Dynamic Analysis Workflow - CAESAR II - Help

CAESAR II Users Guide

Language
English
Product
CAESAR II
Search by Category
Help
CAESAR II Version
12

Before starting and error checking a dynamic analysis, develop dynamic analysis data using the following steps. The steps can occur in any order.

Specify the loads

You do not need to specify dynamic loads if only natural frequencies are to be counted or calculated. Harmonic analysis requires the driving frequencies and forces or displacements to define and locate the sinusoidally varying point loads.

Creating the dynamic loads for spectra or time history analysis requires the most attention. The response spectra or time history profile must be defined, built, or selected. Force sets are built for force response spectra and time history analysis. Response spectra/time history and force sets are combined with other data to build the load cases to be analyzed. Finally, additional load cases may be constructed by combining shock results with static results to check code compliance on occasional stresses. The software provides methods to simplify many of these tasks.

Modify the mass and stiffness model

For dynamic analysis, CAESAR II converts each piping element from a continuous beam element between two nodes to a stiffness between two masses. Additional stiffness is added at the node points to model anchors, restraints, hangers, and other supports in the static analysis model. The masses assigned to each node are one half the sum of all element masses framing into the node. These masses are used as translational inertias only. Rotational moments of inertia are ignored in the dynamic mass model. Their inclusion in the analysis would cause a large increase in solution time without a corresponding improvement in the general accuracy of the analysis.

In many instances, the mass and stiffness established in the static model is used without modification in the dynamic analysis. Some situations, however, can be improved by the deletion of mass points or degrees of freedom. This usually occurs in models with unnecessary masses far from the area of interest or unnecessary degrees of freedom that do not act in the direction of interest. Some piping systems have supports that are installed to suppress vibration and do not affect the static analysis. If these shock absorbers or snubbers were not part of the static model, they can be added to the dynamic model as additional stiffness.

Set the parameters that control the analysis

Options on the Control Parameters tab set the type of analysis to be performed: calculation of natural frequencies and mode shapes, harmonic analysis, spectral analysis, or time history. General settings for the analysis are also defined, such as maximum frequency cutoff, mode summation methods, static configuration for nonlinear restraints, and the friction factor for including friction in the dynamic analysis. The Advanced tab allows you to change the parameters governing the eigensolution which does the modal extraction. These parameters should only be altered under special circumstances.

For more information, see Control Parameters Tab and Advanced Tab.