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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: Seaintonln(--nospam--at)aol.com
- Date: Wed, 11 Aug 1999 16:59:05 EDT
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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