The SECTYPE, SECDATA, and SECOFFSET commands (Main Menu> Preprocessor> Sections> Beam> Common Sectns) are all associated with the BeamTool in the GUI. The appearance of the BeamTool varies depending on the cross section subtype you select:

Figure 12.3: BeamTool with Subtypes Drop Down List Displayed

The top part of the BeamTool relates a section ID number to a subtype (and, optionally, a section name) (SECTYPE). The middle of the BeamTool defines the section offset information, if needed (SECOFFSET). The bottom contains the fields for section geometry information (SECDATA). The dimensions defined by the SECDATA command are determined by the subtype selected. For documentation about a particular variant of the BeamTool, select the subtype that you want information about, and then click the Help button on the BeamTool. The subtype dimensions are also documented in the SECDATA command description.

If you need to define a cross section that is not common, you must create a user mesh file. To create a user mesh file, create a 2D solid model and save it using the SECWRITE command (Main Menu> Preprocessor> Sections> Beam> Custom Sectns> Write From Areas). This procedure is outlined in greater detail below:

When performing a plasticity analysis, you may need to refine the cross section mesh. A cross section consisting of more than one material may be defined to represent layers, reinforcements or sensors. When defining a multiple-material cross section, you will need to specify the material that each cross section cell is made of. You can take a previously-created cross section and modify it.

To create a custom cross section with a refined mesh and/or multiple materials, perform the following tasks:

A composite cross section is a cross section consisting of at least two materials and meshed with a user-defined mesh. When creating a composite section, define the materials before running the SECREAD command. This is necessary to calculate the shear correction factors, material-weighted centroids and the shear centers.

If you redefine a material after creating the section, you must reissue the SECREAD command to recalculate the cross section.

You can save composite cross sections as custom cross sections. For more information, see Using the Beam Tool to Create Common Cross Sections

Using BEAM188, BEAM189 and ELBOW290 elements, you can define tapered sections via SECTYPE command’s TAPER option (Main Menu> Preprocessor> Sections> Beam> Taper Sections). The tapered section varies linearly between two specified locations, (x1, y1, z1) and (x2, y2, z2); therefore, two SECDATA commands are necessary to define the taper:

Specify valid beam section IDs (station1 and station2) for both end points.

sectype,1,beam,rect     ! define cross section at first end point
secdata,.0001,0.5
sectype,2,beam,rect     ! define cross section at far end
secdata,3,0.5
sectype,3,taper         ! new Section ID for tapered beam analysis
secdata,1,0.0,0.0       ! section 1 at location (0,0,0)
secdata,2,0.0,20.0      ! section 2 at location (0,20,0)

Continuing with this example, you can then use 3 as the taper section ID when assigning mesh attributes (SECNUM or LATT). The resulting beam cross section is (0.0001 * 0.5) at end 1, and linearly tapers to (3 * 0.5) at end 2.

The following assumptions apply to tapered beams or pipes defined using this method:

The program performs a number of checks (although not completely comprehensive) to ensure topological equality. The following items are compared:

If both end stations refer to custom cross sections with multiple materials, that material IDs for the cells are the same for both ends.

At a Gauss point of integration, the BEAM188, BEAM189 and ELBOW290 elements will find the closest point on the line defined by station1 and station2. Using this information, a linear interpolation is performed for the cross section mesh. The Gauss point must therefore be located within the end points. A Gauss point within a specified tolerance (SELTOL,TOLER) is considered to be coincident with an end station. The default tolerance value is 1.0E-8.

The tapered section treatment is significantly more computation-intensive than a constant cross section (as recalculation is necessary). If computational cost is a concern, use KEYOPT(12) of the beam element to specify the tapered section treatment:

KEYOPT(12) does not apply to the ELBOW290 element.

Using the ELBOW290 element, you can define a noncircular pipe section via the SECTYPE command’s PIPE option (Main Menu> Preprocessor> Sections> Pipe). The geometry and other data are specified via a subsequent SECDATA command. For a circular pipe, the SECDATA command’s Do value (outside diameter) requires only a constant value. A two-dimensional array represents the outside profile of a noncircular pipe.

*dim,radius,,2,3   ! Declare a 2 * 3 array 
radius(1,1) = 0    ! Angle (in degrees) of the 1st point
radius(2,1) = 1.0  ! Radial coordinate of the 1st point 
radius(1,2) = 45.0 ! Angle of the 2nd point
radius(2,2) = 1.25 ! Radial coordinate of the 2nd point
radius(1,3) = 90   ! Angle of the 3rd point
radius(2,3) = 1.5  ! Radial coordinate of the 3rd point
SECT, 1, PIPE
SECD, %radius%, 0.1, 16 ! Input the array name in the Do field

Because of the symmetry involved, only locations in coordinate phase one (0 < < 90) are required for this example of an oval pipe section:

An illustration of general symmetry expansion rules follows. In these figures, the solid circles represent user-input data, and the open circles are data created automatically based on symmetry assumptions:

The following considerations apply to noncircular pipe sections:

Circular and noncircular pipe sections are topologically identical.

Both circular and noncircular pipes can be used as the end points of a tapered pipe section, allowing great flexibility for defining generalized tubular structures.