C&C Wind Loads

I know for the MWFRS wind pressures the overhang pressure only applies to the windward side (ASCE 27.6.3)

this makes sense as it can't push up under the overhang from the leeward side. I would apply regular negative pressure

@Jerehmy

Agreed, that is why I question my diagram above. However, I can't find any verbiage in Ch. 30 that applies the C&C overhang wind loads only to the windward side hence my uncertainty and bringing this question to the board.

The funny thing is that I've managed to get my hands on some load cases from Mitek and Alpine software. One applies the overhang pressures at both windward and leeward overhangs and the other does not, so I'm not the only one confused here.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

I think the C&C wind loads are a bit confusing as given in the standard. What if the wind is parallel to the ridge? Then does the zone 2 pressures and overhang pressures apply to both side of the truss? Or do only the zone 2 pressures apply and the overhang pressures not apply since the overhangs would not be on the windward side?

A confused student is a good student.
Nathaniel P. Wilkerson, PE

I don't think they'd apply parallel to the ridge. I think the wind, when I think of it as a fluid, needs to run into the wall to cause it to flow up into the eaves.
When it flows parallel to the ridge I don't see any mechanism to cause it to flow up into the eaves. So I'm gonna say no.

Your logic makes sense to me so I am going to go with the zone 1 pressure at the overhang on the leeward side. However, the pressure at the overhang will be slightly different since there is no GC_pi at this location.

Without an overhang there are just zone 2 and zone 1 pressures, overhangs tend to complicate things.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

As a follow up to this question I'm now looking at a open building with a 4:12 pitched roof (gable). Looking at Fig. 30.8-2 in the ASCE 7-10 I have both positive and negative pressures. I will be designing a post frame type structure with site built trusses at 8 ft. o/c. The C&C wind loads will be used to design the member loads of the truss. The standard is not very clear how to take into account wind directionality with regards to the C&C loads. On page 127 of "Guide to the Wind Load Provisions of the ASCE 7-10 in Fig. G10-5 the author states that two load cases are required to account for both wind directions (transverse and longitudinal). Based on this figure it would appear that the positive C&C pressure would only be considered on the windward side of the roof and not the leeward side (Load Case 2), whereas the negative pressure would be considered on both sides of the roof (Load Case 1).

Should there be one other load case where both the windward and leeward side have the positive C&C pressure applied? My thinking is that the positive pressure would only be ever applied to the windward side of the roof but I don't want to mistakenly eliminate a load case from my analysis.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

On a related note I've created a spreadsheet for pitched roof open buildings using Ch. 27 Part 1 of the ASCE 7-10 (MWFRS). I've gone through it carefully but I'm hoping a few others might be interested enough to try it out and help me vett it further.


PDF preview here:


The C&C spreadsheet for the corresponding open building is at:


PDF preview here:


A confused student is a good student.
Nathaniel P. Wilkerson, PE