of  the
beam is just bearing on the column. Just think that the end column is
just
supporting a pre-cast beam.

I can see their argument of using M=wL^2/8 this way. If you have a beam
being supported by the column and design the beam as simply supported
then the
columns will just act as support to beam reactions.

If you design the joint to make sure the moment is transferred from beam
to
column then you should design the column for the moment that is being
transferred.  You only use the ACI provisions for concrete frame if you
design the
beam and column as frame.  In your case you have a concrete  frame with
beams
and interior columns but hinge at the end columns.

Moment are not attracted base on the gross area but based on the capacity
of
the materials as they are being transferred from member to member. Even
how
big your gross area is if your joint is not designed to transferr the
moment
then the moment will just goes back to the beam.  That's the basic
theory
behind moment distribution of frames and continuous beams. That's why as
I said
at the beginning detailing is the key.

ASQuilalaJr., P.E.




In a message dated 12/17/05 2:39:06 P.M. Pacific Standard Time,
ashraf.manjappara(--nospam--at)gmail.com writes:

ASQuilala:
I don't agree with your view and your argument is against ACI 318
S.8.8.1.
What will happen if we are not designing column for the moment it
attracts
based on the gross section area?.






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