On Jun 28, 8:25*am, wrote:
In a free-standing garage, where one wall is almost entirely absent to make
a door opening, what parts of the structure prevent the door wall from
racking?
I am second-guessing myself about my shed plan. *One wall, a gable end wall,
is non-load-bearing but is a shear wall. *Its plan has an 8x8 door opening
in a 12' wide by 9' high wall. *I don't have the background to determine
whether such a wall will have sufficient shear strength. *I'm using
conventional 2x4 framing 16" OC with APA-rated 7/16 OSB sheathing, no
interior sheathing is planned.
The obvious things would be to reduce the door opening, to use heavier
sheathing, and/or sheathe it inside and out. *But first I would like to hear
any advice this august group has to offer, and with my thanks.
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In a free-standing garage, where one wall is almost entirely absent to make
a door opening, what parts of the structure prevent the door wall from
racking?
the bad new is........(the correct / truthful answer) "not much"
but the good news is
a small (I assume yours in small since you're talking about a 8x8
door)
single story free standing garage requires very little lateral
(shear) capacity unless you're in major wind country
I have a 1930 single story garage with a 16' x 7' door, the garage
is about 22 x 20 (with the garage door & a man door on the 20' face).
So my garage has close to zero lateral capacity on the open face. The
other three walls have stucco over 1x6 horizontal sheathing over 2x4
suds at 16" o/c. It also has 2x4 diagonal blocks in each of the
"solid" walls with a stud bay of the corners.
Not exactly an engineered lateral system but good enough to have
survived the following earthquakes:
87 Whitter (~10 miles), 89 Newport (~10 miles), Landers / Big Bear 92
(~50 miles) and the recent Chino (~15 miles) .
For the Chino quake was working on sawhorses on the driveway in front
of the garage when I felt the ground wave arrive.......I stepped away
from the open garage & watched it sway north/south (the weak
direction) as the rest of the motion arrived.
Bottom line...my ancient garage has very little lateral capacity but
it doesn't need much for e/q's.
The code in CA used to allow "rotation" in the design of three sided
structures but they got kinda conservative (overly imo) & I think they
don't allow it.
Rotation depended on having a relatively rigid diaphragm tying the
walls all together, so that the sides & back worked together to make
up for the "zero lateral" of the front wall.
A four sided "box" but open on the top is pretty
strong ..............but take away one of those sides and now its not
so strong.
Add a structural "diaphragm" to tie all three sides together and now
you've got a strong system again.
If you want to play around with the concepts....build a 1/4 model (use
door skin as sheathing) and a brad nailer attach it.
Build it four sided but with the idea of removing one wall, try to
rack it when it has four walls
remove one wall, rack it gently......at the "flat diapghram" (ie the
top of the box), try to rack it again.
If you build it, "nail" the wall sill plates to a 3/4" plywood base
to serve as a foundation.
As Rico mentions....moment resistant systems involving the garage
header are possible but not easy or simple.
Simpson's prefab shearwall may be the best know but they are by far
NOT the best.
I've tested 100's of shearwall configurations as well has the Simpson
& other pre-fab shearwalls.
Check out ShearMax pre-fab panel
www.shearmax.com
concepted, designed & developed by a father / son team; very
inventive but most of all stubborn & persistent....a way better
product than the SImpson
OP-
if you cannot add a flat structural diaphragm to the system to tie the
side walls & the back walls together or if "rotation" is no
allowed.......you can probably use a 4' site built panel to take the
load. But unless you have a local prescriptive code you can use or
your local building dept has a free standing garage "canned design"
like a patio cover, you might be stuck getting it engineered.
If you do get it engineered...ask the engineer how he plans to handle
the concentrated panel hold down loads generated by the a pre-fab
panel. A simple slab is not enoough....most likely require some sort
of grade beam at the open face.
btw, I believe ShearMax has approved panel / header moment resistant
connection details that will avoid the need for grade beams.
As crappy as OSB seems, code affords it the same values as plywood
(plus my testing experience has show no significant difference).
Forget aemeijers suggestion of adhesive, imo not worth the cost,
effort, hassle. And bumping up to 2x6 for a garage is waste of
timber.
Addtionally 7/16 OSB is more than enough for a garage.
cheers
Bob