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Re: Relative Stiffness of Wood Shearwalls

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Comments inline.

> 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.

I don't find anything to support this in UBC 97.  Was it possibly in the
NDS provisions incorporated by reference?  I don't happen to have a copy
handy just now.

> 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.

Going back to your original post, is uniform drift not the nature of
designing according to relative stiffness?  Dividing up the loads
according to design strength/length only works if:  (1) stiffnesses are
equal, or (2) stiffnesses are "close enough" to equal that the error is
absorbed by the conservatism in the code capacities.  It also presumes
that the "design strength" occurs at the same drift.  Besides, it seems
to me that the structure doesn't know how it was designed, so the walls
will take load according to relative stiffness regardless of our design assumptions.

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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