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RE: Strap hold down detailing, Tsunami

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Thor -

My original question was not related to the deflection of the shear wall
when using a strap nor whether using a strap was a good idea at all.

I believe the increased deflection Simpson is referring to (see p. 30 &32)
is the PA or STHD hold down where there is an offset near the base. I agree.
This is not a detail I use. Otherwise, floor-to-floor strap connections
where there is WSP above and below and when using Struct I WSP (G=0.5 as
with DF-L), I don't believe deflection is an issue.

With regards to nailing the straps directly to the studs, as far as I'm
concerned, this is not a practical application because it prevents
installation of SW edge nailing.

I agree with the issue about splitting of the studs with strap nailing in
addition to WSP EN.

Shrinkage is a good point, but not always critical. Some of the multifamily
framing I do consists of floor joists hanging off the top plate. So, the
only thing between the stud above from the stud below is 4 plates and
sheathing.

I believe I've seen the same test results for the diagonal nail deformation
that you are referring to, but the condition I saw was a HD type hold down,
eccentric from the post. I'm not sure if the behavior is the same when the
strap is nailed to the end stud.

My original question/issue is related to purely the structural mechanics of
transferring the shear load from the SW through the end stud/post into the
strap with and without the SW EN in the same region as the hold down strap.

Regards,
	
T. William (Bill) Allen, S.E. (CA #2607)	
ALLEN DESIGNS	
Consulting Structural Engineers	
http://www.AllenDesigns.com	
V (949) 248-8588	 .	 F (949) 209-2509	

-----Original Message-----
From: Thor Matteson [mailto:matteson(--nospam--at)yosemite.net] 
Sent: Monday, December 27, 2004 8:48 PM
To: SEAINT
Subject: Re: Strap hold down detailing, Tsunami

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



 



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