They are not.
What ASCE 7 is trying to say is that you have two separate conditions that must be looked at.
These are sort of like separate wind load cases so each would take its turn as W in your typical Load Combinations.
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Thanks for the reply. My question stems from the contradiction between MBMA manuals (1986 & 2006) and ASCE 7. MBMA give values for Load Case 2 with roof slopes less than 25 degrees, and ASCE 7 doesn't include Load Case 2 for MWFRS below 25 degrees. MBMA 2006 does acknowledge difference between the 2, and recommends using MBMA values. I don't work with MBMA very often, and prefer using ASCE 7. Does anybody know why ASCE values shouldn't be used? The reason looking at both is I've been asked to investigate remove 2 interior columns 25' apart, and install a single column (goalpost) in a 125' wide PEMB rigid frame built in the mid 1980's.
First off, to get a better level of understanding you might be better off looking at Fig. 27.4-1 vs. Fig. 28.4-1. Simplified procedures may have a bunch of extra assumptions thrown in that could make overall understanding of the theory more complicated.
On Figure 27.4-1, the first load case is creating what is basically a wind suction force on the windward roof. This would normally be combined with an internal pressure coefficient to create the largest uplift forces possible. This load is more severe on flat or nearly flat roofs. The second load case is creating a wind pressure force on the windward roof, which is more likely for a steep pitched roof. This would be combined with an internal suction load to get the most severe downward (or inward) loading. The -0.18 coefficient values on the lower pitched roofs in this figure effectively cancel out the internal suction force leaving a net zero wind condition (or one you can ignore) for this second case on the lower pitched conditions.
On Figure 28.4-1, the two load cases A and B have a slightly different connotation. Case A is for a wind load that is primarily oriented as coming at the building perpendicular to whatever ridge orientation exists. Case B is for a wind load that is primarily oriented to occur roughly parallel to the ridge. For a very low sloped roof the two cases give equal roof coefficients and practically equal wall coefficients which makes sense where the ridge effect is minimal. As the roof gets steeper, Surface 2 on Case A starts to shift between the upward suction coefficient to a downward (inward) pressure coefficient in the same manner as happened on Figure 27.4-1. Case B continues the suction only style as the ridge has no effect when the wind blows parallel to it. Both Case A and Case B should be combined with the two internal wind options (pressure and suction) although it should be obvious when internal and external are additive or counter-active.
Leeward roofs are always loaded with wind uplift (suction) regardless of whether you are looking at Chapter 27 or Chapter 28.
The newer MBMA manuals simply discuss how one applies IBC/ASCE wind loadings to typical metal building configurations. There is no separate MBMA design spec any longer. That said the examples in the MBMA manual can be quite enlightening to cover all othe nuances of wind design.
