# Analysis Wizard : Modal Dynamic
For both a 'modal dynamic' and 'modal dynamic p-delta' analysis the user is brought to this page.
# Definition
Number of Modes & Start Mode
Specify the number of modes required from the eigenanalysis. Occasionally it is only higher modes that are of interest. The start mode allows the lower modes to be excluded.
Maximum No. Iterations
All eigensolvers are iterative. This is used to set up the maximum number of iterations that will be allowed. It is rare that this value will need to be increased. The maximum number of iterations can also be set in the advanced analysis settings dialog.
Additional Restraint
For a large class of structure the horizontal and vertical modes are essentially uncoupled. In these situations it may be better to do separate modal analysis for horizontal and vertical modes. The unwanted degrees of freedom can be suppressed using the additional restraints; for example, setting ‘horizontal’ additional restraint will produce only vertical modes.
Frequency Shift
The frequency shift specifies the frequency around which the modes are calculated. For example, a frequency shift of 20 will find the frequencies closest to 20 Hz. The frequency shift currently only works with the subspace iteration eigensolver. If a non-zero shift is added, this will automatically select subspace iteration. See Advanced Solver Settings > Eigensolution for how to change the eigensolver.
Frequency Cut-offs
This option allows for frequencies outside the range of interest to be discarded. These natural frequencies will still be calculated but the results are discarded rather than stored. Note that these are absolute values and not relative to the frequency shift.
Effective Mass Ratio for Filtering Out Modes
There are often a significant number of modes which contribute little to the overall mass participation. This option allows these modes to be filtered from the results on the basis of their effective mass contribution. For each mode the magnitude of the effective mass is compared with the structure mass. Results for the modes where this contribution is below the cutoff get discarded. For example, setting the filter to 0.02 will filter out any mode whose effective mass contribution is less than 2% of the total mass.
Mass Option
This specifies how the mass of the elements is to be included in a dynamic analysis, the following three options are available:
- Lumped mass at nodes - in this option, the total element mass is assigned to the nodes of the element and form the element mass matrix, the proportion of the total mass at each node depends on the length (1D element), area (2D element) or volume (3D element) covered by the nodes for 1D, 2D and 3D elements respectively, e.g. the two nodes of a beam element will each have half of the total mass of the element, the 4 nodes of a rectangular Quad4 element will each have 1/4 of the total mass of the element. The element mass matrix from this option is a diagonal matrix, this option should be used in most cases unless local vibration modes of the elements are interested.
- Calculate mass from the element shape function - in this option, the element mass matrix, called consistent mass matrix, will be calculated using the same shape function for element stiffness matrix calculation. The principle of this method is that the work done by the inertia forces of the distributed mass within the element is the same as the work done by the inertia forces of the calculated nodal masses are equal. The element mass matrix from this option is not diagonal. This option is normally used if element local vibration modes are interested
- Ignore element mass except for nodal mass - if this option is selected, the mass of all elements except explicitly defined nodal masses will be ignored in the dynamic analysis. The nodal masses will be always considered in modal dynamic analysis
A scale factor can be applied to modify the element masses. This is typically used to increase the mass to allow for connections.
Mass Derived from Loads
In addition to the mass of the elements, extra mass can be inferred from loads (Case definition) applied in a particular Direction, with a Scale factor if required. This is used where “dead loads” have been applied to the structure which represent mass in the real structure. Only externally applied forces are included; the effect of gravity (i.e. self weight) is taken into account by the mass option.
P-delta Case
This option is only present if a “modal dynamic p-delta” analysis has been requested.
This is the basis on which the geometric stiffness is calculated. This can either be a load description (e.g. 1.2L1 + 1.2L2) or an analysis or combination case (e.g. A3 or C4). When a load description is specified, GSA first analyses the structure under this load and generates a geometric stiffness from the resulting forces/moments. When an analysis or combination case is specified the geometric stiffness is constructed from the results in this analysis or combination case. Combination cases can only be simple combination cases (i.e. of the form
Calculate modal (material) damping
If a damping value has been specified in materials used by elements in the analysis, this will calculate the resulting damping effect due to the various materials. Note that if only a single material is used in the model then the modal damping will be equal to the material damping.
# Advanced options
For advanced options on dynamic analysis see