Snow depth calculation in metric in ASCE 7-05

[gbuell]

In ASCE 7-05 Section 7.7.1, at the bottom, it says to determine the snow depth (h.b) by dividing the snow load (p.s) by the snow density (gamma). This is all fine and good, makes perfect sense. But then in parentheses there is this note: "In SI: also multiply by 102 to get the depth in m.)

This doesn't make any sense to me, as determining depth by dividing pressure by density is not an empirical formula, and shouldn't need any "coefficients" to correct for different units systems. If I'm in SI, I have density in N/m^3, and I have load/pressure in N/m^2, so pressure/density should give me the depth in meters without multiplying by any special number. What am I missing here? (I thought that maybe the note is assuming that you are going to try to divide psf by N/m^3, but I did the math to see what the conversion factor would be in that situation and it's not 102.)

 

[gbuell]

Well, I figured it out. The 102 factor should not be in the ASCE 7-05. I looked at the ASCE 7-95 to compare, and in the 95, they define gamma as kg/m^3 for SI. When you convert kg to kN, you have to multiply by 102 (which is (1000 kN/N)/9.8m/s^2). So the conversion factor is correct in the ASCE 7-95. In the ASCE 7-05, however, they've changed the definiton of density for SI to kN/m^2, so the conversion factor is no longer needed. They just failed to remove it from the code. So now I've got a new question: what's the best way to go about contacting ASCE or SEI with a correction for the code, so they can fix it for the next version?

 

[Stillerz]

you need to research the "errata" for the code. Might have already been addressed...in my book there is a heavy black vertical bar next to that note.
They usually dont just remove mistakes from books or codes...they note them and reference some sort of errata.

 

[gbuell]

I don't think the black bar means that it's been addressed in an errata. My understanding is that the black bar means the section has been changed since the last code. In the ASCE 7-02, that sentence says "p.f (or p.s)", while in the ASCE 7-05, that sentence says "p.s". I think that is the change being referred to with the bar. I looked at all the errata I could find for the ASCE 7-05 and didn't see this change.

Also, I found a link on the SEI website to a form called "Proposal to Revise the 2005 Edition of ASCE 7":

http://content.seinstitute.org/word/proposal_form_asce_7-05.doc

I'm going to send that in and cross my fingers.

 

[Stillerz]

You might be the first person to ever calculate snow loads in metric!!!
I kid.

 

[Stillerz]

"pf" should definitely be in there also...might want to mention that.
If you were only working with flat roofs, you'd never calculate "ps".

 

[Stillerz]

i retract my last posting.

 

[gbuell]

I'm not surprised this has slipped through the cracks for so long, cause I really MIGHT be the first person to calculate snow loads in metric (using the ASCE 7 anyway)!

I'll consider adding the thing about pf/ps, it is really strange that they removed pf from the 05.

 

[gbuell]

Wait, why do you retract your last post? It makes sense to me that they should include both p.s and p.f in that sentence.

 

[gbuell]

Well, I suppose if you say "p.f or p.s", it could confuse people with sloped roofs, because they would have calculated values for both p.f and p.s and they wouldn't necessarily match. So to eliminate confusion, even if you just have a flat roof, the new sentence is telling you to go ahead and calculate p.s (which would, of course, equal p.f) for this equation. Got it.

 

[Stillerz]

my sentiments exactly.