Irregularity in Seismic Design Categories; D, E and F

The current code provides an “out” for wood framed buildings with wood diaphragms classified as type I or II structures. These buildings may disregard certain irregularities that would normally trigger the design to require dynamic or rigid analysis. At the very least, it would require the engineer to compare the center of mass to the center of rigidity to ascertain if the building can be designed with a flexible analysis.

I recently picked up a copy of Donald Breyer’s sixth edition of Deisgn of Wood Structures ASD/LRFD co-authored by Kenneth J. Fridley, Kelly E. Cobeen and David G. Pollock (McGraw Hill publications) (see Amazon Book for the best price / delivery times).

While Breyer notes that current code ignores Type I irregularities that may cause torsion in a wood frame structure three stories or less, it points to a number of irregularities (Vertical and Horizontal) that would require additional lengthy analysis and possibly the addition of the Over Strength factor Ω _o to be found in ASCE 7 Table 12.3-1 and in sections 12.3.3.4 for combination with E_m . This appears to include “L” or “U” shaped structures with plan irregularities type 2 at re-entrant corners where the irregularity exceeds 15% of the projection (F1) divided by the corresponding side lengths. In other words if you have an “L” shaped structure 100-feet long with one leg projecting more than 15-feet back from the 100-foot length, then the building will be required to be treated as dynamic or rigid.

However, it appears (and this is where I am presently confused) that if the setback of the projection exceeding 15% of the corresponding length can be penalized by the appropriate Ω _o found in the code. Is this correct, or is there more to it? I’ll be exploring this issue after sending out the Alpha version to see if the Omega penalty will allow light-framed structures three stories or less used in residential construction to be exempt from the rigid analysis or comparison of the center of mass to the center of rigidity.

This article is currently still under construction and will be added to by the author in the near future as confirmation to the issues noted can be verified and documented. Please feel free to add your comments below, or to start a discussion on this topic.

~ by structuralist on April 20, 2008.