Mech Dynamics 14.5 WS06 Spectrum

8/11/2019 Mech Dynamics 14.5 WS06 Spectrum

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© 2012 ANSYS, Inc. March 28, 2013 1 Release 14.5

14.5 Release

Workshop 6

Response Spectrum(Suspension Bridge)

ANSYS MechanicalLinear and Nonlinear Dynamics




Workshop 6 - Goals

Our goal is to determine the response of a prestressed suspension

bridge subjected to a seismic load.




Workshop 6 – Project SchematicFrom the project schematic, insert a new Static Structural system.

Drop a Modal system onto the Solution cell of the Static Structural system.Drop a Response Spectrum system onto the Solution cell of the Modal system.

• Import the Geometry file

– simple_bridge.agdb




Workshop 6 - Project Schematic

Edit the Model cell to open the Mechanical application.




Workshop 6 – Units

In Mechanical, set the following units system:

• m, kg, N, s, V, A

• Degrees

• RPM

• Celcius




Workshop 6 - Preprocessing

Set the mesh defeaturing tolerance to 0.5 m.


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Insert a fixed support on the vertex of all four tower foundations.

•the Modal and Response Spectrum systems will inherit this support




Insert a zero-displacement constraint in the Y and Z directions on

the three outer edges at both ends of the bridge deck.




Finally, insert Standard Earth Gravity from the Inertial loads toolbar

button.



Workshop 6 – Modal Solution

Change the Max Modes to Find to 10, then run the Modal solution.

•review the ratio of effective mass to total mass in Solution Information





Review the frequency table for the extracted modes.

Are the frequencies closely spaced?

• Yes

Make note of the highest extracted frequency.

• f = 0.3 Hz





Since we have closely-spaced modes, change the combination

method to something other than SRSS (e.g. ROSE).

• under Analysis Settings of the Response Spectrum branch




Workshop 6 - PreprocessingInsert an RS Acceleration load in the Response Spectrum branch.

• change Boundary Condition to All BC Supports and Direction to Y Axis




Workshop 6 - PreprocessingOpen the supplied seismic data from the Savannah River Earthquake,

copy the spectrum data, and paste it into the Tabular Data.




Workshop 6 - Results

Since the seismic data were supplied in units of G acceleration,

insert a Scale Factor equal to the acceleration due to gravity ofthe working units.

• 9.81 for MKS unit system.





Note the frequency at the spectral peak for this spectrum.

• f SP = 4 Hz

How does this compare to our extracted modes?• all modes have f < fSP

• this means all modes are in the low-frequency region

• therefore, no rigid response or missing mass is needed





Finally, run the solution and insert the result item of your choice.

Note that the bridge deck may need some mesh refinement. Try changing themesh settings and re-solving.

Mech Dynamics 14.5 WS06 Spectrum

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