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Re: Story Drift: 1994 UBC vs. 1997 UBC

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Dennis:

You wrote:
> 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.

The 3Rw/8 factor you are referring to was intended to make critical 
elements STRONGER not stiffer.  The overstrength factor in the 1997 
UBC has the same intended purpose (make critical items stronger) and 
was calibrated to produce results consistent with 3Rw/8.  The 1994 
UBC muddied the waters by requiring deflection compatibility for 
displacements amplified by the same 3Rw/8; that displacement was 
too small.  Now the displacements are approximated at 0.7R times the 
displacement due to the design force.

> 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.

While the expected deflection is not a function of the level of 
inelastic response (read R value), it is a function of the period of 
the structure.  Stiff buildings have low periods and thus less
relative displacement.  The displacement demand on a flexible 
element next to a stiff element is still controlled by the response 
of the stiff element.  The change is that the estimation used in 
the 1997 UBC of the total inelastic displacement is more realistic.


-Mike


> 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.

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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