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RE: Relative Stiffness of Wood Shearwalls

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I'm getting into this discussion a little late but want to add some
thoughts:

1. The issue of relative stiffness using different materials is moot with me
- I simply won't do it. I have been searching for information in the 97 UBC
that I thought stipulated that relative stiffness could not be used in the
design of plywood shearwalls in one common line of shear. Possibly someone
can clear this up - simply, all walls in the same line of shear must be
sheathed with the same materials and the same nailing. Conversely, Hardy
specifically lets engineers know that you can not mix frame or panel sizes
in one line of shear. In Rigid analysis, I believe the opinion is that while
relative stiffness of walls can be balanced, they are not assumed to be
constructed in the same line of shear resistance.

2. Stan's comparison between plywood shearwall and Hardy frame deflections
needs some understanding. The Hardy Panels have a higher capacity than their
Frames due to the light-gauge metal sheathing on one face that is welded to
the 12-gauge columns and top and bottom channels. The deflection of the
panel is also a function of the anchorage to the foundation - a 7/8"
diameter anchor bolt (Holddown) and a 1-1/8" diameter x 25" long anchor bolt
(Holddown) in each 12-gauge column. The capacity of the panels w/ the larger
diameter and longer anchor bolts has a higher capacity and larger
deflection. So the issue is what frame or panel is used and is the plywood
shear wall one highly loaded wall section or a couple of plywood shearwalls
in the same line of shear. 
With this said, I don't feel comfortable designing a high load plywood
shearwall. I have designed walls with sheathing on both side, but rarely
design above 550-plf and typically when I have sufficient dead load to help
resist uplift and the stress at each end of the wood shearwall. 
If push comes to shove, I would feel more comfortable designing a cold-form
steel braced frame or panel in lieu of a plywood wall - the materials are
delivered in monolithic form and are much more resistant to human error
considering the possibility of over-nailing, over-sizing hold-down bolt
holes or incorrectly splicing mudsills.   

3. Last week we talked about creating a hinge normal to the wall by
extending the header over the frames (Shearmax, Strongwall or Hardy). If you
recall, my problem was a 14'-0" wall with a 6-kip lateral load and only 84
inches in which to place my shearwall. The window adjacent to the wall was
12'-0" to the bottom of the header and I thought to extend the header over a
12'-0" frame.
I spoke with Shearmax and with Hardy. Shearmax had a 6'-0" x 12'-0" panel
that would fit in the space and tie into the header. The capacity of the
frame was close to the 6-kip demand but it appeared to be the most prudent
choice. I felt uncomfortable with the Shearmax only because it was a close
call between my demand and the wall's capacity so I called Hardy. 
Hardy suggested that instead of extending the 12'-0" header, I raise the
header up to 13'-0" (or the true height of a 13'X24-inch wide Hardy panel.
Since I had 84-inches, it was suggested that I install (3) of the 24-inch
wide panels - each with a capacity of nearly 3-kips. This worked well as I
used a 2x6 between each frame and 4x6 at the ends of the three panel group
to support the extended header. The header was actually a ripped 6X that fit
above the panels and below the double top plate - effectively reducing the
problem with the out-of-plan hinge effect. 
Hardy's ICC and COLA reports indicate this as a preferred method where
frames and panels don't meet the exact height of the stud wall. The filler
is secured to the frame with 1/4" dia. SDS screws (Simpson or USP
equivalent) The face of the header would be connected to the double top
plate with Simpson LTP-5 plates and the shear transfer from the roof occurs
through the drag truss or through the truss blocking with a Simpson LTP-5
clip at each block to the top 2x of the double plate.
By doing this, the need to sheath the wall from the header up is eliminated
and the use of 4x6 posts and 2x6 studs between the frames helps to reduce
the hinge. To keep the 4x6 posts in place LTP-5 plates are nailed to the
face of the 4x6 posts supporting the header and to the 12-gauge panel posts
with #10 self-tapping screws. 
This is specified in the 40-page ICC report that is posted on the Hardy
Frame website.

The foundation increases to 28-inches minimum below top of slab and 22-inch
below grade. The foundation needs to be designed, but in this case, the
27-feet of wall was beefed up to allow for additional rebar top and bottom
and the 25-inch embedment of the 1-1/8" diameter threaded rods with a 3"
square x 1/4" plate welded to the nuts at the end of the rod and embedded
into the concrete.

