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Re: Timber floor framing

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Brian:

This type of floor is VERY typical in many timber frame homes (i.e. heavy
timber construction with traditional mortise and tenon joints or are made
to look like traditional mortise and tenon joints).  I see it many times
on timber frame homes that the company that I work for designs and
builds.

There is two styles of such wood/timber deckings.  The more "traditional"
is solid timber wood decking.  Such decking would be made of solid 2x (or
3x) timbers.  Usually, such decking is made to be T&G, but there is
nothing to say that it cannot be just butt jointed (most don't like or
want this as when the timber decking shrinks, you will get gaps where
light and other things can come through).  There is also "newer" decking
that is manufacturered wood timber decking.  It will be made of
laminations of solid wood pieces (i.e. not made from layer of plywood or
OSB) much like a glulam member, generally.

As to spacing of support members, you can DEFINIELY span greater than 2'.
In this case my friend Andrew's advice is overly conservative (sorry
Andrew)...more or less so depending on species used.  You can calculate it
out yourself, certainly.  It is just a matter of either analyzing a
simple span beam (most simple & conservative case) or a multiple span
beam (which for the spans you are talking about [you mentioned 4'] would
be the more accurate case) and then checking maximum bending & shear
stresses per NDS requirements and deflection.  There is a table in the NDS
supplement that gives unmodified "base" allowable stresses/values for
visually graded heavy timber decking for most species (although for some
reason they don't have values for the various Oak species).

Now if you are "lazy" and don't want to do hand calculations and have a
copy of the Timber Construction Manual by AITC around, then you can just
use the tables in the back of the book.  There are a series of tables for
determining allowable uniform loading on heavy timber decking in the back.
The tables specifically are called "roof decking" tables, but in reality
they can definitely be used for floor decking too, as the tables are all
based upon a load duration factor of 1.0.  There are tables for
determining max allowable uniform load based upon bending stress and
deflection.

For what you mention (i.e. 2x Oak decking spanning 4 ft), you could
support an allowable load of at about 40 psf for a 6 ft span (the tables
in the Timber Construction Manual only go down to 6 ft) with deflection
govering and using a "controlled random layup" span.  This assumes that
the Oak would have a modulus of elasticity of at least 1,300,000 psi,
which should be the case (but you would have to check with the supplier).
If you convert that to a 4 ft span (i.e. the values should be rougly
proportional of span to multiple power), then you would be limited to
about an allowable of load of at least 60 to 70 psf (total load...i.e.
live plus dead), which for a residence should be fine.  I will note that I
did not do any hand calcs to get "precise" numbers...I leave that up to
you.

While Andrew is a little bit conservative with his response about the 2 ft
span, he is kind of right in that frequently heavy timber floor decking in
homes is done with 3x decking as minimum.  This is largely because most
timber frame homes tend to be "higher end" homes and no one wants a
potentially "squishy" floor for such a home and 2x decking at 4 ft o.c.
has the potentially to maybe be slighty "squishy".

As to supporting the timber desking, it would be supported just like you
would support "stick framed" decks (i.e. decks of 2x or wood I joists) at
walls.  You would build you stud wall just like any normal flooring and
sit the solid timber decking on the stud wall just like you would a stick
framed floor.  In fact, I would argue that you would be more likely to
need to "beef" up a stud wall for stick framed deck systems that for solid
timber decking.  Stick framed decking will have "concentrated" loads at 12
in, 16 in or 24 in on center acting as a uniform load, while a solid
timber decking will behave MUCH more like a uniform load (you have decking
members right up against each other side by side).  I will note that in
general in timber framed homes, the heavy timber decking is typically
supported all be heavy timber members and not bearing walls, although
there is nothing preventing the decking from being supported by a bearing
wall.

HTH,

Scott
Adrian, MI


On Fri, 23 Jun 2006, Brian wrote:

> List,
>
> I am unfamiliar with this type of wood framing and was
> wondering if anyone had experience or could provide
> some information/details on this type of framing.  My
> client would like to use 4x timber beams at 4'-0"oc
> and 2x oak planks as the flooring system to a
> residence.  This home is located in Florida.
>
> I'm not quite sure how the system would go together.
> I believe that the flooring system wouldn't actually
> be 2x planks similar to a patio deck.  I'm guessing is
> that the floor sheathing would actually be multiple
> layers (maybe 3?) of 1/2" plywood that would be
> attached together to make a tongue and groove
> connection.  I'm really not sure what type of nailing
> would be required to connect the floor sheathing to
> the 2x timber members.  I'm also assuming the timber
> beam members would be supported on ganged 2x stud
> posts or timber posts within the bearing wall.
>
> As you can tell, I'm pretty clueless on how this
> system works.  I would appreciate any help that anyone
> could give me.  Or, if someone could direct me to
> where I may be able to find some more information or
> typical details.
>
> Thank you,
> Brian
>
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