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VisualAnalysis 12.0 Help

Dynamic Loads

Include Mass of Model

The mass of member and plate elements is automatically calculated and included in a dynamic analysis. Internally, these masses are distributed to the nodes. For more accurate dynamic modeling, consider splitting members and plates to get a better distribution of mass.

Apply Extra Nodal Mass

Additional mass in your model can be specified through using lumped nodal mass. Use the Modify tab in Project Manager to edit nodes to define additional displacement mass or rotational mass at this location in the structure.

Nodal masses are entered weight units. It may be easier to apply extra mass as static loads that are included through the Analysis Options command.

The self-weight of the model is included automatically in the dynamic analysis.

Use Static Loads for Additional Mass

One way to include additional mass on a model is to specify a single static load case in the Load Case Manager. This may be a Service Load Case or one of the factored load combinations. The direction gravity is acting must also be specified. Only loads with a component in the gravity direction will be included in the mass calculation.

The self-weight of the model is included automatically in the dynamic analysis.

Use Response Spectra

A Response Spectrum Analysis determines the behavior of the structure when subjected to a specified ground acceleration or displacement, e.g., an earthquake. The magnitude of the acceleration or displacement is a function of time and is referred to as a Design Spectrum in VisualAnalysis. Building codes often prescribe a seismic Design Spectrum. Include a Design Spectrum in a Response Load Case using Load | New Response Case. The dynamic response load case associates a specific Response Spectrum with the analysis and provides for direction and scaling factors to adjust how the data is used for each particular analysis.

You also need to define the project-wide modal-superposition method under the Analyze | Advanced Analysis Options. The default (best) method is CQC, but SRSS is also available, and there is a damping factor that you may need to adjust--steel behaves differently than concrete, for example.

Create a Custom Response Spectrum

The data for response spectra used in VisualAnalysis is stored in a text file called spectrum.txt.  A specific earthquake's ground motion or other historical seismic data can be added here as a custom response spectrum. VisualAnalysis includes a few predefined sets of data, and you can generate new data per IBC using the Load | Generate Response Spectrum command.