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Re: Shear wall flanges and eschewed walls[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Shear wall flanges and eschewed walls
- From: "refugio rochin" <fugeeo(--nospam--at)gmail.com>
- Date: Thu, 30 Nov 2006 11:19:11 -0400
I have put a couple of my shear wall programs onto here. these programs you are free to use.
my approach for masonry, is to make a 3-d frame, and model the walls. i model the walls with reduced thickness according to equivalent shear areas for masonry walls as found in masonry tables, ie for an 8" wall w/ grout at 24" o/c. the shear area will be 4.10 inches thick. I increase the density of these walls to accommodate dead load changes, and shuffle my elastic modulus to resemble what I want. Then i look for high stresses in my modeled walls, and input my stresses into my spreadsheet on the page http://efuge.pbwiki.com/STRUCTURAL%20ENGINEERING%20SPREADSHEETS
(you are free to use these, please resend to me if you make modifications.)
in particular this one:
and then I check my loads on the walls and increase or decrease grout and steel as required.
There is a similar spreadsheet for concrete there.
for 2-d frame, I created the following spreadsheets:
these are a little more monstrous to attack. if you would like more discussion, please email me regarding these. for 2-D one story rigid diaphragm, but perhaps a little more time consuming than building a model and doing the previous method.
I realize on my first method, that for infill frames and for vertical shears, I am at a disadvantage when it comes to real modeling. For one, Vertical shear can look like a concrete element when it has distinct joints at grout spacing intervals. Futhermore, I would refer to research using truss member formulations modeled (per research) across the diagonals of the walls. However, if I modify my elements for vertical shear, and somehow release some connections at walls and columns, I think I can create a viable element. I have seen a research article discussing a similar approach in comparison to diagonal elements.: (google this one)
THE BEHAVIOR OF TWO MASONRY INFILLED FRAMES: A NUMERICAL STUDY
Giselle M. Fonseca*, Roberto M. Silva*, and Paulo B. Loure
we don't do wood where we are, but I could see the use of a similar scenario.
i find it would be better if I used elements that are orthotropic but I don't have this availlabe.
I think the most capable open source program is OPEN SEES, although it is not available for commercial purposes. However, I think in the future it will be available. The learning curve is steep. It is a research oriented software. I see it could make a wonderful databank of structures that are built around the world, if people use it and contribute, and the commercial sector contributes to the research and education sectors.
you may find more information on people interested in what you want to do with shear walls in the OPEN SEES community. I would be interested in knowing about your pursuits.
In the past year, I have had communications with users of the shear wall
analysis that was added as an appendix to the open-source wood shear
wall spreadsheet authored by Dennis Wish. The following is a summary of
those conversations. I am very interested how others are designing shear
walls for concrete and wood. Please respond.
David Merrick, SE
(I am no longer a member of SECB)
The following is only for discussion only and may contain errors. Please
respond if you disagree.
The lateral force distribution for one or two story building depends on
assumptions that are different from tall buildings. When a tall building
uses shear walls that have cross-walls acting as flanges, then the tall
building analysis might be more similar to that of the short building. A
short building sheer wall analysis is simplified by making the
assumption of rigid soils and mostly shear strain controls the rigidity.
Cross walls used as flanges can have higher overturning reactions at
intersections for a seismic analysis not in the principle axis with or
without eschewed walls. Both UBC(1633.1) and IBC(1620.2.2) allow the
SRSS combination of loads or add 30% of the other direction's analysis.
In a short buildings, using cross-walls as flanges, assumes that the
soils are soft. If the cross-walls, being used as flanges, are very long
then their full length may not be effective as a flange. The softer the
soil, the longer portion of the cross will can be used as a flange. An
infinitely rigid soil or foundation, results in very little of the cross
wall being used as a flange. Estimates of the effective cross-wall on a
rigid foundation will be less than the height of that cross wall. Wood
shear walls are too flexible to use cross-walls and flanges in bearing.
Wood shear walls not anchored may have some uplift independent of the
foundation and possibly can use a cross wall to resist uplift.
Using cross walls as flanges allows the footing under the shear wall to
be less for resisting the overturning bearing stresses on the soils.
Such a foundation design will create a softer foundation, and lead to an
analysis that may more resemble a tall building shear distribution.
For a flexible foundation, a shear center must be considered, in lieu of
the simple center of rigidity, for a wall set that uses cross walls as
flanges when the wall, does not intersect cross-walls at their centers.
An example would be a set of walls forming the "C" shaped section, when
viewed in plan. Over a soft foundation, the wall's overturning motion
pulls up and pushes down on the end cross walls. That rotates the
cross-walls and forces them to pull laterally on the building in
opposite directions. The shear center is to be used as a center of
rigidity for that set of walls.
For a flexible foundation, the smaller wall rigidity will more depend on
the overturning stiffness of the foundation as well as the flexural
stiffness of the wall.
Using cross walls as flanges for shear walls has its advantages.
However, the simple one or two story building, lateral analysis uses
assumptions that seem to limit the cross wall to act as a flange.
The one or two story shear wall system also has assumptions about the
rigidity of diaphragms and the rigidity of the foundations. For concrete
walls, it is usually assumed that the diaphragms and foundations are
rigid. For wood shear walls, diaphragms are usually considered flexible,
a rigid diaphragm sometimes needs to be considered, and the foundations
are assumed rigid.
Eschewed shear walls are easily misunderstood. An eschewed wall is
oriented diagonally in plan. To test a valid analysis, remove all wall
rigidities in one direction and add a force in the other direction. The
eschewed wall reaction, minus the diaphragm torsion affects must equal
zero. An eschewed wall can only resist an "x" or "y" force when the wall
is braced in the perpendicular direction. The eschewed wall stiffness
diminishes as the perpendicular brace (cross-walls) rigidity is reduced.
To design the walls, use a combination of "x" and "y" forces whose
resultant aligns with the principle axis or combine the "x" and "y"
results using the SRSS (square root of the sum of the squares) method.
Alternatively, the UBC(1633.1) and IBC(1620.2.2) requires 30% of the
perpendicular analysis to be added to the direction being considered.
The following code rules are confusing. For columns supporting
intersecting shear walls, the UBC(1633.1) requires the principle axis
analysis if 20% of the reaction of the other direction's analysis is
added. This seems to be in addition to the omega factor and SRSS. The
IBC(1616.4.1) "the design seismic forces are permitted to be applied
separately in each of the two orthogonal directions and orthogonal
effects are permitted to be neglected." This is not as compelling as
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- Shear wall flanges and eschewed walls
- From: David Merrick
- Shear wall flanges and eschewed walls
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