Components and Cladding with Internal Pressures

[Shrimpfork]

I typically calculate wind loadings on exterior metal wall panels which are considered an element of Components and Cladding. The location of the panels are often attached to the building where the backside of the panels are NOT exposed to the internal pressures. For example, a current design I am looking at is a metal panel cladding over an existing CMU type wall on a high rise building, where the CMU wall will remain in place. The metal panels are simply cladding over the top of the existing wall. In this case, I am thinking that I might be able exclude the interior pressures when I determine the wind pressure for these panels.

Secondly, would this also be valid when these panels are used for new construction walls, where there wall is made up of exterior gyp board over exterior studs. The metal panel is attached thru the gyp board into the studs behind. Again the backside of the metal panel is blocked by the gyp board and is not exposed to the internal building pressures. Can I eliminate the internal pressure loading? What are your thoughts?

 

[JAE]

Hmmm..I think that the internal pressures are due to the difference in pressures between an exterior surface and an interior surface of a building. Having a double layer wall (panel and CMU) seems to separate the two, but I wonder if the metal panel system is tight enough that an increase or decrease in atmospheric pressure would still create the lateral pressure difference on either side of the metal panel (i.e. in the small space between panel and CMU).

I just don't know how the volume of space affects the internal pressures presented in ASCE 7....but pressure is pressure and the volume, technically, shouldn't have an effect on it.

 

[UcfSE]

I would expect to have some sort of pressure differential bewteen the exposed and back side of the panels. It may not be the same as it is for the interior of the structure, but I wouldn't assume that there is simply nothing there to contribute to the wind load effect. If there is zero air space behind the panels, then you probably have a goodpoint for not using internal pressures for the panels themselves.

 

[Shrimpfork]

I can agree to all these points. The panel system type I am running calculations for, is a self contained sealed system, where the joints do not allow air movement to pass from the internal panel cavity to the exterior, so there is some pressure differential, however I don't think it is a same amount as to the what the internal pressure coefficient is intended to calculate. Imagine a shingle siding application in a ship lap fashion on the side of a house with gyp board and a membrane, to calculate the loading on a shingle, can the internal pressure eliminated? Typical a shingle application does have a minute air space behind the material between the laps.

To expand the discussion, the a second panel system I’m looking at is designed with OPEN joints, allowing the air to pass from the internal panel cavity to the exterior. I don't think I need to consider the internal pressures on the back side of the panel in this condition. Your thoughts?

 

[JAE]

With open joints, the internal pressure would be reduced...the big question is, by what amount? I suppose it depends on the rate of pressure drop or increase and the rate at which air can squeeze through your open joints.

If there's uncertainty in design then the typical engineering approach is to increase your level of safety and not to lean towards the less-conservative side of your assumptions.