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UBC'97, Orthogonal Effects

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> 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
> (510) 527-7223 ext. 209

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