Horizontal Force from Sliding Snow

[smb4050]

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.

 

[BigInch]

Do the math. That formula hasn't changed in at least 38 years. Vector addition would still seem to prove the math hasn't changed either.

Independent events are seldomly independent.

 

[BAretired]

I'm not sure I understand the question. Snow is a gravity load. Unless it is sliding, it does not exert a horizontal force on the deck. The gravity load can be resolved into components parallel and perpendicular to the deck but the deck does not feel a net horizontal force from snow.



BA

 

[smb4050]

I have not explained the condition well, and perhaps I'm making it more complicated than necessary. The heart of my question is, does snow at rest exert a horizontal force on a vertical surface or have an equivalent fluid value? I'm not referring a dynamic load nor vertical loading. I have a sliding snow condition (steep slope roof is 24' wide x 34' long with a 1:1 slope). Pf is 23 psf, and the sliding snow value is 220 plf. Approximately 16" from the toe of the steep slope roof, my client wants to install a 30" tall curb to support a skylight. The sliding snow load that is to be spread over a 15' horizontal distance per ASCE 7 will now have a 16" horizontal distance, and will pile up against the 30" curb. My assumption is there are 2 extreme loading conditions, 1) zero horizontal load i.e. a block of ice, or 2) 100% of the snow load i.e. a fluid. It's my belief there should be rationale value between these 2 extremes. I'm trying to determine what will be the horizontal snow load on the 30" tall curb that is 34' long. What is suggested is to break the sliding snow load into normal and longitudinal components to the sloped roof deck, and use the longitudinal value against the vertical curb. Correct?

Thanks for the replies!

 

[EngineeringEric]

I think i understand what you are asking... I would break the weight into horiz/vert components assuming the roof has full snow loading on entire surface and then design my curb for that load... If you look at it as a condition where you can change the slope from level to 90 degree or perfectly vertical and look at the loading in these conditions it will make sense [this is a mental examination not mathematical... use your arm and a pen for reference of the loading ] at 12:12 you are looking at a 45 degree angle so the horizontal load will be .707xTotal weight of snow above the curb... so at say 23 psf * 8 feet of horizontal roof * 1/sqrt(2) = 130plf; but you should include drifts as well.

Again, i think this is what you are talking about but not certain.

 

[BAretired]

Snow at rest does not ordinarily exert a significant horizontal pressure against a vertical wall if the coefficient of friction between the snow and the sloping roof is sufficient to prevent sliding.

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

 

[smb4050]

A brief explanation about the condition, and to thank those who offered suggestions.

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.

 

[RFreund]

I think what smb is asking is - does the snow exert a horizontal pressure like soil would if it were piled against a retaining wall. Or is there an active/passive pressure coeffcient for snow piled against a wall. Correct me if I'm wrong.

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

http://www.HowToEngineer.com

 

[structSU10]

There is a snow load guide produced by ASCE, and in the FAQ section it discusses this. The author references treating the snow as a cohesionless soil to determine horizontal pressure. The author calculates a Ka asuming a friction angle for the snow of 15 degrees, to get a Ka of 0.59. Then he takes the weight at mid depth (which is what you calc per ASCE) and gets the triangular load.

 

[smb4050]

THANK YOU! Yes, this is what I've been looking for. I'm ordering the 2010 snow and wind guides today.