Drag coefficients and shear performance of metal roofing for building diaphragm loading
Drag coefficients and shear performance of metal roofing for building diaphragm loading
(OP)
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.
The roofing is a 26ga PBR ribbed.






RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
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.
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
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
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
http://www.icc-es.org/Reports/pdf_files/5468.pdf
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
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.
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
"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.
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
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?
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
Most profiles, except for the concealed fastener systems, can be lapped and remain watertight.
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
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.
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
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?
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
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
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
You get what you pay for.
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
I have seen these pole buildings up to 60'x300'. They started as farm bldgs, but you see them pop up in commercial locations where the building inspectors are lax. Personally, I think the designers of the ones in my neck of the woods do not consider lateral loads at all.
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
General design minimums per code are 20psf live roof, 10psf dead roof, 75mph cont wind, 90mph gust. Dimensions are 60x96x17 with 3:12 pitch free span, 12ft bay spacing. This presents a unique challenge in the custom roof trusses but I think we have our load in order on the trusses. The design calls for all girts/perlins to be installed in joist hangers, purlins are 2x8 SPF on 24" centers, girts are 2x6 on 36" centers. 26ga PBR roof paneling installed on walls and roof. Main columns are 5 ply 2x8 glulam, embedded 4ft, concrete floor.
Without applying diaphragm action, the columns are undersized but we know from experience in the area that lesser buildings survive here in the midwest.
Technically, we could probably sign off the building to meet the minimums but are looking at further improvements that will increase survivability at high wind ratings. It is inevitable that the structure will see 100+ MPH. We are trying to determine proper drag of the paneling to better estimate loading. We may also install blocking to better handle the shear.
There is to be a wall all the way across the building at the 36ft mark. We have yet to determine if we will need shear capacity in that wall. It is intended to be drywalled on both sides.
There are some lengthy studies available from universities that indeed prove shear ability through the roof panels but we want to stay conservative with that. One element I personally am trying to avoid which is stitching screws which connect the 3ft panel widths. They are not very attractive but do increase shear handling.
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
Design the columns as cantilevers (usually won't work).
Design the columns and trusses as moment frames (usually will require knee braces).
Design the roof as a diaphragm, and the side walls as shear walls (this is the way I have typically done it).
You could also use that interior wall as a shear wall, but it would need to be connected to the underside of the roof diaphragm (alternatively, a drag truss could be positioned over the wall).
DaveAtkins
RE: Drag coefficients and shear performance of metal roofing for building diaphragm loading
Also, to throw a wrench here, I was desiring a "reverse" rolling PBR panel on the walls which, if not familiar, is the exact same panel form, just flipped over so the fasteners are in the valleys. I can see some structural concerns with that but I assume you would typically need blocking on the ends for shear anyway? This would also create an air gap between the frame and panels which may or may not be a good thing. I would like to think I can get some worth while convection cooling behind there but that will probably just introduce dirt into the wall system.