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RE: doubled plywood sheathing on one side of shear wall to increa se capacity?

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Nels:

I do not think your approach is overkill [you would want to pay attention to
framing thickness], but then again it is not in the code either.  It seems
rational that applying two layers on one side will provide some additional
capacity.  The catch is it is not in code and how much.  I talked with Jim
Bowman at AFPA yesterday and he said he thought there was some testing going
on now [CUREE or something] or there was already some testing. In my brief
experience I see guys try to sneak it through my permits a few times a year
so I know it has to be more common since I do only a small amount of the
local wood building plan review.

Nels said:  "I visualize it this way: imagine bolting a deck of cards
together with a
single bolt, and then applying opposing shear forces to the outer cards.
Every card slips a little; the outer cards slip a lot in relation to one
another.  In order to be effective, there needs to be a separate shear
connection across each card-contact surface -- 51 total.  I then suppose
that 51 bolts through the deck would be as effective as 51 separate two-card
connections."

For an anology, I see engineers that design log homes assume all-threads
that go through multiple shear planes resist the shear at each plane.  1997
UBC 2318.5.1.2 says the capacity of drift pins or drift bolts shall not
exceed 75 % of the lateral design values to account for the fact that the
bolts are not fastened on the ends.  Think about a 8' tall log wall with
1/4" overdrilled hole at each 12" diameter log [not allowed by NDS 8.1.2.1]
braced only with full height steel rods.  For an 8 foot tall wall there is
the potential for 2" of drift before the drift bolts even engage. Imagine
where the holes were overdrilled 1/2 " which is probably more likely.

Another example, I see people install steel straps over wood panel sheathing
all the time and this is not exactly how the straps were tested and does not
fit neatly into the NDS yield-limit-equations.


Regards.

Scott M Haan P.E.
Plan Review Engineer
Building Safety Division 
Development Services Department
Municipality of Anchorage
http://www.muni.org/building
phone:907-343-8183  
fax:907-249-7399
mailto:haansm(--nospam--at)ci.anchorage.ak.us



-----Original Message-----
From: Nels Roselund, SE [mailto:njineer(--nospam--at)att.net]
Sent: Tuesday, September 18, 2001 2:03 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: doubled plywood sheathing on one side of shear wall to
increase capacity?


Scott,

The two-layers-on-one-side plywood shear wall assembly needs to be tested in
order to know its strength.  My expectation is that it will not provide
twice the strength of a single-layer plywood shear wall, and, unless
carefully detailed, has the potential of causing undetected [and
undetectable] damage to the framing due to excessive nails.

The nails that secure the second layer of plywood to the framing must pass
through the first layer; they will thus be in bending through the first
layer.  Nails in bending lose much of their effectiveness.  On the other
hand, shear capacity in the first layer adequate to transfer to the framing
the shear assigned to both layers would allow the nails in the second layer
to count  for only the second layer. That requires three times the number of
nails as for a single-layer shear wall.  My practice in transferring shear
through multiple layers [I don't do if for plywood shear walls] is to
provide capacity across each shear plane for the shear that is crossing that
shear plane, and, if a set of connectors is provided for one shear plane, it
may not be also counted on to provide shear capacity in the adjacent shear
plane.

I visualize it this way: imagine bolting a deck of cards together with a
single bolt, and then applying opposing shear forces to the outer cards.
Every card slips a little; the outer cards slip a lot in relation to one
another.  In order to be effective, there needs to be a separate shear
connection across each card-contact surface -- 51 total.  I then suppose
that 51 bolts through the deck would be as effective as 51 separate two-card
connections.

Is my approach overkill?  I'd want to see the results of a testing program
before doing otherwise.

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net



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