Metail building reactions & Rain on snow surcharge

[JStructsteel]

110' wide metal building, 1/4" per foot monoslope frames (col in middle). For rain on snow surcharge, would it be required? Is my W length 110', thus my W/50 is triggered and 5 psf required?

The reactions have snow load called out, and seems to make sense at 20psf, due to Pf min. They also have a 'rain' reaction, which is a little less than the snow load reaction. Im not sure where that is calculated. I would figure it should be about 1/4 of the snow load, since Pf=20psf, and R on S surcharge would be 5 psf.

Should I question them on the R on S surcharge? What is the 'rain' reaction all about?

before asking metal building engineer, want to know if I am making sense.

EDIT: I see that the R on S surcharge is added to the Pf not Pmin. But still want to know what the rain reactions are.

 

[Eng16080]

According to ASCE 7-16, the rain on snow surcharge is applicable where the ground snow load, pg, is 20 psf or less and the roof slope in degrees is less than W/50. W is the eave to ridge distance in feet. In your case, the load would be applicable if pg is 20 psf or less because the roof slope is 1/4" per foot, or 1.19 degrees, which is less than W/50 = 110/50 = 2.2 degrees. You didn't mention what the ground snow load, pg, is for your project, so I can't give a definitive answer.

If the rain on snow surcharge is applicable, it is added to the sloped roof load case, ps. In your case, due to the roof being nearly flat, the sloped roof snow load, ps, and flat roof snow load, pf, will be the same. So, the rain on snow surcharge would be added to pf.

You should also consider the minimum snow load for low-slope roofs, pm, which is covered in section 7.3.4. The rain on snow surcharge, if applicable, is not added to pm, which is its own load case. (I don't know if this is what you're calling "Pf min".)

 

[JStructsteel]

Thanks I did figure it out, (edite my post). Pf (min) is my flat roof min snow load.

Yes, my Pg is 20psf, Pf is 14, and even with the 5 psf rain its still controlled by 20psf.

Thanks again.

 

[Eng16080]

That seems right.