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Wood: Stronger lateral design measures vs. improved construction

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 I have followed the posts on using interior partition loads in the lateral
design of lightweight wood structures. I may have been doing it wrong for
years, but my opinon as to developing lateral load are as follows.
1. Use an Rw of 6 regardless of whether it is a plywood sheathed box
structure (Rw=8) or a mix of stucco and plywood (Rw=6). (personally I don't
provide values to stucco or gypsum after reviewing the documents subsequent
to Northridge and Loma Prieta).
2. Disregard interior partitions unless they are connected to the roof
diaphragm for use to resist diaphragm movement (shearwalls). In most cases
these do not add even a couple of pounds to the roof Dead Load. In addition
to this where roof trusses are used, the interior partition stops at the
bottom chord ceiling joist. Without a vertical strut at this location to
push load to the roof, I can't see how the weight of the wall makes it to
the diaphragm.
3. Always add a miscellaneous load equivalent to about 10-20% of the
materials dead load.

I feel that the miscellaneous load and increase lateral coefficient is more
than sufficient to account for interior partions. It has been my
understanding that partition loads are used in gravity load analysis to
account for the ability to move partitions in commercial and office type
construction, this compensates for partitions perpendicular to joists. (for
that matter, can you reduce the live load to zero where a wall load occurs?)
Because of this, I have never used a partition load in residential
construction since walls rarely are moved.

When providing shearwall calcs, I'll tend to disregard the small tributary
roof load where trusses or rafters are parallel to the wall.

One other issue, It is my opinion that wood framed buildings which are
constructed properly will perform adequately when designed to the most
minimum lateral load standards as compared to increasing design standards
only to have the load paths fail in the field due to a contractor not paying
attention to the details. After Northridge, I visited one home (of hundreds)
that suffered damage to the interior walls. When uncovered it was found that
a heavily sheathed wall with HD20's at each end stopped at the ceiling joist
and was not connected to the roof.
In my opinion, if you were to increase the quality of construction (which we
are now forcing by structural observation) the engineering community would
not have to stiffen design standards.
Now let me preface a few of my comments. I belive that we have to settle on
a design method to accuratly determine wall deflection, minimize plate
spliting, create methods for thickened slab foundations for shear walls that
consider the attached slab to resist uplift,  and more. I don't by the
philosophy that you indescriminantly change the allowable H/b ratio to 2.0
nor do I believe that you accept any H/b ratio without looking at the wall
deflection. As of this date with the 1994 code, the only real design method
that we have for wall deflection is based either on the 1991 UBC Standards
(9-23 I believe) or the methods just published by APA.  Until the APA values
are written into code, the 1991 standard seems to be it.
I have read with interest the desire to use a rigid diaphragm method but
don't feel that this is adequate with a plywood diaphragm.  I do, however
buy the idea of using a stiffness method in a shear line. Some years ago,
Ben Schmidt SE (SEAOSC) suggested this and tried to explain the use in
multi-story buildings. I believe that SEAOSC is working on this now, but
they have not released their design drafts to the general engineering
community for review as yet.
To make matters worse, there was once a movement in the mid to late 80's to
adopt a method of design for flexible diaphragm that coinsided with the
design philosophy of URM construction in the distribution of shear in
multi-story structures related to diaphragm capacity verses proportional
load demand methods. Shear was not distruted by virture of proporional
distribution but was compared to what the diaphragm could handle before
failure occured and the number of interior shear walls was determined by
distributing shear to walls based upon a DCR analysis. (Diaphragm to
Capacity Ratio). The idea (as far as I remember) was that the distribution
of shear in a multi-story structure was not a triangular distribution as
assumed in conventional methods.
The design methods for rigid diaphragm would allow for open front design
based upon distributing shear by rotation into the other three walls. This
is no longer an acceptable method in Los Angeles and Santa Monica and has
spured an open front ordinance in these citys to add lateral support.

My point through all of this rambling is that we should spend the same
amount of time to clean up the actual methods of construction and force
education upon framers and contractors who wish to maintain their licenses
in lieu to developing newer and more complicated design approaches. At this
time, wood framed construction is not really considered heavily in the
experience of engineers who wish to take the SE exam (unless it is an office
building), but soon will be if we make the physics of designing a home more
complicated than it need be. What it boils down to is cost of repair - since
collapse is rarely the issue in wood frame. Will stiffer measures in design
provide an affordable product that reduces the amount of cosmetic damage (or
structural damage) sufficiently enough to compensate for improper field
construction techniques?
I mean to open the can of worms - as a community we need to keep homes
affordable, yet improve the known deficiencies by correcting the contractor,
not penalizing the owner or engineer by more costly and tougher standards.

Dennis S. Wish PE
wish(--nospam--at)cyberg8t.com