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RE: Story Drift: 1994 UBC vs. 1997 UBC

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Sooo.... "Omega sub o" has NOTHING to do with drift, right?

Bill Allen, S.E.
Laguna Niguel, CA

> -----Original Message-----
> From: Ron O. Hamburger [mailto:ROH(--nospam--at)]
> Sent: Thursday, August 12, 1999 9:00 AM
> To: seaint(--nospam--at)
> Subject: Re: Story Drift: 1994 UBC vs. 1997 UBC
> Here's the scoop on 3Rw/8 being changed.
> If we go way back in the blue book  (before 1988), back in 
> the days when V
> was still equal to ZIKCSW, the commentary noted that the 
> design forces,
> given by that equation were significantly lower than the 
> actual anticipated
> resopnse of the structure, and that probably, the real 
> inelastic response
> was 3 or 4 times larger than that calculated under the design forces.
> Remember this was in the days before common use of dynamic 
> analysis, or
> even widespread knoweldge by engineers of response spectra.  
> In the code
> itself, not much use was made of this, other than that 
> attachment of fascia
> panels had to accomodate 3/K times the computed drift under the design
> forces.  Since K varied from a low of .67 t o a high of 1.33, 3/K was
> roughly "3 to 4" times.
> In 1988, the code was "rationalized" using the theory developed in
> ATC-3.06.  ZIKCSW became ZIC/Rw, and the old structural 
> quality factor, K,
> became 8/Rw, so that and K=1 building in the 1985 UBC was an 
> Rw=8 building
> in the 1988 UBC, etc.  At this same time, it was recognized 
> that the design
> forces were reduced from the actual ground motion by the 
> factor R and that
> this was possible due to a number of factors, including overstrength,
> ductility, hysteretic damping, etc. ,   While all this as 
> true with regard
> to the forces induced in a structure, it is not true with regards to
> displacements.  The best estimate of the real displacement 
> induced in a
> structure by ground shaking is that computed with an R factor 
> of 1.0.  By
> 1988 this had been demonstrated many times by Newmark, Hall, Bertero,
> Krawinkler, and many others.  However, SEAOC held on to the 
> "3 or 4" times
> larger myth from the past.  Now "3 or 4" times larger became 
> 3Rw/8 - with 8
> varying between 12 and 6 still resulting in approixmately 3 to 4.
> Now where the 1988 UBC went wrong is that it used 3Rw/8 both 
> to estimate
> the "real" displacement and also the maximum force that could 
> develop in
> teh structure do to overstrength.  It would have been better 
> if it had used
> a different factor for overstrenght than deflection 
> amplification.  3Rw/8
> is a reasonably good factor for overstrength but not for real 
> deflection.
> 8Rw/8 would have been better for deflection.  In teh 1997 UBC, SEAOC
> finally recognized reality and stopped using 3Rw/8 to estimate real
> deflection.  Instead in 1997 we went to  0.7R.  Still not 
> really enough,
> but a lot closer.  The Omega -sub-0 has replaced 3/8Rw for 
> force and is
> approximatley equal to 3Rw/8