ASCE 7 Wind Loads C&C Help

[xez]

I am currently calculating wind loads for an insulated metal panel assembly and had a few questions regarding the effective wind area. The IMPs are fastened (concealed fastening) at every girt/purlin location and every width of the panel and acts as one composite when installed.

In the Chapter 6 wind load commentaries of ASCE7, it states: "For a cladding panel, the effictive wind area may be equal to the total area of the panel. For a cladding fastener, the effctive wind area is the area of cladding secured by a single fastener."

Say I am using a 3' wide wall panel that is 30' long fastened to girts 5' o.c. Do I:

a) Take the effective wind area of the panel (30' x 3') to determine the wind load. Use that wind load and apply it to the tributary area of each fastening location to determine number of fasteners required at each location, or

b) Take the tributary area of each fastening location as my effective wind area to determine the wind load? If this is the case, there will be two different loads in the corner zone - one with a full tributary area and the edge panel with half the tributary area. Why would these areas see different loads if the wind is uniform and acting onto the full panel assembly as a whole?

c) Use the full panel length to calculated the effective wind area when determining whether the panel itself would fail (not fastening) or do I take the span length (girt to girt) to determine the effective wind area?

The code seems open to interpretation when it talks about the single fastener effective wind area and I am not sure which way to design the panels. Any insight would be great. Thanks and I appreciate the help.

 

[frv]

B. And the whole intent of components and cladding is to recognize that you can get very high localized effects over very small areas. A single fastener may see extremely high loads, but a whole panel will not see the same loads (it evens out over the tributary area of the panel).

Very simply, you use whatever tributary area corresponds to whatever you are analyzing: very small trib area for fastener; larger trib area for panel; larger still for whatever is supporting the panel.

 

[WillisV]

Fastener: Determine tributary area to each fastener. Use this area as the effective wind area to determine wind pressure for that fastener. Multiple that pressure by tributary area to each fastener to get fastener load.

Panel: Use span between girts. Multiple that times span/3 to get the effective wind area to determine cladding pressure. Use that pressure to design 1' strip of panel between girt supports.

 

[MrHershey]

What WillisV said.

The different pressures are based on probability of exceedance over the entire tributary area. So much more likely that a single fastener's pressure will exceed a given wind pressure than the whole panel's.

Don't think if it as 'different pressures' think of them as different safety factors or different probability of exceedance. So you have a base pressure (qh using ASCE 7-05) and then a coefficient to account for trib area, zone, etc. (GCp using ASCE 7-05). Everything's at the same pressure, the coefficient modifying the pressure just changes.

 

[JAE]

I do what WillisV says except I wouldn't use 5'/3 = 1.67' as the width - I'd use the 30 inch panel width x 5 ft. girt spacing as the area for the panel design.

Concur with individual area for fastener area.

 

[WillisV]

@JAE - agreed - use actual width where it is beneficial to do so over span/3.

 

[xez]

Thanks for the responses everyone.

So, the effective wind area for the panel design is the (panel width x girt spacing). I'm still not too clear on the following: "For a cladding panel, the effective wind area may be equal to the total area of the panel." Is the code referring to single span conditions in this statement?

 

[JAE]

bcng - yes I would think that is the intent of that statement....single span condition.

 

[spats]

I'm going to have to disagree with JAE... metal wall panels are designed to be continuous (particularly IMP), and even if they weren't, the piece spanning between girts is not isolated. It's part of larger panel. If I decided to throw in girts at 2'-6" on center, I would have to cut the area in half to determine the wind load? I wouldn't. I would take them at their word and use the total area of a panel.

 

[JAE]

spats = makes sense - I just tend to go with the girt distance as that would be smaller than the full panel height and thus a bit more conservative. I don't think the difference between a 30" x 5 ft. area for C&C and a 30" x 40 ft area is all that much in terms of wind psf.

 

[ajh1]

While the concept of using the full panel lengths, or at least some amount larger than a single span makes sense in a theoretical way, ASCE7 is pretty explicit that the effective wind area is based on a single span between supports. We had previously used something in the range of 1.7 times a single span as that seemed to be the influence area for the loads from a single span. Realistically once you get more than one span away from the span in question, you get very little benefit from the continuity of the panel.