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Re: Strap hold down detailing, Tsunami

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My heart goes out to the millions whose lives were forever changed or taken
by the tsunami in the Indian Ocean.  Wood-framed shear walls seem so
inconsequential in light of this recent disaster, yet are important on a
smaller and individual scale.

Bill,

The Simpson catalogs began warning that deflection is increased by nailing
the strap on top of the shear panel.  Notice that all of their illustrations
show the straps nailed directly to the studs.....

Others have expressed concerns about splitting the studs if the strap
nailing is in addition to the panel nailing, but  I have not seen mention of
some recent CUREE/Caltech Woodframe Project results showing that additional
nails in isolated areas of shear panels will degrade the performance of the
wall as a whole.  This would occur if you nail a strap on top of
already-nailed shear panels.

Someone also pointed out that shear panel nailing near shear wall corners
will deform diagonally.  This would conflict greatly with a tie-down strap's
action.  One of those things that may get overlooked, kind of like the
tendency of tie-down brackets on the _compression_ post to get pried up and
split out of posts during shear wall testing--and perhaps, by extrapolation,
during an actual earthquake.  I would tend to use strap tie-downs only on
very stiff walls.  Otherwise the end-post will lever the strap back and
forth during an earthquake, possibly leading to the strap's failure.

Another concern using straps between floors is shrinkage of the floor
platform framing.  Shrinkage will cause slack in the connection between the
lower wall and the upper wall, which will lead to increased wall deflection
and damage.  (I'm having a lot of trouble writing about damage to drywall
when an earthquake has just washed entire villages into the ocean.)

Thor

www.shearwalls.com



Bill Allen wrote:

Currently, I detail straps to be installed over the plywood (oops, Wood
Structural Panel) sheathing. Otherwise, it would be difficult to install =
the
SW edge nailing. Simpson has load tested this condition and they believe =
it
is O.K. to install it like this. I know there may be some who don't like
this installation, but for the purposes of this topic, let's assume it's
O.K.

For CS straps, which are 1-1/4" wide, a single 2X stud may be adequate =
for
anchorage. Similarly, for CMST straps which are 3" wide, double 2X studs
would be a mimumum and may be adequate for anchorage (both for tension =
and
compression). Let's assume that they are for the purposes of this topic.
With this scenario, how do you address the stud which has the shear wall
edge nailing as well as the strap nails? I've heard where some folks =
omit
the edge nailing at the strap. For short straps, this doesn't seem to be
much of a problem. A CS16 has an end length of 11" with 10d nails and =
14"
with 8d nails. However, with a CMST12, the strap is more than half the
height of the shear wall. I'm having trouble reconciling forces with a =
FBD
(load transfers from the SW into the end post back through the SW into =
the
strap).

Of course, one could make the end post wider to allow the edge nailing =
to
run past the strap. For a CS, that would require a double stud. For a =
CMST,
that would require 3-2Xs. I don't have so much of a problem with the =
larger
strap (of course, all of the multiple studs would have to be face nailed =
to
transfer the load from the edge nailing to the hold down strap), but it
seems to me that this *might* be "overkill" for a lightly loaded CS =
strap.
After all, CS20, CS18 and CS16 straps have capacities of 1,030 lbs, =
1,370
lbs and 1,705 lbs respectively.

Opinions and comments are welcome.

Regards,


T. William (Bill) Allen, S.E. (CA #2607)


ALLEN DESIGNS


Consulting Structural Engineers


 <http://www.allendesigns.com/> http://www.AllenDesigns.com


V (949) 248-8588

.

F (949) 209-2509

=20

=20




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