Does Composite Decking Brace The Top Face of 2x12 Joist 2

[Norby_acn]

I am designing a deck for a client and they have told me they want to use 1" nominal thickness Trex composite decking on 2x joist. I know from experience I can count on 1x boards, wood sheathing, or even gyp to brace most 2x material. I've even read literature about using light gage metal wall panels as diaphragms on super-light duty construction. My first thought was that of course I can use composite decking to brace my floor joist (specifically for my C[sub]L[/sub] Beam Stability Factor calculations, just to be clear).

However, through some hearsay in the office, someone heard from somebody on a condo project somewhere, that composite decking caused a deck failure because it wasn't sufficiently stiff to brace some 2x12 joist it was placed on. Well, I've tried to dig into that online a bit... and I can't find any literature one way or the other on the subject. The CCRR and ICC reports don't address it and I can't find anything in the manufacturer's product literature which addresses the issue. Even contacting customer service I can't get ahold of anyone who would be technically competent to tell me one way or another. I almost wish my coworker had never mentioned it because I've gone down the rabbit hole and can't seem find anything to definitively say that composite decking braces, or does not brace, the joist it is fastened to.

So my question is... would you count on 1" nominal thickness composite decking to brace the top flange of a 2x12 that has nothing attached to the bottom face?

Just for reference, I am looking at approximately 12'-13' spans.

 

[phamENG]

I'd say it's reasonable to assume it does NOT brace them, but it will depend on the connection. If you're using the little plastic biscuit hidden fasteners in a full length groove, I'd say the bracing will be dubious at best. As the joist rolls, it would be able to slide along the decking a short distance - probably enough to either induce loading for which the fastener wasn't designed or enough to induce a pretty hefty, secondary torsional load in the joist that keeps it rolling.

Blocking at midspan is cheap and easy. Make sure you account for the fact that there's no diaphragm and supply some sort of bracing if its required.

 

[XR250]

Right or wrong, honestly, I have never given this any thought. Even if it is designed for 5/4 wood, we have no control over future deck finishes.
I have installed those hidden systems and I feel it is reasonable to use them for bracing. I don't recall them sliding too easily and it does not take much to brace a joist.

 

[phamENG]

To be fair, I'm with XR on the past practices part. I've never really considered it either. I hadn't thought of it in my detail and my previous response was my first pass at it.

If installed well I doubt there would be much of an issue as the fit up is pretty tight. Overload the deck or have a less than great install and problems are likely to result. Those spans are well within the range of needing top edge bracing for typical residential live load requirements (40psf).

 

[Ron247]

The deck material I have used would appear to me to be able to brace the 2x12 but the connection is the larger question. Sliding clips and fasteners wedged into slots may not be reliable enough.

 

[Colostruct]

This is a great question. I have no theoretical basis for this. But, I always provide blocking at the third points and figure the decking provides compression zone restraint.

My justification is that I have observed dozens of floors in old houses where the wood flooring is the restraint and I have yet to see joists twisting and failing due to the connection failure of flooring to joists.

 

[LuK13]

What orientation are the deck boards going to be installed in? A lot of the older homes I work in have diagonal board subfloors, diagonal being the key word. The 2015 IRC even allows for diagonal wood board with two fasteners per stud for braced wall panels. Diagonal wood boards are a time tested method to construct a diaphragm.

That being said here are my concerns:

1. Most deck boards are installed perpendicular to the joists, and the boards do not create a diaphragm in this orientation.
2. I share XR250's concern about controlling future deck finishes. What if a new owner 30 years down the road removes the diagonal board and replaces it with perpendicular board? Is the contractor going to pick up on the need for the diaphragm? Is there even a contractor involved or is it just a weekend warrior?
3. I have seen cheaper composite decking deform after being exposed to the elements. How does the material properties of your composite decking compare to the properties of the wood board you would use for a braced wall panel? Even if you get it right, whose to say a bottom dollar replacement isn't installed 30 years later?
4. I share phamENG's concern for the connection detail. The diagonal board that I encounter in the field that has lasted is faced nailed. I have the same reservations about the hidden fastener systems that are popular for decks these days.

So if (1) this is a bit of a science project, (2) the decking manufacturer isn't providing adequate information, (3) there are concerns that future renovations may compromise the structure due to a lack of understanding how this atypical system relies on specific decking details, and (4) blocking and bracing is not overly expensive, then why rely on the decking for bracing in the first place?

All of this to save on some blocking and bracing seems penny wise and pound foolish.

 

[Colostruct]

If I understand the OP's original question he was asking, specifically, about the use of decking to provide unsupported compression restraint of the joists (top of the joist on simple spans). Not the global lateral stability of the deck structure.

 

[bones206]

To me it's analogous to the question of standing seam roofing panels bracing the compression flange of purlins. Unless there is some specific testing that provides a design value, then I would ignore it's contribution. Besides, blocking decks is good practice anyways, for reducing joist twist and sidesway.

I have Trex decking at my house and the builder did not put in any joist blocking. The deck is noticeably bouncier than a I've experienced with traditional lumber decking. It's also super wavy looking since the Trex conforms to all the little variations from joist to joist. Blocking should help reduce those variations.

 

[Colostruct]

bones206

This is an interesting question that comes up often in our field. Restraint of the compression side of structural members. You mentioned specific "testing that provides a design value". Honestly, I cannot find a design value for diagonal decking, wood planks or even plywood that tells me we need "x" amount of lateral restraint for "y" amount of compression stress in a top flange in wood construction. Do you have a source besides that we just know what works?

 

[bones206]

No and I wouldn't think there would be such testing for decking, unless decking manufacturer's felt it would give them some type of competitive advantage. But as far as I can tell, most composite deck manufacturers recommend blocking to help reduce serviceability issues.

