Mesh generation with Gid

Although Tochnog contains some build in mesh generation, here for generality the external Pre-and Postprocessor Gid is used. Start Gid and perform the following steps.

Specify the Tochnog problem type in Gid

Data - Problem type - Tochnog

This takes care that Gid understands that you want to generate a mesh for Tochnog. Some Tochnog specific input is now available, and once the mesh is generated it can be written in a file in Tochnog specific format.

Create points, lines and a surface in Gid

Geometry - Create - Point

Create the points along the entire edge of the slope, thus (0,0), (60,0), (60,5), (40,5), (20,15) and (0,15).

Zoom - Frame

Zoom to the total frame to see all points.

Geometry - Create - Straight Line

Use the points to create the straight lines along the entire edge.

Geometry - Create - Nurbs surface - By contour

Use the straight lines to create a surface. Figure 2 shows the Nurbs surface.

**Figure 2:** Nurbs surface of slope
$\begin{figure}\centerline{\epsfig{file=tutorial_1/gid_pre_lines.ps,width=12cm,angle=0}} \end{figure}$

Assign a material to the surface

Data - Materials - Assign - Surfaces

Now assign to the surface a Tochnog element group (material). Select GROUP1 in the dialog, and set the group_type to -materi. Then select the surface with Assign Surfaces.

Generate a mesh with linear triangles

Mesh - Generate mesh

Gid uses as default element type for surfaces linear triangles, which are OK for the slope stability analysis. Hence we don't need to change the element type, and can go straight on to generate the mesh: Setting the element size to 0.5 gives a fine mesh, good enough for an accurate solution.

**Figure 3:** Mesh of slope
$\begin{figure}\centerline{\epsfig{file=tutorial_1/gid_pre_mesh.ps,width=12cm,angle=0}} \end{figure}$

Save gid data

File - Save As - mesh.gid

Save everything to the directory mesh.gid.

Write file with tochnog elements, nodes and element groups (materials)

Calculate - Calculate

To really obtain the generated mesh in a file that can be used for Tochnog, calculate the Tochnog file. Please realize that this option in Gid only calculates the Tochnog input file for the mesh , and not does the FE calculation itself (solving equations, ...). In the directory mesh.gid you can find the file mesh.dat which contains element , element_group and node data.

The element data records contain the element number, the element name, here -tria3, and finally the node numbers to which the element is connected. The element_group records contain for each element the group number which contains material properties for the element. The node records contain for each node the coordinates.

TochnogProfessional