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Re: Effects of the New Code on Wood structures - good or bad?????

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Seaintonln(--nospam--at) wrote:
> It appears from working through the design examples that under ideal
> conditions the goal is to design the struture so that all walls deflect with
> the same relative stiffness. The code does not suggest what should be done to
> "tweak out" the walls nor does it explain what to do when stiffness differes
> from grid line to grid line or between walls in each grid line.
> It does not seem practical in wood framing to tweak out walls to
> specifically. You may be able to reduce your liablity, but will have a great
> deal of trouble controling the quality of construction in the field. If the
> Building Industry does not take some responsiblity to require those who build
> these structural systems to be certified and educated with special knowledge
> of the code, there can be no quality control that will be be effective and
> you, as the engineer of record, still remains liable under the Strutural
> Observation guidelines.
> There have been many discussions both public and private on the SEAINT
> Listservice regarding these topics 

I wish I had your design examples to refer to.  Is this another case
similar to my anchor bolts where the code is actually promoting a weaker
solution?  If you are forced to reduce the rigidity of a shearwall,
aren't you also reducing the capacity?  I guess it all plays with the
strength vs. demand game we play with masonry.  The '97 UBC is big on
redundancy, so why would it promote weakening a system?  I am unable to
find any reference to rationally relate wall stiffness with openings. 
So how could you calculate (or reduce) stiffness of you don't know what
it is?  The shearwall deflection provision in the code is very basic,
there is no allowance for small windows etc.  At least with masonry,
there is an "equivalent" rigidity of the system.

> .....and most engineers supporting the code
> feel that we need to design both ways and to identify the effects of torson
> on our structures. However, arbitrarily adding stiffness will change the
> distribution of torsion and may not yeild ideal results.

I still feel that whether we like it or not, residential construction
may often act rigidly.  Diaphragms are relatively small and have
reasonable length to width ratios (typically). There will always be
exceptions, but I think you see my point.  By the same token, we can't
accurately calculate wall stiffness', so should we use a "fudge" factor
and just simplify things?  But probably the most important point -
"traditional" residential design has performed better than almost any
other type of structure in the past.  So what does changing the
provisions grant us? Cheaper homes, doubtful.  Simpler design, ya
right.  Better fees, maybe.  Bottom line, it will be a good idea when
more research is available but as of right now it seems the old ways of
wood are still good.

> The code requires the design of wind, and in most cases, wind will still
> govern in wood design. Wind is a uniform load that will not be affected by
> the geometry of the structure as in seismic design. With uniform loading, the
> rotational differences do not appear to be as great as when seismic controls.
> Therefore, one of the underlying questions is "If wind controls, can torsion
> be ignored and the resisting walls designed by rigidity to keep the same
> stiffness in each line of shear?"
> How do we treat field changes when the owner decides he wants to add walls or
> remove walls - or wants to enlarge walls during the course of construction.
> Any minor change such as this will require rebalancing the entire structure
> and a change in one area may affect the torsional shear in another area. How
> do we explain this to our clients?

No matter what the solution, this will always be a problem.  I suspect
it is even a problem now for most of us.
> There are a great number of problems with the new code as it applies to
> residential construction that needs to be addressed and quickly. I am
> interested in any comments from my peers.

What is the formal process for making changes?  Secondly, does anyone
have a better system?  We can all yell and scream all we want, but until
someone has a better method we are stuck.

Jake Watson, E.I.T.
Salt Lake City, UT