12.2. Commands Used for Cross Sections

The general procedure for creating cross sections consists of the following steps:

  1. Define the section and associate a section ID number with the section subtype.

  2. Define the geometry data for the section.

The following commands are available for creating, viewing, and listing cross sections, and for managing cross section libraries:

Table 12.1: Cross Section Commands

Command Purpose
PRESOL Prints section results
SECTYPE Associates section Subtype with SECID
SECDATA Defines section geometry data
SECOFFSET Defines section offset for cross sections
SECCONTROL Overrides program calculated properties.
SECNUM Identifies the SECID to be assigned to an element
SECPLOT Plots geometry of a section to scale
SECWRITE Creates an ASCII file containing user mesh or a custom cross section
/SECLIB Sets default section library path for SECREAD
SECREAD Reads a user defined section library, mesh, or custom cross section
SLIST Summarizes section properties
SDELETE Deletes a cross section

For complete documentation of the cross section commands, see the Command Reference.

 

12.2.1. Defining a Section and Associating a Section ID Number

Use the SECTYPE command to define a section and associate it with a section ID number. For example, the following command assigns a section identification number (2) to a predefined cross section shape (circular solid):

Command(s): SECTYPE, 2, BEAM, CSOLID
GUI: Main Menu> Preprocessor> Sections> Beam> Common Sectns

To define your own cross sections, use the subtype MESH. To define a cross section with integrated properties such as Iyy and Izz, use the subtype ASEC.

 

12.2.2. Defining Cross Section Geometry and Setting the Section Attribute Pointer

Use the SECDATA command to define the geometry of a cross section. Continuing with the example SECTYPE command shown above, note that the CSOLID subtype has two dimensions: the radius and the number of cells along its circumference. Thus, the SECDATA command shown below specifies 5 as the radius of the circular solid beam, and 8 as the number of cells along its circumference. The SECNUM command sets the element section attribute pointer to 2.

Command(s): SECDATA, 5, 8 and SECNUM, 2
GUI: Main Menu> Preprocessor> Sections> Beam> Common Sectns
Main Menu> Preprocessor> Meshing> Mesh> Attributes> Default Attribs

 

12.2.2.1. Determining the Number of Cells to Define

In general, you do not need to set the number of cells when building a cross section. The program calculate values for the cross section such as the area of the section and the moments of inertia about the coordinate axes using default integration rules and will produce results that are numerically exact.

Since the torsion constant is derived from the mesh, the accuracy of the torsion constant is directly proportional to the mesh size of the cross section. The default mesh yields acceptable engineering accuracy.

There are two ways to specify the mesh size for common sections:

  • invoking SECTYPE,,,,,REFINEKEY to set the level of mesh refinement for thin-walled sections (CTUBE, CHAN, I, Z, L, T, HATS, and HREC)

  • specifying the number of divisions using SECDATA for solid sections (RECT, QUAD, and CSOLID)

The thin wall sections have a minimum of two integration points through thickness, so results produced using thin wall sections should be acceptable for materially nonlinear analysis.

However, when doing a plasticity analysis, the cell defaults may need to be changed for the solid sections. Here are examples of program-generated solid section cell meshes and the type of analysis you may wish to use them with.

Figure 12.2: Types of Solid Section Cell Mesh

Types of Solid Section Cell Mesh

 

12.2.3. Meshing a Line Model with BEAM188 or BEAM189 Elements

Before you mesh a line with BEAM188 or BEAM189 elements, some of its attributes must be defined. These attributes include:

  • The material set attribute pointer to be associated with the generated beam elements.

  • The beam element type to be used in meshing the line.

  • The orientation of the cross section with respect to the beam element axis. For detailed information about orientation nodes and beams, see Generating a Beam Mesh With Orientation Nodes in the Modeling and Meshing Guide.

  • The cross section ID to be assigned to the generated beam elements.

Issue the LATT command to associate these attributes with the selected, unmeshed line:

Command(s): LATT, MAT, , TYPE, , KB, KE, SECNUM
GUI: Main Menu> Preprocessor> Meshing> Mesh Attributes> Picked LinesSee the LATT command description for the meaning of MAT and TYPE.

The following arguments are described here in terms of their applicability to beams.

KB

Corresponds to any keypoint number in the model. All beam elements generated will have their beam section oriented such that the beam z-axis will lie in the plane defined by two line end keypoints and this keypoint number.

KE

Corresponds to any keypoint number in the model (optional). This keypoint determines the beam orientation at the end of the line as described above. If KE is given, KB determines the beam orientation at the beginning of the line. This is useful for creating twisted beams.

SECNUM

Corresponds to the beam section defined by the SECTYPE command with the section ID number as given by the SECNUM.