T&G Decking as diaphragm 1

[papa2ten]

Local code official is asking me to submit any research showing the ability of T&G heavy timber decking to act as a roof diphragm.

SBC 2000 says not to use interlocking pieces as diaphragm. However, we are spiking each piece together. We have always used it in the past.

Has anyone ever used spikes to resist the horizontal shear and/or know of any research showing diaphragm using T&G decking.

THanks!!

 

[JAE]

I got the following off a website chat:

Is anyone familiar with methods of using 3x TNG decking as a diaphragm? Shorting of putting plywood on top of or under it and nailing that to the boundaries as the diaphragm, I don?t see any obvious and code compliant route to take. I could envision 3x being a very capable diaphragm with toe nailing or gluing, in theory? but I don?t see a lot of glued assemblies in the code. Anyone have any experience with this?

Paul,

I have a barely ledgible fax from 1997 (still on the old thermal fax paper) regarding the design of diaphragms using laminated timber decking. The fax was from the Filler King Co (now Lock Deck by the Disdero Lumber Co., try

http://www.lockdeck.com)

and was originally written by Robert Hoyle, Jr., P.E. while he was at the Potlach Co. The fax details how to design laminated timber diaphragms and makes reference to ICBO Report No 1379. Mr. Hoyle also has a book, Wood Technology in the Design of Wood Structures which includes detailed design examples. I hope this helps. I can try to xerox the info I have; but, I doubt you would ever be able to read it.

Brian K. Smith, P.E

 

[RARWOOD]

The solution to your problem depends on the thickness of deck you are using. As one of the previous replies indicated with 3" deck you can look at applying a glue bead to the tongue and groove. The problem with this approach is that all the research I am aware of which has been done on glued diaphragms using a 3M glue that is no longer available.I have seen some specifiers call for Miracle adhesive as a subsitu. I might use Miracle for average wind loading, but personally would not use it for high wind or high seismic areas. I am sure though other designers would disagree with me.

For more information contact Gunnar Brink at Disdero Lumber, 503-239-8888.

If you are dealing with 2" deck gluing is probably not an option because the small size of the deck don't allow you to develop an adequate glue bond.

Regarding nailing of the deck, you are some what on your own. Nailing the deck will increase the diaphragm capacity the problem is how effective it will be. With your Building Official you should be able to submit design calculations to justify your approach.

The major problem you have with this approach is that the lateral deflection of the diaphragm may be larger than you predict. You could contact the technical department at Western Wood Products Association for more information.

Also there are a large number of timber buildings in the field with only 2" decking bracing them, which have performed adequaetly over time.

One final thought, based on my experience, is that the cost of glue and labor involved in gluing the deck up to perform as a diaphragm ussually exceeds the cost of a plywood overlay. You might check on your next project what the additional labor cost are for specifing the additional nailing

 

[Taro]

One alternative is to conservatively design it as a horizontal lumber diaphragm, ignoring the interlocking effect of the connections at the T&G. The 2001 NDS wind & seismic supplement gives an allowable diaphragm capacity of 50 plf for seismic and 70 plf for wind. If you orient the lumber diagonally, the capacities jump to 300 plf & 420 plf.

Building codes also allow you to calculate diaphragm capacities by principles of mechanics using values of fastener strength and sheathing shear resistance. In my experience, this is rarely done though because allowable capacities are tabulated for many common diaphragm types.

 

[AlohaBob]

Spaced sheathing like as in gapped 2x6 spaced roof sheathing is allowed and assigned a shear value.

Nonetheless I seem to recall having to overlay plywood over T&G to get high shear values when I couldn't get enough capacity.

See the OLD '88 UBC 2513a You can fly right with this if you have a diagonal controlled random layup. Or else, rational analysis can apply. I've done the calcs RARSWC ands Taro refer to as last resorts.

 

[twells12]

bnannis,
I know you asked this question a while back.. What did you find as a solution?

 

[papa2ten]

Local code official insisted on the addition of plywood deck for diaphragm.

 

[JAE]

On top of the T&G or underneath? Usually T&G is used for aesthetic purposes underneath so I assume its on top.?

 

[jhall]

I have not found any literature on the lateral capacity of T&G decking. I always add sheathing on top of the T&G deck and provide the naling required. I assume that the deck is blocked when required.

 

[teero]

Canadian codes does not alow 1 1/2" deck to be used as diaphragm. We put plywood band or steel plate bracing on top. 3" deck, we use lateral bolts to increase the capacity, if you can not have them running diagonlay. If you need I email you parts of the code.

 

[papa2ten]

Thank You!

 

[boo1]

WCD 2 - Tongue and Groove Roof Decking

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http://www.awc.org/pdf/wcd2.pdf>