SouthFloridaPE
Structural
- Sep 18, 2009
- 3
Hello,
ASCE7 wind provisions have (for low-rise buildings) 3 ways to calculate wind pressures for MWFRS:
Method 1 (if the building meets requirements)
Method 2 for h<60ft
Method 2 for all heights.
Now for a 5:12 sloped roof (Theta=22.6 deg), all three methods give different lateral force components for the roof (lateral drag on a hip/gable roof, normal to the ridge).
Method 1 has (for zones B and D) negative values for 20 deg. and positive/zero for 25 deg. I interpret that as if I should interpolate between the neg. values and zero, so you get a negative lateral pressure (opposite to the windward pressure on the walls).
Method 2 for h<60 ft gives negative coeffs. for GCpf (zones 2 and 3) for 20 deg. and a positive one for zone 2 for 30-45 deg. Linear interpolation makes zones 2 and 3 almost identical, therefore they nearly cancel each other out (the horizontal drag component).
Method 2 for all heights has the positive Cpf starting at 20 deg. on the winward side, and negative on the leeward, creating two cases: one with a small positive drag and one with a larger positive drag. But in both cases there is a drag pushing the roof in the direction of the wind.
Summarizing:
Method 1: Negative drag.
Method 2 h<60 ft: Near zero.
Method 2 all heights: Positive drag.
So what I'm doing is simply neglect the lateral drag on the roof, as permitted by Methods 1 and 2 with h<60 ft, but is this realistic? Is the wind not going to cause a positive horizontal drag on these roofs, as suggested by the general method?
Shouldn't all 3 methods at least agree on the direction of the drag?
ASCE7 wind provisions have (for low-rise buildings) 3 ways to calculate wind pressures for MWFRS:
Method 1 (if the building meets requirements)
Method 2 for h<60ft
Method 2 for all heights.
Now for a 5:12 sloped roof (Theta=22.6 deg), all three methods give different lateral force components for the roof (lateral drag on a hip/gable roof, normal to the ridge).
Method 1 has (for zones B and D) negative values for 20 deg. and positive/zero for 25 deg. I interpret that as if I should interpolate between the neg. values and zero, so you get a negative lateral pressure (opposite to the windward pressure on the walls).
Method 2 for h<60 ft gives negative coeffs. for GCpf (zones 2 and 3) for 20 deg. and a positive one for zone 2 for 30-45 deg. Linear interpolation makes zones 2 and 3 almost identical, therefore they nearly cancel each other out (the horizontal drag component).
Method 2 for all heights has the positive Cpf starting at 20 deg. on the winward side, and negative on the leeward, creating two cases: one with a small positive drag and one with a larger positive drag. But in both cases there is a drag pushing the roof in the direction of the wind.
Summarizing:
Method 1: Negative drag.
Method 2 h<60 ft: Near zero.
Method 2 all heights: Positive drag.
So what I'm doing is simply neglect the lateral drag on the roof, as permitted by Methods 1 and 2 with h<60 ft, but is this realistic? Is the wind not going to cause a positive horizontal drag on these roofs, as suggested by the general method?
Shouldn't all 3 methods at least agree on the direction of the drag?
