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# Re: rho or omega?

• To: seaint(--nospam--at)seaint.org
• Subject: Re: rho or omega?
• From: Paul Crocker <PaulC(--nospam--at)ckcps.com>
• Date: Mon, 25 Oct 1999 11:06:30 -0700

```The equation for Em in 30-2 is typically used for attachments of non-structural
elements, such as a fascia system, large fixtures, etc., or for lateral load
resisting elements that may have a brittle failure mechanisms.  Equation 30-1
assumes a degree of ductility will be present and reduces loads on that basis
(the loads are inherently reduced in the Eh portion of the equation via the use
of R), and thus is not appropriate if deformation is not possible.  The omega
is intended to increase the load since the structure as a whole will likely see
a larger load than implied by Eh.  Notice that omega is smaller for structures
with lower R values; less ductility is being assumed thus non ductile portions
are closer to the appropriate load levels already.  If you did a typical design
approach for the structural elements, and didn't have any particularly brittle
details, using 2.8 in place of 1.5 is probably more than adequate.  This will
depend entirely on the details of the design, of course.  I imagine that
telling the plan checker that you used rho of 2.8 should put the issue to rest
rather quickly.

Darrell Hambley wrote:

> I submitted a set of calculations for a residential home and they said I
> need to show what I used for rho (equation 30-1 in '97 UBC). I used
> equation 30-2 with omega instead because it seemed a worse case (2.8 for
> omega instead of a default value of 1.5 for rho).
> Now I'm confused as I read the UBC further. If anyone can answer a few of
> these questions I'd be really grateful!
> 1. When do you use 30-2 instead of 30-1? "E" (in equ 30-1) is for elements
> according to 1630 but, according to 1630.3.1 omega (in equ 30-2) is for
> "specific elements" also. They're both for elements. What's the difference?
> 2. Now please look at 1632, equ 32-1(elements). Fp=4*Ca*I*W which is 1.44W
> in zone 3. That's a big number! Paragraph 1630.1.1 defines Eh as equal to V
> or Fp. I don't like that word "or". Which one do I use? Is it implied that
> one value is for the structure as a whole and the other is for a specific
> element?
> 3. The 1.4 factor...  E/1.4 from 1612.3.1 will lower the value of E of equ
> 30-1, (E=pEh) but now equation 30-2 is still larger so question 1 above
> still confuses me... When do I use equ 30-2?
> Signed,
> Darrell Hambley PE
> or
> Confused in Seattle
>
>

```