Seismic Horizontal Irregularity

[arcrdm]

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.

 

[dougantholz]

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.