Dynamic frequency analysis results

The natural frequencies, periods, mode shapes and mass participation factors calculated during the static analysis can be included in a report. They can also be viewed graphically in animated diagrams superimposed over the undeformed frame as described in "View dynamic mode shapes".


Mode shape displacements are relative only. They define the mode shape, not its magnitude. You can’t compare the displacements of different mode shapes in an attempt to determine which mode will result in the largest displacements. The scale factor for the displacements of each mode shape is unique to that mode.


The mode shapes in SPACE GASS can be reported as mass normalized or unit normalized. Mass normalization means that each mode shape is scaled or normalized to the mass matrix, resulting in a generalized mass of 1.0 for each mode. Unit normalized means that the each mode shape is scaled so that its maximum translation is 1.0. Unit normalization makes it easy for you to relate the displacement of a particular node to the maximum displacement within a mode shape. For example, a normalized displacement of 0.60 indicates that the node moves by an amount which is 60% of the maximum displacement in that particular mode shape.


Mass participation factors (MPFs), which are also calculated during a dynamic frequency analysis, represent the contribution of each mode to the overall dynamic response of the structure. Each mode has its own MPF.


The total MPF for each direction is a reliable indicator of how well the modes you have analysed represents the overall dynamic response of the structure. If all possible modes have been analysed then the sum of the MPF’s (the total MPF) will be 100%, however if the total MPF is 80% for example then this indicates that other significant modes exist that haven't been included in your analysis. If you wish to increase the total MPF then you should repeat the dynamic frequency analysis with a larger number of "Dynamic modes" requested.


A total MPF that exceeds 100% indicates that the mode shapes from the dynamic frequency analysis are not accurate enough. If this happens, you should repeat the dynamic frequency analysis using a smaller tolerance.


If you wish to use the dynamic frequency analysis results to perform an earthquake analysis, refer to "Dynamic spectral response analysis".