×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

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

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.

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

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.

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

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.

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

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.

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

"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

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

(OP)
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.

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

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.

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

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.

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

(OP)
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?

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

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.

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

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.

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

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.

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

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?

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

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.

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

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

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

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.

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

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

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

How large are these "post frame buildings"?

You get what you pay for.

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

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.

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

For some reason, I didn't think fastline was talking about farm buildings. Maybe his "Aerospace" area of interest led me to think aircraft hangar.

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

I think he is talking about a personal project he is making some sort of glulams for.

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

(OP)
Maybe I can explain more to see if you guys can offer some design ideas? The building is for personal use, not commercial or aerospace related.

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

There are various ways to justify lateral loads on these buildings:

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

(OP)
Dave, just as a general, I am curious, are you typically able to validate the roof paneling over the purlins as a sufficient diaphragm?

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources