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Re: IBC response spectrum analysis: Is accidental torsion required? See IBC 1618.8.

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You can get seismic nodal forces from the initial dynamic analysis.
Then you multiply 5% of building dimension to the nodal loads at the floor level. That is the torsional loads, and run static analysis to get additional shear force and overturning moment. Those will be added to seismic loads for loading combination. 
If you consider 5% accidental torsion in the modal analysis model, the results may not be reliable dynamically.

When the actual mass and stiffness distribution is incorporated in the dynamic model to account for the torsional effects,
you don't need to consider additional 5% eccentricity torsion.

Kim, Young Ho P.E
KOPEC, Korea 
  

----- Original Message ----- 
From: Haan, Scott M. <HaanSM(--nospam--at)ci.anchorage.ak.us>
To: Se-Practice List (E-mail) <se-practice(--nospam--at)polhemus.cc>; Seaint list (E-mail) <seaint(--nospam--at)seaint.org>; IBC list (E-mail) <IBC_IRC(--nospam--at)listbot.com>; Alan Carr (E-mail) <carr(--nospam--at)icbo.org>; Scott Gruhn (E-mail) <sgruhn(--nospam--at)bbfm.com>; He Liu (E-mail) <afhl(--nospam--at)uaa.alaska.edu>
Sent: Tuesday, March 20, 2001 6:14 AM
Subject: IBC response spectrum analysis: Is accidental torsion required? See IBC 1618.8.


> 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
> 
> 
> 
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