 

[phamENG]

Colostruct - I asked a question on these forums a few years back. There's nothing hard and fast for traditional wood construction. Steel design is the only field that really provides highly detailed research into discreet bracing of flexural and compression members, so I've looked to Yura and others for guidance in wood as well. Turns out, it works pretty well. Most of the raw theory in their work is material independent, or easily transferable. You have to take care in the detailing, since wood is a fundamentally different material, but the basic theories of stability still hold.

By the way, I take issue with your statement that perpendicular deck boards, when properly face nailed, do not provide a diaphragm. If that's the case, about 90% of the decks in the US would be classified as unstable. The SDPWS has diaphragm tables for lumber decking in diagonal and perp. arrangements. There have been tests performed on traditional wooden deck structures to determine diaphragm strength even when there is a small gap between the boards as is customary to provide drainage. They found that the face nails (of which you need at least 2 per support per board) act as a moment-connection-in-miniature to resist the lateral load. So you have hundreds of these little force couples keeping the deck stable. Once these fail, there's a large lateral shift that then stops as the deck boards come in contact while the frame racks. The displacement stops briefly as the load increases again, the resistance coming from lots of friction and shear in the remaining connections (this part is pretty tough to quantify). Then total failure occurs as the last of the connections give out.

 

[Colostruct]

PhamENG

Thanks. I've used a lateral restraint "technique" similar to steel restraint on a tall glulam beam restrained by floor sheathing. I just have not run this calc (which was not something I would want to run for every deck) on the design of a deck joist. My point was that 2x decking is something we often use without considering exactly what it provides...just less than composite decking...doesn't seem like a great answer for the OP.

BTW. I did not state that decking did not provide a diaphragm...I believe that was a different poster. I've designed dozens of decks and depending on the situation I have assumed lateral diaphragm action to the outside braced columns. However, I do not believe that open decking should be relied upon for decks (especially above grade) that are in rotation. So, if the posts are not braced vertically, I will add horizontal bracing (often times straps in tension) at tops of joists.

I have never seen a deck joist or interior joists fail due to loss of top face restraint due to rot and I live in snow country where light decks often see loads much larger than they were designed for. The decks that fail, mostly, fail due to pull out from the buildings (I'll get on my high horse about that some other time). I have seen a trex deck fail due to ice loading on the joists. The trex remained nicely fastened to the joists. I have witnessed a truss roof fail due to twisting where the exposed purlins were flat 2 x 4 @ 32" O.C. with the roofing spanning between them. There was not sufficient top compression lateral restraint.

 

[LuK13]

phamENG,

Noted, although I just grabbed the 2005 copy of the SDPWS that was in reach and it looks like horizontal lumber sheathing has a v[sub]s[/sub] that is 1/6 of diagonal and a G[sub]a[/sub] that is 1/4 diagonal.

I guess I had just built this perception from seeing diagonal lumber sheathing in the field, or when there is horizontal board sheathing there is typically a second form of bracing, such as let let-in bracing was provided.

I skimmed the article you linked. The conclusions seem to be mostly about the joist-to-ledger connection being a weak point. I found the part that discusses the failure mechanism of the deck board fasteners, but I couldn't find the load at which this occurred. I'll have to revisit this later.

Good info to have going forward, the difference in capacities and stiffness are definitely something to be aware of.

 

[phamENG]

Colostruc - touché. My apologies. And I agree - we don't give it much thought. I only look at quantifying the bracing for wood beams specifically when I'm dealing with a significant beam in a large structure, which is somewhat rare. Otherwise, I take a more qualitative approach.

I generally try to avoid knee braces to the columns. The loads end up quite high and the details a bit more complex than your average deck builder can reasonably be expected to execute in the field. Not so say there aren't some great carpenters out there building decks, but my general impression has always been that the good guys are in the house while the cut rate hacks show up later and slap on the deck. What do you use for lateral? I've read some other articles/studies that concluded the maximum lateral load that a crowd of people can sustain on a deck (due to resonance and human vibration response) is roughly 1/4 of the live load. Given the propensity of these things to collapse, I've taken that to heart and use 10psf lateral traction loading. (You're not alone on that high horse.) But 5psf would likely be sufficient in most single family residential settings.

LuK13 - you are correct. Horizontal is waaaay more flexible than diagonal. But it's there and can do the job in light duty situations.

 

[EdStainless]

Given how soft and flexible Trex is (and very low modulus) even if it was screwed into the 2x it wouldn't provide much bracing. Use plenty of blocking.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Colostruct]

Phameng

I am fortunate in that I design mostly structures in high snow load that has a bit of seismic. So, my typical snow old is 75 PSF and I use an average lateral load of 8% which is conservative. That calcs out to about what you use...about 7 PSF.

You are correct that the deck bracing at the columns...knee braces are often not designed or built properly.

This thread is timely. I’m designing a sun deck at the local community center and they want to use trexish for decking. That deck is only a few feet above grade but 150 PSF with live loads and partial snow. There are lots of little pieces to a “simple” deck. “It’s just a deck” I am sure I will hear when I submit my design.

 

[XR250]

Around here, 1/2"Ø lags are used for the knee braces that end up right between the plys of the double band. Pretty great detail . I only specify X or V bracing for columns or "brace per code" if it is a low consequence deck.

 

[Norby_acn]

Wow, I really appreciate everyone's input on this. There has really been a lot of discussion surrounding the issue, probably more on this thread than on all other posts I've ever made combined.

That said, I would like to remind people that this is not an eqo measuring contest, neither is it a competition with winners and losers.

This has really helped me in terms of how I think about it and how I will proceed moving forward with this and other decks.

Thank you all!