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Creating Openings
To create openings in an FE surface, the first step is to ensure that the boundary of the opening in modelled by using linear elements. The boundary openings are then created by selecting the perimeter members. There are no restrictions on the size of openings, nor on the number of members forming the boundary. However, boundary members cannot have once or more edges on the boundary of the FE surface itself; the openings must lie inside the boundary of the FE surface. Instead of creating an opening which lies on the boundary of an FE surface, the boundary of the FE surface should be adjusted to include the opening edges as boundary members.
The software limits the number of openings to 20 per FE surface. However, for very large surfaces where the number of openings is grater than 20, the surface can be split into multiple FE surfaces with a limit of 20 openings per surface, allowing for a larger number of openings in a single structural element. The division of any element into multiple FE surfaces does not affect how the element behaves.
Openings are added under the Openings Tab within the FE menu. To access the FE menu from within MasterFrame, select Modify Geometry>Edit FE surface, or, alternatively, access the FE menu from any of the other menus. It is not recommended to use Create>Add Slab, Wall, Ramp (FE surface) since this automatically creates a new FE surface and opens the FE area on the Main Border tab, awaiting the input of an FE surface boundary.
An opening can be added by either selecting the individual members which make up the opening boundary, or by picking a point within a "panel" using the "Auto Pick Opening" option. The opening number is selected from the drop down in the right hand pane.
To select the individual members that form an opening, select the .png){kind=small}
icon and then select the individual members by hovering the mouse pointer over the members in turn and select them by left clicking with the mouse buttons. Each selected member will highlight in red and the member number will appear in the right hand pane. A typical screen layout is shown below, with a single opening added.
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When a valid planar, closed boundary has been selected, the right hand pane will not that the opening is valid. Selecting the 3D view, the opening will display in the FE surface.
To select an opening by selecting a single point internal to the opening, first check the "auto Pick Opening" check box and then ensure the icon is active and select a point internal to the opening boundary. This will automatically select the relevant boundary members, which will highlight in red in the graphics window and the member numbers will also be displayed in the right hand pane.
Openings located close the the boundary of an FE surface can lead to issues with the meshing of the surface. The problem lies where the distance between an edge of the opening and an edge of the FE surface is very small - the FE mesh needs to significantly reduce in size to reduce the element size to get the elements to be compatible with the physical gap between the edges. But this means the mesh algorithm has to locally reduce the mesh size while still trying to maintain the global mesh size away from this physical constriction. This can lead to mesh errors. Even where errors do not occur, the very small mesh size leads to a significant increase in the number of nodes and so the analysis be adversely affected.
Where an opening is located close to the boundary of an FE surface, it can often be better to modify the FE boundary to include the opening, omitting the small width of element from the model. The loss of this small part form the model does not usually adversely affect the accuracy of the analysis.
As similar issue can occur where openings are located in close proximity leading to a thin strip of structure between the two openings. Again, a simplification of the model is usually desirable with minimal effect on the accuracy of the analysis.