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# Re: rho or omega?

• To: seaint(--nospam--at)seaint.org
• Subject: Re: rho or omega?
• From: Robert Shaffer <rkdn(--nospam--at)cruzio.com>
• Date: Mon, 25 Oct 1999 13:16:29 -0700

```Darrell,

For light frame construction such as a house,  you are probably designing based
upon working stress (ASD).  That being the case E= rho *  Eh/1.4.  Ev=0 per
definition and the 1.4 factor is per eq 12-9 thru 12-11 for ASD.
Omega applies to collector elements and replaces the previous 3Rw/8 factor in
some circumstances,  but in your case you are probably exempt per 1633.2.6.
r.i = (Vtrib * 10/lw)/Vtotal   If your house is a simple box, then Vtrib is
half Vtotal and  Vtotal can be calculated per eq 30-11 in section 1630.2.3
which is plenty conservative when compared with eq 30-5.  The lw is your length
of shearwall,  so if you have less than 10 feet of shearwall you get
penalized.  Rho is then calculated per eq 30-3 and applied to Vtotal if it is
greater than 1.0 .
The equations 32-1 thru 32-3 for Fp are only for elements of structures and
usually do not apply in a house ( ie wall anchorage in a tilt-up or masonry
building ).
ICBO has a very good seminar on the new 97 UBC lateral requirements.

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?