Drag coefficients and shear performance of metal roofing for building diaphragm loading

[fastline12]

Trying to finalize some numbers for wind protection of a building. I need to account for diaphragm action of the building to better estimate wind load ratings. I am looking for any standards or help in determining the proper drag coefficient to use here and how I might account for the shear performance of the corrugated metal roofing that will be applied to the walls and roof. Ultimately, I need to drive this back to building standards for final approval.

The roofing is a 26ga PBR ribbed.

 

[hokie66]

Roofing is for weather protection, not a structural diaphragm. It is sometimes used for diaphragm action in very small buildings like farm sheds, but not large buildings, which should be braced by structural elements, not by thin gauge roofing.

 

[JedClampett]

Fastline, I see you're an aerospace engineer. For wind speeds in the structural world, we don't worry about drag coefficients for corrugated roofs.
And for the shear performance of corrugated roofs, you need to get the manufacturer's published values from an ESR report or other code compliant document. And their catalogue values are not reliable.

 

[hokie66]

fastline, what wind loading code are you using? At least one code, the Australian Standard, does give values for drag on corrugated roofing, perpendicular to the corrugations and in the roofing plane, but that component of loading is often ignored by designers due to its relative insignificance.

 

[DaveAtkins]

"Post Frame Building Design Manual" has diaphragm tables for corrugated metal roofing.

hokie66: fastline12 is referring to corrugated metal roofing, not standing seam metal roofing. If properly nailed to wood purlins, it serves as a diaphragm.

DaveAtkins

 

[haynewp]

This may also help for the shear information if it matches your deck profile:

http://www.icc-es.org/Reports/pdf_files/5468.pdf

 

[fastline12]

Thank you! Sounds like the metal roofing supplier should be able to provide shear performance data?

Haynewp - Can you possibly clarify the "span" description in these tables? I was not able to make sense of it. Our roof design right now utilizes roof trusses on 12' centers with a dimensional lumber purlin system in joist hangers between trusses on 24" centers. This would mean the purlin top edge would be flush with the top of the top cord of the trusses.

 

[DETstru]

The "span" is simply the distance between the members that directly support your roof deck. Assuming the deck is anchored (screwed) to every wood purlin, your span is 2'. The tables go down to 4' so just use those values.

"No. of spans" is how many supports (purlins) is a single section or sheet your roof deck is secured to. Lets say the roof deck came in 8' long sections, it would be secured to 5 purlins, making the number of spans = 4.

 

[hokie66]

The tables which haynewp linked assume valley fixing for those profiles. Where I am, we always use crown fixing for better watertightness, and ignore diaphragm action.

 

[fastline12]

Hokie, we are in the midwest so wind, uplift, etc must be considered here. Without consideration to diaphragm action, are you saying you use alternative shear components? We are looking at the structure right now without diaphragm action and would have to grossly oversize the poles or columns from what is typically done in this area.

Can you better explain "crown fixing" vs "valley fixing"

The sheet size will be custom ordered and in some areas be as much as 30ft long. We are right now trying to determine the practicality of handling such pieces but the least amount of seams, the better this diaphragm action looks.


With consideration to drag coefficients, some mention "we don't worry about it" but surely you are calculating some values to determine performance in the wind?

 

[hokie66]

I assume by Midwest, you mean in the Midwest USA. I am in Australia. Here, we do not rely on the cladding to resist shear, but rather typically use diagonal bracing trusses in the given planes. By "crown fixing", I mean the screws go through the top of the corrugation, while if "valley fixed", they would go through the bottom, where the water runs.

Most profiles, except for the concealed fastener systems, can be lapped and remain watertight.

 

[haynewp]

In the US, we usually have roofing material over the metal deck so the water is not run in the lower deck valleys. Some agricultural buildings (barns) don't have this and the water tightness may be more questionable.

 

[hokie66]

haynewp,
Yes, that is an important distinction. Metal deck and metal roofing are not the same thing. You usually use metal deck under other types of roofing, not metal roofing.

 

[haynewp]

hokie66,
In Australia, I would assume then that you do normally rely on diaphragm action when you have metal deck fastened in the valleys instead of using bracing, correct?

 

[hokie66]

No, we don't use metal deck, and to my knowledge your type metal deck is not available here. Built up roofing is rarely used. Most large buildings have metal roofing, domestic structures have metal roofing or clay/concrete tiles. Our buildings typically use diagonal bracing trusses in the roof. Sometimes diagonal bracing in the walls, sometimes masonry or concrete shear walls.

 

[DaveAtkins]

Everyone is muddying the waters here.

Post frame buildings (in the U.S. at least) have corrugated metal roof panels (NOT wide rib metal deck), with the screws in the valleys, fastened directly to wood purlins. The screws often come with silicone or rubber washers to prevent water intrusion. There is no other roofing material over the metal roof panels. This type of system has been well tested to determine diaphragm capacities.

DaveAtkins

 

[Brad805]

Dave, they are a poor quality roof, but they are a lot cheaper. The other downside, is you need to review the roof screw pattern, and rarely do they call for such a review. In his case I would put in 3/8 diameter rods just like the pre-eng guys would do. It makes squaring up the roof framing a little easier.

 

[jayrod12]

I agree with Dave. When I am doing preliminary design I have found that you can get 100 plf of shear capacity fairly easily

 

[hokie66]

How large are these "post frame buildings"?

You get what you pay for.

 

[jayrod12]

Surprisingly hokie, they can get pretty large the biggest I've done is 40'x120'. But they are in the end a cheap storage building. They rule the majority of the farm storage market around here.