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IBC response spectrum analysis: Is accidental torsion required? S ee IBC 1618.8.

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IBC 1618.8 says horizontal shear distribution is required per 1617.4.4
except the amplification of torsion per 1617.4.4.5 is not required for the
portion of torsion included in the modal analysis model.  

IBC 1617.4.4.4 requires masses to be offset 5% of the building dimension
from the center of mass to account for accidental torsion.  It appears for
"response spectrum analysis" the center of masses should be offset in the
computer model to account for accidental torsion or torsion moments could be
determined from story forces and be used in load combinations.  UBC 1631.5.6
specifically required accidental torsion effects per 1630.7 to be
considered.


For a response spectrum design, does the IBC require masses to be offset 5%
of the buiding dimension from the center of mass to account for accidental
torsion?  



Thanks!

Scott M Haan  P.E.
Plan Review Engineer
Building Safety Division http://muni.org/building, 
Development Services Department,
Municipality of Anchorage
phone: 907-343-8183   fax: 907-249-7399
mailto:haansm(--nospam--at)ci.anchorage.ak.us