MASIL analysis
Modal dynamic analysis using MASIL is a feature that will be available in a future release of GSA. Please reach out to Oasys if you are interested in trying it out in the meantime.
MASIL is a new form of modal dynamic analysis that focusses on the modes which are significant in the understanding of the dynamic characteristics of a structure and that provides information for further dynamic calculation such as time-history, harmonic, response spectrum (RSA) or footfall analysis.
In any real model it is impractical to calculate all the modes and so MASIL allows a subset of significant modes to be targeted. It is important to have confidence that a sufficient number of modes have been found and to help assess this we use effective mass as a measure. There are three effective masses and three effective inertias for each mode – one in each of the global directions. If the sum of these effective masses in each direction is close to the mass/inertia of the structure it can generally be assumed that the modal analysis has captured the dynamic characteristics of the structure.
When looking at seismic code there is typically a requirement on the effective mass captured - typically around 90% when carrying out RSA. Equally any mode contributing more that one or two percent effective mass can generally be considered significant.
Modelling implications
In a MASIL analysis there is a structure mass matrix and structure stiffness matrix, just as with the conventional modal analysis.
A MASIL analysis is set up using the Analysis Wizard from the Analysis > New analysis task menu command. Select the MASIL analysis option. The next page allows the user to specify the task name (a name to associate with this modal analysis), the target effective mass in the x, y ands z directions vectors, a limit to the maximum number of nodes and an option frequency cutoff. If the required target cannot be reach in the specified maximum number of modes the solver give a warning that the solution has completed but that the target has not been reached.
Results of MASIL analysis
The results for a MASIL analysis are identical to those for a modal analysis.
Details of results available and how they can be viewed are in the Results display options section.