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RE: Structural Irregularity

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I feel your pain. I'm designing a 3-story residence (2-story with daylight basement) having cantilevered upper floors. To make matters worse I fall under LARUCP.  From what I understand so far, and I have not been able to confirm this with any plan checker, but most all structures I will design (SDC D/E) from now on would need to be designed as rigid diaphragm structures.
-----Original Message-----
From: Dennis Wish [mailto:dennis.wish(--nospam--at)]
Sent: Sunday, April 20, 2008 11:17 PM
To: seaint(--nospam--at)
Subject: Structural Irregularity

I just picked up Breyer’s “Design of Wood Structures” and noticed that in Chapter 2 and in Chapter 16 he has a very interesting discussion of geometric (vertical and horizontal) irregularity that would prevent the structure from being designed as a flexible analysis.  I was under the impression that if you had a vertical irregularity, you could introduce a shear element that did not stack below the shear element above and resolve the irregularity as long as you can create a collector that would drag the shear from above into the new shear wall in the line of shear at the lower level. According to Donald Breyer this is not the case and would trigger a Vertical Irregularity. What I am not clear of in his chapter 16 is whether or not you can still use flexible diaphragm design if you introduce the Overstrength Factor  - Omega into the seismic shear calculation. If this is not the case, then the assumption leads to the conclusion that in SDC D,E and F the diaphragm will have to be designed as rigid unless all shear walls stack and all exterior walls have shear walls in them and are not set back but stack as well.


Any comments on this since most residential structures could not comply with this unless they were simple box designs.  


Dennis S. Wish, PE


Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant

La Quinta, CA 92253

760.564.0884 (Phone, Fax and Answering Machine)