Vol. 4 | Issue 2 | Feb. 2017

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Erin Hopple
Digital and Content Marketing Manager

Finding Modes and Frequencies of Vibrations Using GT STRUDL®

1. Create a structural model using GT STRUDL’s GT MENU, Model Wizard, or CAD Modeler, or another structural design tool like Intergraph Smart® 3D or CADWorx® Structure.

2. In the GT Command Mode, click on "Modeling -> Dynamic properties -> Joint inertia" menu command. The "Direct Specification of Joint Inertias" dialog box will appear.

3. In the "Direct Specification of Joint Inertias" dialog box, select the "Replace all existing joint inertias" radio button.

4. Check the "Mass Distribution Check Box."

5. Check the “Consistent” ratio button.

6. In the “Direct Specification of Joint Inertias” dialog box, select the “Ok” button.

7. In “GT Command” Mode, click on “Analysis -> Dynamic Analysis -> Eigenvalue (frequencies)”menu command. The “Eigenvalue Analysis Data” dialog box will appear.

8. In the “Eigenvalue Analysis Data” dialog box Check the “GTLanczos” radio button.

9. Check the “Number of Modes” check box and type 5 in the “Number of Modes” text box.

10. Check the “Solve (ISSUE Dynamic Analysis request)” check box.

11. Press the “Ok” button. The “Eigenvalue Analysis Data” dialog box will close and the modes and frequencies will be listed in the command output screen.

These modes can also be visualized graphically.

12. In “GT Command” Mode, click on “Modeling -> Graphical Modeling (GTMenu)” menu command. The “GTMenu” Interface will appear.

13. In the “GTMenu” interface, click on “Results -> Mode Options -> Mode Shapes” menu command. The “Mode Shape” data input pane will appear on the right hand side.

14. In the “Mode Shape” input pane, provide the appropriate inputs and click on the “Animate” button. The animation of the current selected mode shape will appear in the graphics window.

Stay up to date and in the know with our webinars!

February 28 - GT STRUDL Webinar: Clarifying Nonlinear Analysis per the AISC 14th Ed. Code. Learn more.

View all past GT STRUDL webinars here

Is "equivalent lateral load analysis" to simulate the response of a structure to ground motion acceptable? Absolutely NOT when an Eigen solution shows that no single mode contributes a large enough modal mass participation (say around 90%) that it can be justified to be the mode that predominates the response of a structure to ground motion.

This is often the case for a variety of industrial structures where a large number of modes are required to achieve an acceptable value of total dynamic mass participation (>=90%) in connection with seismic analysis and where each mode makes a relatively small contribution to the total mass participation. For these cases, ONLY a rigorous Response Spectrum analysis is acceptable.

Did you know that GT STRUDL was used to analyze the 40 foot high Chick-Fil-A Cow in Atlanta?