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RE: Plwd: Rigid Diaphragm Analysis - Opinions Wanted

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The omega factor applies to collector elements which is 2.8 in most
cases. There is an exception in the code section that states that this
force is limited by the amount of force the system can deliver or resist
which may be less than the 2.8 factor. Example, for wood diaphragms,
they are expected to yield at 2x their allowable capacity (according to
Ron Gallager in a report on recommended tilt-up building wall anchorage
upgrades) and this would limit the design to an increase less than the
Omega factor. In the Vol I seismic design manual, they state that
overturning of a shearwall also can limit the force developed in
collectors. This makes sense ... once the wall starts to overturn... the
force goes somewhere else. This can greatly reduce the collector design
when compared to the Omega Factor. I design my shear walls for the Rho
(redundacy) times the working stress shear in that line and design the
hold down. Then I back calculate the shear that can be withstood using
the hold down capacity. It has worked out well and always governed so
far in the 3 buildings I have done using 1997 UBC.

The redundacy is not effected by the short period clause ... It still
must be check and is a pain. 

As far as the Vertical & Horizontal earqthquake loading ... it is my
interpretation that they do not occur simultaneously in ASD but do occur
at the same time when using LRFD. I do think more examples are warranted
on this. For example, In Seismic Design of Building and Bridges (one of
the recommended texts in the back of Seismic Design Manual Vol I) the
tilt-up wall panel example does not include the vertical earthquake
component even though it is designed using concrete strength design. It
appears that any load combination containing E in strength design is
where this applies. Also, the 0.85 D overturning load combination is not
clearly referenced anymore. It would seem that this is one important
place where the vertical earthquake component could be a problem. Since
most overturning analysis was previously done with working stress loads,
it makes for confusing practice to keep switching from working stress to
strength design depending on what you need to check.

My 2 cents

Gerard Madden
CRJ Associates
Design Engineer, Structural Department

> -----Original Message-----
> From:	Seaintonln(--nospam--at) [SMTP:Seaintonln(--nospam--at)]
> Sent:	Tuesday, July 13, 1999 8:02 PM
> To:	seaint(--nospam--at)
> Subject:	Re: Plwd: Rigid Diaphragm Analysis - Opinions Wanted
> 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) 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.
>  Laguna Niguel, CA >>