Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Story Drift: 1994 UBC vs. 1997 UBC

[Subject Prev][Subject Next][Thread Prev][Thread Next]
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


> From:          "Bill Allen" <Bill(--nospam--at)AllenDesigns.com>
> Subject:       RE: Story Drift: 1994 UBC vs. 1997 UBC
> Date:          Wed, 11 Aug 1999 11:13:10 -0700
> Thanks for the history lesson.
> However, this does little to make me feel comfortable about determining the
> required stiffness of the flagpole. Based on the 1997 UBC, Ix is about 1/3
> that required by the 1994 (that is, IF I'm reading this section correctly).
> Is THAT the intent of this section of the code?

> > > "Flagpole" columns (i.e., inverted pendulums)
> > > h=8 ft.
> > > D=Delta
> > > Ds=Delta sub s (service)
> > > Dm=Delta sub m (maximum inelastic)
> > >
> > > 1994 UBC:
> > > Ds < 0.005*h=0.48 in
> > > Dm=Ds*3*Rw/8=0.48 * 3 * 3 / 8 = 0.54 in.
> > >
> > > 1997 UBC (1630.9.2)
> > > Dm=0.7 * R * Ds < 0.025 * h
> > > Ds < 0.025 * h / 0.7 * R = 0.025 * 8 * 12 / (0.7 * 2.2) = 1.56 in
> > >
> > > Is it true that we can now design flagpoles (and steel
> > frames for that
> > > matter) for an allowable drift of three times than before
> > or have I missed
> > > something???

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
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