Standing Seam Metal Roof On A Bar Joist Roof

[spats]

I found a few old posts on this subject, but they weren't completely helpful. In my situation, the owner's contractor has requested that we use a standing seam metal roof on a new 53' x 64' on a clearspan bar joist structure with CMU bearing walls. The building is about 20' tall. I know that the metal roofing cannot be used as a diaphragm to brace the building. In addition, the metal roof slopes in the direction of the 53' bar joist length, so a sub-purlin system system is needed perpendicular to the joists at 5' on center max. But then there's the matter of providing a roof diaphragm. Reading the literature will tell you that you either need to provide horizontal x-bracing in the plane of the bar joists, or to use a plywood or metal deck substrate. If you use a substrate, it seems that the sub-purlins would no longer be required.

I feel that the best course of action would be to use a metal deck diaphragm/substrate. It runs perpendicular to the joists and the metal roofing... perfect! However, what happens in the cavity space between the roof and the deck? It seems like you'd have to ventilate the space, both from a thermal and moisture standpoint. Do the standing seam clips simply screw into the deck? Has anyone ever been involved in using such a system? I also know that telling the owner/contractor that they need metal roofing plus metal deck is going to be a hard sell. This is a "cheap" unconditioned building. I need some good advice.

 

[KootK]

This is Nucor's CFR system which is similar to what you've proposed. Their business is economical shelter so I'd have to think they'd be knowledgable.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[spats]

Thanks KootK, I'll look into it. I don't know why they use the extra "z-bars" on top of the deck, rather than attaching the roofing clips directly to the deck. It adds not only the z-bars, but an additional layer of insulation. It's starting to seem that the best system might be to use horizontal x-bracing at the top of the joists for bracing. I don't want to tell the owner/contractor that they need metal deck plus filling the cavity with insulation, for an unconditioned building. What do others use for horizontal bracing in such a case? Welded steel straps (will they sag)? Rod bracing with clevises and turnbuckles? Thanks.

 

[KootK]

I go with the steel straps. They don't sag appreciably over ~6'. Lots of info and details here: Link

XR250 did something similar recently where the roofing was able to be the diaphragm: Link. PBR roofing or something. Perhaps you could steer your client in that direction.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

Spats..don't forget that you will likely have an energy code requirement, so the insulation as shown might be necessary.

Several of the metal roof system manufacturers have systems that will do what you need. Be careful with which materials you specify (are you near the ocean? Do you have high uplift resistance requirements?). Tall clips with long panels don't perform well over the long term. I have investigated fatigue failure of such clips just from thermal cycling. Also be concerned with pull-out and pull-over strength of fasteners when fastening into metal decking.

 

[spats]

Thanks guys for the info. KootK: the Vulcraft publication makes no mention of how the top of the exterior wall is able to span horizontally for a full bay. With perimeter steel framing it would be easy to use a girt near the top of the wall. In my situation, with exterior load-bearing concrete masonry, I can put poured tie beams at the tops of the walls, but they can't span laterally for the full width and length of the building. Besides, my building is essentially just one big bay in both directions. If I create one horizontal truss in the plane of the roof in each direction (say 1/3 of the building length & width in depth, crossing at the middle of the building), then I can use 2 joists for chords/struts in one direction, but need to create two chords/struts in the other direction perpendicular to the joists. I'll need to have continuity in these perpendicular chords/struts as they cross the other non-strut joists. This is starting to get a lot more complicated than I had hoped. Any suggestions?

 

[KootK]

Could you post a plan? With some dimensions and a joist layout, I can be considerably more creative. At least, I think that i can...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

The generic answer is that the bracing should be restraining the lateral movement of the joists such that your bond beam only span from one joist to the next.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[spats]

Thanks KootK. See the attached preliminary roof plan.

 

[KootK]

Yeah, things get a little trickier once you've got no diaphragm at all. This is the default way of things in Hokie66's neck of the woods. Perhaps he can suggest improvements.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[hokie66]

Those plans look a bit familiar, with trusses in the plane of the roof. KootK is correct, in Australia, we don't use metal decking or roofing as diaphragms, but rather always provide discrete bracing. But then, we don't use bar joists either, so bracing would tend more toward angles, hollow sections, etc. rather than straps.

 

[KootK]

When I look at the bracing and imagine the effort involved, it makes me wonder if it's really s true cost savings over having the deck. I am often surprised by the cost of installed deck however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[spats]

The problem with using deck is the cavity you create between the standing seam and the deck that must be insulated/vented.

Another curious thought about the Vulcraft publication. Every joist & joist girder bay, by itself, has no internal diaphragm, yet somehow transfers lateral loads to the bracing/column lines. Are the joists to be considered braced only at the bridging lines?

Again, I basically have a single bay building, and my single bay isn't magically OK internally like all the bays in the Vulcraft publication... not much help!

Thanks for the bracing concept KootK.

 

[KootK]

Another curious thought about the Vulcraft publication. Every joist & joist girder bay, by itself, has no internal diaphragm, yet somehow transfers lateral loads to the bracing/column lines. Are the joists to be considered braced only at the bridging lines?

I haven't read through it in a while but I believe that all of the examples in the Vulcraft publication are situation where you still have a steel deck but it's capacity is augmented with the strap bracing. So, in those scenarios, I suspect that the deck is being used to brace the joists and transfer their contribution to lateral load out to the strap bracing system.

Again, I basically have a single bay building, and my single bay isn't magically OK internally like all the bays in the Vulcraft publication... not much help!

To stabilize the system, I'd think that you'd need:

1) A strap bracing system like those we've been discussing and;
2) The ability to count on your sub purlins to brace your joists and move roof seismic mass around.

I'm not sure that I've ever seen on OWSJ system that had no deck diaphragm whatsoever. This may be why.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[spats]

The Vulcraft horizontal bracing section 4.3 begins with the sentence "There are occasions when diaphragm action cannot be used to provide lateral stability for the structure. This most frequently occurs in single story structures where standing seam roofs are used. In these cases, lateral forces can be resisted in the plane of the roof with a horizontal bracing system". It doesn't sound like there is any deck there to me, but thanks for the effort.

 

[KootK]

So it seems. I imagine that they would have anticipated using the purlins for truss bracing as discussed above. At the end of the day, we have a no-deck scheme that would work if someone's willing to pay for it,right?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.