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[SEAOC] Re: [SEAOC] Wood Screw Capacity

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In a message dated 96-07-18 12:04:27 EDT, you write:

<< Subj:	[SEAOC] Wood Screw Capacity
 Date:	96-07-18 12:04:27 EDT
 From:	jimdane(--nospam--at)ix.netcom.com (JAMES ARDEN DANE)
 Reply-to:	seaoc(--nospam--at)power.net
 To:	seaoc(--nospam--at)seaoc.org
 
 I have a retrofit application where I have been asked to attach drag 
 strap blocking to 5/8" plywood using a series of A35 clip angles.  I am 
 trying to find a wood screw with an established shear capacity when 
 attached to 5/8" plywood.  The intent is to avoid the removal of light 
 weight concrete and carpet when the office is in use.  Has anyone used 
 a similar application?
 
 ... >>

I have seen a few jobs where an A35 was used to screw to 3/8" plywood.  I am
not sure I believe in this detail.  According to NDS and the 1994 UBC a screw
needs to have 7d in order to have full design values for shear.  This limits
you to a 6d (0.138" diameter) and 7d (0.151" diameter) screws at
approximately 50% of their full capacity when screwing into 5/8" plywood.  I
know that APA  has some published values for screws in plywood which would
have to be reduced for working stress by some factor of safety.  I think that
you would be better off telling the owner they need to remove the lightweight
concrete.  Another approach might be to epoxy a flat block to the underside
of the plywood, you could still screw  A35's to the block and plywood, but I
would have more faith in the epoxy glue.  The glue is good for 50-75 psi in
shear (typical failure of plywood is rolling shear = 75 psi) and the usual
failure is the wood and not the glue.  Fire consideration is probably the
downfall in using strictly epoxy.

                           ___________   glued interface.
    ____________/_________
    ___________/__________    <--------plywood sheathing
                [_______]                <------ flat blocking
                     |  | |                     <------ A35  for  shear
transfer between blocking
                     |  |                      <------ vertical blocking
                    ===                     <------ drag strap (assumed
nailed to blk'g) 

   (hope the above illustration is readable)


Michael Cochran
Brian L. Cochran Associates, Inc.
Los Angeles, CA

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