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Unbalanced Snow

Unbalanced Snow

Unbalanced Snow

(OP)
ASCE 7-98 has the greater value of Unbalanced Snow placed at the eave.

ASCE 7-06 & 7-10 have the greater value placed at the ridge.

Nature did not change – our interpretations did.

Side-note: In Wisconsin, we have an allowance to use what is called an “Alternate to the requirements of IBC section 1608.1”, which reduces the resultant unbalanced snow load pressures obtained by the IBC and ASCE method.

Since there is an apparent difference of interpretation, conservatively and for simplicity and expediency, I apply the greater snow pressures over the entire width of the structure. Although this seems overly conservative, which may result in higher cost, since my supervisor has instructed me to pursue this direction, and it does exceed minimal requirements of IBC and ACSE, I am comfortable with this decision, in addition, since experienced estimators of the general contractor client have rarely scoffed at any structural member sizes I have specified over the past years, since the adoption of IBC, that are related to unbalanced snow.

What are your opinions or wisdom regarding this?

What practices do you use? How do you apply unbalanced snow?

Thank you all!

RE: Unbalanced Snow

The three versions of the over-ridge snow are evolutions. The ASCE7-98 (IBC 2000) was the first instance of this phenomena being codified as compared to the classic unbalanced snow load for steep pitches. Over time with some additional research and rational thinking the -05 and then -10 versions were created. The inherent problem with the initial pass on this was that it presupposes that ALL of the snow that had existed on the windward side of the ridge is going to end up in the drift on the leeward side of the ridge. None of the snow would remain on the windward roof (although you are expected to design for 30% of the roof load on that roof anyway) and none of it would blow farther than the edge of the roof and fall on the ground or the neighbor's structure, etc. This assumption was made despite the fact that the typical stepped snow drift (high roof to low roof) which has been around for a long time was based on approximately 35% of the snow ending up in the drift and the remainder either staying on the high roof or blowing off onto the ground. The stepped snow drift has over the years been shown to be quite accurate in terms of the drift size and formation, so the 35% value appears to make a lot of sense. This magnifies the issue with the over-ridge drift as some of the low pitches on which it applies could be questioned as having any drift requirement let alone a 100% requirement. The latest -10 provisions actually use the same basic mechanism to calculate the load as the stepped roof condition, with some rules added for how to proportion it for this condition. In actuality, the drift should probably be a tapered drift rather than the uniform block that the code states, but the uniform block was chosen for simplicity without creating a huge penalty in terms of overall loads.
You are obligated to use at least as much load as specified for the code edition in question, but realistically a large percentage of the country is now on IBC 2012 or 2015, both of which follow the ASCE 7-10 version of the Specifications.

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