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Re: Report on Wood Diaphragm Issues

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Ron Hamburger wrote:

"4- There are some cases when flexible diaphragm assumptions can result in
potentially unsafe conditions.  In particular, the committee was concerned 
with
cases where cantilever column elements were used as vertical elements of 
lateral
force resisting systems, and multistory structures where shear walls landed on
beams or other flexible elements.  In such cases, relatively little of the 
load
that would be assumed to accrue to these elements would actually be resisted 
by
the elements, forcing more load to go to elements which may be undersized for
these loads."



BY:

1-Has there been any verification that the damaged columns, in softstory 
apartments alluded to, were actually designed and checked for lateral forces 
including drift requirements? Most of the damage that I saw in the Valley was 
in buildings with flimsy 3" steel tube columns that were highly unlikely to 
have met the code requirements for the time of construction, let alone the 
current code. And I have tried to find any published reports that would 
validate the above argument and have yet to get my hands on one.

2-By assuming a flexible diaphragm the designer is required to provide 
substantial resistance in an open front of a mutilstory wood framed building. 
Whereas if rigid diaphragm is assumed, one can dump all the load in the back 
wall and provide virtually no resistance in the front. Is this a better 
design knowing that in wood construction there would be substantial give in 
the elements even though the UBC deflection calcs for the diaphragm may show 
a fraction of an inch?

3- There should be a simplified method similar to the current code provisions 
that would require larger base shear to compensate for a possible underdesign 
of some of the more rigid elements. I still think that flexible wood 
diaphragm assumption could result in a better laid out and safer wood 
structure than trying to use diaphragm in rotation.

Ben Yousefi, SE
San Jose, CA