asce 7-05 figure 6-18b

huangyhg

asce 7-05 figure 6-18b
asce 7-05, page 67, figure 8-18b (net pressure coefficient, cn for pitched free roofs) shows load case a & b. what are these cases?
thanks,

hmm..looks to be just the maximum variation in loading due to gusts at different times.  it says to check all load cases, so i assume you check case a and case b for each situation.  i don't see any text reference to indicate the letters actually correspond to a specific design condition, and don't see anything to indicate you'd use case a this time and case b for some other situation.
i think they are the lower and upper bounds for all possible wind flows or pressures over a pitched roof. this is similar for a house with a gable roof. try calling asce to get the explanation. good luck.