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Re: Plwd: Rigid Diaphragm Analysis - Opinions Wanted

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It is my interpretation of section 1630.3.1 that Omega sub o is an 
amplification factor which is to be applied to elements of a structure in the 
same manner as the past code 3Rw/8.  In neither design example from SEAOSC or 
the ICBO Volume II design problems is the Omega factor addressed on the 
entire structure. E is calculated for allowable stress design to be simply 
V/1.4. 

the seminar notes from ICBO go a bit further in the discussion of Em which 
contains an Exemption for light-framed construction:
"The exception recognizes that the design for Em would result in larger 
collector elements and a substantial increase in number of fastners. Such a 
design could make construction difficult and actually decrease capacity due 
to splitting of wood members with large numbers of fasteners.
Light-frame construction generally includes multiple lines of resistanc 
carrying fairly low loads.. and earthquake damage properly designed and 
constructed wood collector elements has not been reported. Hence, higher 
design force levels for light-frame collectors does not seem warranted." 
(page 182 of Document #109 - "1997 UBC Earthquake Regulations" ICBO 
(copyright 1998)). 

The only real reference to Omega sub 0 is in section 1630.3.1 which 
references specific elements within the building rather than applied to the 
entire structure. 

It is also my interpretation from the ICBO notes (from the seminar listed 
above) that most residential structures are considered "short period" 
structures and the base shear will be controled by 0.24 Na. In this case the 
redundancy factor rarely becomes an issue as the structure is considered 
"well behaved".  

Dennis Wish PE

In a message dated 7/13/99 10:59:54 AM Pacific Daylight Time, 
Bill(--nospam--at)AllenDesigns.com writes:

<< Off topic but relevant (to me, anyway).
 
 When using the Simplified Static Procedure, where in the code does it say
 that you do not have to calculate Rho or Omega?
 
 Under section 1630.2.3.4 Applicability (of the simplified static method), it
 says that sections 1630.1.2, 1630.1.3, etc. shall not apply but it does not
 say that section 1630.1.1 does not apply. Section 1630.1.1 is the section
 where Rho and Omega are defined and specified in E=Rho*E sub h + E sub v and
 E sub m = Omega sub 0 * E sub h.
 
 Is there another section that voids 1630.1.1?
 
 From what I've read, the only things that the simplified static method saves
 is looking at the fault maps to determine Na and the vertical redistribution
 calculation.
 
 TIA,
 
 Bill Allen, S.E.
 ALLEN DESIGNS
 Laguna Niguel, CA >>