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Minimum Snow Loads

Minimum Snow Loads

Minimum Snow Loads

(OP)
There seem to be two conflicting interpretations out there regarding minimum snow loads as defined in ASCE-7-98.
   Pg = ground snow load
   Pf = flat roof snow load = Pg*I*Ce*Ct
   Ps = sloped roof snow load = Pf*Cs

The "minimum" Pf is defined in ASCE-7-98 as:
   I*Pg when Pg is less than or equal to 20psf
   I*20 when Pg is greater than 20psf

So here's the point of contention:
  Do you apply that minimum to Pf ONLY if the actual roof in question is flat (or low-sloped?).

I've always applied it to EVERY roof... even steep ones.  Now I'm second-guessing that decision.  Please give me your thoughts.

RE: Minimum Snow Loads

KevinCinNC:

++++++++++++++++++++++++++++++++++++++++++++++++++++++
   Pg = ground snow load
   Pf = flat roof snow load = Pg*I*Ce*Ct
   Ps = sloped roof snow load = Pf*Cs

The "minimum" Pf is defined in ASCE-7-98 as:
   I*Pg when Pg is less than or equal to 20psf
   I*20 when Pg is greater than 20psf

++++++++++++++++++++++++++++++++++++++++++++++++++++++++


The "minimum" that is specified is for cases when the Ce*Ct<1.00.   For those cases, use the "minimum Pf" value you state.  Use that directly if you have a flat roof, and with the Cs factor if you have a sloped roof.  For example, a structure with a fully exposed roof, with upwind terrain characteristics of Exposure D has Ce = 0.8. The strucure is located in, say Cape Gireardeau, MO, where Pg =15 psf.
( for exposure D, it must be on the river.)Assume Importance factor is I = 1.0. If the structure is heated, then Ct = 1.0.  Then Ce * Ct = 0.8.   Then Pf by the equation given is 15 * 1.0 * 0.8 * 1.0 = 12.0 psf.  But the lower limit of Pf is 15 * 1.0 = 15 psf.  So use 15 psf on a flat roof, or 15 psf * Cs on a sloped roof.

The same structure located in Duluth, MN where Pg = 60 psf, (this one must be on Lake Superior), then Pf is by the formula  
   Pf = 60 psf * 1.0 * 0.8 * 1.0 = 48 psf.


the min allowed Pf = 20 psf * 1.0 = 20 psf.

So for a flat roof, use Pf = 48 psf, for a sloped roof, use
Ps = 48 psf * Cs.  

Clear?

Regards,

chichuck

 

RE: Minimum Snow Loads

Both of you guys are leaving out the 0.7 in the equation for pf (Eq. 7-1).  Am I missing something? If you agree it makes the question all the more pertinent.  I'm currently polling my "source" to see if he knows what the intent was.  Hopefully, I'll get back to you.

RE: Minimum Snow Loads

(OP)
Thanks, Dozer.  My understanding (which may be flawed) was that you DON'T apply the 0.7 multiplier when calculating MINIMUM Pf... assuming that the minimum applies.

When the MINIMUM doesn't apply then, of course, Pf=Pg*Ce*Ct*I.

RE: Minimum Snow Loads

Okay, I just got the word from "a highly placed source" that the minimum snow load provision only applies to low slope roofs.  So you do not multiply Cs by the minimum pf for sloped roofs. You multiply it by whatever equation 7-1 gives you.

BTW, I think there is some confusion on what I'm saying about the .7 factor.  All I'm saying is this factor appears in equation 7-1: pf=0.7*Ce*Ct*I*pg but you two were leaving it out.  I'm not sure if it was a typo or you really think it doesn't belong.  I'm just spittin' back out what I read in the book.  I would agree that you do not use the 0.7 for determining the minimum pf value for flat or low-slope roofs.

This whole discussion brings up another point that would probably be worthy of a new thread.  That is I don't think building codes should be written in a narrative style for the technical aspects.  Look at this issue and how different people interpret it differently. Rather stuff like this should be written like you write a computer program.  "If roof slope is less than 5 degrees then . . . blah . . . else blah blah.'' And I don't mean in paragraph form, but rather like a flow chart or pseudo-code (in the programming sense).

RE: Minimum Snow Loads

(OP)
Thank you, Dozer.  That's very helpful.

And I couldn't agree more.  The snow-load provisions in ASCE-7-98 are obtuse.  A flow-chart with some algebra in the decision-nodes would eliminate misinterpretation... and would be simple enough to include as a figure in the IBC.  Based on your "highly placed source", i will probably write a simple macro in excel to do the work for me.

It's worth noting that I looked at a couple of commercially available packages to see what assumptions they used.  One of them applied the minimum Pf to all roofs... even very steep ones.

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