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In general, interstory drifts CANNOT be used to gauge the story 
stiffness, regardless of the pattern of loading used.  Consider the 
two simplest systems with uniform story stiffnesses.  Moment frame 
structures typically assume the "shear building" type of deformation 
noted below.  Shear wall structures typically assume the "cantilever 
building" type of deformation noted below.

Shear building
   .                     o
   .                     o  Almost zero interstory drift at top
   .                    o
   .                   o
   .                  o
   .                o
   .             o
   .          o
   .     o  Maximum interstory drift at bottom

Cantilever building
   .                         o Maximum interstory drift at top
   .                    o
   .                o
   .            o
   .         o
   .      o
   .    o
   .  o
   . o
   .o Almost zero interstory drift at bottom

Even when the story stiffnesses are identical, there is a significant 
variation in the interstory drifts.  In fact, if there is a 
significant stiffness discontinuity near the bottom of a shear wall 
building, the interstory drifts might be IDENTICAL due to the 
cantilever mode of response.

With respect to the comment you quoted from the Design Manual, if the 
structural configuation is so complex that a calculation of the story 
stiffnesses is difficult, I believe that modal analysis should be 
used anyway. 

This is just one reason of many that I almost always use modal 
analysis for multistory buildings.


> From:          "Mark Oakford" <oakfordm(--nospam--at)>
> To:            "Post SEAINT" <seaint(--nospam--at)>
> Date:          Sat, 9 Oct 1999 16:16:38 -0700

> Does any one have an opinion for or against substituting inter-story drift
> for stiffness in determining whether a building has a soft story or not?
> UBC 1997 Table 16-L, Vertical Irregularities Type 1 defines a soft story as
> one in which the lateral stiffness is less than 70 percent of that in the
> story above or less than 80 percent of the average stiffness of the three
> stories above.  If a structure has a soft story the 97 UBC requires use of
> the dynamic analysis procedure in Section 1631.
> Example 4 in the SEAOC Seismic Design Manual uses inter-story drift to
> determine if a soft story exists. The example goes on to say: "Generally, it
> is not practical to use stiffness properties unless these can be easily
> determined.  There are many structural configurations where the evaluation
> of story stiffness is complex and often not an available output from
> computer programs.  Recognizing that the basic intent of this irregularity
> check is to determine if the lateral force distribution will differ
> significantly from the linear pattern prescribed by Equation (30-1) which
> assumes a triangular shape for the first dynamic mode of response, this type
> of irregularity can also be determined by comparing values of lateral story
> displacement or drift ratios due to the prescribed lateral forces."
> Wouldn't a unit load at the top level of the frame provide the information
> required to determine if a soft story exists?  Why apply a different force
> at every level using equations 30-14 and 15?
> Mark Oakford, P.E., oakfordm(--nospam--at)
> RSE Consulting, Federal Way, WA  98093-1417
> T 253-927-6169   F 253-838-3823

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Michael Valley                                   E-mail: mtv(--nospam--at)
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201