zackngineer
Structural
- Jan 31, 2024
- 16
I have a question on calculating effective wind areas for metal roof panels.
Parameters:
ASCE 7-22
Florida
160 Vult
Exposure C
MRH: 21'
97' W x 146' L
Primarily Hip Roof
ASD pressures
Components & Cladding
Definitions:
Effective wind area per ASCE 7-22 is: "Effective wind area is the area of the building surface used to determine (GCp). This area does not necessarily correspond to the area of the building surface contributing to the force being considered. Two cases arise. In the usual case, the effective wind area does correspond to the area tributary to the force component being considered. For example, for a cladding panel, the effective wind area may be equal to the total area of the panel. For a cladding fastener, the effective wind area is the area of cladding secured by a single fastener. A mullion may receive wind from several cladding panels. In this case, the effective wind area is the area associated with the wind load that is transferred to the mullion." for case 1.
For case 2: "The second case arises where components such as roofing panels, wall studs, or roof trusses are closely spaced. The area served by the component may become long and narrow. To better approximate the actual load distribution in such cases, the width of the effective wind area used to evaluate (GCp) need not be taken as less than one-third the length of the area. This increase in effective wind area has the effect of reducing the average wind pressure acting on the component. Note, however, that this effective wind area should only be used in determining (GCp) in Figures 30.3-1 through 30.3-6. The induced wind load should be applied over the actual area tributary to the component being considered." This case does not apply to the scenario I have described below.
Scenario:
If we were to worst case our roof, we would look at the smallest effective area. On a standard hip roof, that would be the panel installed in the outermost corner of the roof. The effective area for this panel typically ranges from 3-6 SF (3' wide panel * panel length). For Zone 3, OHS (Overhang w/ soffit), </= 10 SF., the uplift in this area is -109 PSF per MecaWind. This number is not unusual or higher than expected. That being said, the issue I run into is the metal roofing product approvals always have VERY low design pressures relative to the building design pressures. In this example, the client wants to use "29 GA. Pro-Panel II" by Metal Sales Manufacturing Corp (See attached product approval). The design pressures allowable are 41.6 PSF for 24" fastener spacing and 71.5 for 12" fastener spacing. Neither of these meet our required design pressure. I run into this very frequently on metal roofing jobs. We do a lot of metal roofing jobs. Usually, I send it back to the contractor and say they need to pick a different panel. Usually they do and usually the new one they pick will meet the design pressure requirement. That being said, in this case, I have seen this panel installed on similar structures in the same general area. This gives me pause. So, my question is, am I analyzing this problem incorrectly or does this product simply not meet the requirements of the building I am evaluating?
Parameters:
ASCE 7-22
Florida
160 Vult
Exposure C
MRH: 21'
97' W x 146' L
Primarily Hip Roof
ASD pressures
Components & Cladding
Definitions:
Effective wind area per ASCE 7-22 is: "Effective wind area is the area of the building surface used to determine (GCp). This area does not necessarily correspond to the area of the building surface contributing to the force being considered. Two cases arise. In the usual case, the effective wind area does correspond to the area tributary to the force component being considered. For example, for a cladding panel, the effective wind area may be equal to the total area of the panel. For a cladding fastener, the effective wind area is the area of cladding secured by a single fastener. A mullion may receive wind from several cladding panels. In this case, the effective wind area is the area associated with the wind load that is transferred to the mullion." for case 1.
For case 2: "The second case arises where components such as roofing panels, wall studs, or roof trusses are closely spaced. The area served by the component may become long and narrow. To better approximate the actual load distribution in such cases, the width of the effective wind area used to evaluate (GCp) need not be taken as less than one-third the length of the area. This increase in effective wind area has the effect of reducing the average wind pressure acting on the component. Note, however, that this effective wind area should only be used in determining (GCp) in Figures 30.3-1 through 30.3-6. The induced wind load should be applied over the actual area tributary to the component being considered." This case does not apply to the scenario I have described below.
Scenario:
If we were to worst case our roof, we would look at the smallest effective area. On a standard hip roof, that would be the panel installed in the outermost corner of the roof. The effective area for this panel typically ranges from 3-6 SF (3' wide panel * panel length). For Zone 3, OHS (Overhang w/ soffit), </= 10 SF., the uplift in this area is -109 PSF per MecaWind. This number is not unusual or higher than expected. That being said, the issue I run into is the metal roofing product approvals always have VERY low design pressures relative to the building design pressures. In this example, the client wants to use "29 GA. Pro-Panel II" by Metal Sales Manufacturing Corp (See attached product approval). The design pressures allowable are 41.6 PSF for 24" fastener spacing and 71.5 for 12" fastener spacing. Neither of these meet our required design pressure. I run into this very frequently on metal roofing jobs. We do a lot of metal roofing jobs. Usually, I send it back to the contractor and say they need to pick a different panel. Usually they do and usually the new one they pick will meet the design pressure requirement. That being said, in this case, I have seen this panel installed on similar structures in the same general area. This gives me pause. So, my question is, am I analyzing this problem incorrectly or does this product simply not meet the requirements of the building I am evaluating?
