load case a b, asce 7-05 figure 6-18a

huangyhg

load case a & b, asce 7-05 figure 6-18a
i am designing a lean-to roof (no walls) against an existing metal building to cover equipment that serves the existing building.  figure 6-18a in asce 7-05 provides windward & leeward coefficients for calculating wind pressure(s), but the load cases a&b are not illustrated or defined for open buildings explicitly.  can anyone shed some light on whether section 6.5.12.4.4 (parapets) might be the correct guidance for which load case to use?
i'd also like to know the consensus of the community if this qualifies as an open building or is it partially enclosed (the existing metal building is the only solid wall of the lean-to.
thanks,
jmm