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Garage Door Detail

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You have described....

>a garage door detail,
>which consists of a GLB spanning the garage opening, supported by two very
>narrow (16" min) walls, which are sheathed on two sides. The ply sheathing
>is nailed to the GLB in a grid pattern.  Tiedown straps are added between
>the GLB and the walls, with HD's as the footing.

>What we are doing is creating a wood moment frame. Is this wise?? If wood,
>over time, shrinks, or takes on a permanent deflection, wouldn't the nails
>lose their shear resistance capacities?
>
>Talk to me, people
>
>Kate O'Brien, P.E.
>Simi Valley, CA
__________________
Others have said:

>As far as I know, the Code does not recognize the existence of plywood
>"frames." 

> It goes without saying that the owner must
>be made fully aware that this is not code-approved construction. 

>Assuming that this detail is meant to be a portal frame!
__________________
I say:
        Of course it is a portal frame. Same as always, only right side up.
The familiar way is inverted, with an indifferently detailed concrete beam
in the ground and two plywood columns sticking up, depending on hold-downs
for most of their moment connection. This is true even if these plywood
columns are wider than the 2:1 ratio now touted as the panacea for most of
their shortcomings.  

        While the building code does not expressly provide a methodology for
rigid frames of wood, it has done very little more for customary wood shear
wall construction, especially in matters of their moment connections. 

        The code, by its own terms, does not purport to discriminate against
uncommonly used ways of assembling commonly used materials to satisfy
code-specified loading conditions and serviceability expectations. (See Sec.
104.2.8 in the '94 ed.)  The P.E.Act in this state defines the work of a
professional engineer, in part, as "creative" (Sec 6701). The code gives an
all-purpose requirement for every engineered design: "a rational analysis in
accordance with well-established principles of mechanics." (Sec. 1603.1)  
(It's amazing how often urgently-enacted code changes fail to admit of any
rational analysis-- a spectacle most easily seen while serving on a change
-perpetrating committee.) 

        This interesting design problem appears to be amenable to use of
conventional statics and strength of materials. It has been for me, anyway,
and in too many permutations to remember. APA has useful but seldom-invoked
strength values in their Plywood Design Specification. Nail "slip" values in
UBC Standards show a non-linear stress/strain relationship and lots of
resistive value beyond the loads corresponding to allowable shear wall
shears. Understressing this nailing offers disproportionate advantage in
reducing the nail slip component of assemblage deflection. And who says you
are limited to 2x studs or a single row of nails?  

        Due caution is among the design tools also. As mentioned by many,
envisioning the effects of moisture change and the effects of play,
slippage, and deformation in the connections would be essential. Effects of
workmanship on the components is another important consideration. There's a
host of things to discover and deal with, but isn't that what our education
and training was supposed to equip us to do?  Isn't that ability part of
being a professional?

        The next several years are going to be entertaining, as the 2:1
ratio limit goes into effect, and results in all sorts of creative
circumventions and evasions, for better or otherwise.  Too bad no fixes or
mitigations for 3.5:1 ratios were allowed.  Kind of like controlling Teen
Pregnancy only by demands for abstinence, which few will obey, and you won't
allow condoms.  

Charles O. Greenlaw, S.E.     Sacramento  CA