Horizontal Force from Sliding Snow
Horizontal Force from Sliding Snow
(OP)
The attached cross section shows an existing 12:12 slope structure (x 24' run), solar panels installed a few years ago with standoffs fastened to the sloped roof, and a proposed new skylight system installed on a 30" high side curb at the base of the sloped roof. There are 4 existing skylights openings x 34' long. The original building was built in the early 1970's with existing skylight installed at that time which have leaked since the early 70's. The sloped roofing membrane is an adhered EPDM. The ground snow load is 30psf, exposure category is B, Ct = 1.1 and ridge h = 100'.
In researching for information I found the following formula for a horizontal force (Fs) on a vertical projection e.g. chimney, parapet: Fs = Fv(x)/square root of {(rise squared) + (run squared)}, and Fv = L(0.5*L + B)*Pf for up to 6' width. I have no idea if this formula is applicable, or the basis of it. I'm hoping engineers in mountainous regions have run across it in the past, and know the basis of the formula or can suggest a reference to use.
ASCE 7-10 provides some information for vertical loading. Section 7.9 Sliding Snow gives a weight to be spread across 15', and Section 7.6.3 Unbalanced Snow Loads for . . . Sawtooth, lists Fig 7-6 which shows 2*Pf as the valley. I did not see anything on horizontal forces in ASCE 7.
Any thought or insight someone is willing to share will be welcomed.
In researching for information I found the following formula for a horizontal force (Fs) on a vertical projection e.g. chimney, parapet: Fs = Fv(x)/square root of {(rise squared) + (run squared)}, and Fv = L(0.5*L + B)*Pf for up to 6' width. I have no idea if this formula is applicable, or the basis of it. I'm hoping engineers in mountainous regions have run across it in the past, and know the basis of the formula or can suggest a reference to use.
ASCE 7-10 provides some information for vertical loading. Section 7.9 Sliding Snow gives a weight to be spread across 15', and Section 7.6.3 Unbalanced Snow Loads for . . . Sawtooth, lists Fig 7-6 which shows 2*Pf as the valley. I did not see anything on horizontal forces in ASCE 7.
Any thought or insight someone is willing to share will be welcomed.






RE: Horizontal Force from Sliding Snow
Independent events are seldomly independent.
RE: Horizontal Force from Sliding Snow
BA
RE: Horizontal Force from Sliding Snow
Thanks for the replies!
RE: Horizontal Force from Sliding Snow
Again, i think this is what you are talking about but not certain.
RE: Horizontal Force from Sliding Snow
If melting occurs such that the coefficient of friction is lost between snow and roof deck, the snow will slide off the roof if there are no obstructions. However, if a curb is blocking the free sliding of snow, there will be a horizontal force against the curb. The magnitude of the force can be determined using principles of statics.
If snow is sliding from a high part of the roof and is suddenly stopped by hitting a curb, the force on the curb will be dynamic and will depend on the mass and velocity of flow.
It would be prudent in the present situation, to provide a reasonable resistance to potential horizontal forces acting on the curb. It is not specifically covered by code, so some engineering judgment will be required.
BA
RE: Horizontal Force from Sliding Snow
The vertical wall/curb I wrote about is on the horizontal roof 16" from the bottom of the sloped roof. It's not on the sloped roof. I was trying to find out if other engineers specifically those in higher snow load regions have had a similar situation, and how they approached it or knew a rule-of-thumb to use. I understand on a sloped roof how to breaking the snow load into the normal and parallel components.
BA answered my question in the first sentence of his 2nd reply: "Snow at rest does not ordinarily exert a significant horizontal pressure against a vertical wall . . ."
To be conservative, I intend to use 0.707 * Sliding Snow load/linear foot and add 20% for dynamic impact.
Thanks to those who took time to share their ideas and knowledge.
RE: Horizontal Force from Sliding Snow
If this is the case you could assume that it is similar to a hydrostatic pressure and just it's density assuming the 'active pressure coeffcient' is = 1, similar to water. However during my fort building days I seem to remember snow having some cohesion.
EIT
www.HowToEngineer.com
RE: Horizontal Force from Sliding Snow
RE: Horizontal Force from Sliding Snow