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

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Scott:

You asked:  "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?"

Answer:  No, the designer has two options as follows.

1) Compute the accidental torsion by displacing the mass in the
computer model by 5% of the building dimension, in which case
(according to Sec. 1618.8) the accidental torsion need not be
amplified in accordance with Sec. 1617.4.4.5.

2) Compute the accidental torsion by applying static torsional moments
corresponding to the story forces times an assumed 5% eccentricity, in
which case amplification of the accidental torsion in accordance with
Sec. 1617.4.4.5 is required.

-Mike

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

-----Original Message-----
From: Haan, Scott M. [mailto:HaanSM(--nospam--at)ci.anchorage.ak.us]
Sent: Monday, March 19, 2001 1:15 PM
To: Se-Practice List (E-mail); Seaint list (E-mail); IBC list
(E-mail);
Alan Carr (E-mail); Scott Gruhn (E-mail); He Liu (E-mail)
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