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Re: Effects of the New Code on Wood structures - good or bad????? -Part 2

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PART 2 OF 2

Dennis - You admit that just because most engineers design wood buildings
based on tributary areas ("flexible diaphragms") doesn't make it right.  We
have just been lazy and not thinking, but acting like sheep, instead of
engineers.

What is happening is that the professional community is only now catching up
to what has been in the code for over 30 years.  The design examples you
cite are just the first step in that catch-up process, but it is not a
result of any new code provision.  As I understand it, these new examples
simply perform both a rigid diaphragm and flexible diaphragm analysis, and
simply use the worst case for each, so no consideration of diaphragm
stiffness is required.  For the flexible case this is simple, and is the way
it has been done typically in the past.

The problem is that to do a rigidity analysis, the designer needs to know
the stiffnesses of the shear walls, which are poorly defined.  The code has
an equation in Volume 3 (UBC Standards) for calculating shear wall
deflection (and thus rigidity), but it is only for an isolated cantilevered
shear wall, with no adjacent piers or spandrels.  This is not rational for
the vast majority of wood buildings, but the problem is that we are stuck
with it since it is in the code.  There is no equivalent code-language
provision for other materials.  For instance, for concrete walls we use
engineering judgement and basic statics to analyze the stiffness properties
of wall with openings, etc.  We all know that it is an approximation, but
it's OK because we are not violating the code, we are being engineers.  For
a steel braced frame, the stiffness is well-defined by statics, and thus
needs no code provision.

However, having the plywood shear wall deflection equation in the code,
without a commentary in code language stating that it is for ISOLATED
CANTILEVERED SHEAR WALLS ONLY, is what will lead to misinterpretation by
designers, plan checkers, expert witnesses, juries, etc.

The comments on this list service are an important tool to help in greater
understanding of the inconsistencies and lack of rational methodologies to
achieve the goals outlined in the code, or to effect a change to a more
rational document.  I welcome any responses to, or critiques of, my little
expose here.

Mark Swingle, SE
Oakland, CA

PS these are my views and not necessarily those of my employer.