Soffit Wind Loads (C&C) on an Open Structure

[WEJ83]

I am looking at a large open structure, trying to determine C&C wind loads on the soffit. Roughly 100'x164' with a low side eave height of 35.5' and a roof slope of 1/4":12". It is a PEMB structure with a lower soffit approximately 5' below the upper roof surface. There are separate purlin systems supporting the roof and the soffit. Designing under ASCE 7-10.

Figure 30.8-1 gives Cn values for positive and negative pressures, noting that the Cn values represent the contributions from the top and bottom surfaces. The commentary (p. 572) states that, for surfaces separated structurally from the top roof surfaces, the pressure coefficients from the top and bottom surfaces may be separated (or conservatively each surface could be designed using the full Cn values.)

My question is, is there anywhere to look for guidance on how to separate out the top and bottom coefficients? 7-22 now has a section on loads on the bottom surface of an elevated structure, but my prelim calcs indicated these loads are higher than the "conservative" combined loads from Fig. 30.8-1. (I still need check and make sure I am apples to apples between the two codes.)

 

[SandwichEngine]

From what I'm envisioning, it's basically an enclosed structure that's suspended above the ground. However, it really wouldn't meet the code definition of enclosed if it has zero vents or other openings. If that's the case, it seems like you could use the loading from the standard equations for an enclosed building but omit the internal pressure coefficient. The commentary for 26.12 states that openings are defined as holes that allow air through the building envelope during a designed wind event. If there are no such holes, how would you get an internal pressure coefficient.

Alternatively, Figure 30.9-1A gives different values for the upper and lower surface of attached canopies. There could be considerable difference in how your structure is behaving compared to a canopy when the wind is blowing into and around a canopy supporting wall but I would think your structure would behave exactly like one of these canopies when the wind is blowing in and out of the page per the diagram. Because this portion of the code doesn't differentiate between loading direction you should be able to use this section and feel confident your loading is conservative.