MWFRS Wind Force ASCE7-10

[EngSD]

I am trying to figure out why if you used the envelope procedure in ASCE7-10 on a gable roof your windward roof will always have a negative pressure (suction) but if you use the directional procedure the windward roof can have either positive or negative pressure on the windward roof.

I'm comparing Figure 27.4-1 (Directional) and Figure 28.4-1 (Envelope).

Thanks

 

[mike20793]

The envelope procedure is more conservative, so the positive pressures equate to a higher total MWFRS force than the directional procedure.