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Re: Wood framed walls

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Joe:

In my attempts to penetrate the residential market with the assistance
of zealous building officials, I've actually developed a spreadsheet
which automatically calculates panel stiffness based on UBC Standard
25.923, and then input all the shear wall lengths and offsets from
the origin to calculate center of force and center of resistance.

The usual assumption of 10% incidental torsion (what you call rotation)
has RARELY been valid for residential construction, where more often
the view wall is entirely or substantially glass, and the rear lower
wall is often (buried) concrete retaining wall. A more appropriate
value of incidental torsion for engineers not wanting to go to all
the trouble is 30%. It can make a huge difference on holddown design.

By the by, I've found nailing is not all that critical as UBC 25.923
might imply. That is, as you increase nailing and the stiffness of
the individual panel, you increase it's contribution to load distribution,
and the solution diverges instead of converging (e.g. a wall with 2"
nail spacing will drag down more than it's share of lateral loading),
so it's rarely been necessary to re-iterate a solution more than once.

If you'd like a copy of the spreadsheet for wind and seismic lateral
analysis of plywood shear walls, let me know. I'm trying to get time
to put together a user's manual, if for no other reason so I can remember
how to use the program. Me, I'm trying to get entirely out of residential.

At 10:04 PM 8/17/97 -0700, you wrote:
>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
>>
>>
>>
>>
> 
>
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>
>
>
Regards,
Robert Marmaduke PE
POB 28995, Bellingham, WA 98228
360.738.0854 VOX/FAX
avi(--nospam--at)az.com    http://www.az.com/~avi