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FW: column detailing

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I'll preface this response by saying that I don't, thank goodness, use
UBC97, since we our juridiction is going directly from UBC94 to the IBC.

However, looking over the sections you mentioned, I noted the following:

1)  1921.7.2.2 states that the maximum spacing of hoops is "one-half the
least cross-sectional dimension of the column", NOT d/2 --> in your case
this would require smax<= 12/2= 6" as opposed to 4.5" or 3".  Of course, the
spacing will depend on many other factors, including the size of both long.
and trans. reinf, bar configuration etc.

2)  I agree that 1921.8.5 is in fact less stringent than 1921.7, since it
allows 2x6"=12" max spacing at midheight, whereas 1921.7 states 6" max full
height.

But I feel that the real question pertains more to the actual expected
performance of the columns, and less to literal Code requirements.  If in
fact the building is VERY stiff in that there is a lot of shear wall area,
then you probably don't need to be quite so concerned.  However, shear wall
buildings can STILL be limber, if it is tall or there are slender shearwalls
etc., so it would depend entirely on the case at hand.

However, I think it is important to, without knowing about your particular
case, remember that once the building is constructed and subjected to an EQ,
it will act as it exists, and SAYING that a column is not part of the LLRS
doesn't mean anything.  If it is stiff enough to compete for load with the
designated LLRS, it will take load, if not, it won't.

Perhaps you could analyze the structure for the expected lateral
deformations, accounting for the fact that the LLRS members will likely be
cracked (lower Ieff) much more than the "non-participating" members such as
your column.  If you really reduce the shear wall stiffness, and keep the
column stiffness the same, and the column is still limber enough (relative
to the shearwalls) such that it doesn't take much load, you're home free.
If not, then perhaps some level of ductile detailing is warranted.

Anyway, you asked for comments so there you go!  Hope this helps.

T. Eric Gillham PE

-----Original Message-----
From: Evan Mizue [mailto:eam(--nospam--at)baseengr.com]
Sent: Friday, June 08, 2001 9:36 AM
To: seaint(--nospam--at)seaint.org
Subject: column detailing


BACKGROUND INFO:  I have 48"x12" reinforced concrete gravity-load-only
columns.
The columns are not part of the lateral force resisting system.
The lateral force resisting system is reinforced concrete shearwalls.
The building is in UBC Seismic Zone 2.

QUESTION:  Does the UBC code require that I space stirrups @ d/2 min. for
the entire length of the column?

Section 1921.2.1.3 requires that the columns be detailed per Section 1921.7.
I interpret the minimum requirements of Section 1921.7 to be that transverse
reinforcing must be spaced at d/2 for the entire length of the column. For a
48"x12" section, d/2 is about 4 1/2."  Additionally, if the column is
moderately or heavily loaded. The required stirrup spacing is even tighter
(3").

This detailing requirement seems to be excessive in light of the fact that
detailing requirements for actual seismic frames in Zone 2 are required to
conform with 1921.8, which is LESS stringent than 1921.7.  Also ACI 318-95
does NOT require ANY special detailing for non-lateral-force-resisting
columns, as long as you are not in a high seismic design category, i.e. Zone
2. The columns only need to conform to the basic requirements of Chaper 7.

I also don't understand the reasoning for detailing a gravity-load-only
column to perform with a highly ductile response, given that the lateral
force resisting system is r/c shearwalls.  Provided the shearwalls are
designed correctly, in a design seismic event, there is no way the
gravity-load-only columns will experience large deflections or large
deflection-induced forced.

Please comment, thanks!
Evan


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