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Re: Shear Wall Design

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Mam,
 
Do talk with CSI, its been awhile but they did have me modifying some of the element individual properties for bending, shear for the different axis (2-2, 3-3, etc.) of the member to help reflect the expected behavior of the model.
 
In a message dated 11/22/2006 3:23:40 PM Pacific Standard Time, mam(--nospam--at)cyber.net.pk writes:
Thanks Michael.
Strangely, yours is the only response to my query although it must
have been be a very commonly recurring problem in the design of highrises.
You seem to be right in your diagnosis. Let us see what the CSI has to say
about that. There could be some ways to circumvent the problem.
Like defining slabs as "membrane" instead of as "plate", or defining
the slabs as "plate" but using a very minimal value for the "bending
thickness" and actual value for "membrane thickness", etc. In this
way we may require two models, one for the design of vertical elements
and the other for the design of horizontal elements. But I would prefer
a solution which does not compromise the actual behavior of the structure
under applied loads. It will be good to hear some more responses from
experienced professionals.
Thanks,
mam

From: Mlcse(--nospam--at)aol.com
Subject: Re: Shear Wall Design
To: seaint(--nospam--at)seaint.org


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I would talk with CSI about this and they can tell you how to modify your
floor elements. 

Since the floor element has out-of-plane stiffeness, it is now acting as a
beam spanning between your shear walls, which results in a moment frame like
behavior (bending moments in walls and slabs, and the slab moments result in
horizontal shears being generated into the shear walls).  There are horizontal
shears that likely are developed in the shear walls due to the flexure of the
slab under its selfweight that typically wasn't observed in earlier computer
models since the floor element didn't have out-of-plane stiffeness.  Now that
floor elements can be modeled with out-of -plane stiffness you see horizontal
shears being developed in to the shear walls from gravity loads.  The question is
do these shear actually exist to the maginitude reported by the computer
program, or due to the typical floor level at at time construction methods used in
a building erection you don't actually see these types of horizontal shears
occur.  The computer program applies gravity loads instantaneously to the entire
structure for dead and live loads for all floor levels of the building, and
doesn't reflect gravity load redistribution due to the nature of construction
where floors are constructed a floor level at a time, and loads are added a
floor level at a time for the height of the building. The actual framing
detailing used of course would impact this as well (pinned connection, fixed
connection, steel partially yielding under gravity loads, etc.).

CSI can better describe this than I have.

Michael Cochran SE.

In a message dated 11/20/2006 10:43:45 AM Pacific Standard Time,
mam(--nospam--at)cyber.net.pk writes:
In the design of reinforced concrete high-rise buildings, with shear
wall-frame dual system,
seismic zones 2 or 3, using ETABS.  I frequently encounter the situation
where the ETABS
analysis results show quite large horizontal shear forces in shear walls
under the action of
dead loads (and also live loads). This is so even for regular buildings. In
fact, in some shear
walls, the shear forces under dead loads only are more than the shear forces
obtained under
seismic loads, especially in lower floors. The situation is further
aggravated if there are basements.
Designing these walls for the usual Code (UBC) specified ultimate load
combinations
(dead +live +seismic) results in very high shear reinforcement values, and
sometimes the
design shear exceeds the max allowed. If we neglect the shear due to gravity
loads, the walls
can easily resist the shear due to seismic effects, using appropriate
horizontal reinforcement.
Usually, floors are modeled as plate elements with rigid diaphragms and shear
walls are modeled
as shell elements, all with proper meshing and line constraints.  Flexure is
generally under control.
It is strange that almost all the publications I have seen do not mention
the horizontal shear
forces in walls due to gravity loads and only consider the shear forces
under seismic loads
while designing the walls for shear. I would very much like to hear the views
and experiences
of others in this regard. Please note that I am talking about buildings of 
25 stories and above.
Thanks,

  mam


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