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[SEAOC] Re: [SEAOC] Rw factors

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We typically use Rw = 6.0 for plywood shear wall systems when combining with
steel frames and masonry shear walls.  Just because the loading is
distributed by tributary area does not mean you could argue using a different
Rw value for that lateral resisting element.  Depending on the aspect ratio
of the plywood diaphragm, the actual loading to the lateral resisting
elements may not be distribute by tributary area, especially if you consider
deflection compatability of the lateral resisting elements.  

I observed damage in two 5 story office buildings after the Northridge
Earthquake, which had flexible diaphragms and truss moment frames on the
front of the building and masonry pier/spandrels on the back side of the
building.  Since the deflection of the two lateral resisting elements was
significantly different (ex: the masonry pier/spandrel system  was more rigid
than the truss moment frames) when the truss frames deflected, the floor
diaphragm  acted as a drag transfering all load to the masonry pier/spandrel
beams at the back of the building overstressing them to failure.  In this
case, the distribution of lateral load was more 90% to the masonry system and
10% to the moment frame system, instead of the 50 - 50 distribution of load
normally assumed by tributary area for the two lateral resisting systems as
allowed by the code.  There was no damage to the truss moment frames or the
glass curtain wall which it supported.

Based upon these observations,  I would still be hesitant of using stictly
tributary area without considering deflection compatibilty, especially in
building more than one story.  Obviously this is more work,  and hopefully
you can get a fee to cover this.

My  two cents worth.

Michael Cochran
Brian L. Cochran Associates
Los Angeles,  CA

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In a message dated 96-11-08 13:50:09 EST, you write:

<< Subj:	[SEAOC] Rw factors
 Date:	96-11-08 13:50:09 EST
 From:	MGFran(--nospam--at)
 Reply-to:	seaoc(--nospam--at)
 To:	seaoc(--nospam--at)
 I would like to know what other engineers have been using for Rw with the
 following conditions: Flexible diaphragms using plywood shear walls mixed
 with steel frames on both residences and commercial structures.
 We have conservatively used the lowest value for Rw for all lateral force
 resisting elements in the structure in the past, but it seems that one could
 justify using an 8 for the plywood shear walls and a 6 for "Ordinary" steel
 moment frames, for example, since the diaphragms are flexible and the shears
 are distributed based upon tributary area.