GiD ProblemTypes for 2D Slope Analysis Input File Generation

Example posted by: Christopher McGann, University of Washington

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This article describes the use of the tool GiD as a pre- and post-processor for a 2D analysis of a soil continua in OpenSees. Two separate procedures are made available for download, one which creates an input file for an effective stress analysis using nine node quadrilateral elements, and one which creates an input file for a total stress analysis using four node quadrilateral elements. Only the former procedure will be described in this article, as the procedure for the four node elements is very similar, but less complex. The nine node element procedure discussed in this article was used to develop the input file for the Dynamic Effective Stress Analysis of a Slope example posted in the practical examples section of this manual.

GiD can be used for many purposes, one of which is as a pre- or post-processor for a finite element package. For pre-processing, the interaction between GiD and a particular outside program is fully customizable via a series of files called a problem type. The files which make up the problem type define things such as which materials can be used, which conditions such as nodal degrees of freedom or fixities can be assigned, and the formatting and syntax of the input file which is generated from the GiD mesh. Further information on pre-processing using GiD, or the use of GiD in general, can be found in the GiD manual.

The files which define the nine node element procedure discussed in this article include:

• The GiD problem type, UWnineQuad2Dup.gid, which consists of the following individual files:
  • UWnineQuad2Dup.bas, a template file which defines how the OpenSees input file is generated from the GiD input
  • UWnineQuad2Dup.mat, a file which defines materials for use in GiD
  • UWnineQuad2Dup.cnd, a file which defines certain conditions necessary for the analysis for use in GiD
  • UWnineQuad2Dup.bat, a batch file which renames the file output by the procedure as directory_name.tcl
  • UWnineQuad2Dup.tcl, a file containing two tcl procedures necessary to the creation of the input file

The files which define the four node element procedure include:

• The GiD problem type, UWquad2D.gid, which consists of the following individual files (descriptions correspond to those listed above):
  • UWquad2D.bas
  • UWquad2D.mat
  • UWquad2D.cnd
  • UWquad2D.bat

All of the files mentioned above can be downloaded by clicking here.

The UWnineQuad2Dup.gid Problem Type

The UWnineQuad2Dup.gid problem type can be used in conjunction with GiD to generate an OpenSees input file a dynamic analysis of a 2D soil continua underlain by an elastic half-space. The problem type is set up for nine node quadrilateral elements with an additional pore pressure degree of freedom at the corner node, and uses a Lysmer-Kuhlemeyer (1969) to account for the finite rigidity of the underlying elastic half-space. Each file in the problem type performs a specific task, and together they allow GiD to be used as an effective pre-processor for this type of problem in OpenSees.

It is important to note that using this problem type to create an input file for a particular analysis does not guarantee that the generated input file will function perfectly. It is a useful tool which greatly simplifies the creation of a mesh and the assignment of materials and conditions for 2D problems, but it does not automatically create perfect input files. The user must make sure that the input file was properly generated and must exercise engineering judgment in the creation of the analysis.

Conditions File - UWnineQuad2Dup.cnd

This file defines the conditions which can be applied to the mesh or geometry in GiD. These conditions are then used by the template file, UWnineQuad2Dup.bas, to generate the OpenSees input file from the GiD mesh. The format of the condition file is described in the GiD manual.

Essentially, this file is a list of the conditions that apply to this particular problem. These conditions are used to define the degrees of freedom and the point constraints (fixities) for the nodes, to define the sets of nodes which are tied in equalDOF to create the free-field columns discussed in the slope analysis example, to identify the slave node for the Lysmer-Kuhlemeyer dashpot element, and to specify the locations of nodal masses used to represent the dynamic effects of a body of water. The value assigned to each question in each condition is the default value for this condition. These values can be changed when assigning the conditions in GiD.

Material File - UWnineQuad2Dup.mat

This file specifies the nDMaterials which can be assigned to the elements in the GiD-generated mesh. As with the condition file, the material file is a list of materials using the format discussed in the GiD manual. The materials which are included in the material file correspond with the intentions of this problem type to be used for geotechnical analyses. The available materials for this problem type are:

• ElasticIsotropic
• PressureDependMultiYield
• PressureDependMultiYield02
• PressureIndependMultiYield
• J2Plasticity

Each of these materials are supplied with default values which can be changed when assigning them in GiD. There are two tabs for each material type, one for the material parameters (e.g. shear modulus, mass density), and one for the parameters which are assigned in the nine node quadrilateral element (undrained bulk modulus, fluid mass density, horizontal and vertical permeabilities, and body forces). The problem type is set up so each material has a corresponding set of element parameters to allow for the analysis of layered soil profiles.

New materials can be made in GiD using these base materials as templates. For example, in a soil profile in which there are several layers of sand, different versions of the PressureDependMultiYield wich have separate names and sets of parameters can be created and assigned to the individual sand layers.