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RE: Relative Stiffness of Wood Shearwalls

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Charley,
I'm sorry if this seems to have gotten confused. I really don't have a
problem using relative rigidity to calculate shearwalls in the same line of
shear. In fact, I think I would prefer it IF it could be considered an
accurate rather than approximate method. 

With that said, the author of the ICBO Seismic Design Manual Volume II
Design Example 2 (a Wood Framed 3-story Structure) stated:
"Testing on wood shear walls has indicated the above deflection formula is
reasonably accurate for wall aspect ratios (h/w) lower than or equal to 2:1.
For higher aspect ratios, the wall drift increases significantly, and
displacements were not adequately predicted by the formula. Using the new
aspect ratio requirement of 2:1 (UBC 1997) makes this formula more accurate
for determining shear wall deflection/stiffness than it was in previous
editions of the UBC, subject to the limitations mentioned above." I believe
the author of this was Doug Thompson, SE or Bill Nelson, SE. 

Historically, engineers rarely paid attention to shearwall deflection not to
mention the design of walls by relative rigidity. As noted above, the use of
relative rigidity in wood design is "relatively" new to the more restrictive
wall aspect ratios. The difference between code designed walls and
proprietary walls that exceed the H/b ratio (aka; h/w) is that the
proprietary walls are tested for capacity and deflection within working
stress models. This allows for more creative solutions to engineering
problems that are compounded by the resistance of Architects and Designers
to be more cognizant of the structural design and performance restrictions
in the code. We need creative solutions so that we can perform the magic
most Architects expect of creative engineers. 

With that said, the discussions we have here are great. I don't want to give
the impression that I am against any methodology - I make choices that I am
comfortable with and may be guilty of taking the conservative route that
also simplifies some of my design problems under time stresses. For example,
1) Keep all shearwalls in one line of shear either plywood shearwalls with
the same thickness and nailing as all other walls in the line of resistance;
2) never mix dissimilar shear resisting materials - I wouldn't want to put a
plywood shear wall in the same line as I have a moment frame or masonry wall
(although I have attempted on one occasion to design a plywood wall to
0.0025H drift when placed in the same line as a URM wall out of necessity as
a repair after Northridge) and 3) Never mix different products in the same
line of proprietary shear walls such as Hardy Frames.

Now, let me deviate a bit as Hardy's ICC Report now allows for the mixing of
different frames and Panels in the same line of shear so long as the
engineer in responsible charge considered the distribution of forces by
relative rigidity. When I asked about the change in philosophy I was
reminded that Hardy is now owned by Mitek Industries and that extensive
testing (including cyclic testing) was done to accommodate their ICC
Research Report PFC5342. Specifically, section 2.7.7 states:

"Any Hardy Frame (r) Panel or Brace Frame can be used in a single, double or
multiple Portal Frame System. The allowable design value for the system is
the cumulative allowable load for each Panel or Brace Frame used. When two
or more identical Panels or Brace Frames of the same size are used, then the
allowable design value of the system is equal to the sum of the
corresponding allowable design values for each individual Panel and/or Brace
Frame. WHEN DIFFERENT SIZE PANELS AND/OR BRACE FRAMES ARE USED IN THE SAME
LINE, THEN THE APPLIED LOAD CAN BE PROPORTIONED BASED ON RELATIVE
STIFFNESS."

Do I feel comfortable with the later? Not really - but this is because I
have nearly thirty years of experience in wood and have developed my
professional intuition on historic issues and experiences. The issues
related to more restrictive H/b ratios or proprietary shearwalls prevents me
from trusting the numbers (a linear interpolation for empirically tested
proprietary walls such as the Hardy's). In time I may start to experiment
more, but at this point I use Hardy Frames exclusively, but feel most
comfortable convincing my clients to allow me to entirely wrap the home in
plywood or OSB. Why? Call it a psychological factor. Only one home in the
last ten years has been designed with virtually no plywood where Hardy's
were used and while the numbers work I don't have the faith in the field
Quality Control. I have more faith in a cold-form proprietary shearwall such
as the Hardy's because they are much more difficult to bastardize then
proprietary plywood walls like the Shearmax, Strongwall or new Trus-Joist
wall. I've seen too many plumbing contractors cut first and ask questions
later. It takes some serious thought to cut throw cold-form steel in a wood
home and this is the wake up moment for the contractor to pick up the phone
and call for an alternative solution.

Charlie, I agree with you on the topic of QC/QA with some reserve. I don't
think you can compare a commercial/industrial construction with a
light-framing. The days of craftsman are essentially over. The unions don't
control residential construction unless you are dealing with the largest of
home builders and even then I don't like much of the quality I see on high
end community living. Homes in the $600,000.00 plus range in gated
communities are simple tract homes with expensive finishes (hard surface
floors, stone exteriors, granite counters, bull nose corners, plaster
finishes and gated communities. Close scrutiny of the quality of
Construction leaves much to be desired and I am concerned about this in
areas of high risk such as here in the Palm Springs region. 

I finished a 5000 square foot home in a new community that will sell for
over $1.8 million. The property was developed ten years ago and the area
subdivided, utilities put in and building pads created. Yet when the owner
of this home purchased the lot the developer of the land failed to disclose
that he never properly graded the lot. The owner who is also the architect
found that he had to tear out the plumbing and steel for the slab on grade
and remove two feet of soil and replace with certified fill. The soil report
only indicated that this needed to be done during the development of lots -
this lot was assumed developed and the other owners of lots yet to be built
up don't know that it will cost them $15,000 more to prepare their pad for
building. Why? The city explained that developers decided to maximize their
profits by preparing the lots for utilities, but that they decided not to
compact the site and failed to disclose this to the buyers. It is indicated
on the master grading plan and soils report on file with the city, but were
not give to the property owner when he purchased his lot. In fact, the
survey and grading plan he received specifically failed to indicate the same
notes that occurred on the original tract plan.

