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Re: rho or omega?
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- 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? > Thanks for any help you can offer. > Signed, > Darrell Hambley PE > or > Confused in Seattle >
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