RAM Elements Dynamic Modal Analysis FAQ

How do I get RAM Elements to perform a Response Spectra Dynamic Analysis?

1. After the geometry of the model is defined, place nodal masses using the Nodes>Masses spreadsheet input,

2. Define a dynamic load case for each X and Z direction,

3. On the Gen>Earthquake acceleration spreadsheet input, enter a scale factor, direction and damping percentage,

4. On the Gen>Response spectrum spreadsheet, enter some response spectrum,

5. Run the analysis and choose the desired combination method (e.g. CQC),

6. Click the View menu>Finite elements toggle (if your model uses automatic member or shell segmentation)

7. Use the View>Modal deflection results to see the mode shapes or Output>Analysis> Dynamic analysis to get periods and modes info.

Note, when viewing Modal Displacements or Modal Defections on screen, the bottom right hand selection normally for "Condition:" changes to "Modes:" where you can select the desired mode to see.

For more details, refer to Help>RAM Elements Manual>Chapter 10: Dynamic Seismic Analysis.  To see example files go to C:\ProgramData\Bentley\Engineering\RAM Elements\16.2.0\Data\Samples and open "Dynamic1.etz" or "Dynamic2.etz".

How can I change the number of modes used in the dynamic analysis?

Click the Process menu>Analyze model toggle, and under Dynamic and Response Spectra change the number of modal shapes to calculate.

Note, you can never solve for more modes than you have mass degrees of freedom. For example, if you have a 2 story structure with two lumped diaphragm masses with TX, TZ and RY masses defined, then the maximum number of solvable mode shapes is 6. 

What scale factor should I apply to the response spectrum?

See RAMSS Dynamic Modal Analysis FAQ

Can RAM Elements perform an Eigenvalue solution to determine mode shapes without the response spectrum data?

RAM Elements will perform an Eigen solution without the response spectrum curve data. The program only requires that the Mass is entered in the Node>Masses spread sheet. Once the mass is defined the program will perform the Eigen solution and the View>Model shapes options and the Output >Analysis >Dynamic Analysis results will be available for viewing.

How do I generate the data for the response spectrum curve?

The data can be generated using ASCE7-05/10 section 11.4.5.  Sample curve data can be loaded using the "Open response spectrum file" toggle.  The following spread sheet shows how to generate this data given a site specific S_DS and S_D1.

http://communities.bentley.com/products/structural/structural_analysis___design/m/structural_analysis_and_design_gallery/255161.aspx

What loads are included in the automatic mass generation and how does the program determine which members or shells to include per rigid floors?

Here’s what’s included in the automatic mass generation (from selected dead and live loads with their percentage):

• Point loads in nodes
• Members self-weight
• Members distributed loads
• Pressure in members
• Shell self-weight
• Shell pressures

The requirements to consider a member in a specific rigid floor are:

• Both nodes have the same floor number
• One node belongs to the floor and the other one is below it

The condition to consider a shell in a specific rigid floor is:

• All its nodes have the same floor number

See Also

RAMSS Dynamic Modal Analysis FAQ

STAAD.Pro Response Spectrum FAQ