Advanced FEA for Large Telescope Truss - Geometry

Step 2: Geometry

The Geometry of the Tertiary Truss is the most complicated part of the design process.  Many hours were spent creating the Solidworks geometry for the truss. This requires much time and practice in Engineering Design. However, this tutorial focuses on properly analyzing the structure, not creating it from scratch, so we will provide the created geometry.  ANSYS is able to import Solidworks files directly, however, it also requires the Solidworks program to be installed on the same machine, so we provide the file in .STEP format, the "neutral" format for computer modelling.

[Click here to download the geometry file.|^Truss K_SF_2.75_surf_Flex.STEP]

Unzip the folder, and open up ANSYS.  Use the import button to open up the file. (Remember to select "geometry file" from the drop down menu, so you can see .STEP files.)

After opening and generating the import, you should see the entire geometry has been detailed in the window.  You should take this time to familiarize yourself with the structure.

Line Body Truss

1. Parts

Part groupings are ways for ANSYS to automatically group pieces together and create automatic connections. We should create 2 part groups: the Truss K-1, and the Tertiary Subframe Surface.

Truss

Subframe

The remaining Flexure mounts and Rigid mount pad should remain separate entities.

Thickness

Once the geometries have been assigned into parts, there are a few thicknesses that need to be applied.

The shell models of the entire Subframe Part all need a thickness of 2.75  mm, (2.75e-3 m).

2. Coordinate Files

If we were to directly evaluate the imported geometry, ANSYS would consider the Truss as solid body objects, and generate a highly complex mesh on each of the Truss members by creating a mesh around each circular and rectangular tube.  This is wasted computational time.  Each of the Truss members is a simple beam, and can be evaluated using beam theory.  In order to tell ANSYS to compute it as such, we need to convert these members into line bodies and assign them a cross section.

The first step is to create a coordinate file detailing the end points which the curve of the line bodies will be applied to.  We have split this into four files in order to visualize each section easier, rather than having it be a jumbled mess in one file. [The files are provided here.|^Truss Coordinate Files.zip] Unzip them and place them in an easy-to-find place.

(extra) Deciphering the Coordinate Files

If you open up the files, it might be confusing at first what is going on. Each row is a different point, and each column (separated by a tab) is a property of this point.  The property columns are as follows:

1. The Group Property

This property links points together. All Group 1's will be considered in one line or plane, all group 2's in another line, etc.

2. The Point Number

This property denotes the order of the points in the group. It starts at 1 and proceeding upward. For example, a line will start at point 1 and end at point 2. All points in a group need to have a different number, and the order of the numbers affects the directionality of the geometry.

3-5. The Coordinates

These numbers are the x,y,z coordinates of the points.  The most straightforward, but also the most tedious part of creating the files.  Each point was modeled on the coordinate systems.

3. Creating Line Bodies

Now that the coordinate files have been created, we can use them to create the line bodies.  Attach a Static Structural module to the Geometry, and open up the Model.  Remember to activate the cross sectional view for line bodies. View> Cross Sectional Solids.  Suppress the existing Truss Part, so there is no overlap.

Concept>3D Curve allows the import of lines from the coordinate file.

You will need to change the Definition to "From Coordinates File", as well as the Operation to "Add Frozen," and then select your file and generate.  Adding Frozen means that the bodies created will not merge with existing bodies.

Remember to select all the created line bodies and create a new Part out of them. I recommend naming it "Truss Assembly."

4. Cross-Sections

Once the line bodies have been created, we need to assign cross-sections to them. There are two types: the Cylinder Tube and Rectangular Tube.

Circular Tube

Rectangular Tube

Apply the Circular Tube cross sections to the outside and conical line bodies, and the Rectangular Tube cross section to the horizontal line bodies.

Circular Tube Application

Rectangular Tube Application

 Rectangular Offset

If you notice, the rectangular line bodies are not touching the Flexure mounts.  We need to offset the cross sections 40mm (.04m) toward the subsurface and Flexure mounts.

For the 12 Rectangular Tube cross sections, select Offset Type and change it to User Defined.  Due to the directionality of the lines that we input from the coordinate files, the X coordinate has been defined perpendicularly toward the Flexure mounts.  Thus, all we need to do is add in .04 to the X Offset.

As you can imagine, there are times and geometries that this simple result will not happen. Always check the individual coordinate systems of the line bodies.

5. Geometry Connections

For later connection-definitions, we need to define surface areas for the line bodies to connect to.  The Flexure mounts have a defined surface area already, so we need to create a projection on the solid mount pad.

(video)

[Go to Step 3: Mesh]
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