I have a Meshed (Finite Element) Concrete wall supported on a beam or slab. Why do I not see a moment in the beam/ slab?
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Tekla Structural Designer
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Question
I have a Meshed (Finite Element) Concrete wall supported on a beam or slab. Why do I not see a moment in the beam/ slab?Answer
Where Meshed Concrete walls are spanning between (i.e. supported at their ends by) beams, columns, or other walls, the upper wall can effectively act as a deep beam. This is the nature of Meshed walls which, analytically, are an istotropic elastic plate. This can both span horizontally and develop in plane axial forces (at all angles).Thus the load path in the wall can in effect act in a 'strut and tie' manner (also termed a 'truss analogy' - as illustrated in the picture below), as well as in flexure (spanning horizontally about a vertical axis), distributing load directly to supports at its ends and practically none to a (much less stiff) beam underneath it. The 'strut and tie' or 'truss analogy' is a traditional 'hand analysis' method used for the design of deep beams, 'squat' walls and pile caps, all of which have a low span (length)/depth (height) ratio.
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Below, we have two small simple example models. Each has the same loading applied. In one case, the wall is "supported" (apparently or 'conceptually' but not in practice) by the meshed slab, in the other, by the steel beam under it.
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If we look at the contour forces in the wall (Fxy shear and Fy (vertical) axial force), we can see the load is shed to the edges of the wall:
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Thus the load on the wall is taken directly to the supporting primary beams (at its ends).
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If we now look at the example of the wall "supported" by the steel beam, we see a similar effect. There is very little moment in the steel beam - it is being supported by the (much stiffer) meshed wall, not vice-versa.
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Looking at the load analysis view for the steel beam we also see that there is very little moment in it.
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If you wish to idealize the behavour of the wall as having no (horizontal) bending stiffness, or 'strut and tie' action, and just loading the supporting beam/ slab with a UDL - so the beam 'does all the work' - then you can use a Bearing wall. Bearing walls have only vertical axial stiffness, and zero lateral or horizontal bending stiffness. For more information on the analysis model for bearing walls see the Help Topic How bearing walls are represented in solver models .
We now consider the effect that this will have in a similar model.
In this model, the Meshed shear walls have been changed to Bearing walls. As the Help topic referred to above explains, these act as a series of pinned columns that will apply only vertical loads to the supporting beam/ slab below it, thus approximating a UDL. If we now look at the moment occurring in the beam that is supporting the bearing wall, we see the expected moment profile for a UDL. The beam is 'doing all the work' of supporting the wall above it and the loads applied to this.
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It should not be assumed from this discussion that the appropriate thing to do is to use bearing walls for all walls. Bearings walls are - deliberately - not self-supporting or laterally stable. They must be supported by something that is laterally stable. If you wish walls to contribute to the lateral stability of the structure, they sholuld be concrete walls (either meshed or mid-pier). There is no way to have a wall that has both the idealized behaviour of a bearing wall, but that can also be self-supporting.