ASCE 7-05 Snow and Wind Load Questions

[2457F]

1) On page 51 of ASCE 7-05 the figure lists the external pressure coefficients Cp for arched roofs of enclosed and partially enclosed buildings and structures, however, I could not find coefficients for arched roofs of open buildings and structures. Does ASCE 7 have any recommendations/suggestions when designing arched roof buildings that are open?

2) This next question involves both wind and snow loads. I understand that unbalanced snow loads develop when wind is applied in the transverse direction of a gabled roof structure. This unbalanced snow load develops on the leeward side and not the windward side. We have one load combination that includes DL+Unbalanced Snow Load + Transverse Wind Load (I've left the factors off) which is a legitamate combination. However let's say the wind changes direction the next day so that it is now acting in the longitudinal direction. We still have an unbalanced snow load that has accumulated on the roof but now all of a sudden the wind load has changed direction. Does the load combination of DL+Unbalanced Snow Load+Longitudinal Wind apply? Longitudinal wind will not create an unbalanced snow load on a gabled roof structure so is the load combination valid? Another argument is that the transverse wind can reverse itself and therefore you would have to consider unbalanced snow loads on each surface of the gabled roof but the code indicates that we do not have to consider this. I believe I've gotten a bit long winded on this but I just want to make sure that I am clear.

 

[Splitrings]

This is just my thoughts and opinion, take it for what it is worth. If I understand the wind provision correctly, very few buildings meet the open criteria. I do a lot of agricultural type gable roofs on poles. It would seem these are "open" buildings but by definition they are not. As soon as you sheath the gable end they typically don't meet the requirement that each wall be at least 80% open. As an enclosed building you must apply the wind load to the full horizontal projection, even though there is no wall there to care the loads to the MWFRS. I am guessing this is the reason there are no coefficients for an arched roof considered to be open. Certainly if the arch is "springing from ground level", as there would be 0% open on these two sides. If your arch is on posts but has a gable that is sheathed, again I would guess it does not meet the open criteria.

As far as wind and unbalnced snow loads, I always use the unbalanced snow and apply the the 4 wind cases as shown in figure 6-9 (method #2).