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SOFT STORY, SEAOC SEISMIC DESIGN MANUAL #4

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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)RSEC.com
RSE Consulting, Federal Way, WA  98093-1417
T 253-927-6169   F 253-838-3823