Working with Shear Walls

Dr. Frame3D includes basic real-time shear wall modeling. This page outlines how to create, modify, view, and remove shear walls. There is also a Notes and Caveats section providing implementation background and a discussion of limitations of the current release.

The image below shows an example of a typical shear wall analysis in progress.

One of the walls shown includes a cut-out, and the figure on the right includes a display of internal stresses and the displaced shape. The introduction of walls leads to discretization of the associated bounding beams and columns, with the wall mesh points acting as shared nodes connecting the components.

Creating Walls

To create a shear wall, first select the shear wall tool ( ) and note that the Inspection Table will fill in with default values as indicated below:

Unlike most operations with Dr. Frame3D, for walls it is necessary to set the desired parameters before creating the object. In this respect, the creation of a wall is akin to automatically generating a frame or truss.

Once the desired parameters are set, use the tool to click at a starting joint, support, or member point, and this will cause a tracking rectangle to be drawn similar to the tracking line that appears with the other member tools:

Click a second time at the desired end location, and a wall will be created:

To create a free-standing wall (i.e., one without bounding beams or columns) begin with a surrounding frame with suitable dimensions, add the wall as described above, and then delete the bounding members.

Modifying Walls

To modify a shear wall, the important thing to note is that once created, the various nodes and elements making up the wall are automatically grouped. Thus, to select and modify individual wall components, one must hold down the alt/option key while selecting. Once selected, individual nodes and elements can be edited and manipulated as usual. For example, although not drawn unless selected, there are node objects located at each mesh point, and once selected, the usual node operations can be performed on these nodes (e.g., loading, relocating, etc.).

To remesh it is generally necessary to delete the existing wall and re-install a new wall with desired mesh parameters.

To remove a shear wall, simply select it and use the delete key to erase it. To delete an individual element or set of elements, select them via alt/option-clicking, and again use the Delete key. Among other things, this can be used to generate cutouts in walls. The figure below shows a set of selected wall elements (and the associated nodes). These elements were selected using an area drag with the select tool, but individual shift-selection can be used, too.

Hitting the delete key with the above selection leads to the following model:

Using the SupportTool, we can add additional supports by clicking in the usual fashion:

Viewing Results

There are various options for viewing shear walls. The figures presented so far have had displacements and stress plotting turned off, but Dr. Frame handles shear walls in real time, just like everything else, so you can observe stress fields and displacements interactively. The figure below shows the displaced shape and internal stress field:

There are various ways to view stress fields. Using the standard Show Axial Force Values command (type 's') causes stress glyphs to be plotted as shown above. The background shading colors reflect the intensity of the in-plane maximum shear stress in the element, while each glyph is a small cross with an orientation aligning with the principal axes of stress at the point in question. Each leg of the cross is drawn with a length proprtional to the corresponding principal stress. Red indicates tension, and blue indicates compression. The size of the glyphs can be scaled using the standard stress scale buttons.

The figures below illustrate alternative stress field views:

In the figure above, stresses are shown by plotting lines perpendicular to the maximum in-plane tensile stress direction, with the line lengths proportional to the maximum tensile stress magnitude. Such a depiction can help indicate where cracks might tend to form in materials sensitive to tension. Similarly the plot below shows stress glyphs aligned with the directions of maximum shear stress. This depiction can indicate yielding tendencies in ductile materials.

Displacement values for internal wall nodes can be obtained by selecting them using the Select Tool. This will cause the relevant results to appear in the Results Pane (for multi-selection; for single selection, the results can also be viewed in the Inspector Pane):

Notes and Caveats

• The image below shows the results of a common test of the in-plane response of shell elements. The target ("exact") value for this case is 0.356 inches, and the Dr. Frame result can be seen to be 0.297 inches. This essentially matches the constant strain triangle response for this configuration, with the associated overly stiff bending behavior.

  

• The next result considers simple out of plane bending. In this case the target value is 14.46 inches, and it can be seen that even with a relatively coarse mesh one can obtain useful results. A single element mesh gives a result of 14.3 inches. Again, it should be emphasized that it is necessary to maintain aspect ratios near unity for these elements to perform well.

  

• The final result considers another case of out of plane bending. In this case opposite corners have been given a unit displacement, and the target reaction force value is 0.016 kips. The 5x5 mesh result is shown to be 0.015 kips; the single element reaction force for this configuration is 0.014 kips.

  

The following are additional limitations and issues associated with using these elements:

Currently there is no way to extract numerical stress values from a wall element. While this partially limits their usefulness, this is consistent with the main purpose of the current shear wall modeling capabilities: to capture load-displacement behavior. More general capabilities will be included in a subsequent release, and this will make the tool more suitable for doing quantitative plane stress analysis.

Depending on your hardware, you should be able to maintain good performance with reasonably refined meshes. Realize, however, that memory and computational requirements grow nonlinearly with mesh size so it is not too difficult to create meshes that will bring Dr. Frame to its knees. Just how big is too big depends on your processor and RAM, so you will need to experiment.

Distributed area loads for transversely loading walls and floors have not been implemented as of this release. Manually distributed nodal loads can be used as a workaround.

Shift-dragging with the pan tool to relocate a structure with grid snapping on can cause wall nodes to coalesce at grid points leading to an unrealistic (and unexpected) model. Use undo to restore the prior state and disable grid snaping before dragging.

The connection between wall elements and the associated bounding beams and columns is discrete and does not account for beam on elastic foundation behavior. This can lead to discretization artifacts in moment and shear diagrams such as in the figure shown below: