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Re: Masonry Skewed Shear Walls
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- Subject: Re: Masonry Skewed Shear Walls
- From: David Merrick <MRKGP(--nospam--at)winfirst.com>
- Date: Sat, 21 Apr 2007 07:55:39 -0700
Aldo GonzalesA 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% ruleAll 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 methodComponent 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(--nospam--at)gmail.com> To: seaint(--nospam--at)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(--nospam--at)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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