Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

RE: Plywood rigid diaphragms

[Subject Prev][Subject Next][Thread Prev][Thread Next]

This really isn't a new topic.  I know that when I worked at the Office of 
the State Architect (San Francisco office) over twenty years ago, we 
considered the rigidity for wood when plan checking school and hospital 
construction.  It didn't often change the results due to the regularity 
that schools and hospitals had at that point in time but did a time or two 
when the architect got a little crazy with the layout. We didn't require a 
rigid diaphragm analysis by the structural engineer unless we thought there 
would be a problem (which we had usually checked for before asking for such 
an analysis.  The conditions where it is important are usually very 
obvious).  Likewise, we didn't always consider concrete diaphragms rigid 
depending on the relative stiffness of the shear walls & diaphragms.  I 
don't know what their current practice is.

The speaker (Ed Diekmann) at the SEAONC seminar was referring to the '97 
UBC requirement (Section 1630.6) which states, in part, "Diaphragms shall 
be considered flexible for the purposes of distribution of story shear and 
torsional moment when the lateral deformation of the diaphragm is more than 
two times the average story drift of the associated story." and Section 
1605.2-Rationality which states, "Any system or method of construction to 
be used shall be based on rational analysis in accordance with 
well-established principles of mechanics.  Such analysis shall result in a 
system that provides a complete load path capable of transferring all loads 
and forces from their point of origin to the load-resisting elements.  The 
analysis shall include, but not be limited to, provisions of Sections 
1605.2.1 through 1605.2.3."

This becomes important if you have an irregular building where the center 
of mass and the center of rigidity are separated by a significant amount. 
 The key point is that the relative deflections of the shear walls and the 
diaphragms are what matter.  A deep diaphragm of short span (say a ratio of 
0.5) will not deflect as much as a shear wall with a 3.5:1 ratio supporting 
it.  Thus, the principles of mechanics are being violated if you use a 
tributary area approach because the diaphragm wants to rotate and will drag 
the shear wall along for the ride.  A "skinny" wall remote from the center 
of rigidity, in particular,  could be loaded much more than would be 
determined by tributary areas.

As Ben Schmid has shown in his papers on narrow shear walls, they tend to 
deflect a lot under a given  unit shear.  Compare that to wider walls. 
 Think about that when you are trying to visualize how the building will 

Regarding the Tri-services Manual approach, remember that the diaphragm and 
shear walls don't know that the government has declared them flexible. 
 They will just try to distribute loads according to their relative 

For reasonably regular buildings with shear walls reasonably balanced about 
the center of rigidity, the differences between rigid diaphragm theory and 
flexible diaphragm theory are less than the precision by which we know the 
applied loads (i.e., how close do we really know earthquake loads?) and 
tributary area is much quicker.

The best way to understand where the rigidity of the diaphragm is important 
is to set up some simple examples with a long (stiff) shear wall at one 
end, a skinny (flexible) shear wall near the center and a skinny (flexible) 
shear wall at the other end.  You'll quickly get a feel for when you need 
to consider rigidity.  Calculate (estimate) the deflections of the 
diaphragm and the shear walls.

Bill Cain, S.E.
Oakland, CA

-----Original Message-----
From:	Lynn [SMTP:lhoward(--nospam--at)]
Sent:	Friday, August 28, 1998 7:46 AM
To:	seaint(--nospam--at)
Subject:	Re: Plywood rigid diaphragms

I have always designed plywood diaphragms as flexible.  There are some
instances where they may become stiff enough to be considered rigid (very 
blocked diaphragms, or plywood with concrete over the top), but most are
considered flexible.
I know the military considers plywood as flexible (see the tri-services
manual), and DSA and OSHPD consider plywood diaphragms as flexible.  If 
has some new information on this issue, I would like to see it.
If this is true, we may be looking at seismic retrofit of millions of 
with plywood diaphragms !!!


John Buchanan wrote:

> Hi-
> Just finished seaonc wood design seminar. An interesting issue was raised
> regarding the design of shear walls and diaphragms.
> According to the presenters, design engineers should consider the
> horizontal wood diaphragms as rigid elements, and distribute the 
> shear to the shear walls based on a rigid analysis.
> Of course at this time no one has any published data on the relative
> rigidities of the shear walls other than longer walls are stiffer than
> shorter walls!!!!
> Does any one have any comments on this analysis approach for light frame
> wood construction?
> John Buchanan