Slab Stiff Column/Wall Mesh Regions

Since the Masterframe model is made up of 1D line elements and 2D FE surfaces, where columns or FE surfaces intersect another FE surface, that interaction takes place at a point or along a line. This can lead to localised high stresses in the FE results. These localised high stresses can result in higher error results in these areas along with increased reinforcement (which is calculated from the stresses).

To account for the fact that walls and columns have a cross sectional area over which the interaction of the interconnecting surfaces interact, the software allows for the modification of the FE Mesh over the footprint of the intersecting members. This modifies the mesh local to the point of intersection and modifies the stiffness of the FE elements within this intersecting area. Thus the use of slab stiff regions will better represent 2-dimensional nature of the intersection, but at the cost of modifying the shape and size of the mesh thus increasing the number of nodes within the model, which may lead to an increased analysis time.

To define stiff regions per FE surface, go to Analysis>FE Surface Meshing>Slab Stiff Column\Wall Mesh Regions. The layout of the screen is shown below.

The stiff region options are selected from the drop down. The options are as follows:

As Global - sets the stiff region settings for any selected FE surface to match those of set globally.

Yes (all walls and columns) - automatically modify the mesh to create stiff regions at all intersections of the currently selected FE surface

No stiff regions - if the global stiff regions have been set, this option will stop stiff regions being created in the currently selected FE surface.

Selected column and wall regions - allows specific wall or columns to be selected to create stiff regions at their intersection with the currently selected FE surface.

When the "Selected columns and wall regions" option has been selected, the members to create the stiff region for are selected graphically. To select a member, activate the selection mode by clicking on the  icon. To add a stiff region to the area of a column, use the mouse pointer and select the column itself. To create a stiff region at the intersection of two FE surfaces, the intersection selected is the member along which the intersection occurs. As each member is selected, the member number will be added to the panel on the right hand side of the screen.

A typical layout of the screen is shown below. Here, a stiff region has been applied to wall around the void in the slab, along with a number of columns. The selected members are shown in red.

The effect on the mesh can be seen in the following screen capture.

Note: The use of the stiff region causes a modification of the FE mesh, which can lead to a significant increase in the number of nodes within a model. Therefore, it is generally advisable to use the stiff regions only where required. As such, the use of the Global Stiff Region is not advised. It is more advisable to carry out an initial analysis with no stiff regions and thoroughly interrogate the model results, then apply stiff regions only where required.