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Re: Shear Wall Design

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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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