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Re: Effects of the New Code on Wood structures - good or bad????? -Part 1

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PART 1 OF 2

Dennis - I sincerely appreciate the efforts you are making to promote
rationality with respect to seismic design of light-frame buildings with
plywood shear walls.  However, I would like you (or anyone else) to explain
exactly what has changed in the 97 code that leads to your ranting and
raving.  I am unable to find the major changes of which you speak.  You keep
blaming the new code and speak with urgency about it due to the fact that it
took effect in CA this July 1.  The relevant portions of which you complain
the most have not changed at all since the 1994 UBC, and have not changed
significantly since 1967.

Since at least the 1967 version (that's as far back as I have looked),
consideration of the following three items has been required for earthquake
design (and wind design) in the UBC: 1) The relative stiffnesses of the
various shear walls with respect to one another, 2) The relative stiffness
of the diaphragm vs the shear walls in each line, and 3) The eccentricity
between the center of application of the forces and the center of rigidity
of the resisting elements.  These three items have not changed, and these
are the main issues that you imply have changed so drastically in the 97
UBC.

There is no provision in the code that I can find where it says you must
balance deflection in each line, or balance the stiffness of each element in
a line with respect to the others.  Just because someone does a design
example that way doesn't mean that the code requires it.  If the stiffness
don't match, then more or less force will be delivered to that wall,
depending upon its relative stiffness, assuming that the diaphragm has
sufficient stiffness to redistribute the forces.

If the diaphragm is so stiff that its deflection is miniscule relative to
the deflection of the walls, then the shear distribution will be based
purely upon a "rigidity analysis", with no consideration of the deflection
of the diaphragm.  In that case accidental torsion must be added to the
actual torsion.  If the diaphragm is so flexible that the deflection is very
large relative to the shear walls, then no significant redistribution can
take place, and the forces are based upon tributary area, and no real or
accidental torsion is considered.  If the diaphragm is somewhere in between,
then only partial redistribution takes place, and then the forces in the
shear walls fall somewhere between the "rigid" and "flexible" cases.

The problem is that most wood buildings fall somewhere in between, and there
is no technically justified way of modeling the diaphragm or the shear walls
to find out the actual forces in the walls.

END OF PART 1 OF 2