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[SEAOC] Re: [SEAOC] Re: [SEAOC] Rw factors

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If I read your comments correctly, you have essentially justified the code
requirement to use the most restrictive value leading to the greatest shear
in a combined system. 
We have been taught that good engineering leads to the safest structure
using the least materials at the lowest cost. Unfortunately this has lead
to may problems which are not as yet considered in the code.
IMHO when we push to the limit for ecconomics using codes that are more
than likely to change with the next seismic event, we need to begin
weighing the design with our engineering intuition as to how the system
will move and how the shear is transfered to resisting element - not by
diaphragm flexibility, but by the stiffness of the resisting element.
I fully agree with your assessment. 
Dennis S. Wish PE

At 09:21 PM 11/8/96 -0500, you wrote:
>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
>------------------------------------- separator
>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.