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Re: Garage Door Detail

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"Wood diaphragms in structures that support floors or roofs above shall not
be allowed to transmit lateral forces by rotation or cantilever. However,
rotational effects shall be accounted for when unsymmetric wall stiffness
increases shear demands." (L.A.C.C. Section 91.9305.10 Horizontal
"EXCEPTION: Diaphragms that cantilever 25 percent or less of the distance
between lines of lateral load resisting elements from which the diaphragm
cantilevers may transmit their shears by cantilever provided that rotational
effects on shear walls parallel and perpendicular to the load are accounted
for." (Same section).

The prohibition of open-front design by use of rotation came about after the
Northridge earthquake. Retrofit provisions in the code were required for
soft-stories with living units above.

Dennis Wish PE

-----Original Message-----
From: Mark A. Scott <MAScott(--nospam--at)>
To: seaoc(--nospam--at) <seaoc(--nospam--at)>
Date: Tuesday, February 10, 1998 6:41 PM
Subject: Re: Garage Door Detail

>everyone eems concerned about the frame on either side of the door opening.
> What about your diaphragms?  Does the roof diaphragm have anchorage to the
>rest of the structure?  Are the boundaries sufficiently fastened?  Is there
>a need to cross tie the ceiling or use it for a diaphragm?  Why not use the
>two side walls and the back wall?  The load is resisted by the stiffest
>path.  Is that path adequately tied and anchored?  Do the engineering and
>let the code be your guide!!
>Thanx, Mark Scott SE
>> From: Charles Greenlaw <cgreenlaw(--nospam--at)>
>> To: seaoc(--nospam--at)
>> Subject: Garage Door Detail
>> Date: Sunday, February 08, 1998 10:40 PM
>> You have described....
>> >a garage door detail,
>> >which consists of a GLB spanning the garage opening, supported by two
>> >narrow (16" min) walls, which are sheathed on two sides. The ply
>> >is nailed to the GLB in a grid pattern.  Tiedown straps are added
>> >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
>> >over time, shrinks, or takes on a permanent deflection, wouldn't the
>> >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
>> The familiar way is inverted, with an indifferently detailed concrete
>> in the ground and two plywood columns sticking up, depending on
>> 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
>> their shortcomings.
>>         While the building code does not expressly provide a methodology
>> rigid frames of wood, it has done very little more for customary wood
>> wall construction, especially in matters of their moment connections.
>>         The code, by its own terms, does not purport to discriminate
>> uncommonly used ways of assembling commonly used materials to satisfy
>> code-specified loading conditions and serviceability expectations. (See
>> 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
>> all-purpose requirement for every engineered design: "a rational analysis
>> accordance with well-established principles of mechanics." (Sec. 1603.1)
>> (It's amazing how often urgently-enacted code changes fail to admit of
>> rational analysis-- a spectacle most easily seen while serving on a
>> -perpetrating committee.)
>>         This interesting design problem appears to be amenable to use of
>> conventional statics and strength of materials. It has been for me,
>> and in too many permutations to remember. APA has useful but
>> strength values in their Plywood Design Specification. Nail "slip" values
>> 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
>> 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
>> workmanship on the components is another important consideration. There's
>> host of things to discover and deal with, but isn't that what our
>> 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
>> 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
>> allow condoms.
>> Charles O. Greenlaw, S.E.     Sacramento  CA