The Building Industry Association (BIA) is now developer's organization and
no longer concerns themselves with actual construction quality issues. Local
engineers offered to help educate framers and general contractors on the
issues that they should consider when building. The offer was simply
dismissed by BIA who wants as little control over those who perform the work
as possible.

I can go on and on about the problems the separate the quality issues that
those who work in steel, concrete and masonry are familiar with and those
who work in wood or cold-form steel. They are two different breeds with the
light-framing industry technically advanced, but left in the Stone Age when
considering the quality of labor and craftsmanship. There are always the
finished carpenters, but studs are laid by the board foot and plywood by the
square-foot of panels erected and the same applies to gypsum. The faster the
laborer works, the greater the earning potential - quality is ignored by all
but the oldest and more expensive general construction companies who avoid
tract developments and focus on single high-end custom home construction.

As to the issue of QA/QC for proprietary shear elements (and metal
connectors as a whole); from my own experience, whether it is Simpson,
Hardy, Shearmax or Trus-Joist - each company has a competent field
representative that services the area. I know that Scott Champion of Hardy
will come to the job site to instruct a framer and his crew who have never
worked with Hardy Frames. His approach is hands-on in the proper
installation of the products. Furthermore, as the products (generically)
evolve so does the ease of installation by use of templates. 

Those of us in Southern California know names like Tim Foster, PE or Kent
Carlson, PE. I bring up these two engineers because I knew them since their
days as representatives for Trus-Joist. During the 80's Tim and Kent left TJ
to start Specialized Testing - a well known testing lab that did most of
their work in unreinforced masonry testing. Tim and Kent went their separate
ways as the URM work slowed down and now Tim is representing Hardy Frames
(Mitek) while Kent is working with ShearMax. There are competent engineers
working closely with these companies to watch QC as well as to help
engineers solve the problems that we have in the field the same as Karen
Coloneus of Simpson (sorry if I spelled Karen's name wrong) helped me make
choices on connectors I wanted to use and others I wanted to avoid. Steve
Pryor with Simpson, Buddy Showalter with AF&PA, Tom Skaggs with APA - I can
go and on, but the fact is there - professionals in the structural
engineering communities are their to help us with products and work done in
the evolution of wood products. 

Think about that first metal connector that started a company and what it
evolved into. Can anyone say that this was only a financial venture or was
it an idea that evolved into a financial venture? 

Watching how a product is installed in the field is part of structural
observation on light-framing. We are there to inspect the building before it
is wrapped and our responsibility is to insure that the QC is maintained or
a fix is prepared.

If I had to put my money somewhere I would rather put it into a proprietary
building product that helps to eliminate construction defect and convince my
client to spend the extra money. One last comment; Most engineers will omit
truss blocking for shear transfer and use flat 2x blocking with plywood
sheathing. In my book this is a waste of money in labor and can be better
served if the truss blocks are ordered with the trusses. However when
bidding some truss companies lose out when they include blocking while their
competition does not and the owner doesn't understand the advantage. It's
the little things and the hardest thing to convince a client is that you get
what you pay for.

Sorry for the diatribe, but I hope that I answered the questions more
clearly (if not verbosely).

Best Regards,
Dennis


 


Dennis S. Wish, PE


California Professional Engineer

Structural Engineering Consultant

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

http://www.structuralist.net

 


-----Original Message-----
From: Charley Hamilton [mailto:chamilto(--nospam--at)uci.edu] 
Sent: Wednesday, May 19, 2004 11:03 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Relative Stiffness of Wood Shearwalls

Returning to the current post:
What is your objection to using relative stiffness in a single line,
Dennis?  I'm not taking sides on the matter, per se, but I am curious
about your rationale.  What is wrong with having walls in the same line
designed to take load based on their relative stiffness?  How is this
different from the design procedure if there are multiple column sizes
in a steel moment frame line?  Do they not "draw" load according to their 
stiffness?  I do not believe that the physics prohibits the practice of
differing lateral stiffnesses in a single line of resistance.  If it did,
what on earth would we do about dual LFRS systems such as shear wall/moment 
frame structures?

 > 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.

This sounds like the distributed reliability argument.  It has valid
points, in that you eliminate the "single point of failure" mechanism.
However, if one of the walls fails, the structure still has less
strength and stiffness than originally designed.  I'm a distributed
reliability guy myself, but there are limitations to what the distribution
buys you.

 > 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.

I can't argue about the potential to minimize on-site construction error
due to the fact that the components are delivered in a "monolithic" form,
but the same thing could be ensured with good QC/QA on site.  The only
thing that minimizes poor construction practices in the
commercially-assembled
units is good QC/QA at the plants.  Unless these proprietary specimens are
installed by the manufacturers (and some of them are), there is still the
strong
potential for construction flaws related to the anchorage (sill bolts or
HD bolts).  Misplaced anchors, bent anchors, incorrect materials are all
still outside the realm of many of these systems.  They still require
inspection to verify quality construction and observation to motivate it.

Just my $0.02,

Charley

-- 
Charles Hamilton, PhD EIT               Faculty Fellow
Department of Civil and                 Phone: 949.824.3752
     Environmental Engineering           FAX:   949.824.2117
University of California, Irvine        Email: chamilto(--nospam--at)uci.edu




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