Roof Pressure in Design Wind Load Cases

P? ASCE 7-05 Figure 6-9 only references the windward and leeward pressures (Pw, Pl) not a pressure (P), which are added together. Either for each direction separately or simultaneously in both directions at 0.75 the separate case.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

Sorry, my use of P was misleading. I realize the diagram only shows windward and leeward, but roof pressure still exists on the building. I have read thru past threads and there seems to be some confusion here. I was hoping there has been some developemnt on it, maybe a change or commentary in ASCE 7-10 (I don't have it yet).

I cannot remember who suggested it, but I will proceed with Case 3 as if wind was coming at 45 degrees to the building. Roof pressure will start at that windward corner and get lower as it moves across the roof, NOT adding the roof pressure that would result from the two orthogonal directions.

@PMR06 - Ref ASCE 7-05, Refer to Figure 6-6 for calculating external pressure coefficients Cp for walls (windward, leeward, and side wall) and roofs (windward and leeward) of all heights, for MWFRS purposes. For roofs, the external pressure coefficient would depend on the distance from windward edge as given on page 49. The design wind pressure on the walls and the roof would be combination of external + internal pressures.
After getting design wind pressures on the walls and the roofs using Fig 6-6 for each of the directions X and Y, do combinations of wind loads using cases 3 & 4 of Fig 6-9. These combinations are applicable to both, the walls and the roof.

Thanks DST148. I think you have actually implied my question. In each of the x and y directions I get roof suction say 20psf at the windward edge. 0.75 of that from each direction would be 15psf + 15psf = 30psf total suction at the windward corner. It is my argument that the roof pressure is not calculated as 0.75 from each orthoganal direction. One should start at the windward corner with the appropriate suction (20pf in my example), then work along the building diagonal length reducing as needed h/2, h, etc. I wanted to verify my claim though with what others are doing.

@PMR06 - Case 3 is Case 1 & Case 2 acting simultaneously at 75% wind loads.
If Case 1 is WX, Case 2 is WY, then Case 3 is 0.75 WX + 0.75 WY.
You would have to use the method above even if you are interested in calculating pressures along the diagonal of the building for MWFRS purposes.
For designing the roof elements, it would be C & C pressures as per Fig. 6-17.
You may also refer to commentary on page 297.