Roof Diaphragm - Sheathing over T&G decking

[JAE]

There's been one other post before this on this topic (thread507-106592) but we just faced this situation and I thought I'd ask out there if there are further comments or input.

The IBC doesn't recognize 2x tongue and groove decking (we have 2x6's) as providing roof diaphragm shear behavior. I've heard in the past that placing Structural I plywood or OSB over the top of the decking can provide the necessary diaphragm action.

My questions, however, are this:

The IBC recognizes the sheathing but specifies nailing on the "supports" - edge nailing and field nailing. We have gluelam beams at 6'-9" o.c. and thus there are no roof members spaced at 16" or 24" o.c. to specifically provide for the nailing.

If we use the 8d nailing with 1 3/8" required penetration, there is no standard length of nail that works - 8d or 10d nails are too long and would poke through the decking - ticking off my architect.

Is the sheathing considered as "blocked" under the IBC diaphragm capacity table?

The IBC states that we can use the NDS nail capacities and manually calculate diaphragm shear capacities from them. Is there a specific technique that shows how to do this?

What have any of you done with respect to this?

 

[msquared48]

6d won't give you the value you need?

Technically, in normal wood construction, wherever there is nailing, there is blocking. Just use the same nail pattern you would use for normal construction here, but with 6d for either a blocked or unblocked diaphragm.

I've done this in the past and it has worked for me.

Mike McCann
MMC Engineering

 

[JAE]

7/16" sheathing over 2x T&G decking - total thickness = 1.9375 inches.

Per IBC table, 6d nails are required to have 1 1/4" penetration into the support members. 6d nails are 2" long which is too long.

 

[Ron]

JAE...have you considered adhesive fastening for the decking over T&G? Wood-to-wood adhesive fastening is usually quite good in lamellar shear, thus developing the diaphragm should be easy.

 

[msquared48]

Then use 16 gage staples as they are only 1.5" long, and rated up to 405#/ft, depending on the lumber.

Mike McCann
MMC Engineering

 

[msquared48]

Or go to 5/8 or 11/16" ply with the 6d. Still works with the required penetration and overall nail length.

Mike McCann
MMC Engineering

 

[JAE]

All good ideas.

Ron, how do you "justify" the adhesive under the IBC? What are the load values (plf)?

 

[msquared48]

In Section 2305.3.10, adhesives are specifically not allowed for as a SUBSTITUTE for mechanical fasteners in sheathed shear walls in SDC's of D, E, or F. The implication here is that it can be added, but not relied upon.

It is interesting to note that a similar section is not found in the structural diaphragm section of the IBC (2305.2), which to me would imply two things: either

1. It is not to be relied upon structurally in structural diaphragms, or

2. By omission from the section, it is permitted if it is tested and verified.

Perhaps there is more guidance in ASCE7 or the NDS...

Mike McCann
MMC Engineering

 

[msquared48]

Nothing in ASCE7, but I did find an old article from March, 1981 in the Journal of the Structural Division - "Experimental Evaluation of Composite Action".

The general consensus was that diaphragm deflection was decreased with the glue used, Scotch 5230 (which I believe is no longer available), but I could not find any quantification of any load per foot with the glue. It only listed the applied test load of 200#/ft used in the test.

I thought there was an old ICBO Report on Scotch 5230 that gave the application rate and some tested values. Similar adhesive manufacturers today should have similar data.

Mike McCann
MMC Engineering

 

[Ron]

JAE..I couldn't find anything that helped with code approval on adhesive use for this, but didn't find anything against either. I found a relevant journal article ...I'll email it to you...can't post it.

Mike...I searched the Evaluation Reports but didn't find anything on that adhesive under either "Scotch" or 3M. All of the Eval Rpts on adhesives apply to manufactured panels that have separate Eval Rpts.

I would agree with both mechanical and adhesive fastening. I'm not sure how much area you have to deal with, but you might consider screws instead of nails. You can, obviously, get the screws in any length you want. Automated screw guns work well for large areas. Adhesive fastening will cut down on squeaking that you'll get under load.

