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Re: Story Drift: 1994 UBC vs. 1997 UBC
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- Subject: Re: Story Drift: 1994 UBC vs. 1997 UBC
- From: "Ron O. Hamburger" <ROH(--nospam--at)eqe.com>
- Date: Thu, 12 Aug 1999 09:00:05 -0700
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 Seaintonln(--nospam--at)aol.com on 08/11/99 01:59:05 PM Please respond to seaint(--nospam--at)seaint.org To: seaint(--nospam--at)seaint.org cc: (bcc: Ron O. Hamburger/EQE) Subject: Re: Story Drift: 1994 UBC vs. 1997 UBC If I understand what you're saying, the old method allowed the designer to design the column based on deflection matching an adjacent shear element, when in reality, the column was expected to deflect much more. Therefore, the results of prior codes produced columns that were much more flexible than the adjacent walls and could, conceivably produce greater damage from shear produced by torsion in the diaprhagm. I was always under the assumption that the calculated shear was magnfied by 3Rw/8 time Pe (if you use the '94 UBC Rw for cantilevered columns of 3 from Table 16-P) or 1.125 times. Next I would design the deflection in the column using a fixed column deflection formula. I now fail to see how the defelction can be so much greater in the new code if the actual load applied (assuming working stress design) was simply increased by this factor and static formulat's to determine deflection were used to calculate the bending or deflection in the cantilevered beam. Now I'm confused??? I thought the 3Rw/8 term was to add a level of safety into the design of the column by purposely making it stiffer. Dennis In a message dated 8/11/99 12:17:17 PM Pacific Daylight Time, mtv(--nospam--at)skilling.com writes: << Bill: Yes, the required elastic stiffness is different. However, because it is now based on 0.7 times the elastic displacement for the unreduced forces (because R cancels out), the values can be more or less stringent than in previous editions of the UBC. That is, the old drift limit was based on REDUCED "elastic" design forces that are not really related to the total inelastic displacement that will occur. It has been observed that the total displacement is almost unrelated to the level of inelastic response (whether R is 1 or 8). This has been dubbed the "equal displacement rule", although "rule" is too strong a word. In the example you provided (cantilever columns), more of that fake "service" displacement is now allowed because the displacement that really matters (for seismic response) is the total displacement including inelastic action. For special steel moment frames, less "elastic" displacement is allowed using the 1997 UBC. The total inelastic displacement allowed for both systems is the same using the 1997 UBC. Here's a (non-dimensional) way to calculate the required elastic stiffness (to meet the drift limits): Kmin = F / d Under the 1994 UBC, F was 1/Rw and d was 0.005. Therefore, Kmin is 1/(0.005Rw) which varies from 66.7 to 16.7 as Rw varies from 3 to 12 respectively. Under the 1997 UBC, F is 0.7 (because the check involves 0.7R times the displacement with F = 1/R; R cancels out) and d is 0.025. Therefore, Kmin is 28 regardless of the value of R. The answers would be the same for Rw = 7.14. However, the new way is more consistent with the displacements that cause seismic performance problems. One implication is that displacements that are really due to service-level loads (like wind) may now control. -Mike >>
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