ASCE 7 Rain-on-snow surcharge

[enfsgi]

Does anyone know the rationale behind this - W/50 slope? Example: 1/4:12 roof pitch = 1.19 degrees. W = 60 eave to ridge, W/50 = 1.2. Slope is less than 1.2 so surcharge is added. Same roof, W =55, W/50 = 1.1. Slope is now greater than 1.1 thus surcharge is not required. Seems there is a magical length for each roof pitch when this must be considered. Is this correct or is there something I have misinterpreted?

 

[woodman88]

Be nice to know which book you are talking about.
But if from the ASCE 7 10, the Commentary states the below.

"C7.10 RAIN-ON-SNOW SURCHARGE LOAD
The ground snow-load measurements on which this standard is based contain the load effects of light rain on snow. However, because heavy rains percolate down through snow packs and may drain away, they might not be included in measured values. Where pg is greater than 20 lb/ft2 (0.96 kN/m2), it is assumed that the full rain-on-snow effect has been measured and a separate rain-on-snow surcharge is not needed. The temporary roof load contributed by a heavy rain may be significant. Its magnitude will depend on the duration and intensity of the design rainstorm, the drainage characteristics of the snow on the roof, the geometry of the roof, and the type of drainage provided. Loads associated with rain on snow are
discussed in Colbeck (1977a and 1977b) and O’Rourke and Downey (2001).

Calculated rain-on-snow loading in O’Rourke and Downey (2001) show that the surcharge is an increasing function of eave to ridge distance and a decreasing function of roof slope. That is, rain-on-snow surcharges are largest for wide, low-sloped roofs. The minimum slope reflects that functional relationship.

The following example illustrates the evaluation of the rain-on-snow surcharge. Consider a monoslope roof with slope of 1/4 on 12 and a width of 100 ft with Ce = 1.0, Ct = 1.1, I = 1.2, and pg = 15 psf (0.72 kN/m2). Because Cs = 1.0 for a slope of 1/4 on 12, ps = 0.7(1.0)(1.1)(1.0)(1.2)(15) = 14 psf (0.67 kN/m2). Because the roof slope 1.19° is less than 100/50 = 2.0, the 5 psf (0.24 kN/m2) surcharge is added to ps, resulting in a design load of 19 psf (0.91 kN/m2). Because the slope is less than 15°, the minimum load from 7.34 is I × pg = 1.2(15) = 18 psf (0.86 kN/m2). Hence the rain on snow modified load controls."

It was the same in the ASCE 7 05 I think. Hope I type it without mistakes.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[enfsgi]

Current code references 2005. The input is appreciated.