As for the adhesive, I would suggest an epoxy, since it is not solvent cured. Solvent cured adhesives will maintain an odor longer than epoxy. You could use an acrylic adhesive, I'm just not sure you can get the strength you want without creep; though for diaphragm action, the load is not continuous so it probably doesn't matter.

 

[OldPaperMaker]

JAE,
You might go to

http://www.apawood.org

and find Form No. TT-097B "Designing Diaphragms over Existing Board Floors or Roofs". The three page article has some suggestions about the subject of your post.

I hope that this helps.

 

[msquared48]

I don't believe that screws are rated for diaphragm construction in the IBC Ron, but I also feel that they would be better than nails if rated.

Just labor $$$ here I guess.

Mike McCann
MMC Engineering

 

[Ron]

Mike...not that familiar with IBC diaphragm requirements, but the primary focus has to be prevention of buckling of the overlay sheets by maintaining a good bond between the sheets and the boards. Screws will do that better than nails, as you noted. The adhesive will further enhance that composite action.

JAE...the paper noted by OPM is a re-statement of the code requirements. It addresses the nailing pattern as you previously noted, but does not address adhesives.

 

[msquared48]

JAE:

Found this by googling Scptch 52130 Adhesive. Appeared to be an Australian site, but it seems like it is still around.

Product Catalogue for Maintenance and Repair (MRO) Solutions Adhesives 3M™ Miscellaneous Adhesives 3M™ Scotch-Grip™ Wood Adhesive 5230



3M™ Scotch-Grip™ Wood Adhesive 5230 Printer-friendly format

Additional Information


Meets the requirements of plywood fabricator service specification AFG-01 and complies with the provisions of FHA "Use of Mateirals bulleting UM-60". Conforms to I.C.B.O. Research Recommendation No. 1379 concerning diaphragm construction when using "Lock-Deck" (Reg. T.M. of Potlach Corp.) Laminated decking, roofing and
flooring system or equivalent; to I.C.B.O. Research Recommendation No. 2964 for attaching plywood sheathing to support members prior to nailing in accordance with Chapter 25 of the Uniform Building Code. GALLONS AND LARGER - NOT INTENDED FOR CONSUMER SALE OR USE.

Note that there are two reports - ICBO Research Recommendation #2964 and #1379.

Mike McCann
MMC Engineering

 

[msquared48]

#2964 appears voided, and 1379 is still listed, but I don't think it is really applicable here. Shucks...

Mike McCann
MMC Engineering

 

[swsengineer]

We typically use a 1 3/4" long 11 gauge roofing nail...the diameter is a little larger than a 6d nail but smaller than an 8d. Gluing the T&G is very time consuming and the glue is expensive. Nailing down the OSB is the preferred method of every glulam erector I know and is the method recommended by most deck maufacturers.

You can obtain a small amount of diaphragm using the NDS guidelines for a horizontal lumber diaphragm, but it ignores any contribution from any toenailing or shear friction in the tounge & groove.

Why use a plywood or OSB thicker than 5/16"? Anything thicker will require a larger nail and more nail penetration....penetration you can't obtain with a nominal 2" T&G decking unless you special order some 1 3/4" long 8D nails.

 

[JAE]

Simpson appears to have 10d nails that are 1 1/2" long. That would work for us.

 

[swsengineer]

1 1/2" nails only gives you 1 1/16" penetration with 7/16" o.s.b. and you need 1 1/4" minimum. with 5/16" o.s.b. you'll get 1 3/16" which is close enough but you'll just have to make sure they use 5/16 and not 7/16.

 

[JAE]

What we are doing is using the reduced capacity of a 10d nail (based on the NDS allowance for reduced penetration) and showing that the 10d nail with reduced p is stronger than the 8d nail with 1 3/8" penetration.

 

[swsengineer]

That sounds like a good idea but I don't get how you show the reduced P is stronger than the 8d nail with 1 3/8". I agree it's more than the 6d nail at 1 1/4". But with a .71 reduction factor (1.0625/1.5) the reduction takes you below the 8d values which are roughly 84% of the 10d values.

I'm not trying to be arguementative, I'm just trying to understand too. I may be looking at a different chart than you.