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Re: Seismic Upgrade.... Blue Book Commentary on wood diaphragm

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After reading and reviewing the wood diphragm discussion, I think that some
of the vocabulary is being over looked.  The description of "rigid" vs.
"flexible", was discussed during the committees after the Norhtridge
earthquake with great flavor.  I like to think of these differences as
"continuous beam" type versus "simple span'.

It was clear from the damage of the Northridge earthquake that the simple
span or flexible diaphragm assumption, or half the width tributary area
assumption abeing supported by rigid supports was not correct.  True the
single family residence building is not the type building that is of
concern, it is the light frame timber commercial or multi family
residential.  It is also true that the 30-40 year old buildings are also
not the concern, it is the 80's more windows and less shearwalls, higher
aspect ratio shear panels, shear panels with door and window openings that
are the concern.

The 80's commerical and large single family or larger multi-family
structures that are the concern.  The diaphragms in these types of
buildings are clearly more continuous than simple, and the diaphragm is
supported on differing stiffness, is this consistent with half the
tributary width being supported by the wall, I am not sure.

Oh, the discussion of the more rigid diaphragm in wood also include the
discussion of limiting the expected drift, just like in steel, and not only
limit the expected drift but to balance the expected drifts.

I do not think it is a matter whether the plywood is glued or provided a
1/8" gap between sheets, it has to do with the aspect ratio of the
diaphragm and the stiffness or drifts of the supporting walls.

Williston "Bill" L. Warren, IV - S.E.
Newport Beach, California



From: Byainc(--nospam--at)aol.com
Subject: Re: Seismic Upgrade.... Blue Book Commentary on wood diaphragm
To: seaint(--nospam--at)seaint.org

Paragraph 1:

-"Most lateral force resisting systems using wood diaphragms in combination

with 
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 
situation worse.  

Paragraph 2:
 
- "Observations of light frame construction in past earthquakes suggests
that 
a 
life safety performance has occurred using this practice for structures 
having..."

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 
more rigid.

Paragraph 4:

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.

Regards,

Ben Yousefi, S.E.
San Jose, CA