seismic horizontal irregularity

huangyhg

seismic horizontal irregularity
in table 12.3-1 of asce 7-05 there are five sections referenced for the type 4 horizontal irregularity (out-of-plane offsets irregularity).  all of the sections referenced are the same as asce 7-02 and the source document fema 450-1/2003 edition except section 16.2.2.  this section requires that a 3-d mathematical model be created of the structure.  does anyone know the reason that this section was added to asce 7-05?  i can find no discussion of this requirement, which adds a significant amount of cost/time to the engineering of structures with this horizontal irregularity compared to previous code editions.  
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well unfortunately the nature of the code is that it is written by different people.  asce 7 committee takes the fema/nehrp documents and modifies them; nehrp takes atc documents and modifies them and so on and so forth.  so what ends up happening is that if a member of the asce 7 committee has a strong opinion that horizontal irregularity should require a fem model of the structure, then it will be in the code.  it isn't particularly fair or accurate, but it is the way it is.  i believe, but haven't looked to make sure, that simple structures such as those with a flexible diaphragm are exempt from horizontal irregularity provisions.  so with hope the more complex structures would already have structural modeling.