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# Re: Story Drift: 1994 UBC vs. 1997 UBC

• To: seaint(--nospam--at)seaint.org
• 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 >>

```