Fall 2009, MET 425 - FEA Applications II

Prof. Dave Johnson, psuprofdj@psu.edu
Penn State - Erie, The Behrend College

HW-8B:  Modal Analysis and Beam/Plate elements

Concepts: Modal Analysis Using Beams and Shell elements in Workbench

The figures below show a machinery base (or skid) created by welding I-beams, channel, and plate.  All materials are structural steel.  The overall base dimensions are approximately 23.6 ft length x 7.1 ft width.

This figure shows the I-beam and channel layout under the steel plate decking.

This figure shows that the steel plate deck edges align with the web of the I-beam frame

This assembly was modeled in ANSYS.  You may click here to access the IGES File   
Suggestion: Use a Right-click on this link, then "Save Target As...", then "Save as type: All Files", and use a file name like: "skid.igs" 

Open the IGES geometry file in Workbench, DesignModeler.  
Make sure the part is ~23.6 ft x ~7.1 ft.  If not, use a "Body Operation" to scale the model to that size.  (In Simulation check, surface area of plate should be ~167.4 sq. ft.)

Use Concept > Line Bodies from Edges for the beam framework under the plate.  
Use different Line Bodies for the different cross-section shapes.  "Freeze" Line Bodies to prevent them from merging.  View > Cross Section Solids to visualize the assembly

Use Concept > Cross-Sections to define the I-beam and channel shapes

For each LINE body:

• Assign the correct cross-section
• Define the Offset Type: Origin
• Then, select ALL the LINE bodies, with Right Mouse, Select all UN-aligned edges, and use a VECTOR (0, -1, 0) to orient the beam and channel cross-sections.  This way they will be placed below the deck plate

The thickness of the steel plate is 0.0375 ft.  Assume the surface body supplied in the IGES file is the mid-surface for the shell elements of the deck plate

Select ALL bodies and Form New Part to create a "multi-body" part

Move the model to Workbench, DesignSimulation.

All materials are structural steel

The mesh should show all beam elements with the proper offset and alignment

There are NO constraints on the model

Modal Analysis Settings

• Max Modes to Find: n ?
• Limit Search to Range: Yes
• Range Minimum: 1 Hz
• Range Maximum: 70 Hz

Turn in:

• a geometry plot showing proper orientation of the beam elements
  (DesignModeler: View > Edge Joints, Cross Section Alignments, and Cross Section Solids)
• a plot showing the mesh you used:
  Document the number of nodes and elements and element types
• Estimate the total mass of the skid and compare it to the total mass reported by ANSYS in the Solution Information object
  Estimate mass: [(length x area of I-beams) + (length x area of channel) + (length x width x thickness of plate)] x density of steel
• A tabular listing of all natural frequencies between 1 and 70 Hz
• Mode shape plots of any modes which show twisting and/or bending of the skid