Horizontal Snow Pressures 1

Hi Fred

Interesting question. On the one hand, when I think of snow, there is that fluffy moldable white stuff of our childhood where we built snow forts with vertical walls, on the other hand with additional moisture or pressure does this condition change? Here is what Bernhard Muessen, an expert in snow thermo-mechanics has to say on the subject. Glean what you will.

"The exploration and exploitation of polar regions requires the ability to work in and with the firn layer, which is the topmost layer of the continental ice sheet. A firn layer consists of meteoric snow which changes by metamorphosis from original snow flakes to a stable firn body. As the firn is compressed by subsequent layers of snow, a porous granular material evolves with density that is increasing with depth. This leads to polycrystalline ice in the bottom layers. The porous structure of the firn gives rise to volumetric viscous behaviour. Its granular build-up leads to an increase of volume under shearing conditions (dilatation). Furthermore, firn exists at high homologous temperatures, thus it shows creep behaviour.
A material law for snow and firn is proposed, which accounts for these characteristic features of the material. Following the approach by von Mises, it is based on the assumption of an elastic and a viscous potential. The dilatation is described by the third invariant of the stress tensor to model a strain orthogonal to the shearing direction. Creep experiments are used to evaluate the material parameters."

So....it appears that poisson's ratio may be in evidence for polar snow regions such as these, but for practical purposes I'd guess that lateral creep may not be present or not sufficiently large under normal seasonal conditions to warrant building design consideration in the lower US.

Hope this helps.

PE in NE

If you knew the density - you could proably treat it the same way you would with soil loads.

I would think it is minimal unless there are layers of ice/sleet involved.

Snow placed in tall piles by front end loader adjacent, (six to nine feet away), to an existing metal building caused crushing and bending of the corrugated metal skin of the building both from rolling down the pile and ending against the wall and by sliding off the roof and ending against the wall. This matches no theory of drifting or falling snow but occurs every winter. Estimate 40 psf horizontal load to a six foot height.

Hi Civil

Good observation. Goes to show that a single train of thought is not ususally conclusive.

I was only thinking about lateral pressures due to a precipitation event. As you mentioned, stored potential energy due to a mass of snow falling from a loader would certainly demonstrate viscous behavior immediately upon being deposited, but would it also exhibit any creep effects after in its final position without being subject to further surcharge loading?

PE in NE

The bending and deflection of the metal skin continues after snow placement on thawing and raining days.

Also consider sliding snow from a high roof such as a Pre Eng structure. Generally we provide extra wall girts at the base for the horizontal load that would other wise buckle the light weight sheeting.

Sorry for the late reply but I just found this info this morning. Lateral pressure due to snow is addressed in ASCE's "Snow Loads: Guide to the Snow Load Provisions of ASCE 7-05". There is an example on page 137.