ASCE 7-05 Type 4 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.

 

[josta]

Table 12.6.1 of the ASCE 7-05 (pg. 128) seems to have an exemption for this 3D modeling; Seismic Design Category D,E,F, in irregular structures having only horizontal irregularities 2, 3, 4, or 5, the Equivalent Lateral Force method may be used.