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Re: Garage door detail

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>> Subject: Garage door detail
>>
>> >...1997 (many of you may also have gotten it).  It is 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.
>>
>> >My initial thoughts on this are:
>>
>> >We could resolve the moment at the GLB/wall connection by distributing
the
>> load to the nails in the grid pattern (this was a basic statics problem
in
>> school). The nails would all be in shear.
>>
>> We could determine the uplift force by resolving the moment into a couple
>> (another basic statics problem). We could use standard Simpson products
(or
>> equal) for the straps and holdowns.
>
>
>>
>> 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
>>
>> Assuming that this detail is meant to be a portal frame!
>>
>> The main shrinkage will be across grain.  The glulam and the plywood
should
>> show little shrinkage.  The amount of nail grip loss will therefore be
>> minimal.  If that is considered a problem then allow for it in the
treatment
>> factors Kt or Kst.
>>
>> I have a problem with the base detail.  With such a short "wall" the
moment
>> transfer to the foundation is going to be relatively high.  The detail as
>> shown would not adequate for the moments possible.  Alternatively the
lower
>> connection should be detailed as a "pinned" condition.  Also, serious
>> anchors may be needed into the foundation and the glulam connection
>> depending on the load accumulation at this joint.
>>
>> In order to reduce the load(s) to such a portal, consideration of the
>> supported floor to act as a diaphragm might work.
>>
>> Thor Tandy  P.Eng
>> Victoria  BC


>I need help.  If the moment connection is between the GLB and the
>"Wall", I considered that only a pin may exist at the base of this
>frame.
>
>
No reason why you can't take the whole thing as a fixed, pinned, friction,
whatever, frame.  The fixed case will give the most conservative answers at
the base fixing especially if the portal is considered elastic or even
plastic as in the case of the ideal portal - a steel frame.  Rigidities are
directly proportional to the nail slip which in turn is relative to the nail
load.  This suggests that your idea to use a procedure that utilizes only
nail shear is close to the mark.  The behaviour of the frame as a whole will
have similar contributions from both simple shear and simple bending ( in
the columns anyway) and the equations in any standard frame book can give
the forces to be dealt with.  Detailing of the nailed joints will be similar
to that of a plywood portal (only the relative rigidities of column and
Glulam will differ to any extent) and considerations of adequate ductility
will drive your final nail array and detail.  Straps in lieu of a nailing
pattern is also practical but I find that the load transfers between beam
and column may result in incompatibilities that simplistic "moment = force x
arm" causes at the strap locations.  I prefer more "intimate" continuity at
beam column junctions.  The overall effectiveness of the design is always in
the nail, tooth or bolt slip (no pun intended :o)) so it is easy to be
fooled by any of the analytic methods that come from pure elastic
considerations.

I am similarly uncomfortable with the the change in ductility in the wood
connection with number of cycles.  But that is another debate!

Thor