Illustrating the Masonry Wall with a Linear Material Model
Full Shear Coupling
The downstand beam and the rib in the 3D model are supported by a force pair. For typical bending resistance, a compression component is created in the web and a compression component in the slab. If, in this context, the masonry wall above the downstand beam is connected to the floor with its full shear coupling, the entire structure acts together. That does not reflect the reality. There is the risk that the rib will be significantly underdesigned.
No Shear Coupling
To avoid a common effect of rib and wall, you can define a line release. You have to release the degree of freedom Cux . Furthermore, you should prevent the rib from hanging against the wall with the vertical component. Utilizing a nonlinearity allows display of the behavior (fixed if vz positive). This nonlinearity is also the reason to select a line release instead of a line hinge.
Compared to the full shear coupling, the downstand beam now has a much higher load. The reinforcement has almost quadrupled.
However, the question remains open whether the distribution of internal forces in the wall is realistic, and whether there are effects in this context that may affect the loading of the rib.
Illustrating the Masonry Wall with a Nonlinearity
Illustrating with "Membrane Without Tension"
Another approach in this article is to display the masonry wall as a surface of the "Membrane Without Tension" type. It should be mentioned here that it is important to ensure that the wall cannot absorb tensile forces. During the subsequent rib design, you will determine approximately the same reinforcement results. When considering the axial forces, you can see the distribution of the compression struts in the wall. It is noticeable that there is a horizontal compression strut on the bottom side of the wall now.
Displaying with "Isotropic Masonry 2D" Material Model
In order to check the model results with the "Membrane Without Tension" surface type, we will create another model using the "Isotropic Masonry 2D" material model. The material model is adjusted in such a way that the masonry cannot absorb any tensile force.
The two results are approximately identical. However, this model also has a horizontal compression strut at the bottom edge of the masonry wall.
Figure Showing the Construction Progress
The construction progress may be affected, depending on when the downstand beam and the connected slab are stripped. If the slab is stripped before the masonry wall above is constructed, there may be no loading on the story or in the masonry wall due to the permanent loads of the story below. The masonry wall would not exist at this time. To check this correlation, it is necessary to perform a calculation considering the construction phases. Figure 06 clearly shows that no compression strut is present at the bottom edge of the wall (see Figure 05).
When designing the rib based on the internal forces determined while considering the construction phases, the result shows a reinforcement increase of about 20%.
Summary
When you display a masonry wall using a rib, you have to make sure that the wall takes no loads from the rib. You can ensure this in part by using line hinges and releases. Furthermore, you have to find out if the construction phases have an influence. It is important to prevent the masonry wall in the model from absorbing stresses at a point in time when it is not yet available.
