Subject: Re: Seismic Upgrade.... Blue Book Commentary on wood diaphragm
Date: Sat, 22 May 1999 12:02:51 EDT
-"Most lateral force resisting systems using wood diaphragms in combination
wood shear walls would not meet this definition of flexible."
Comment: Has there been substantial verification/documentation that
this is the case? By making such a broad statement we may be making the
- "Observations of light frame construction in past earthquakes suggests that
life safety performance has occurred using this practice for structures
I suggest wording as follows:
"Observation of light framed wood construction in the past earthquake does
not indicate any negative implications on the life safety performance of the
structures due to this practice."
Also, I don't think the regularity and the additional partitions really makes
a direct contribution to a wood diaphragm acting as flexible (although they
enhance the building performance overall). On the contrary the presence of
the interior walls only limits the diaphragm deflection, hence making it act
The conclusion of relative flexibility of moment frames having contributed to
failure of tuck-under garage may not be quite accurate. The majority of these
building had no moment frames in the open front, only a series of small pipe
columns designed for gravity only, which were subjected to substantial drift
that ultimately caused their failure. And actually by promoting the idea of
diaphragm rigidity for wood framed construction we may end up seeing more of
these types of structures as some unscrupulous designers may tend to resist
all lateral loads by the back walls and provide no resistance in front of
these types of buildings. Is this really a good idea?
This brings me to my last comment. In the past, for light framed wood
structures the common practice has been to ignore drift calculations even for
buildings up to 4-5 stories high. The rationale for this has been to assume,
since the shear walls meet the prescribed aspect ratios, no excessive drift
would be anticipated. But there has never been any code language that
specifically states this. Does this mean that next item that we may be
discussing at depth is: should we verify drift requirements on such
buildings? And we all know that those strange formulas for calculations of
wood diaphragm and shear wall deflections leaves much to be desired. Please
check the postings during the past few days regarding this subject.
Ben Yousefi, S.E.
San Jose, CA