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Re: Quick/Easy Question for Seismic List

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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 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 Business: 
Email for Personal: smaxwell(--nospam--at)

On Wed, 22 Mar 2000 EphHirsch(--nospam--at) 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