Components and Cladding Wind Pressures - Permeable Facade

[sticksandtriangles]

Hello,

I am in charge designing some supports for a facade.
In calcing the C&C wind loads per ASCE 7-05 Method 2, the question arose as to if I should be including +/- 0.18 for internal pressures.
The cladding is permeable so wind can get behind the panels and maybe create pressure behind the panel, but this is definitely not internal pressure.
I think I feel comfortable using +/-0.0 for my internal pressures and just using GCp in my wind load calcs. See below

Let me know if you know any good references on the topic as well.
Thanks against for your opinions in advance.

 

[brut3]

Sticksandtriangles,

Great question, it really made me think about the way I design my cladding systems.

My first thought was to go to ASCE 7-10 Chapter 30 commentary C30.1.5 which deals with air-permeable cladding. To summarize it states that the applied pressures across the assembly will vary drastically so using the net pressure (including the internal pressure) is recommended. However it does leave the door open saying the designer may used an alternate design methodology.

I personally will still use the internal pressure coefficient when designing my cladding elements however I can follow and understand your rationale if you choose not to include it. Just my opinion. I would love to hear what others have to say regarding this topic.

 

[sticksandtriangles]

Thanks for the thoughts brut3,

I got myself into a bind by using the wrong wind speed, local jurisdiction called for a higher basic wind speed than ASCE 7 did. Typically I include the internal pressure, but if I chose to not include it, I would not have to revisit the calculations.

 

[Guastavino]

I think it could be argued either way.

Don't Include Internal Coeff. Argument:

1. Assumptions for the 0.18 are likely VERY different than for this case.
2. Wind interaction will be much more complex for this particular application than simple code assumptions.
3. Jurisdiction is already over ASCE 7.
4. Without a wind tunnel test, who really knows.
5. Very redundant structure, IE lots and LOTS of ties.

Include it Argument:

1. Code appears to recommend it for these gray areas.
2. Conservative to include it.
3. Easier to argue to include it.

Ultimately, I'd do some gut checking and think through the added value as compared to cost to upfit/change order back in.

 

[sdz]

The wind that will cause +ve internal pressure is also going to cause +ve external pressure. Similarly with -ve pressure. The worst design case is to ignore it.