Directional vs Envelope Wind Pressures

[P1ENG]

I've always used envelope because my projects are mostly small rectangular, modular buildings. I've been practicing for 15 years and I rarely use directional. However, today I have a job that, for simplicity, is 60'x60' with 9' walls and gable roof with a 4:12 pitch. Normally, it doesn't happen, but they are going to put 60' long trusses on-site after the modular units are set. I was getting much higher uplift reactions per my envelope analysis than the truss print was showing based on the direction method. Because of the span and the pitch, this roof is about as tall as the building (again, unusual for my type of work). When considering the directional procedure, this results in a low h/L ratio in both the transverse and longitudinal directions.

Long story, short: I am getting maximum 10.4 psf (transverse) and 14.8 psf (longitudinal) uplift using the directional procedure and 21.6 psf uplift using the envelope procedure. ==> 21.6/14.8 = 1.46. Is this common to see a 46% increase in pressure when using envelope for large footprint, short structures? Based on "parallel to ridge" values, my h/L being less than 0.5 means my Cp values peak at -0.9. However, if my h/L were > 1.0, then the Cp value would max at -1.3, which would bring my pressure up to 20.1 psf (much closer to the envelope value). So the envelope must be conservatively assuming (including) structures with h/L > 1.0?

Can anyone let me know if I'm crazy? It's Friday and I might be struggling.

Juston Fluckey, SE, PE, AWS CWI
Engineering Consultant