Modeling and Determining Internal Forces for a T-Beam with Masonry Wall Above

Technical Article

When modeling a reinforced concrete rib with a masonry wall above, there is the risk that the rib is underdesigned if the structural behavior of the masonry is not correctly considered and the connection between masonry wall and downstand beam is not modeled sufficiently accurate. This article deals with this issue and shows possible modeling options of such a structure. In this example, the reinforcement is determined only from the internal forces and without any secondary minimum reinforcement.

Illustrating the Masonry Wall with Linear Material Model

Full Shear Coupling
The downstand beam and the rib in the 3D model is 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 with its full shear coupling to the floor, the entire structure acts together. That does not reflect reality. There is the risk that the rib will be significantly underdesigned.

Figure 01 - Required Reinforcement for the Rib and Axial Force Distribution of the Wall

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 avoid that the rib hangs against the wall with the vertical component. Utilizing a nonlinearity allows to display the behavior (fixed if vz positive). This nonlinearity is also the reason for selecting a line release instead of a line hinge.

Figure 02 - Parameters for Line Release

Compared to the full shear coupling, the downstand beam has now a much higher load. The reinforcement has almost quadrupled.

Figure 03 - Required Reinforcement for the Rib and Axial Force Distribution of the Wall

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 Nonlinearity

Illustrating with "Membrane Without Tension"
Another approach in this article is to display the masonry wall as a surface of the type "Membrane Without Tension". 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.

Figure 04 - Required Reinforcement for the Rib and Axial Force Distribution of the Wall

Displaying with Material Model "Isotropic Masonry 2D"
In order to check the model results with the surface type "Membrane Without Tension", we will create another model by using the material model "Isotropic Masonry 2D". 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 has also a horizontal compression strut at the bottom edge of the masonry wall.

Figure 05 - Required Reinforcement for the Rib and Axial Force Distribution of the Wall

Figure Showing the Construction Progress

Depending on when the downstand beam and the connected slab are stripped, the construction progress may be affected. If the slab is stripped before the masonry wall above will be 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).

Figure 06 - Required Reinforcement for the Rib and Axial Force Distribution of the Wall Considering Construction Phases

When designing the rib based on the internal forces determined while considering the construction phases, it results in 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 partially ensure this by using line hinges and releases. Furthermore, you have to find out if construction phases have an influence. It is important to avoid that the masonry wall in the model absorbs stresses at a point of time when it is not yet available.

Keywords

Entire model Masonry Downstand beam Rib Design BIM

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