A simplified model to finding either of the eschewed wall component
stiffness is to add a horizontal spring to the top middle of the
eschewed wall and perpendicular to the stiffness component to be
determined. Set the spring stiffness to the sum of other walls parallel
to that spring. For a flexible diaphragm, limit the walls whose wall
line crosses the eschewed wall. If there are no wall lines crossing the
eschewed wall then nearby walls need to be considered but reduced in
stiffness due to the flexibility of the diaphragm.
Test your model!! If the y walls are reduced, so will the x stiffness
component of the eschewed wall. If your analysis does not respond in
that way, something is wrong!
Recommended code wording.
"The stiffness components of a lateral brace, eschewed in plan, shall be
determined by a rational analysis that includes the influence of other
lateral bracing not parallel to the eschewed lateral bracing."
David Merrick, SE
Sacramento, CA
Notes....
100%+30% rule
All codes allow significant eschewed wall to be used if the building is
designed by using 100% shear plus 30% of shear affects from the
perpendicular direction.
Component method
Component stiffness of an eschewed wall approaches the correct answer
when parallel and perpendicular affecting walls have sum stiffness of at
least 20 times greater than that of the eschewed wall, in both
directions. For a flexible diaphragm, those parallel and perpendicular
walls are limited to those whose wall lines cross at the eschewed wall
location.
Consider the eschewed wall where no parallel nor perpendicular walls are
in line with the eschewed wall. Push in one direction and the eschewed
freely tilts. When there are moment frames in x direction and shear
walls in y direction, and then any eschewed wall will have a
much-reduced stiffness in the y direction, possibly the component
stiffness, or less, in the x direction.
The real solution is in between the two cases above. To envelope the
worst possible condition is easily done by designing the building with
the 100%+30% rule. I have yet to consider if the "component method" is
conservative for the eschewed wall with the 100%+30% rule. It might be
best to consider two cases one with a reduced or zero eschewed wall
stiffness.
Code writers of IBC 2006 apparently have given up on most being able to
understand the above and now require a building that is significantly
influenced by an eschewed wall must use the 100%+30% rule or be analyzed
by a dynamic analysis procedure. I believe the IBC2006 defines what
eschewed walls are significant. A flexible diaphragm can be computer
modeled if the flexure stiffness (E) can be set low and the shear
modulus be set to a high value (G>>E). Such a manipulation will need to
be check for errors due to large differences in stiffness values and
significant figures in the stiffness matrix operations.
I was looking at S-Frame and found it is possible, but I have yet to run
sample problems.
The 100%+30% rule alone may not be conservative for some structural
elements near the effectively low stiffness of an eschewed wall. A
dynamic analysis can wrongly model the eschewed wall with two walls to
represent the stiffness components. Eschewed wall stiffness components
are dependent on the other walls.
One person requested to produce a cleaned up version of the spreadsheet
for determining the eschewed wall reaction. Any more
>From: "refugio rochin" <fugeeo@gmail.com>
>To: seaint@seaint.org
>Subject: Re: Masonry Skewed Shear Walls
>
>Skewed Shear Walls
>Break components of load acting on shear wall into perpendicular and
>parallel loads.
>for flexible diaphragm, load acts on wall by tributary area.
>look at ACI 530 for out-of-plane loads, and in-plane shear wall loads
>and analyze by the appropriate equations.
>
>
>2007/4/20, Aldo Gonzales <allmin02@yahoo.es>:
>
>
>>I'm working on a project with Masonry skewed shear
>>walls and flexible diaphragm. Can someone help me with
>>some example from some book, references to papers, or
>>let me know how should I analyze it? I would
>>appreciate any comments. Thanks.
>>
>>
>>
>>
>>
>
>
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