ASCE Sliding Snow, Enclosed Sloped Roof

[hillaryBCTI]

My question is regarding ASCE7-05's snow provisions for sloped roof and how and when to apply them.

See attachment for drawing.

I have a building that has a 6:12 sloped roof. On the North, South, and East sides there are higher buildings and on the West side (for simplicity) there are low buildings.

My question mainly comes into play on the East side. The 6:12 roof slopes downward to Building 9 (shown on the bottom of the drawing) The valley that is created from the sloped roof is 10 feet below Building 9. Essentially, this creates a bucket.

I have calculated my snow loads:
p.f = 42psf - flat snow load
p.s = 42psf - sloped snow load
density of snow = 20.5 pcf

Unbalanced Snow:
Windward = 0.3 * p.s = 12.6psf
Leeward = p.s = 42psf
Leeward Surcharge = h.d*gamma/ sq.rt.(S) = 37 psf
Leeward Surcharge horizontal extent from ridge = 9.75ft

Sliding Snow:
p.sliding = 0.4*p.f*W

When looking at loading the roof members, it is my understanding that my load cases would be:
Balanced Snow Load
Unbalanced Snow Load
Balanced Snow Load + Drift from surrounding buildings (ASCE7-05, section 7.7)
Balanced Snow Load + Sliding Snow

My questions are:
1. How do I approach sliding snow when it will sit in a valley on the same roof vs. sliding off completely onto a lower roof? ASCE states that the sliding snow will act over 15' (assuming a lower roof) will the snow in this case sit 15' horizontally on to the roof in the drawing?

2. Since the building to the East is higher, I am assuming there will be drift coming onto the sloped roof. How will this interact with the sliding snow? Should they be added together?

3. Should I add together the sliding snow with the balanced snow load on the sloped roof? I percieve the snow as all sliding down to the valley and accumulating there. And thus, there would be no more flat/sloped snow on the roof (barring another snow storm). I am assuming the comment in ASCE that it be added to the balanced snow load was to account for the balanced snow load on the lower roof as well as the snow from the higher roof.

4. Would it be a valid assumption to fill the valley between the roof and the East wall with snow 10' deep to account for if the snow was just to sit there?

Any help/insight you can offer would be appreciated.

If pictures or calculations would help, please let me know and I can post them.

Thank you!

Hillary

 

[SteelPE]

I found your picture/plan a little confusing:

I thought ASCE 7 had a provision where sliding snow could be reduced if the sliding snow was blocked by other snow on the roof. So in a valley you wouldn't really have sliding snow but you may have drift as you indicated above because of the higher roof up against your valley.

You may want to take a look at the "saw toothed" roof requirements. Although not a real saw tooth you have some similar features.