## 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.

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

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

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

## RE: Minimum Snow Loads

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

## RE: Minimum Snow Loads

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

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.

## RE: Minimum Snow Loads