Need a book? Engineering books recommendations...

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

Re: Wood framed walls

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Residential floors and roofs SHOULD be investigated to determine if they are
"rigid" diaphrams.  If I was blindfolded and forced to "choose my method of
analysis" for residential construction, I'd pick the rigid model every time.
SEAONC "By The Code" by Kelly Cobeen in January 1996 makes a great argument
for considering rotation in wood framed LFRS, because in fact, they rotate.
A common, but perhaps deficient method of analysis in my area is quite
simple, yet considers the diaphragm rigid:  The total load in the diaphragm
is distributed to the shear walls based upon wall length.  

Three assumptions are tacitly made:
1)  The relative rigidity of shear walls is directly proportional to their
length.  
2)  Rotation does not substantially increase unit shears.  
3)  The floor or roof is rigid enough and connected well enough to support
any rotation required to distribute shear to walls based upon the wall's
relative rigidity.  

In Oregon, it seems that engineers are often asked to do the LFRS for a
house when the house fails to meet a prescriptive path for lateral, but
makes it for vertical.  The market seems to have produced a standard and
expected rate ranging from about $400 to $600 per LFRS design...you can't do
a lot of engineering for that fee, so the typical design assumptions above
are often "run with" without much scrutiny.  It is rare that any structural
observation takes place on home construction in my area (Western Oregon was
upgraded to Zone 3 only as recently as 1992...construction is only beginning
to adjust now).

Is the story the same in other areas across the States?  I get the feeling
that in CA, engineers spend a lot more time and put out a great deal more
effort and care in the residential industry than we do in Oregon.  What is
the typical scope of work on residential projects?  What are the typical
fees collected?  Is observation included/required?

Respectfully,

Joe McCormick  

At 11:45 PM 8/14/97 -0700, you wrote:
>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
>
>
>
>