Modal Analysis of a Steel Frame

Name: Sushanth Kumareshwar Panchaxrimath

Department: Mechanical Engineering

Course: Powertrain NVH of Electrified Vehicles

Date: 11/26/2016

SUMMARY

A dynamic modal analysis of a steel frame is conducted

using two CAE tools from Altair HyperWorks, namely

Optistruct and solidThinking Inspire.

The geometry of the steel frame from the given engineering

drawing is constructed using the CAD sketcher tool that

comes pre-built with the Inspire software or using

HyperMesh.

Modal analysis of the steel frame using free-free condition is

performed in both, Optistruct and solidThinking Inspire.

Results obtained using Optistruct and Inspire are compared

with the given experimental modal analysis results.

CONTENTS

SL.NO Topics Pg. No

1 CAD Creation 1

2 Model Set-up 6

3 Analysis & Results 11

4 Results Comparison 25

5 Conclusion 31

1 | P a g e

1. CAD Creation

Engineering Drawing:

The given engineering drawing of the CAD geometry (steel

frame) is as shown below:

2 | P a g e

Geometry Creation using solidThinking Inspire:

The above CAD geometry of a steel frame can be

designed using the CAD sketcher tool that comes pre-

built with the solidThinking Inspire, an Altair

HyperWorks product.

The sketching ability of the software is commendable

and makes it really simple to build the geometry.

The geometry can be then either imported into

HyperMesh for preprocessing and analyzed using

OptiStruct, or analysis can be performed by using the

Inspire software alone.

Steps followed:

The solidThinking Inspire software is started and

the geometry option is selected form the menu.

3 | P a g e

Rectangles option is selected and the 2D

geometry of the steel frame is built as per the

given engineering drawing.

After the 2D design is completed, the geometry is

extruded for a given thickness (1 Inch). The final

geometry of the steel frame is shown below.

4 | P a g e

CAD Creation in HyperMesh

The CAD geometry can either be built from scratch or

imported into HyperMesh software in many ways.

One of the methods is explained below:

Nodes and lines are created at intervals as per the

given dimensions on the X-Y plane.

Next, the 2D elements are created within the

boundary of the lines, using the spline option.

5 | P a g e

The 2D elements are then offset by 1inch, with 4

layers of 3D elements. Later, the 2D surface

elements are deleted. This completes the creation

of geometry and meshing in HyperMesh.

6 | P a g e

2. Model Set-up

Model Set-up in HyperMesh:

Once the geometry and meshing is done, the material

properties need to be assigned to the model.

Please note, if the geometry is in different units say,

inches and has to be converted to millimeters, the

entire model must be scaled by a factor of 25.4, before

meshing.

Right click on the model browser, create material.

Material properties: Young’s Modulus, E=2.06 E11;

Poison’s Ratio, NU=0.29; Density, RHO = 7.80E-009

7 | P a g e

Next step is to create properties. Right click on the

model browser and select create properties. Select

the card image as PSOLID. Assign the steel material to

this property.

8 | P a g e

Next, create a load collector. Right click on the model

browser and select create load collector. In the load

collector, select EIGRL as the card image to request

the number of modes. V1 and V2 are the initial and final

frequencies.

So, enter the value for V1 as ‘0’ and V2 as ‘1000’.

Next, assign the material property and the property

created to the solid elements that are generated.

Next step is to define the load step to perform the

normal mode analysis. Right click on the model browser

and select Create Load Step. Click on the drop-down

menu for Analysis type and select Normal modes.

9 | P a g e

For METHOD (STRUCT), click Unspecified Loadcol.

In the Loadcol dialog, select EIGRL and click OK.

10 | P a g e

Model Set-up in solidThinking Inspire:

Once the CAD geometry is created in the built in

sketcher of Inspire software, material has to be

assigned to the model.

Click on structure menu, and select material. A

default material or a new material with the properties of

the user’s choice can be created and assigned to the

model.

In our case, we can define a new material named,

‘New Steel’ with the given properties.

11 | P a g e

3. Analysis & Results

Analysis & Results in OptiStruct:

Click on the Analysis page and select OptiStruct.

Click the OptiStruct button on the right side of the

panel.

12 | P a g e

The OptiStruct solver performs the analysis and

provides the results.

The analysis is completed once you see the window as

shown below. The window can now be closed.

