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Re: Wood framed walls

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Bill Cain wrote....
> 
> [Bill Cain, SE]  I observed (from outside the perimeter fencing
that had
> been set up) the apartment building that collapsed in the
Northridge EQ.
> It appeared that the first floor walls consisted solely of the
stucco
> system applied directly to the studs without any sheathing (Maybe
those who
> got a closer look can verify if this was the case) and probably
gypboard on
> the interior. 

Having the opportunity to inspect Northridge Meadows for many days, I
can say that the first floor shear elements were a combination of
stucco shear walls, gypboard shear walls, plywood shear walls, and
pipe columns.  Generally speaking, the main problem with the stucco
was the old code's lack of stapling at the plates.  The stucco slid
right past the sill plates still intact, pulling the studs along with
it.  Sill plates were visibly stripped clean of studs and stucco. 
Current code requires the plates to get lath stapling now.

Another potential problem was the combination of gypboard and stucco
shear walls on the first floor.  The second floor concrete topping
created a somewhat rigid diaphragm, and deformation incompatibility
of the two shear wall types may have contributed to a progressive
failure (ie: the stiff stucco failed before the flexible drywall ever
saw the load).  The standard of practice then was not to assume the
diaphragm as rigid.

How many out there use an approach other than a strictly flexible
diaphragm (tributary area) analysis for residential with or without
concrete topping floors?

John Lawson S.E.
Kramer & Associates Structural Engineers, Inc.
Tustin, CA
jlawson(--nospam--at)Kramerinc.com