# RE: Quick/Easy Question for Seismic List

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: RE: Quick/Easy Question for Seismic List
• From: "Ritter, Mike" <mritter(--nospam--at)lgt.lg.com>
• Date: Thu, 23 Mar 2000 08:35:50 -0500
```Scott,

Thanks,  Mike

> -----Original Message-----
> From:	Scott E Maxwell [SMTP:smaxwell(--nospam--at)engin.umich.edu]
> Sent:	Wednesday, March 22, 2000 11:12 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Re: Quick/Easy Question for Seismic List
>
> I believe that the methods used in BOCA and UBC are different which
> would
> potentially explain the difference you noted.  I am not 100% sure of
> this
> since we don't use the UBC that much around here.  For the BOCA, one
> checks the story drift.  As you and others noted, this means that the
> story height not the building height should be used and the story
> drift
> for each level of the building should be checked.  BOCA gives an
> allowable
> story drift (delta "a") in a chart.  The values range from 0.007 to
> .025
> of the story height (there is one possibility of unlimited drift)
> depending of the type of building system and the "Seismic Hazard
> Exposure
> Group".  The allowable drift is compared with the value from analysis.
> The analysis value is the actual story drift from an ELASTIC analysis
> multiplied by a value called Cd.  The Cd value is used to "convert"
> the
> elastic story drift to an approximate inelastic story drift.  This is
> similar to what the R factor does when determining forces.  The Cd
> values
> depend on the type of lateral system and range from 1.25 to 6.5.  If
> you
> take a value of allowable drift of 0.02 and divide it by 4 or 5 (a Cd
> value) this would be similar to the values that you mention from the
> UBC.
>
> Hope this helps.
>
> Scott
>
> ----------------------------------------------------------------------
> ----
> Scott E. Maxwell, SE, PE
> Structural Engineer
>
> BEI Associates Inc.
> 601 West Fort Street
> Detroit, MI  48226
>
> Telephone: (313) 963-2300
> Fax:       (313) 962-4269
>
> Email for Personal: smaxwell(--nospam--at)engin.umich.edu
> ----------------------------------------------------------------------
> ----
>
>
> On Wed, 22 Mar 2000 EphHirsch(--nospam--at)aol.com wrote:
>
> > Story drift is relative displacement between each story level, not
> overall
> > building height.  It shouldn't matter whether you're in feet or
> inches, as
> > the factor should be dimensionless (i.e. a %age) so that the
> resultant drift
> > would be in the same proportional units. (Unless BOCA has some other
> way of
> > stating the factor, since .02 seems unusually high.  UBC seismic
> drift is on
> > the order of .004 or .005 times the story height depending on the
> natural
> > period.)  The value calculated using the total building height would
> not be
> > story drift but total cumulative horizontal displacement at the
> roof.  By the
> > way, a 3 story building 74 feet high?!
> >
> > E. G. Hirsch, SE
> >
> >
>
>

```