13 | P a g e

In order to view the results (post processing), click on

the HyperView tab in the OptiStruct panel. HyperView is

the post processor for all the Altair HyperWorks

products. It provides the results and animation for the

modal analysis of the steel frame.

14 | P a g e

In the HyperView window, click on contour plot, and click

apply. All the animation results along with the respective

values for each mode is displayed upto 1000 HZ (79

modes), as per the given data.

15 | P a g e

Analysis Results (Mode v/s Frequency)

The results are available in the output file created by

the solver deck.

Another key aspect to note is that, in OptiStruct, the

first six modes are the rigid body modes, hence they

are filtered, i.e., the first mode in experimental analysis

is the same as the 11th mode in OptiStruct.

16 | P a g e

17 | P a g e

Analysis Results from OptiStruct (Modes v/s Frequency)

0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

1.00E+03

1.20E+03

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79

Fre

qu

en

cy (

Hz)

Modes

Frequency

18 | P a g e

Analysis & Results in solidThinking Inspire:

Click on the play button present in the Analyze menu.

Select the number of modes for the normal mode

analysis.

Select the ‘more accurate’ option, to perform the

second order meshing to obtain more accurate results

and click on the run icon.

19 | P a g e

Now the job gets submitted to the built-in solver.

20 | P a g e

Once the analysis is complete, a green flag appears on

the analyze icon. Click the green flag to view the

animation results for the various modes.

The main advantage in using this software is that, once

the geometry is constructed and the material is

assigned, it performs automatic meshing and provides

the analysis results which saves a lot of time thus,

making it a very efficient product.

21 | P a g e

Animation results:

22 | P a g e

Result Values:

The first mode in experimental analysis is the same as

the 5th mode in Inspire.

23 | P a g e

24 | P a g e

Analysis Results from Inspire (Modes v/s Frequency)

0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

1.00E+03

1.20E+03

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73

Fre

qu

en

cy (

Hz)

Modes

Frequency

25 | P a g e

4. Results Comparison

OptiStruct and Inspire analysis results, are compared with the

experimental analysis results, for the purpose of validation. This

includes two sub sections, namely, comparison of the numerical

results and the comparison of the animation results (mode

shapes comparison).

Comparison of Numerical results:

Modal Mode Frequency (Hz) OptiStruct Mode Frequency (Hz) Inspire Mode Frequency (Hz)

1 31.9734 11 31.0502 5 31.1098

2 43.7648 14 43.1358 8 42.9046

3 80.3804 18 80.6851 12 80.8074

4 94.9917 20 94.5433 14 95.1742

5 136.9682 24 135.8208 18 136.6552

26 | P a g e

Comparison of Mode Shape Animation Results:

1. Modal Mode 1 v/s OptiStruct Mode 11 v/s Inspire Mode 5

Experimental (given)

OptiStruct Inspire

27 | P a g e

2. Modal Mode 2 v/s OptiStruct Mode 14 v/s Inspire Mode 8

Experimental (given)

OptiStruct Inspire

28 | P a g e

3. Modal Mode 3 v/s OptiStruct Mode 18 v/s Inspire Mode 12

Experimental (given)

OptiStruct Inspire

29 | P a g e

4. Modal Mode 4 v/s OptiStruct Mode 20 v/s Inspire Mode 14

Experimental (given)

OptiStruct Inspire

30 | P a g e

5. Modal Mode 5 v/s OptiStruct Mode 24 v/s Inspire Mode 18

Experimental (given)

OptiStruct Inspire

Modal Mode OptiStruct Mode Shape Inspire Mode Shape

1 Checked Checked

2 Checked Checked

3 Checked Checked

4 Slightly off Slightly off

5 Slightly off Slightly off

31 | P a g e

5. Conclusion

The modal analysis of a steel frame was performed and the

results were compared.

From the analysis, it is observed that the results obtained from

OptiStruct and Inspire CAE tools, match the given experimental

results for the first three modes.

When the experimental results are compared to the CAE

results, a mismatch occurs in the mode shape animation of the

4th and 5th mode,

Hence, it can be observed that as the mode number and

frequency increases, it is difficult to obtain matching results

between the CAE tools and experimental analysis.

Finally, when the pre-processing, analysis setup, simplicity and

post processing is taken into consideration, Altair’s

solidThinking Inspire tool proved to be the most efficient.