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RE: Story Drift: 1994 UBC vs. 1997 UBC

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check how your variables are defined.

Ds=design level response displacement(s=strength design, not service)

drift defined per 1994 UBC = Ds/1.4 

-----Original Message-----
From: Bill Allen [mailto:Bill(--nospam--at)AllenDesigns.com]
Sent: Wednesday, August 11, 1999 11:13 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Story Drift: 1994 UBC vs. 1997 UBC


Michael,
Thanks for the history lesson.
However, this does little to make me feel comfortable about determining
the
required stiffness of the flagpole. Based on the 1997 UBC, Ix is about
1/3
that required by the 1994 (that is, IF I'm reading this section
correctly).
Is THAT the intent of this section of the code?

Regards,

Bill Allen, S.E.
ALLEN DESIGNS
Laguna Niguel, CA


> -----Original Message-----
> From: Michael Valley [mailto:mtv(--nospam--at)skilling.com]
> Sent: Wednesday, August 11, 1999 10:19 AM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Story Drift: 1994 UBC vs. 1997 UBC
>
>
> Bill:
>
> The short answer:
> Your "flagpole" designed using the 1994 UBC would really deflect 1.5
> to 2 inches in the design event; the UBC was lying about the expected
> deflection.
>
> The long answer:
> The 3Rw/8 multiplier significantly underestimates the expected drift.
> This has long (probably for more than 30 years) been recognized by
> researchers, code writers, and many designers.  The UBC
> continued to indicate smaller displacements because a realistic
> estimation of the expected drift concerned code writers.  ATC 3,
> which is the source document for modern seismic design in the U.S.
> was published in 1978 and in it the expected drift is calculated
> as Cd times the drift from code-level forces.  The NEHRP Recommended
> Provisions always carried this same calculation method (based on Cd).
>
> UBC code writers have continued to drag their feet for two reasons:
> 1) the big numbers are "scary" and 2) they generally don't believe
> that practicing engineers are capable of anything that isn't dirt
> simple.  Given increasing concerns about the actual performance of
> buildings in earthquakes (which can be quite sensitive to the imposed
> displacements), it was decided that the first reason was no longer
> compelling.  That's why the 1997 UBC finally reflects knowledge
> concerning displacements that was state of the art in the late
> 60s and early 70s (not bad; only 30 years too late).  On the basis
> of the second reason, it was decided to "simplify" by using 0.7 R
> instead of Cd.  I suppose this "simplification" is justified for two
> reasons: 1) we would all somehow be baffled if we had to look up Cd
> in Table 16-N, and 2) this way the deflection calculation procedure
> (in the western U.S.) can change in the 1997 UBC and change again in
> the 2000 IBC where Cd values are used.
>
> -Mike
>
> *******************
> > From:          "Bill Allen" <Bill(--nospam--at)AllenDesigns.com>
> > Subject:       Story Drift: 1994 UBC vs. 1997 UBC
> > Date:          Wed, 11 Aug 1999 06:08:27 -0700
>
> > Can this be true?
> >
> > "Flagpole" columns (i.e., inverted pendulums)
> > h=8 ft.
> > D=Delta
> > Ds=Delta sub s (service)
> > Dm=Delta sub m (maximum inelastic)
> >
> > 1994 UBC:
> > Ds < 0.005*h=0.48 in
> > Dm=Ds*3*Rw/8=0.48 * 3 * 3 / 8 = 0.54 in.
> >
> > 1997 UBC (1630.9.2)
> > Dm=0.7 * R * Ds < 0.025 * h
> > Ds < 0.025 * h / 0.7 * R = 0.025 * 8 * 12 / (0.7 * 2.2) = 1.56 in
> >
> > Is it true that we can now design flagpoles (and steel
> frames for that
> > matter) for an allowable drift of three times than before
> or have I missed
> > something???
> >
> > Regards,
> >
> > Bill Allen, S.E.
> > ALLEN DESIGNS
> > Laguna Niguel, CA
>
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> - - - - - -
> Michael Valley                                   E-mail:
> mtv(--nospam--at)skilling.com
> Skilling Ward Magnusson Barkshire Inc.
> Tel:(206)292-1200
> 1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:
>       -1201
>
>