Snow Drift loads in the Sierras

[cmhski]

I am trying to determine the drift loading for an attached deck, but my numbers using the UBC 97 Appendix 16 equations seem very high.

Site info:
Pg = 230 psf
Pf = 161 psf
Wb = 30 ft
Width of deck = 10 ft
hr = 8.5 ft (the deck is up against a gable wall with eave height = 8.5 ft & ridge height = 15 ft, how does this affect the drifting? (there is clearly no sliding snow load)).

Our local snow is known as "Sierra Concrete", so drifting is not that common. Is it possible to make a judgement call based on local conditions?

Any comments/sugestions much appreciated,
cmhski

 

[whyun]

With strict interpretation of the code, you must design for the drift load per Section 1644. Your basic ground load seems high, but presumably it is determined by the local building official. Using this high Pg value, I understand drift loads will be very high.

In lieu of making a judgment call, you should contact the local building official and determine whether they require drift loads.

 

[REBSE]

It appears that your situations resembles Fig. A-16-4 and that the hr cannot be greater than the ridge height otherwise there is no longer a structure for which the snow can drift against. This is of course the maximum drift load and it would change linearly down to the eave height using the same rational. I had a similar condition in Truckee where we used this rational. But I agree with whyun to check with the building official and get his/her opition.

Good luck,

Ryan Erick Broomé, P.E.
Jacobs Consulting Inc.

 

[cmhski]

Thanks for your input. I called the building department, and they said that they usually do not require drifting to be taken into account when dealing with decks & landings, but that the individual engineer could decide otherwise. Their argument was that the snow drift section of the UBC 97 is in an appendix, and not in the main body of the code, so individual jurisdictions could decide how/when to apply it. To summarize, in this situation where sliding/impact is not an issue, it would be ok to design for Pf only.

cmhski

 

[whyun]

Did you talk with the building department about what percentage reduction can be taken for snow load as a part of the seismic weight?

This is related to 2001 CBC Section 1612A.3.2 Exception number 2.

What is your building department's requirement in how to determine the percentage reduction? It appears to be a total judgment call. For example, flat parapetted roof presumably can't take ANY reduction where as a high-pitched roof where snow can slide may take the full 75% reduction.

 

[cmhski]

Hi whyun,

Their website (where snow drift is not mentioned even once) states that: "One-third of the design snow load shall be added to the deadload for seismic design. (Ord. 93-05 §1(part), 1993: Ord. 88-01 §1(part), 1988: prior code §15.06.020)"

Yesterday, I also learned that they do not allow the 1.15 load duration factor for snow loads as they reasonably argue that the local snow load duration is longer than 2 months.

cmhski