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Re: Story Drift: 1994 UBC vs. 1997 UBC

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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.


In a message dated 8/11/99 12:17:17 PM Pacific Daylight Time, 
mtv(--nospam--at) 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 
 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 >>