consider it as mwfrs or cc

huangyhg

consider it as mwfrs or c&c?
i am recently participating in a job to estimate a building roof wind damage (gable roof).  the roof is caved in at windward and ballooned out in the leeward and the roof rafters and joists are buckled.   the estimation report will directly affect the determination of which side insurance company will pay for the repairing. the calcs show that if the roof is considered as a main wind force resisting system and use asce7-05 figure 6-6 to determine the wind pressure coefficient cp, the conclusion will be that the roof design meets the code requirement. but if use asce7-05 figure 6-11 to determine the wind pressure coefficient gcp (consider it as component and cladding), seems the conclusion will be the opposite way.
my question is for the roof rafter and joist buckling failure, should i consider it as  main wind force resisting system to do design checking or should i consider it as component and cladding to do design checking?
appreciate your oppinion, thanks.
it depends upon the tributary area of the rafter/joist members.  
mwfrs is typically used for the overall building system design - the roof diaphragms, shearwalls, collectors, wall x-bracing, and since it is for the whole building per se, the area of building associated with the wind load is quite large.  thus, local peaks and valleys of wind pressure are averaged out over the whole area and this results in a mwfrs pressure.
thank you very much for your information, jae. it is really very helped.
but now one more question comes.
the scenario is like this: the tributary area of one rafter is smaller than 700 s.f.,  but the tributary area of the multi-buckled rafter is larger than 700 s.f. so one can argue  that the whole failure area is larger than 700 s.f., and all those buckled rafters failed at same time by wind load exerted on this larger than 700 s.f. area, therefore it should be considered as mwfrs. on the other hand, one can also argue that the failure occurred  at one rafter first, then it caused a "domino" effect resulting all rafters to fail, therefore a smaller than 700 s.f. tributary area should be considered and the design checking should be based on c&c.
for above two argues, what are you guy's thought?
thank you very much.

each rafter would have the wind load based on the rafter area (less than 700 s.f.) so the domino effect may have occurred....or at various times, each rafter's applied wind peaked at the higher amount due to its smaller area.

i was always under the impression that mwfrs members were receiving load from more that one surface.  granted there are exceptions to this definition.
here is a thread from some time ago that hashed this issue out.
one way to think of c&c vs. mwfrs is what's visible and what's not visible.
components and cladding are generally visible...they're the outside portions that transmit loads to the "inner" structure.  to give an example, the sheathing on a shingled roof is a cladding (yes,it's "clad" with shingles, but the shingles serve no purpose other than watershedding...like paint).  the sheathing serves other purposes such as lateral bracing for the top chords of trusses or rafters, and the sheathing can be used as a diaphragm, when it can become part of the mwfrs.
one key, as jae has noted, is the tributary area.  for fasteners, the tributary area is quite small, so the factors go up.  as you increase the tributary area, the localized influence decreases...as jae says, it "averages" out.
in my opinion, trusses are clearly mwfrs, even though the influencing tributary area might be quite small.
one other point of c&c vs. mwfrs....the c&c function is to tranfer load....the mwfrs function is to share and distribute load.
i think the commentary says trusses can act as both.   i would say the truss chord between the panel points is c&c, while the entire truss and uplift reactions are mwfrs
lets put it this way, rafters and joists are not part of main wind force resisting system so they automaticaly fall in to components and cladding category. but asce-702 allows to design any cladding element with area tributry area greater then 700-sqft for mwfrs loads. please see 6.5.12.1.3
structgen's statement is what is correct.  it's all about the tributary area.

when i first saw this post, the question that came to my mind was, when was the building designed and what code was criteria?  if the building is less than 6 or 7 years old, than the "i" codes were probably (but not certainly) the current design.  but if building is ten years old and the code was the old ubc (sbc, cbc, etc.), why should the asce 7 be the gauge?  if the design was close enough that mwfrs vs cc makes a difference, then maybe it's fine per ubc.