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RE: Quick/Easy Question for Seismic List

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You appear to be absolutely correct regarding the units.  Doesn't
matter.  I was just confused about the magnitude of the drift.  I think
somebody else on the list hit on something this morning.  This is an
inelastic drift limit.  Elastic deflections from our computer analysis
should be amplified by Cd (4.5 in this case) for comparison.  If we take
the 1.48' or 17.76" divided by 4.5, we get an elastic drift limit of
around 3.9".  I understand from the reponses that we need to look at
story drift, between stories.  However, based on our previous
discussions, a total height of 74', etc...our allowable elastic drift
limit would be around 3.9" which seems more realistic.  Inelastic drift
would then be the 17.76".  I read that the UBC uses something like 0.005
times story height.  If you take the 0.02(hsx) value of drift limit for
BOCA, and divide by the amplification factor (Cd = 4.5), then we get
something close to what UBC has.   

Sorry for the previous confusion on height of the building, number of
stories, etc.  

Thanks Paul,  


> -----Original Message-----
> From:	Paul Crocker [SMTP:PaulC(--nospam--at)]
> Sent:	Thursday, March 23, 2000 1:44 PM
> To:	seaint(--nospam--at)
> Subject:	Re: Quick/Easy Question for Seismic List
> > 17.76 inches is wrong. Your answer of 1.48 inches appears to be
> correct,
> > although little less than I am used to see. (0.166%)
> >
> > The height used should be in feet not in inches. The formula uses
> feet. See
> > then definition of the story height.
> Why do you assume that if you put feet into the equation that it
> yields something
> other than feet as its result?  It is also acceptable to use meters
> for story
> height.  Surely this could not be so if the 0.02 was calibrated to
> make a feet to
> inches conversion.  If the EERI web site still has the field report
> from the
> earthquake in Columbia last year (?) posted, it might be worthwhile to
> read this,
> as it briefly discussed observed correlations between drift and
> damage/failure,
> which largely agreed with the pre-existing body of research.
> Depending on your
> anticipated loads and building use, getting too close to the 1.48'
> drift may not
> be appropriate, but I do not see it as a code violation.  Also, if you
> get too
> close to the drift limit, drift will exceed the threshold beyond which
> P-Delta
> analysis cannot be ignored, which may cause problems of its own.
> Overall,
> though, if a maximum inelastic drift limit approach is taken, a limit
> of 1.48'
> for a 74' building is not surprising.  Surely moment frames would
> never be
> possible if a 1.48" level of stiffness was code required.  In fact,
> only the
> longest shear walls could possibly comply with that.  Try to visualize
> 1.48" in a
> 74' building under extreme loading and imagine if that would be
> possible.
> Paul Crocker