snow loads arch roofs

huangyhg

snow loads & arch roofs
i have some questions regarding snow loading around a arched roof.
we have a school we are working on which is essentially square in plan with a rectangular gymnasium in the middle of the square.  the gymnasium walls project a minimum of about 2.5 ft above the 1.5:12 sloping low roof (which runs all around the gym) and the gymnasium roof consists of glulam arch trusses (total depth of approx. 10 ft), wood framing and a corrugated steel roof deck.
i am trying to calculate the snow build up around the perimeter of this high arched gym roof as per nbcc (cdn code) and am a little unsure of the best way to do this.
whenever you have two roofs separated by a step height, you get a snow build-up on the low roof, which is a function of the difference in elevation from the upper roof to the lower roof.  now, what about for arch roofs?  if i take the top of the arch as the step height, then the snow build up load is very high, but if i use the low end of the arch the snow build up is much lower.  
in addition, the tricky part is that this is a slippery metal roof, so sliding snow comes into effect as well on two sides of the arch.  would i consider the sliding snow from the unbalanced load case to all end up on the low roof, or do i use the basic snow load from half the upper arch?  how would it spread out along the low roof after sliding?
ps - go flames go!

awwww come on guys...no bites, not even jae or boo1?  don't you get any snow down there?
well first of all i would trash the slippery roof thing, however i would add the sliding load onto the lower roof.  in eastern ontario during the ice storm we had about 6 inches of ice and it connected to the screw heads on all the 'slippery roofs' and wouldn't let go ...then we got a foot of wet snow and then it rained. ont. bldg code is proposing to increase snow loads by about 15%, ie 1/50 year return period, based some what on the loads resulting from this winter storm.  besides its amazing how many times you go back and find snow gaurds mounted along the eave.
whats the diff. higher lower roof along the sides of the arch and the gable ends?  whats the span of the arch?
i think the mid height of the arch would be a good starting point for the gable ends of the arch.  you may want to consider using the full height of the arch for the middle third and then the outside mid height for the outside thirds.  i always start this way and the divide this load by 3 kn/m^3 to see if it makes sense... the l* and w* lengths could have a significant effect here.
the sides the same thing only i would use the mid-height of the arch.
unbalanced load, the figure in obc is clear, however depending upon the exposure, ie north or south,
really need to know the geometeric configuration of the arch to comment further, length, width? and geometry of lower roof.  melt water from the arch if it has a southern exposure, were does it go? into the snow of the lower roof?
if i understand your geometry correctly, the step height is that at the interface of the low roof with the high roof, without regard to the height of the arch.  the width (span) of the arch also affects the drift height on the lower roof.  the drift can then be applied with a triangular load distribution on the low roof.  check both windward and leeward conditions.  (the example i gave is windward).
connect2, ron - thanks for your answers.
the arch is spanning approximately 50 ft.  in addition, the walls supporting the arch trusses project about 2.5 feet above the low roof.  the dimension from the peak of the arch to the low roof is about 10 feet.  the plan dimensions of the arched roof are 74' (22.6 m) by 53.5' (16.30 m).  the 74' dimension runs north south.
i ran the snow build up loads for the low roof at the gable ends (for wpg, ss = 1.7, sr = 0.2), with l = 22.6, w = 16.3 for the following step heights

a) 11 ft -> so = 5.17 kpa (108psf)!! with 6.0m long drift
b) 2.5 ft -> so = 2.48 kpa (52psf)!! with 1.533m long drift.
quite a big difference!
i'm thinking i should reduce l,w for the step height of 11 ft or use an average step height for the full w,l?
sorry, the dimension from the peak of the arch to the low roof is about 11 ft instead of 10 ft.