Let me refine my questions.
Question 1. The definition of torsional irregularity in Table 16 M includes
is the following "when the maximum story drift, computed including
accidental torsion at one end of the structure TRANSVERSE to an axis is more
than 1.2 times the average of the story drifts of the two ends of the
Does that mean that I have to calculate the drift in Y axis for seismic
input in X axis in order to determine whether or not torsional irregularity
exists in X axis?
Question 2. Section 1633.1 requires the use of orthogonal effects provisions
in each of the followings case
a) The torsional effects exist in BOTH major axis
b) The non parallel systems
c) A column of a structure forms part of two or more intersecting lateral
force resisting systems.
In my project the item a) may or may not be applicable. I calculated that
the torsional irregularity as defined in Table 16M (if I interpreted it
correctly) exists only in one of the principal axis.
Do I not need to use orthogonal effects provisions because torsional
irregularity doesn't exists in BOTH axis?
Question 3. The amplification factor is a multiplier for the minimum
accidental torsional effects. The structure may be irregular in one of the
principal axis and not irregular in the other.
Can the torsional amplification factor Ax (UBC 1630.7) be different for two
axis of the building or do I use the worst case for both axis? In other
words do I penalize the lateral resisting system in both directions equally
when only one direction is irregular?
I appreciate you response.
Alexander Sasha Itsekson
INGRAHAM DEJESSE ASSOCIATES INC.
(510) 527-7223 ext. 209
> -----Original Message-----
To: "INTERNET:seaint(--nospam--at)seaint.org" <seaint(--nospam--at)seaint.org>
Subject: RE: UBC'97, Orthogonal Effects
From: merrick group <merrickgroup(--nospam--at)compuserve.com>
Date: Fri, 7 Apr 2000 14:19:55 -0400
Maybe it would easier to find
the principle axis. Look at the attached rigid diaphragm spreadsheet
to the back of the Dennis Wish Program. The check for principle axis is
a force is applied and there is no perpendicular deflection, other than
affects of torsion,
then that is the principle axis. Forces could be greatly reduced.
question 1.do what with x and y drift for one force?
I would use drift as the total deflection of the x and y components.
question 2. Can orthogonal affects be in a single directional?
I thought orthogonal affects are due nonparallel walls. That affects both
not in line with the principle axis.
question 3 Is Ax directional?
The factor is required for an irregularity definition that is not
directional specific. A building is an irregular building or not.
The Ax factor is to be for both directions. Specific-to-direction are
Accidental-torsion and actual eccentricities, and the deflections used to
Until there is a better argument, I would calculate.
Ax in both directions and use the
> From: Alexander Sasha Itsekson [mailto:sitsekson(--nospam--at)ida-se.com]
> Sent: Friday, April 07, 2000 8:27 AM
> To: List server
> Subject: UBC'97, Orthogonal Effects
> I am confused... Aren't we all.
> I design a rehab for a 3 story flat slab concrete building 250ftx80ft in
size. The architect is chopping 3 out of 4 exterior walls to allow for new
windows. New concrete shear walls are being added at the interior. The
building is sitting on the Bay Mud and is supported on drilled caissons. I
increased seismic forces in my model for orthogonal effects, but I am
looking back at my design trying to cut construction costs due large new
pier cap with piers at the location of new shear walls. New shear walls
create a "C" shape with the web of the "C" located in the center line of the
building and oriented along the building's long side. There is some torsion
to deal with.>
> Section 1633 in part states that in zones 2,3 and 4 one should consider
loads acting in the direction other than principal axis of the building if
the building has torsional irregularity for BOTH major axis. One can use
SRSS method or 100% of seismic force in one direction acting concurrently
with 30% of seismic forces in the other. It increases the loads on the
lateral resisting elements significantly.
> What I am confused about among other things is the definition of torsional
irregularity. Table 16-M states it shall be considered when the maximum
story drift at one end of the structure TRANSVERSE to an axis is more than
1.2 times the average of the drifts of two ends of the structure. The blue
book commentary (page 132) talks about the special concern of the structures
with plan irregularities that can cause the structure to deform in a
direction PERPENDICULAR to the direction of the input ground motion.
Unfortunately, example 10 in Seismic Design Manual does not indicate the
direction of applied load.
> Here are my questions:
> 1. Do I use for my calculations drift along Y-axis when I apply seismic
force along X-axis plus accidental torsion?
> 2. If I have this irregularity only in one of two principal axis, am I
required to consider orthogonal effects?
> 3. Can the torsional amplification factor Ax (UBC 1630.7) be different for
two axis of the building or do I use the worst case for both?
> Thanks in advance,
> Alexander Sasha Itsekson
> INGRAHAM DEJESSE ASSOCIATES INC.
> (510) 527-7223 ext. 209