The capacity of the three panels 13'-0" tall x 24-inch wide each is nearly
9-kips and I only need 5.8 Kips based on the demand.

The advantage of the Hardy system over a very good Shearmax system is that
this particular type of installation has been tested and is clearly detailed
and calculated in the ICC and COLA reports (I believe there is a reduction
of load in the City of L.A. Reports as I am not in their jurisdiction. 

I also want to point out that while I make positive connections to the end
posts, I make sure to strap from the 12-gauge columns with self-tapping
screws and extending it over the header where it is nailed. This adds to the
rigidity of the connection of each frame when deflecting.

The depth of the embedment and size of anchor bolt at each end is a function
of the uplift calculated on the frame. The greatest depth that I indicated
in this example will resist nearly 25-kips of uplift (assuming that the
foundation concrete and reinforcement has been substantiated.

Finally, the numbers of proprietary shearwalls that are available are out
there to help us. I don't believe that they are brought to market without
appropriate consideration for both in-plane and out-of-plane loading.
However, I will admit that I had not done as extensive a study prior to this
demanding project in the past. I did a lot of reading in the last week and
looked at the alternative - cantilevered columns or steel frames. In short -
the cost to use steel is simply too costly when there are good alternatives.

Don't pass up the new Trus-Joist proprietary shearwall system either.
Trus-Joist is a late comer but they are actively going after this market.
Shearmax uses a very interesting hold-down system and Strongwalls use
something heftier than their standard Holddowns. I think the addition of
take-up devices will eliminate the fear of shrinkage effects and I firmly
believe that there is a lot to come from technology that is not only safe,
but preferable.

One last comment; While I do use proprietary shearwalls where most cost
effective, I also contract with the home owner and will sit them down and
discuss the additional assurance of sheathing the exterior of the home with
15/32" plywood to assist in creating some redundancy in the lateral
restraint system. I do this more for the finish than for the security or
performance. The client feels that the frames are a lot to place their faith
in when I discuss their use. The plywood sheathing adds a comfort zone that
makes them feel safer in their new home and this must be considered.

When looking at a custom home over $300,000.00 the additional 1% added to
the typical 17-20% of the construction cost for framing is a small price to
pay for the additional hardware and sheathing.

Dennis S. Wish, PE


California Professional Engineer

Structural Engineering Consultant

dennis.wish(--nospam--at)verizon.net

http://www.structuralist.net

 


-----Original Message-----
From: Stanley E Scholl [mailto:sscholl2(--nospam--at)juno.com] 
Sent: Friday, May 14, 2004 3:38 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Relative Stiffness of Wood Shearwalls

For what it is worth, in LA and Santa Monica it is not permissible to use
proprietary systems in the same line as wood shear walls. I jockey the
shear wall nailing (and sometime use plywood on two sides on the shorter
walls) so that the drift of all walls in the same line are approx. equal

And then in  SM use the rigid diaphragm analysis as prescribed. In LA it
is premissible to use the flex. diaphragm analysis and add a 20%
surcharge.

Stan Scholl, P.E.
Laguna Beach, CA


On Fri, 14 May 2004 12:36:29 -0700 "Paul Crocker" <pcrocker(--nospam--at)reidmidd.com>
writes:
> "I've always believed that it is best to calculate a uniform drift if 
> you
> design plywood shearwalls according to stiffness rather than simply 
> to take the diaphragm shear and divide it uniformly into the total 
> length of wall on hand."
> 
> My understanding is that the implications of the research behind the 
> new code provisions in the '03 IBC 2305.3.3 for aspect ratios 
> between 2:1 and 3.5:1 is that assumed uniform plf shear 
> distributions for walls of different lengths isn't too bad when used 
> for reasonable aspect ratios.  It isn't exactly right, but it seems 
> to work out well enough.  The meaning of the word "best" in your 
> statement is tough to quantity, but it looks like the traditional 
> assumptions work well for many cases.  If you start mixing and 
> matching light-framed walls and proprietary systems along the same 
> line, or perhaps mixing light-framed walls with different plywood 
> thicknesses and nail spacings along the same line (not sure why you 
> would do the latter if the plf can be taken to be the same), then 
> the outcome might be different.  As with most things, the more 
> "cook-book" the method you want to use the more you have to limit 
> the method's use to fairly typical cases, and not to very unique 
> cases.  
> 
> Paul Crocker, PE, SE
> 
> 
> 
> 
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