asce 7 wind loads

huangyhg

asce 7 wind loads
what am i missing?  as i look at fig. 6-10 to determine the pressure coefficients on mwfrs for low-rise buildings, i'm wondering why the coefficients would be any different when looking at reference corner a versus reference corner b.  the building figure looks symmetrical.  with different roof pitches on either side of the ridge, i would understand why each corner needs to be looked at differently.  are there other reasons that all four corners need to be looked at which would help me understand this better?  asce 7-98 was so simple, which makes me wonder what i'm missing when the figures go from two to eight when looking at the 7-02 or 7-05 version, and they seemingly depict nearly the same thing.  
also, what is, and what is the significance of the zone 2/3 boundary as mentioned in note 7 of figure 6-10?
thanks for your help.   
postframese
the only thing you are missing here is the fact that the building shown is perfectly symmetrical.  most buildings that you encounter in design will not be perfectly symmetrical as shown in figure 6-10, so unfortunately the new code requires that all 8 loading patterns must be considered as you will develop different wind pressures for each direction resulting in different lateral loads to your mwfrs for each case.  (see note 3 asce7 p.53)  i know its a pain, i don't understand the logic in making such a complicated procedure for wind loading that still ends up being very conservative, not saving anyone any resources. if your building is perfectly symmetrical, then you can use your engineering judgment and realize that your pressure will be the same and you only need to consider four different load cases.  also note that once you determine the worst case loading scenario from figure 6-10 you must consider the design wind load cases in figure 6-9.   
dear postframe
i've got a "freeware" spreadsheet for asce7-05 wind written by a good friend of the engineering community that might help speed things along.  take a look and tell me if you think it might help you.
pe in new england