Modelling shortcuts in LimitState:GEO
LimitState:GEO allows users to rapidly define problem geometries for many standard applications using the built-in wizards. However, it is rare that the problem being encountered in real life exactly matches that which has been specified in the software. One of the many benefits of LimitState:GEO is that the model can be altered easily within the viewer environment. With just a little experience and understanding of how the software works, users can build models to their exact specifications with ease.
This application note outlines a number of tips and shortcuts that can be used to that can make achieving your ideal model even easier.
A number of selection shortcuts exist in the software that uses may not be aware of:
- Select Filter - Accessed through the Select menu, the selection filter allows the user to specify the type of object that can be selected in the viewer; either Vertices, Boundaries, Solids or Slip-lines. Similar fine-grained selection may be achieved by selecting the appropriate object type from the dropdown menu in the Property Editor. It should be remembered to switch the filter back to 'Select All' once the desired actions have been undertaken, as the inability to click on e.g. solids can cause confusion!
- Rectangle Select - The rectangle select tool allows the user to quickly select a number of objects at the same time. These can then be differentiated from each other in the Property Editor or moved by dragging. By drawing from left to right, the tool will select all objects enclosed within the rectangle (marked with a red line). However, by drawing from right to left, the tool will select all objects that are within or touched by the rectangle (marked with a green line). This latter functionality is especially useful when selecting in particularly complex parts of a model.
- CTRL Select - Holding down the CTRL key allows the user to select multiple objects. This is useful if properties (material, nodal density etc.) need to be changed for many objects at the same time. This selection mode can be used in conjunction with both rectangle and click select.
When solving problems using LimitState:GEO, one issue that can sometimes occur is the critical failure mechanism touching the edges of the problem domain [Figure 1a]. While this may be entirely acceptable (e.g. in cases where the material to the sides or beneath the model is strong, like bedrock) in other situations it is a sign that that the real critical failure mechanism is larger than the currently defined problem size. In such cases it is prudent to extend the domain in order to fully encapsulate the mechanism and therefore obtain a more accurate solution. However, in complex geometries this can be tricky. Simply dragging an external boundary can cause e.g. the slope of different strata in the model to alter. Obviously the model is now unsuitable as it differs from the original. However, ‘Custom’ construction lines, the boundaries can be moved and the geometry of the slope maintained.
- Using the ‘Custom’ construction line tool, draw diagonal construction lines along each of the soil layers. Use the snap functionality to locate the start and end points of the line automatically [Figure 1b - construction lines highlighted in purple].
- Using the ‘Rectangle Select’ tool, select the entirety of the boundary that you wish to extend outwards.
- With ‘Click Select’ and the ‘Ortho’ option chosen, drag the selected boundary in a horizontal or vertical direction to a location of your choosing. You will see that the soil layers are now misaligned with their original slopes (the construction lines) [Figure 1c].
- Using the ‘Custom’ construction line tool again, draw a vertical (or horizontal) construction line along the extended boundary. Again, use the snap functionality to locate suitable start and end points.
- With ‘Click Select’ chosen, select one of the vertices that are out of position and drag it to the appropriate intersection of the construction lines. The snap will show a red ‘X’ over the intersection point [Figure 1d].
- Repeat step 5 for all the vertices that you need to move.
Solve the problem and check that the critical failure mechanism now lies within the problem domain [Figure 1e].
|Figure 1 - Extending boundaries in LimitState:GEO whilst maintaining slope geometry|
Sometimes a solid zone or boundary may need moving to an alternative, but precise location – for example, if a basic model has been created quickly using a wizard or imported DXF file. This could be achieved by recalculating all the vertex positions and entering them in the Geometry Editor. However, this is likely to be time consuming for complex shapes.
One easy way of ensuring that geometrical features are relocated to an exact spot is to select and drag them by a vertex, rather than by a boundary or arbitrary point within a solid zone. To do this:
- Open Draw > Settings and set the grid size and spacing to appropriate values.
- Ensure that Snap is active.
- Ensure the grid is displayed in the viewer [Figure 2a].
- Select the boundary or solid object(s) that you wish to move – ensuring that the vertices are selected too. This can be done using the ‘Rectangle’ select or by holding down CTRL when click-selecting objects.
- With ‘Click’ select chosen, click an appropriate vertex and hold the left mouse button [Figure 2b].
- The object can now be dragged to grid points or snapped to other objects (e.g. vertices or boundaries). Move the object to the desired location [Figure 2c].
- Release the left mouse button. As long as the geometry is still valid, the object will now reside in the required location [Figure 2d].
Note that, if a vertex or other appropriate location does not already exit in the model, one can be generated prior to moving the object and snapped to.
|Figure 2 - Moving an object (pipeline) to a specified location within LimitState:GEO
Deleting Unwanted Artefacts
During the process of modelling a problem, objects (mainly solids) can sometimes be generated that are not actually required. Superfluous boundaries and vertices in a model can attract nodes during solve and therefore reduce solution accuracy (as there are now fewer nodes to be used in the rest of the problem). Removing these artefacts from the model is not always straightforward but, with a little practice, can generally be achieved in a short space of time.
The two main points to remember are:
1. Two boundaries cannot be merged along their length if that action tries to make a zero-area solid.
If any action tries to make a solid object possess zero (or negative) area, LimitState:GEO will rightly recognise this as creating an invalid geometry and prevent it. If you wish to remove a solid and the associated boundaries, you must first delete the solid, so that merging the boundaries is then permissible.
2. It is sometimes not possible to delete a group of vertices and boundaries en-masse because of the order in which the software tries to remove them.
When a group of objects are selected and the user tries to delete them, the software will perform delete actions on each in sequence, starting with an arbitrary object. If the deletion of any of these objects, at the point of trying, would cause an invalid geometry to occur, LimitState:GEO will stop the process of deletion, revert to the pre-delete state and display an error message. However, this does not mean that the objects cannot be deleted at all. Often the user can visually determine the most appropriate order of deleting quite easily (generally starting with vertices, then boundaries). In such cases, selecting the objects individually, in the logical sequence, and deleting one-by-one, is the best technique.
To illustrate the above, consider the case of removing a zone that has unwittingly been created within a larger body of soil [Figure 3a]. To remove the unwanted objects:
- Select the solid zone and delete it [Figure 3b].
- Select one corner vertex of the exposed internal boundary and delete this to create a triangular hole [Figure 3c].
- Select the remaining exposed vertex and drag this to meet the vertex immediately above it [Figure 3d]. A straight boundary with two internal vertices will now remain for deletion.
- Select and delete each of the internal vertices in turn [Figure 3e].
- Select the remaining internal boundary and delete this [Figure 3f].
- The unwanted artefacts will now have been removed from the model [Figure 3g]. Note, however, that two vertices (the ends of the final internal boundary) remain on the external boundaries. These can be selected and deleted if needed.
|Figure 3 - removing an unwanted solid and associated boundaries from a LimitState:GEO model