ASCE7-10 snow drift?

[SteelPE]

I have a question regarding snow drift loading in ASCE7-10.

In ASCE7-05 for gable roofs with a width to ridge less than 20 feet, you are required to design for an unbalanced load of IPg. This requirement is repeated in ASCE7-10 but, ASCE7-10 now places an upper limit on the roof slope for the application of unbalanced snow loads of 7:12. How is this limit is to be applied? Is this limit only to be applied to roofs with a width to the ridge greater than 20 feet, or can this limit be applied to all roofs? I believe it applies to all roofs but I am uncertain.

 

[ajh1]

The steeper than 7:12 exclusion would apply to all roofs regardless of size. The same exclusion existed in ASCE7-05.

 

[SteelPE]

ajh1,

Thank you for your response.

As far as I can tell, the exclusion has changed from ASCE 7-05 to ASCE 7-10. ASCE 7-05 had a limit of 70 degrees, a 7:12 pitch is equal to a limit of 30.2 degrees.

 

[ajh1]

You are correct. It was too early in the morning yesterday when I checked on the two provisions.

 

[Splitrings]

Read the commentary on page 431 of ASCE 7-10. It explains the rational.

 

[RFreund]

Good question and good answers.

I have a side question here - the commentary page 431 C7.6.1 first paragraph: "Recent research suggests that the size of this nominally triangular gable roof drift is comparable to a leeward roof step drift with the same fetch."

What is fetch?

EIT

 

[SteelPE]

Rfreund

hahahahahaha I though I was and idiot because I didn't know what a fetch was either. I was having a conversation with another engineer a few months ago about flood loads on the side of a building. He was talking a good game and used the word fetch.... I had to ask him what it meant and felt like an idiot. I lost that battle but won the war... although he would never admit to it

Sorry about that, here you go the definition of fetch:

fetch: the distance in the direction of the prevailing wind that air or water can travel continuously without obstruction

 

[ajh1]

In the case of the unbalanced snow loads it is basically the length of the roof on the opposite side of the ridge. This is the area that the snow is in theory traveling from to form the ridge drift. In some of the older editions of the code it was muddied up a bit because there was discussion of wind-borne snow traveling at orientations not exactly perpendicular to the ridge. Obviously that can happen, but to keep the equations simple they specify basically a perpendicular dimension.

Fetch is also what you do after you throw the ball and your dog decides to ignore you.

 

[RFreund]

Haha, thanks guys!

EIT